NBS SPECIAL PUBLICATION 250

1980 EDITION

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Calibration and

Related Measurement Services

of the National Bureau of Standards

L
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ot Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities' — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national,

problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering- — Mechanical

Engineering and Process Technology' — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. ^Some divisions within the center are located at Boulder, CO 80303.
Calibration and Related Measurement Services

of the National Bureau of Standards

1980 EDITION

L. J. Kieffer, Editor

Office of Measurement Services

National Bureau of Standards

Washington, DC 20234

Supersedes NBS Special Publication 250 - 1978 Edition

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1981
Library of Congress Catalog Card Number: 63-60099

National Bureau of Standards Special Publication 250 - 1980 Edition

Nat. Bur. Stand. (U.S.), Spec. Publ. 250 - 1980 ed., 105 pages (Mar. 1981) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $4.50

(Add 25 percent for other than U.S. mailing)

d U.S. GOVERNMENT PRINTING OFFICE : 1981 O-340-997 (1547)
FOREWORD

NBS Special Publication 250 provides detailed descriptions of the currently

available NBS calibration services, measurement assurance programs, and other

measurement services. This revised edition reflects the services available as of the second

quarter of 1980 and reflects a number of important changes since the 1978 Edition of

SP250 was published. Future editions of SP250 will be published periodically as NBS

services change.

An Appendix to SP250 is published every 6 months (June and December) that lists current prices for the services described in this publication and the NBS points of contact

(addresses and phone numbers) from whom additional information can be obtained. NBS

will notify users of SP250 of changes in services or proposed changes in services by means

of announcements in the Appendix. If you are not already on our mailing list for SP250

Appendices and wish to be, fill in and return the card at the back of this document.

Many users of this publication find it convenient to keep SP250 and the current

price list (Appendix) in a three-ring binder. It is important that you refer to the current

issue of the Appendix in order to have up-to-date information with respect to NBS points

of contact. We call your attention to the availability of a variety of new Measurement

Assurance Program (MAP) services. These are carefully designed quality control

programs for critical measurements that allow the user to achieve a high level of

confidence that the measurements being made in the user's laboratory are consistent with

national standards and adequate for their intended use. As noted in the text, Measurement

Assurance Program services are available for some traditional units (e.g., mass) and for some units associated with new technologies (e.g., laser power and energy).

The Office of Measurement Services welcomes suggestions on how this publication can be made more useful to those who rely on NBS measurement services. Suggestions

are also welcome concerning needs for new calibration services, measurement assurance programs, or other measurement services. BRIAN C. BELANGER, Chief

Office of Measurement Services

III
ABSTRACT

This publication provides detailed descriptions of the currently available NBS

calibration services, measurement assurance programs, and other measurement services. In

addition, each section describing specific services contains references to additional

publications giving even more detail about the measurement techniques and procedures

used. This revised edition reflects the services available as of the second quarter of 1980. NBS Special Publication 250 was last issued in 1978. The Appendix to SP250 is reissued

every 6 months (June and December). It hsts current prices for the services described in

this publication and the NBS points of contact (addresses and phone numbers) from whom

additional information can be obtained.

Key words: calibration; measurement assurance; measurement services; standards;

traceability.

IV
CONTENTS

Page

Foreword HI

Abstract IV

Chapter I

I. General Information 1 A. Introduction 1 B. How to Use This Publication 2 C. Request Procedure 3 D. Shipping, Insurance, and Risk of Loss 4

E. Priorities and Time of Completion 5 F. Use of NBS Reports 5 G. Units 5 H. Measurement Assurance Programs 6

I. Traceability 7

J. Policy on Fees for Services 7 K. Legislative Authority 8

Chapter II

II. Mass and Dimensional Metrology 9

A. Mass, Volume, and Density 9

B. Length 12 C. Dimensional Metrology 15 D. Flatness, Roundness, and Angular Measurements 17 E. Surface Texture 18 F. Office of Weights and Measures 19

Chapter III

III. Mechanics and Acoustics 21 A. Force 21 B. Acoustic Measurements 22

C. Vibration Measurements 24

D. Structural Engineering—High Capacity Testing Machine 25

E. Fluid Flow 25

F. Hydraulics 26

G. Aerodynamics 27

Chapter IV

IV. Electrical Measurements—DC and Low Frequency 29

A. Resistance Measurements 31 B. Precision Apparatus 35

C. Impedance Measurements 36

D. Voltage Measurements 39

E. Electrical Instruments (ac-dc) 42

F. Instrument Transformers and Comparators 44

G. High Voltage and Energy Measurements 45

V
Chapter V

Page

V. Electromagnetic Measurements at Radio, Microwave, Millimeter

Wave and Laser Frequencies 49

A. Introduction 49

B. Attenuation Measurements 50

C. Electromagnetic Fields and Microwave Antenna Measurements 52

D. Impedance and/or Reflection Coefficient.... 53

E. Laser Power and Energy 57

F. Noise Temperature Measurements 58

G. Phase Shift 59

H. Power Measurements 60

I. Voltage Measurements 63

J. Baseband Pulse Parameters 65

K. Electromagnetic Interference Measurements 67

Chapter VI

VI. Time and Frequency 69

A. NBS Frequency Standard 69

B. NBS Time and Frequency Dissemination Services 69

C. Direct Signal Source Calibration Services 70

Chapter VII

VII. Thermodynamic Quantities 71 A. Thermometry 71 B. Pressure and Vacuum Measurements 74

C. Humidity Measurements 76

D. Cryogenic Measurements , 77

Chapter VIII

VIII. Optical Measurements 79

A. Radiometry and Photometry 79

B. Spectrophotometric Standards 81 C. Image Optics and Photography 82

Chapter IX

IX. Ionizing Radiation 85

A. Neutron Sources and Dosimetry 85

B. Radioactivity 87

C. Dosimetry of X-Rays, Gamma-Rays, and Electrons 88

D. Radiochromic Dosimetry (High-Dose Gamma-Ray Calibrations) 90

Chapter X

X. Computer Science—Magnetic Disk Calibration 91

VI
Chapter XI *

XI. Miscellaneous Services 93

A. Standard Reference Materials 93

B. Proficiency Sample Programs 95

C. National Voluntary Laboratory Accreditation Program 96

D. Standards Information Services 96

E. Standard Reference Data 97

F. Technical Information and Publications 97

G. NBS Measurement Seminars 97

Index 99

VII
i
Calibration and Related Measurement Services of

the National Bureau of Standards

CHAPTER I

I. General Information

A. Introduction

A critical portion of the mission of the National Bureau of Standards, a major

technical arm of the Department of Commerce, is to provide the basis for a complete and

consistent national system of physical measurements. The Bureau accomplishes this in a

variety of ways, the calibration of instruments and devices being one of the most familiar. This publication describes the measurement services that NBS provides to industry, other

government agencies (federal, state, and local), and the general public, and explains how

to obtain these services. Although it has the legislative authority, NBS does not routinely

provide services to foreign organizations or individuals. The legislative authority for these

services is described in section K.

The calibration of standards and instruments is a widely used method of providing

tie points to national standards. However, a number of other kinds of services relating to physical measurements are available, including:

(1) The broadcast of time and frequency signals

(2) Technical reports, monographs, and other publications

(3) Precision Measurement Seminars, talks, and other training aids

(4) Consultation and advisory services

(5) Use of NBS facilities in special cases*

These services are discussed in the main body of the text of this publication.

The Bureau also provides services in the following areas not covered in depth by

this publication (see ch. XI for further details):

Standard Reference Materials

Standard Reference Data

Interlaboratory Testing Program (for engineering materials)

Product Standards (Specifications) Information

Engineering Tests Laboratory Accreditation

*Opp>ortunity is afforded for collaborative work in the NBS facilities primarily through two programs. The

Research Associate Program is a plan which enables scientists and engineers from industrial, professional, trade, and other organizations to work for specified periods (usually 1-2 years) on a full time basis under the sponsorship of their employers. Participants perform non-proprietary research compatible with NBS interests and activities. For further information, contact:

Industrial Liaison Officer Room A402, Admin. Bldg. National Bureau of Standards Washington, DC 20234 A similar program, the Guest Worker Program, permits collaboration on an individual basis not necessarily

involving organizational sponsorship. As with the Research Associate program, the individual has the use of NBS facilities to pursue work benefiting NBS objectives. The Guest Worker must waive compensation and government liability for injury, death, or personal property damage.

1
Those who must make precise measurements consistent with national standards have

available a wide variety of NBS measurement services from which to choose. NBS staff will provide assistance to users regarding the appropriateness of particular NBS services

for individual measurement problems.

The Headquarters of the National Bureau of Standards is located in Gaithersburg,

MD, approximately 25 miles northwest of Washington, DC. In addition to the large

office/laboratory complex in Gaithersburg, the Bureau maintains a major facility at Boulder, CO, which houses the offices and laboratories for electromagnetics, cryogenics,

time and frequency, and quantum physics. Certain calibrations are performed in Gaithersburg while others are performed in Boulder. Accordingly, it is necessary to determine at which location the particular calibration desired is available, so that your

instrument or device can be shipped to the proper location. In the Appendix to this publication it is clearly indicated whether a particular calibration is performed in Boulder

or in Gaithersburg.

SP 250 is available at the following places:

Superintendent of Documents, Government Printing Office, Washington, DC

20402.

Office of Measurement Services, National Bureau of Standards, Washington, DC

20234.

Field offices of the Office of Field Services, Department of Commerce.

Program Information Office, National Bureau of Standards, Boulder, CO 80303.

Federal Depository Libraries.

The Appendix listing current services and fees is issued twice yearly (June and

December) and is available free from

Office of Measurement Services

National Bureau of Standards

Washington, DC 20234

Program Information Office

National Bureau of Standards

Boulder, CO 80303

B. How to Use This Publication

The chapters in this publication describing the calibration services are grouped

according to technical disciplines, which to some extent parallel the organizational

structure of the Bureau. An index is provided to assist in locating particular services. The following four steps provide a checklist for obtaining a calibration service from

NBS:

STEP 1: Determine whether or not an NBS calibration or measurement service is the

best solution to your measurement problem. If necessary, contact the appropriate person

in NBS to discuss your particular needs. (Refer to the Appendix for NBS points of

contact.) For some measurement problems, the use of a Standard Reference Material,

Standard Reference Data or other Bureau services may be the best solution. Upon request

NBS will provide detailed information on the services described in chapter XI. Private metrology laboratories offering services to the general public can be found

throughout the United States It is not uncommon to find laboratories which, in certain areas of specialty, are capable of making measurements on a par with those made at NBS.

Generally speaking, NBS' services are provided for special calibrations which are not

readily available elsewhere and for calibrations which require direct reference to national

standards. Accordingly, employing the services of a reputable private calibration

laboratory may be a perfectly adequate and cost effective solution to many commonly

2
encountered measurement problems. Two national organizations that can provide

information regarding names and addresses of private calibration and test laboratories are:

National Conference of Standards Laboratories

c/o NCSL Secretariat

National Bureau of Standards

Boulder, CO 80303

(303) 497-3787

(Note: NCSL Directories are also available from the Office of

Measurement Services, NBS-Gaithersburg.)

American Council of Independent Laboratories, Inc. 1725 K Street NW.

Washington, DC 20006

(202) 659-3766

STEP 2: Locate the description of the service in this publication. Having determined that an NBS calibration is warranted, the next step is to use the index in this book to find the

description of the particular service of interest. If specific technical questions are not answered by the appropriate section of this book, call the telephone number listed for that service for further information or write to the NBS technical point of contact indicated in the Appendix.

Many calibrations are listed in the fee schedule (Appendix) as being performed "At

Cost." At-cost calibrations are those for which the required amount of NBS labor,

materials, etc., is so variable as to make it impossible to state a fixed price. Further

information on the factors which influence the actual cost of such calibrations can be

obtained by telephoning the technical point of contact listed. Often a cost estimate or a range of cost can be provided over the telephone if the NBS staff member is given a description of the particular device and the desired calibration. STEP 3: To request a calibration from NBS, read carefully the material in section C. The

purchase order should be sent to either Boulder or Gaithersburg depending on the

location at which the calibration is to be performed. Many users of NBS services find it convenient to schedule in advance the date of calibration by phone or letter so that the

instrument or device need not be shipped to NBS until the time of its scheduled

calibration approaches.

STEP 4: Prepare the instrument/device for shipping to NBS and ship. Follow the

instructions in section D. Note that the fee for calibration does not include shipping to or from NBS.

C. Request Procedure

A formal purchase order for the calibration or test should be sent before or at the

time the instrument or standard is shipped. This should provide clear identification of the

apparatus being submitted and give separate instructions for return shipment, mailing of

report, and billing. To minimize the time during which equipment is out of service,

usually one can arrange to delay shipment until the test is scheduled to start. Requests

from Federal agencies, or from State agencies, for calibrations or tests on material to be

used on private or Federal contract work, should be accompanied either by purchase

order or by letter or document authorizing the cost of the work to be billed to the agency.

The Bureau's acceptance of purchase orders does not imply acceptance of any provisions

set forth in the order contrary to the policy, practice, or regulations of the National

Bureau of Standards or the U.S. Government. The purchase order should clearly state special or necessary conditions of test where appropriate (i.e., operating frequency,

temperature, etc.).

3
Requests for measurement services should be directed to the address given in the

appropriate section of the Appendix (Fees for Services), except that requests from foreign sources should be sent to the Office of Measurement Services (see section A).

D. Shipping, Insurance, and Risk of Loss

Shipment of apparatus to NBS for calibration or other tests should be made only

after the customer has accepted the estimate of cost and the tentative scheduling. Repairs

and adjustments on apparatus submitted should be attended to by the owner since NBS

will not undertake them except by special arrangement. Apparatus not in good condition

will not be calibrated. If defects are found after calibration has begun, the effort may be

terminated, a report issued summarizing such information as has been found, and a fee charged in accordance with the amount of work done.

The customer should pack apparatus sent to NBS so as to minimize the likelihood of

damage in shipment and handling. In every case, the sender should consider the nature of

the apparatus, pack it accordingly, and clearly label shipments containing fragile

instruments or materials such as glass. Care should be taken in selecting the best mode of

transportation.

To minimize damage during shipment resulting from inadequate packing, the use of

strong reusable containers is recommended. As an aid in preventing loss of such

containers, the customer's name should be legibly and permanently marked on the outside. In order to prolong the container's use, the notation REUSABLE CONTAINER, DO

NOT DESTROY should be marked on the outside.

Shipping and insurance coverage instructions should be clearly and legibly shown

on the purchase order for the calibration or test. The customer must pay shipping charges to and from NBS; shipments from NBS will be made collect. The method of return transportation should be stated and it is recommended that return shipments be insured,

since NBS will not assume liability for their loss or damage. For long-distance shipping it

is found that air freight provides an advantage in reduction of time in transit. If return shipment by parcel post is requested or is a suitable mode of transportation, shipments will be prepaid by NBS but without covering insurance. When no shipping or insurance

instructions are furnished, return shipment will be made by common carrier collect and

uninsured.

The risk of loss or damage in handling or testing of any item by NBS must be

assumed by the customer, except when it is determined by the Bureau that such loss or damage was occasioned solely by the negligence of Bureau personnel. In such cases, the owner may apply for reimbursement.

Shipments from foreign countries which are to be transported to NBS for test, must

be prearranged with a customs broker either in the country of origin or in the United

States for entry of the instrument, with or without bond as may be necessary, and prepaid

transportation to and from the ports of entry and exit. Entry in bond is required for all foreign made instruments shipped to NBS for calibration, whereas instruments made in the

United States may be entered without bond. If arrangements are made with a broker in the country of origin the broker should, in turn, have a customs broker in or near the port

of entry arrange for entry of the instrument and its transportation to the National Bureau

of Standards. Direct arrangements can be made with customs brokers located in the

Washington, DC/Baltimore, MD area. These brokers will arrange for entry of

instruments, prepaid transportation from the port of entry to the Bureau, and

transportation to the port of exit after measurements have been completed. This

recommendation does not preclude the use of customs brokers at ports such as Baltimore,

MD or New York, NY if such an arrangement is preferred by the owner of the

instrument. An alternative method of shipment which eliminates the need for the services

of a customs broker is by air freight to the National Bureau of Standards. When shipments

are made by this method the Bureau will pick up the instrument at the airport, arrange for

4
entry in bond when necessary, and after test obtain release from bond if required and

deliver the instrument to the airport for return shipment. The instrument will be returned

collect with transportation charges payable at destination. Transportation by air is much

more expensive than by ship but the difference is largely offset by customs broker's

charges. An added advantage of air transportation is the very great decrease in the time

the instruments are away from the owner's facility. The Bureau's charges for calibrations or tests will be billed separately from those of a customs broker. Prepayment of all NBS

charges is required, with respect to instruments being shipped to NBS from outside the

United States.

E. Priorities and Time of Completion

Scheduled work assignments for calibrations and other tests generally will be made

in the order in which confirmed requests are received. However, Government work may

be given priority. For the regular services, the workload is usually such that the

turnaround interval, between the date a customer's apparatus is received and the date it is prepared for return shipment, will be not more than 45 days. Some types of instruments

may require a considerably longer time, particularly if their abnormal behavior requires reruns to check reliability. Others can be calibrated and returned within 10 days. The

customer who can spare his instrument for only a short time usually can arrange by letter or phone call for shipping it to the Bureau just as his assigned starting date approaches.

Generally, the acknowledgement of the purchase order gives the expected completion

date.

F. Use of NBS Reports

Reports on calibrations or other services rendered to a customer are regarded as the property of the customer. Copies are not supplied to the parties except under applicable

Federal law. The results shall not be used to indicate or imply that they are applicable to other

similar items. In addition, such results must not be used to indicate or imply that NBS

approves, recommends, or endorses the manufacturer, the supplier, or the user of such devices or specimens, or that NBS in any way "guarantees" the later performance of items after calibration or test. NBS does not approve, recommend, or endorse any proprietary product or

proprietary material. No reference shall be made to the Bureau or to reports or results furnished by NBS in any advertising or sales promotion which would indicate or imply

that NBS approves, recommends, or endorses any proprietary product or proprietary

material.

G. Units

The National Bureau of Standards develops and maintains the national standards of

measurement including the U.S. National SI standards (for the base units, supplementary

units, and numerous units derived from these). The International System of UNITS (SI) was defined and given official status by the 11th General Conference of Weights and

Measures, 1960. A complete listing of SI units is presented in detail in NBS Special

Publication 330. SI is now the dominant system used throughout the world and its use in the United States is growing. The NBS practice is to express data in SI units unless this makes communication excessively complicated. For example, commercial gage

designations, commonly used items identified by nominal dimensions, or other commercial

nomenclatures or devices (such as drill sizes, or commercial standards for weights and

measures) expressed in inch-pound units are an exception from this practice. However

even in such instances, when practical and meaningful, SI and inch-pound units may be

given in parallel. Users of NBS calibration services may specify the units to be used in the

calibration, especially for commercial devices and standards using inch-pound units or

5
units having some legal definition. For additional information on NBS metric practices

and on SI see: NBS LCI 120, NBS Guidelines For Use of the Metric System

ISO—International Standard 1000, SI Units and Recommendations for Use of

Their Multiples, American National Standards Institute (ANSI), 1430 Broadway,

New York, NY 10018

E380-79—Standard for Metric Practice, American Society for Testing and

Materials, 1916 Race Street, Philadelphia, PA 19103 (Also available from ANSI

as Z 210.1 or IEEE as 268).

Reference E380-79 is particularly valuable where precise conversion from

customary or other units to SI must be made. NBS Special Publication 304A,

Measurement Systems, provides a brief history of measurement systems along with a chart

(in color) on the basis of the modernized metric system.

For information on metric units, direct inquiries to the National Bureau of

Standards, Washington, DC 20234, (301) 921-3304.

H. Measurement Assurance Programs

In recent years, NBS has offered to the public a new measurement service—the so- called Measurement Assurance Program ("MAP"). This section briefly explains the

concept.

All measurements are made for a purpose—our health, safety, the goods we produce

or use and all our technology depend on measurements. It is important that the errors of

measurement be small enough so that actions taken are only negligibly affected by these

errors. The overall objective involves the end result of the performance of measurement

systems—the quality of measurements made by systems which include instruments,

operators, environments in which they function, procedures, and special characteristics of

the objects being measured. One must always carefully distinguish system performance

from element performance—that is, the performance of an instrument, operator, or any

other single element of the system. Adequate instrument performance may be a necessary

condition for adequate system performance but obviously it cannot be a sufficient condition. It is the performance of instruments under actual practical conditions of use which must be considered in approaching the problem of adequacy of the measurement

effort.

It is essential to recognize that when the quality of measurements is at issue,

standardization must take into account all factors influencing the uncertainty of

measurements. In general, conformity to good instrument design or good calibration,

while important, is not sufficient to guarantee "good" measurement.

The uncertainty of measurements will, in general, depend on the instruments, the

environment in which they are used, the procedures used, the training of the operator, the treatment of measurement data, and other factors. It must be remembered that uncertainty

is a property of measurements—it is not a property of instruments.

Measurement assurance (like quality control in industrial production) is the means

by which one guarantees that whatever errors occur in the measurement process are kept

sufficiently small to be adequate to the need. There are several elements in a measurement

assurance program:

It has its beginning in a requirement on the limit of measurement error which

can be tolerated (e.g., radioactivity to ±1%)

There must be a reference base to which the allowable error is referred (e.g., NBS Volt, NBS Kilogram)

There must be documented technical proof of measurement uncertainty

6
° There must be some continuing redundancy in the system so that one is able to

^ "sample" the quality of the measurements being made (e.g., by remeasuring a test item, or by the use of well characterized artifacts).

This redundancy should be truly indicative of system performance. If a test item is used

for routine checking, it should have the characteristics of the regular workload and should

be run at intervals sufficiently large to permit a full range of the factors which cause random variation. The importance of continuing verification that the measurements are "in control" is obvious in re'gulatory situations where action may be taken on the basis of only a few

measurements. In any legal contest one would have to show that the system had

predictability—i.e., that the announced error limits were applicable at the time of

measurement.

NBS provides the users of these services the means by which the offset of their measurement process relative to the national reference unit can be determined. Usually

this involves on-site measurements of an NBS transport standard or suitable artifact so that

all elements of the users process are involved (not just the standard as is the case when the

instrument is shipped to NBS for calibration).

All who measure need to know that the errors of their measurement relative to the

accepted reference are less than the allowable limit. The procedures by which this is achieved constitute a measurement assurance program whether it be the result of

extensive redundancy or from performance data from produced items or similar measures

of operational success. Currently available NBS Measurement Assurance Program services are described in the main body of the text of this publication.

I. Traceability

Traceability is a term which a number of contracting and regulatory agencies have

invoked to specify the standards used in the calibration of instruments.

Perhaps the first and most far-reaching traceability requirement has been that of the

Dept. of Defense MIL-STD-45662, "Calibration Systems Requirements," which states

(sec. 5.7.1): "Measuring and test equipment shall be calibrated by the contractor or a commercial facility utilizing standards whose calibration is certified as being traceable to the National Bureau of Standards, has been derived from accepted values of natural

physical constants, or has been derived by the ratio type of self-calibration techniques."

This specification defines the term "traceability" as: The ability to relate individual

measurement results to national standards or nationally accepted measurement systems

through an unbroken chain of comparisons.

NBS can provide technical advice on how to make measurements consistent with

national standards, but NBS cannot legally state what must be done to comply with a contract or regulation calling for "traceability to NBS" to which NBS is not a party. For

additional information on traceability and its implications, refer to "Traceability: An

Evolving Concept," B. C. Belanger, ASTM Standardization News, Vol. 8, No. 1, January

1980, pp. 22-28. (Reprints available from the NBS Office of Measurement Services.)

J. Policy on Fees for Services

The basic congressional directive concerning charges to the public for services is stated in 31 U.S.C. 483a, reading in part:

"It is the sense of the Congress that any work, service publication, report,

document, benefit, privilege, authority, use franchise, license, permit, certificate,

7
registration, or similar thing of value or utility performed, furnished, provided, granted,

prepared, or issued by any Federal agency...to or for any person (including groups,

associations, organizations, partnerships, corporations, or businesses).. .shall be self- sustaining to the full extent possible..."

Services for which charges are to be made are those which provide special benefits to individuals and small segments of the public above and beyond those which accrue to the

general public. Special benefits obviously accrue in the case of calibration services. A number of guidelines have been set up in Department of Commerce

Administrative Order 203-5, dated August 20, 1973, as amended including the following:

° Department of Commerce policy is to recover full cost of performing a special

service, not the value to the recipient.

° However, charges or fees should not be established if they would seriously

impair the objectives of programs or are against public policy.

° Department policy is to refuse to furnish special services to individuals,

groups, or companies when the service would be considered in competition with

commercial enterprise.

Cost elements for inclusion in charges to the public should include but not be limited

to:

1. Direct costs, such as labor, accrued leave factor, personnel benefits, supplies,

etc.

2. Direct and indirect supervisory costs

3. Overheads, including supporting services, depreciation on equipment,

maintenance, etc. Calculations of fees for NBS calibrations are based on the above guidelines.

K. Legislative Authority

Part 200, Title 15, of the Code of Federal Regulations, states the legislative authority for and describes the measurement services NBS provides to the public. That issuance in the

Federal Register of August 19, 1980 pursuant to the authority contained in 15 U.S.C. 275a

and 277, incorporates by reference the full text of NBS Special Publication 250 (SP 250),

"Calibration and Related Measurement Services of the National Bureau of Standards."

The recognition of the need for a consistent measurement system led to the

formation of the National Bureau of Standards and in particular to the responsibility (15

U.S.C. 272) for "The custody, maintenance, and development of the national standards of

measurement, and the provision of means and methods for making measurements

consistent with those standards, including the comparison of standards used in scientific investigations, engineering, manufacturing, commerce, and educational institutions with

the standards adopted or recognized by the Government."

Section 2 of the NBS Organic Act (15 U.S.C. 272) specifically authorizes activities related to calibration and measurement to be undertaken when the need arises in the

operation of Government agencies, scientific institutions and industrial enterprises such as the testing, calibration, and certification of standards and standard measuring apparatus;

the study and improvement of instruments and methods of measurements; the compilation

and publication of general scientific and technical data resulting from the performance of

the functions specified in the Organic Act or from other sources when such data are of

importance to scientific or manufacturing interests or to the general public, and are not

available elsewhere.

8
CHAPTER II

II. Mass and Dimensional Metrology

The National Bureau of Standards maintains the national standards for mass in the

form of the prototype kilograms (K4 and K20) and has established a number of

realizations of the length unit relative to the international definition of length in terms of

the wavelength of radiation from a Krypton source. This section describes the services

available to those needing tie points to the measurement system maintained by the Bureau.

A. Mass, Volume, and Density

The Bureau provides services to support the segments of the national measurement

system which rely directly or indirectly on mass measurements. These services include the

calibration of reference standards of mass, of glass and metal volumetric reference

standards, and of reference standard hydrometers, also the density determinations of solids and liquids, and are offered only if suitable service is not available otherwise. In order to provide prompt and useful service, the acceptance of the items for calibration or test is based on discussions with each user to determine details necessary to meet measurement

and delivery requirements, and on inspection of the item at the Bureau with reference to

its suitability for the usage intended. The section in the Appendix entitled "Mass, Volume,

and Density" lists the most common services. The services are not limited to these specific

items and inquiries are invited concerning other measurement problems in the above areas. Services are available to enable a user to establish a measurement assurance program

for certain measurement processes. This may involve developing procedures for establishing and maintaining a state of statistical control for the measurements, the

determination of the offset of the process from the national system, and assisting in the

determination of the uncertainty of measurements made by the user's process. The

philosophy behind this service is given in:

References

[1] Measurement philosophy of the pilot program for mass calibration, P. E. Pontius, Nat.

Bur. Stand. (U.S.), Tech. Note 288, 41 pages (May 1966).

[2] Notes on the fundamentals of measurement and measurement as a production process, P. E. Pontius, NBSIR 74-545, 65 pages (Sept. 1974). Order from NTIS as COM 74- 11656.

Arrangements for Calibration or Test

Arrangements for calibration (or test) must be completed before shipping apparatus

to the Bureau. While all of the services related to mass, volume, and density are on an

actual cost basis, subject to a $25 minimum charge, a mutual agreement on the work to be

performed generally results in substantial savings for the user. Detailed packing and

shipping instructions are available on request. Items not accepted for calibration or test will be returned, the cost of inspection or the minimum charge will be applicable.

The results of a calibration or test will be reported either in a National Bureau of

Standards Report of Test (which in many cases is prepared by a computer program), a continuation report, or a letter report. In each of these, the values reported are accompanied by an appropriate estimate of uncertainty (allowance for random and

systematic errors) as determined by an analysis of the specific measurement process. A

9
continuation report is used for those items submitted for recalibration on which

preliminary tests indicate that no significant changes have occurred since the last calibration. Usually a letter report is used to report a test for compliance with a

specification which states limits for the departure of the actual value from nominal.

Charges for these services are listed "At Cost." Upon receipt of a request for

services, an estimated cost will be given along with a firm date for completion. An effort will be made to discuss the measurement requirement with the customer so as to give

proper service at minimum cost and delay.

1. Reference Standards of Mass

The Bureau's calibration of reference standards of mass provides extensions of the mass unit embodied in the NBS standards of mass. A normal calibration consists of

establishing a mass value and the appropriate uncertainty for that value for each weight

which has been designated to be a reference standard. It is desirable, but not necessary,

that a weight meet the adjustment tolerances established for Classes A, B, M, or S-1 prior

to submission. Normally weights are available from manufacturers, many of whom can

furnish directly documentation suitable for meeting quality assurance contracts and

requirements.

Individual weights or sets of weights in the range of 30 kg to 1 mg or 50 lb to 1 /tilb in decimal subdivisions, which are designated as reference standards, must be of design,

material, and surface finish comparable to but not necessarily limited to present Classes A,

B, M, S, or S-1. Design, material, and surface finish of large mass standards (over 50 to 50,000 lb) must be compatible with the intended usage. For these large mass standards an

adjustment with reference to a nominal or desired value can be included as a part of the

calibration procedure.

The values of true mass (and an apparent mass correction) included in the report

will be determined by using computed volumes based on the manufacturer's statement of

density of the material, on the density computed from measured volumes, or, in the

absence of this information, on estimated density values. The apparent mass corrections

are computed for 20 °C with reference to Normal Brass (density 8.4 g/cm^ at 0 °C, volume

coefficient of expansion 0.000054/°C) in an ideal air density of 1.2 mg/cm\ Apparent mass

corrections to any other basis can be furnished if requested.

For periodic recalibrations of reference mass standards, the user need measure only

differences between weights or groups of weights within a set and compare them with

computed differences. As long as the agreement is within allowable limits, the values can

be considered constant within the precision of the comparison process. Mass standards

which are submitted to the Bureau for recalibration frequently are tested in this manner. If these tests indicate that no significant changes have occurred, a continuation report so

stating and referring to the previous NBS Report of Calibration will be issued.

References

[1] Mass and mass values, P. E. Pontius, Nat. Bur. Stand. (U.S.), Monogr. 133, 39 pages

(Jan. 1974).

[2] Realistic uncertainties and the mass measurement process, P. E. Pontius and J. M.

Cameron, Nat. Bur. Stand. (U.S.), Monogr 103, 18 pages (Aug. 1967).

[3] Designs for the calibration of standards of mass, J. M. Cameron, M. C. Croarkin, and

R. C. Raybold, Nat. Bur. Stand. (U.S.), Tech. Note 952, 64 pages (June 1977).

2. Reference Standards of Volumetric Apparatus

The procedure used almost universally for testing glass volumetric apparatus is to weigh the amount of distilled water contained or delivered with reference to the

10
graduations marked on the instrument, the volume being computed from the density of

the water (for tables see NBS Circular 19, available on request). The quality of the

markings and the care exercised in reading or setting the liquid level are major factors in

test calibration and usage. Normally the Bureau will accept instruments for calibration

which have volumes in the range 1 ml to 1 gal and which conform essentially to requirements contained in NBS Circular 602, "Testing of Glass Volumetric Apparatus,"

Federal Procurement Specifications NNN-B-(X)789 (Buret, straight, precision), NNN-F00289a (Pipet, volumetric), NNN-P-00350a (Pipet, measuring), or NNN-F-00289a (Flask,

volumetric), if such instruments are to be used as reference or transfer standards.

The following comments relate to commonly used volumetric apparatus:

° Groups of blood pipets which are used as factor standards for quality control

will be accepted for test. Values will be furnished in a letter report.

° Suggested test procedures for the verification of the compliance of precision

grade glassware with specifications and tolerances are available on request.

° The usual calibration procedure for metal volumetric apparatus consists of

determining the value "to contain" or "to deliver" by either gravimetric means

or by the use of transfer standards. Normally the Bureau will accept instruments

for calibration which have values in the range 1 gill to 1000 gal, which comply

essentially with the specifications contained in NBS Monograph 62, "Testing of

Volumetric Standards," and which are free from dents, bumps, or scratches.

° While it is preferred that the zero index or the gage scale be adjusted and

scaled prior to calibration, these operations can be incorporated into the

calibration procedure if requested. Slicker plate type standards should be

adjusted by the manufacturer.

References

[1] Procedures for the calibration of volumetric test measures, J. F. Houser, NBSIR 73- 287, 24 pages (Aug. 1973). Order from NTIS as COM 73-11928.

[2] Calibration of small volumetric laboratory glassware, J. Lembeck, NBSIR 74-461, 34

pages (Oct. 1974). Order from NTIS as PB246623.

[3] The equivalence of gravimetric and volumetric test measure calibration, R. M.

Schoonover, NBSIR 74-454, 16 pages (Feb. 1974). Order from NTIS as COM 74- 10988.

3. Density Determinations ofLiquids and Solids

The usual procedure for determining the density of solids is hydrostatic weighing.

At the Bureau liquid densities usually are determined by gravimetric methods. Other

methods are available depending on the requirements. The Bureau will accept requests for density determinations if the need is critical, as in the support of scientific studies or standard sample programs. Limitations on the mass, physical dimensions, or volume of the

sample are available on request.

Specific gravity hydrometers covering the range 0.62 to 3 and proof spirit

hydrometers in the range 0 to 200 proof, which are designed reference standard

hydrometers (used to test other hydrometers), normally are accepted for calibration. A

limited number of other types of reference standard hydrometers will be accepted for multipoint calibration subject to a discussion of detailed requests. Instruments accepted

must comply essentially with the requirements of NBS Circular 555, "Testing of

Hydrometers."

11
References

[1] A density scale based on solid objects, H. A. Bowman, R. M. Schoonover, and C. L.

Carroll, /. Res. Nat. Bur. Stand. (U.S.), 78A (Phys. and Chem.), No. 1, 13-40 (Jan.-

Feb. 1974).

[2] Reevaluation of the densities of the four NBS silicon crystal standards, H. A. Bowman,

R. M. Schoonover, and C. L. Carroll, NBSIR 75-768, 36 pages (Aug. 1975). Order

from NTIS as PB247943.

[3] The utilization of solid objects as reference standards in density measurements, H. A.

Bowman, R. M. Schoonover, and C. L. Carroll, Metrologia 10, 117-121 (1974).

[4] Liquid level instrumentation in volume calibration, R. M. Schoonover, H. H. Ku, J. Whetstone, and J. F. Houser, NBSIR 75-900, 37 pages (Oct. 1975). Order from

NTIS as PB250859.

B. Length

Length measurement services offered by NBS are intended to serve several

purposes: (1) to disseminate the length unit to the industrial and scientific community, (2) to offer guidance in establishing measurement assurance programs in length, (3) to provide

assistance where there are special problems in measurement techniques, or where extreme

accuracy requirements exist, and (4) to resolve disagreements between parties involving

length measurements.

Arrangements for Calibration or Test

Charges for services in length are listed "At Cost" in the Appendix. Upon receipt of

a request for services an estimated cost will be given along with a firm date for completion. An effort will be made to discuss the measurement requirement with the

customer so as to give proper service at minimum cost and delay.

1. Gage Blocks

All gage blocks submitted for test should be in substantially new block condition

and each block should be marked with an identification number.

In shipping gage blocks extreme care should be taken both against corrosion and

damage by contact with other gage blocks. All defining steel surfaces should be greased

and the blocks padded with waxed paper or volatile rust inhibitor treated paper. A

greased steel surface coming in contact with newspaper, wrapping paper (unwaxed), or

excelsior is very likely to corrode. Sets of gage blocks should have packing inside the case and the case should be bound shut as the clasps open or break frequently during shipment.

References

[1] Measurement assurance for gage blocks, C. Croarkin, J. Beers, and C. Tucker, Nat.

Bur. Stand. (U.S.), Monogr. 163, 75 pages (Feb. 1979).

[2] Measurement assurance program—A case study: Length measurements. Part 1. Long

gage blocks (5 in to 20 in), P. E. Pontius, Nat. Bur. Stand. (U.S.). Monogr. 149, 75

pages (Nov. 1975).

[3] Gage block flatness and parallelism measurement, J. S. Beers and C. D. Tucker, NBSIR

73-239, 12 pages (Aug. 1973). Order from NTIS as PB273962.

[4] Intercomparison procedures for gage blocks using electromechanical comparators, J. S. Beers and C. D. Tucker, NBSIR 76-979, 23 pages (Jan. 1976). Order from NTIS as PB248992.

[5] A gage block measurement process using single wavelength interferometry, J. S. Beers, Nat. Bur. Stand. (U.S.), Monogr. 152, 34 pages (Dec. 1975).

12
[6] Preparations for gage block comparison measurements, C. D. Tucker, NBSIR 74-523, 14 pages (July 1974). Order from NTIS as COM 75-1 1 126.

2. Linear Thermal Expansion of Length Standards

A coefficient of linear thermal expansion will be determined for length standards

such as gage blocks and line scales only where the manufacturer's stated value is inadequate or unknown. Coefficients will be determined in a temperature range near 20 °C

(68 °F).

3. Gage Block Comparator Stylus Tip Radius

If gage blocks of different materials are being compared it is important to know the

stylus tip radius so a penetration correction can be determined. Tips received for measurement will be examined for flaws and imperfect geometry. Only tips which are

spherical in the contact area and are free from cracks and chips will be measured because

imperfections prevent predictable performance.

[1] Contact deformation in gage block comparisons, J. S. Beers and J. E. Taylor, Nat. Bur. Stand (U.S.). Tech. Note 962, 46 pages (May 1978).

4. Line Standards ofLength

Graduated scales up to 6 m (20 ft) in length and having suitable graduations and

cross-sectional shapes will be measured interferometrically. The length values will be

reported at 20 °C (68 °F).

5. Surveying and Oil Gaging Tapes

The calibration of surveying tapes and oil gaging tapes will be made with the tape

under tension and supported on a horizontal flat surface in a laboratory controlled near 20 °C (68 °F). Unless otherwise requested the total length and each 50 ft or 15 m

subinterval will be measured and reported at 20 °C (68 °F). For oil gaging tapes the height

from the tip of the plumb bob will also be reported at 60 °F. Each interval calibrated on a surveying tape will have a table of computed lengths

for two (single catenary), three, four, and five equidistant points of support. The

computed values will be based on the measured lengths and the average values for weight

per unit length and AE (cross-section area of tape times modulus of elasticity) determined

during the test and given in the report.

The following thermal expansion coefficients will be assumed unless another value is provided by the customer or the tape manufacturer:

Horizontal tension will be applied to the tapes by weights hanging from a pulley

device. For various materials and lengths of tapes the following weights will be used in the calibration:

Reference

Steel: Stainless: Invar:

0.00001 16/°C (0.00000645/°F)

0.0000106/°C (0.00000589/°F)

0.0000004/°C (0.00000022/°F)

13
Equal or less than Longer than

Material 30m 100ft 30m 100ft

steel 5 kg 10 lb 10 kg 20 lb stainless 5 kg 10 lb 10 kg 20 lb Invar 15 kg 201b 15 kg 201b

An NBS serial number will be engraved on each calibrated tape for identification.

6. Surveying Leveling Rods

Surveying leveling rods having invar graduated strips will be calibrated in a

laboratory controlled near 20 °C (68 °F). The 1, 2, and 3 meter intervals will be measured

from a zero point on the footpiece established by the intersection of the centerline of the

invar strip with the plane of the bottom of the footpiece. A thermal expansion coefficient

of 0.00000 15/°C will be assumed for the invar strip unless another value is provided by

the customer or the manufacturer. The lengths of the intervals will be reported at 20 °C

unless otherwise requested.

7. Sieves

The precision seal of the National Bureau of Standards on any sieve indicates that the sieve has been tested at the Bureau and found to conform to specification. Except by

special arrangements, the testing of sieves at the Bureau is limited to No. 3 1/2 to No. 400,

inclusive. The following Standard Reference Materials are available for use in sieve

calibrations.

Glass spheres for particle size

Size Wt/unit

SRM* Type (/xm) Sieve Nos. (g)

1003 Calibrated Glass Spheres 5-30 40-45 1004 Calibrated Glass Beads 34-120 400-140 63 1017a Calibrated Glass Beads 100-310 140-50 84 1018a Calibrated Glass Beads 225-780 60-25 74

1019 Glass Spheres 890-2590 18-8 100

*See section on Standard Reference Materials (ch. XI) for ordering procedure.

8. Haemacytometers

Each haemacytometer chamber and each cover glass which conforms to the

specification is marked with a National Bureau of Standards precision seal. In general,

work will be discontinued when a substantial number of items in a lot fail to comply with

specifications and the fee will be computed on the basis of the number of items tested plus a special handling charge.

9. Precision Circles

Graduated circles or encoder discs having suitable graduations and being not less than 50 mm (2 in) in diameter will be tested for maximum angular error in the mean of

opposite angles.

14
C. Dimensional Metrology

For controlling dimensions in the manufacture of all products, various types of

dimensional gages are used. Gages which are used for precise size control of critical dimensions are measured in our laboratories as a service to the public. The wide variety of

such gages measured includes end standards for length, cylindrical plug and ring gages

and balls for diameter, and screw thread gages. Angle gage blocks, polygons, angular

indexing tables, and other such items are calibrated for controlling angular dimensions. In addition, various instruments, measuring tools, and accessories are calibrated such as optical flats for measuring flatness, surface roughness specimens for calibrating measuring

instruments, cylinders used for the measurement of threads and gears, micrometer and

vernier calipers for measuring size, and many other specialized components. Consultation

services with respect to dimensional and angular measuring problems are available also. The length and diameter standards and gages, with the exception of thread and gear

measuring wires, are in general intended to be used as comparison masters. In addition to the items listed, high quality components requiring gage tolerance inspection of length or diameter may be calibrated as special measurements. In addition to the items listed in the

Appendix, other items such as gage block accessories and Knoop indenters are calibrated

upon special request. Sizes other than those listed in the fee schedules also are calibrated on special request.

A 3-axis measuring machine is available for calibrating 2 and 3 dimensional ball plates, glass grid plates and other devices of complex shape. The machine, built to state-of- the-art specifications, is housed in a stabilized environment and can be operated either

manually or under full computer control. It has a working volume of 48 X 24 X 12 in and

can detect a length difference of 0.0250 nm (10 fiin). Bulk disc storage of computer

programs is provided and a line printer capable of graphic presentation is available. The

machine will provide digital data in three dimensions or two dimensional graphic

projections of complex three dimensional shapes. Reference to the standard of length is insured by laser interferometry.

Reference

[1] Three dimensional metrology, R. Hocken, J. Simpson et al.. Annals of the CIRP, Vol.

26-1, 1977.

[2] Unified three-dimensional program—Two useful noncontacting probes, J. A. Simpson,

NBS Report 10597 (May 1971).

Shipping Instructions

In the shipment of gages, extreme precautions should be taken against both

corrosion and damage by contact with other gages during transit. All defining steel surfaces should be greased and protected with rust inhibiting paper or a suitable strippable

plastic coating. A greased steel surface coming in contact with newspaper, untreated

wrapping paper, or excelsior is very likely to corrode. Small gages suitably wrapped may

be fastened in place in a strong container so that no movement is possible. Plug and ring

gages ordinarily should not be shipped mated. In the case of large-size threaded plugs and

rings, however, mating is permissible as a means of protecting the plug threads. In such

cases a grease must be used that will prevent corrosion between the mating gages.

/. End Standards ofLength

The listed end standards, having spherical, flat, or pointed ends normally are used as instrument or machine tool setting standards. End standards up to 20 ft in length can be

calibrated. As a special test, end measuring bars with flat ends of gage block quality can

15
be calibrated with higher accuracy than listed below. Lengths are reported with a 2.5 lb measuring force unless otherwise requested.

2. Step Gage Standards ofLength

Step gages having flat parallel faces along a common center line are calibrated as special measurements. The test is made horizontally in lengths up to 40 in. Step blocks

having flat parallel adjacent faces also are calibrated as special measurements.

3. External Diameter Standards and Gages: Plug Gages

In addition to the scheduled plug and wire gages, external diameters of other

products may be accepted for measurement.

4. Measuring Wires for Threads and Gears

Thread measuring wires for 60° and 29° threads are tested for compliance with the

latest specifications in commercial use given in NBS Handbook H-28 and American

National Standards Institute, Inc. Standard B1.2. These wires, which are supplied in 3-wire sets, are calibrated and the pitch diameter correction factor is computed for the

average diameter of the 3 wires in the set. Special thread measuring wires for unusual

sizes and for threads finer than 80 tpi are measured in a manner consistent with current commercial practice as a special test. The published fee is for a single set of 3 wires. Gear

measuring wires in the l-92"/P, l-728"/P, l-44"/P, and l-68"/P series supplied in sets of 2 wires are tested for compliance with the latest specifications in commercial use and the mean diameter reported. Reference master wires for threads and gears are supplied as

single wires which are intended as transfer standards of diameter for the calibration of

working sets of thread measuring and gear measuring wires. Wires are measured using

standard measuring practice.

Reference

[1] On the measurement of thread mea^Suring wires, B. N. Norden, NBS Report 10987 (Jan.

1973).

5. Spherical Diameter Standards: Balls

Balls used in precision bearings and master balls used as transfer diameter standards

are calibrated according to current commerical practice. The ball diameters reported are the undeformed sizes as calculated with the Hertz relations to maintain a consistency in reported sizes.

6. Internal Diameter Standards: Ring Gages

In addition to ring gages, holes in precision products can be calibrated as special

measurements.

7. Plain Conical Plug and Ring Gages

Plain conical plug and ring gages are tested for angle and diameter at a specified

position.

8. Threaded Plug and Ring Gages

Threaded plug and ring gages are tested in accordance with current commercial

practice as outlined in NBS Handbook H-28 and American National Standards, Inc. Standard B1.2. Threaded plug gages can be tested for pitch diameter, major diameter,

16
flank angle, lead, and taper. Threaded ring gages must be submitted with the setting plug

gage for test and are set for fit and tested for minor diameter, flank angle, lead, and taper,

and standoff if applicable.

American Petroleum Institute (API) reference master thread gages are tested as required by the API specifications. The gages submitted should be marked with the API

monogram and the API registration number if required. If not so marked, the information

should be submitted with the gages and they will be marked. Gages which meet the

specifications will be marked as specified in the API standards. All thread gages must be

submitted in sets of plug and ring. The name of the gage owner should be given for inclusion in the report. Copies of all reports are sent to the American Petroleum Institute. API standards may be obtained from the American Petroleum Institute, 211 N. Erray,

Suite 1700, Dallas, TX 75201. Fee is for single set of plug and ring.

9. Length and Diameter Measuring Instruments

Instruments accepted for calibration include micrometer calipers, vernier calipers,

plain snap gages, micrometer screws, dial micrometers, vernier calipers, and other similar

devices.

10. Length Measuring Elements

Length measuring elements such as micrometer screws, dial micrometers, and other

length-transducing systems are included in this schedule.

IL Instrument Components Requiring Dimensional Control

Some instruments such as penetration needles, polariscope tubes, and Knoop

indenters contain elements which require dimensional control of lengths and angles.

Penetration needles and cones are tested for compliance with ASTM specifications

and supplied with individual labels so stating. All needles must have individual

identification numbers; needles received without identification will be marked for a fee.

D. Flatness, Roundness, and Angular Measurements

This section deals with the calibration of reference surfaces of flatness and

straightness such as optical flats, surface plates, and straight edges.

1. Optical Reference Planes: Flats

Optical reference planes are tested interferometrically, horizontally supported with

test surface supported on three equally spaced pads located at 0.7 of the radius from the

center. The measurement is performed along two marked diameters at 90° to each other on each surface when each diameter is parallel to two of the support pads.

References

[1] A survey of the stability of optical flats, C. P. Reeve and R. C. Veale, NBSIR 73-232,

27 pages (June 1973). Order from NTIS as PB273947.

[2] The calibration of an optical flat by interferometric comparison to a master optical flat, C. P. Reeve, NBSIR 75-975, 40 pages (Dec. 1975). Order from NTIS as PB253113.

2. Surface Plates and Straight Edges

The straightness of specified lines on surface plates and the straightness of straight

edges are calibrated by optical and mechanical techniques.

17
3. Roundness

In addition to the measurement of the deviation from roundness of round parts,

instrument cahbration standards for roundness measurement equipment can be calibrated.

4. Roundness Measurement

The deviation from roundness at eight or more specific positions around nominally

round standards is determined. The size of the calibration step or deviation from

roundness on calibration standards is determined. The departure from roundness of

components and gages is measured and the results are reported in graphical form.

5. Angular Standards

Fixed angular standards having flat defining surfaces are calibrated by

autocollimation and interferometric techniques. Angle gage blocks are calibrated using

autocollimation techniques.

The calibration of polygons is done with autocollimation techniques. It consists of

the determination of flatness of each face, variation of the angle between each face and

the base (where possible the polygon will be adjusted for minimum variation), and the

angle between faces. The solid angle calipers and step mirrors are calibrated by autocollimation or

interferometric techniques as to the angle between faces. Wedges are calibrated by

autocollimating or interferometric techniques at a specified wavelength for deviation

angle.

Mechanical angular references such as cylindrical squares and machinist's squares

usually are calibrated by mechanical techniques.

Reference

[1] A survey of the temporal stability of angle blocks, R. C. Veale and C. P. Reeve, NBSJR

74-601, 22 pages (Nov. 1974). Order from NTIS as PB273948.

6. Angular Measuring and Setting Instruments

Instruments and tools used for the precise measurement of angle, such as precision

angular rotary and indexing tables, autocollimating telescopes, or angle generating

equipment, are calibrated at specific angular settings.

Reference

[1] The calibration of indexing tables by subdivision, C. P. Reeve, NBSIR 75-750, 38 pages

(July 1975). Order from NTIS as PB249934.

E. Surface Texture

Precision surface roughness specimens and test measurements on other surface

finishes can be made in accordance with American National Standards Institute, Inc. Standard B-46.1 (1978). Since 1 July 1973, these measurements are made with an

interferometrically calibrated minicomputer/stylus instrument system rather than by

comparisons against master specimens. (See November 1978, "NBS Dimensions," p. 18 and ref. [6].)

18
/. Instrument Calibration Specimens

Instrument calibration specimens for surface roughness measuring equipment having

regular geometrical surface profiles are calibrated with the use of the interferometrically

calibrated minicomputer/stylus instrument system (see ref. [6].)

2. Surface Roughness Measurements

Roughness measurement of roughness comparison specimens or of other surfaces are measured in accordance with American National Standards Institute, Inc. Standard B46.1,

using our instrumentation.

3. Step Height Measurements

Thin film step heights are measured with the use of the minicomputer/stylus

instrument system and interferometrically calibrated reference step heights (see ref. [6].)

References

[1] Surface microtopography, R. D. Young, Phys. Today 24, No. 11, 42-49 (Nov. 1971).

[2] Precision reference specimens of surface roughness: Some characteristics of the Cali- Block, R. D. Young and F. E. Scire, J. Res. Nat. Bur. Stand. (U.S.), 76C (Eng. and

Instr.), Nos. 1 and 2, 21-23 (Jan.-June 1972).

[3] The topografiner: An instrument for measuring surface microtopography, R. Young, J. Ward, and F. Scire, Rev. Sci. Instrum. 43, No. 7, 999-1011 (July 1972).

[4] Eight techniques for the optical measurement of surface roughness, R. D. Young,

NBSIR 73-219, 38 pages (May 1973). Order from the author at National Bureau of

Standards, Washington, DC 20234.

[5] Surface finish, friction and wear; the need for more than one parameter, D. A. Swyt,

NBSIR 73-196, 28 pages (May 1973). Order from NTIS as PB273941.

[6] Evaluation, revision and application of the NBS stylus/computer system for the

measurement of surface roughness, E. C. Teague, Nat. Bur. Stand. (U.S.), Tech. Note

902, 151 pages (Apr. 1976).

[7] Evaluation, revision and application of the NBS stylus/computer system for surface

roughness measurement: Minicomputer software, E. C. Teague, NBSIR 75-924, 79

pages (Apr. 1975). Order from NTIS as PB248686.

[8] Surface Hnish measurements: An overview, E. C. Teague, Soc. Manuf. Eng. Tech. Pap.

IQ75-137, 1-21 (1975).

[9] The measurement and characterization of surface finish, R. D. Young and E. C. Teague, Chapter 2 in Properties of Electrodeposits, Their Measurement and

Significance, R. Sard, H. Leidheiser, and F. Ogburn, Eds., pp. 22-49

(Electrochemical Society, Princeton, NJ, 1975).

[10] Instrumentation for the chemical analysis of manufactured surfaces, R. D. Young,

Ann. CIRP2A, No. 2, 549-554 (Aug. 1975).

[11] Measurements of stylus radii, T. V. Vorburger, E. C. Teague, F. E. Scire, and F. W.

Rasberry, Wear 57,39-49 (1979).

F. Office of Weights and Measures

The role of the Office of Weights and Measures (OWM) is to provide leadership and

those technical resources that will assure accuracy of the quantity representatiions in all commercial transactions for all buyers and sellers in the United States, and to promote a uniform national weights and measures system.

In fulfilling its mission, OWM engages in a wide range of activities, including

providing the secretariat and other technical input for the National Conference on

19
Weights and Measures. Foremost is the assistance offered to the States in the following

areas:

(1) The development of model weights and measures laws and technical regulations

for the States and local jurisdictions.

(2) The development and dissemination of design and performance specifications for various standards of mass, length, and capacity for use as State and local reference,

laboratory, and field standards.

(3) The design of testing equipment and the development of testing procedures for weighing and measuring devices.

(4) The examination of prototype commercial weighing and measuring devices and

equipment submitted by manufacturers for conformance with Handbook 44 requirements.

(5) The calibration of State standards. State weights and measures laboratories

perform calibrations and tolerance tests of mass, volume, and length secondary standards

for industry and service agencies.

(6) The conduct of technical training in weights and measures enforcement and

laboratory metrology. OWM serves as a central resource for metric coordination for the

states and local jurisdictions, and prepares and disseminates information on standards,

testing equipment technical procedures, technical investigations, and standard practices.

/. Prototype Examination of Commercial Weighing and Measuring

Devices, Reference and Field Standards

OWM operates a Prototype Examination Program which provides for an evaluation

of (1) prototype weighing and measuring devices to determine compliance with the

requirements of NBS Handbook 44, "Specifications, Tolerances, and Other Technical

Requirements for Commercial Weighing and Measuring Devices," (2) standards to determine compliance with the requirements of NBS Handbooks 105-1, 105-2, 105-3,

"Specifications and Tolerances for Reference Standard and Field Standard Weights and

Measures." This program may be used by manufacturers and weights and measures

officials in determining the acceptability of devices for commercial use or the suitability of

reference and field standards.

Equipment will be examined at any stage of development on request. The

examination may be made in the laboratories of the National Bureau of Standards, at the

factory, or in the field. When a device is found to be in compliance with Handbook 44, or standards in compliance with H-105-1, H-105-2, or H-105-3, a report of test will be issued to the

submitter. When equipment is found not to be in compliance, the submitter will be notified

by letter and the discrepancies fully explained. The equipment may then be modified and

resubmitted. If it is the submitter's decision not to make any modifications, a report of test will be issued detailing the areas where discrepancies exist. Copies of all reports of test will be sent to each state weights and measures office. To obtain a prototype examination:

Address a letter giving a reasonably complete description of the equipment, its operating characteristics and instructions, and its intended application, model number,

capacity, size, and shipping weight, to the Office of Weights and Measures, National

Bureau of Standards, Washington, DC 20234, requesting an examination.

20
CHAPTER III

III. Mechanics and Acoustics

That broad area of the physical sciences generally prescribed by the action of forces on bodies, materials and matter is quantified through measurements based on mass, length,

and time. Length and mass are the capstones of this measurement system usually

characterized by the discipline of mechanics. In addition to the base units of mass and

length, a wide range of derived quantities and multiples or submultiples of all these units are employed by science and technology to implement the affairs of industry, commerce,

and government. Among the mechanics-related derived quantities maintained and

disseminated by NBS are force, volume flowrates, air and water velocity, acoustic

quantities, and vibration. Well-characterized instruments are compared with these

standards through calibrations and measurement assurance programs to provide

measurement traceability and to ascertain the quality of measurements in the field. Supporting research and development programs are conducted to extend the range and

quality of these types of measurement and to increase the efficiency of the measurement

system.

A. Force

A force measuring system consists of two elements, an elastic device and a means to sense the distortion, or deflection, of that device under the action of applied forces. The

calibration procedure consists of applying known forces, either tension or compression, to the elastic device and recording the sensed deformation. An analysis of the data provides

correspondence between the system indication and the magnitude of the applied force. In some cases, force can be applied to two or more systems in series or series parallel

arrangements using a hydraulic press. In this case, the correspondence for one system is established relative to the known correspondence for one or more previously calibrated

systems.

For the range of 10 Ibf to 10^ Ibf, the NBS force generators are "dead weight"

machines with fixed load increments. For loads in excess of 10*' Ibf, a large universal

testing machine is used as a press. Compression calibrations from 10*" Ibf to 12 x 10*" Ibf are

relative to calibrated load cell systems. Tension calibrations from 10^ Ibf to 6x 10* Ibf are

relative to the indicating system of the testing machine.

Subject to the conditions that the instruments submitted are sufficiently stable as to warrant the calibration effort, and that an adequate calibration service is not available

elsewhere, NBS will perform the following tests:

(1) Characterize a transducer-indicator system in any accepted force units for the

correspondence between indication and applied force, uncertainty estimates, and estimates

of other parameters as identified which may be of importance relative to the manner in which the instrument is to be used. Direct current powered transducers can be

characterized in a similar manner in which case the correspondence is expressed in a dimensionless ratio (mVA'^).

(2) Perform calibration tests as specified by accepted voluntary standards or by

companies or individuals. Test data will be processed in the manner specified or

requested. Reports, however, may include a disclaimer with regard to the validity of the

uncertainty estimate.

(3) On a time available basis, NBS will conduct tests to support developmental

work on force measurement devices and systems in excess of 1000 Ibf. Devices must be

21
proof tested and test procedures must be reviewed prior to submission of the device.

Resulting test data will be returned in a letter report, normally without further analysis.

Where intermediate parties are involved, the reports will be issued to the ultimate user. NBS will, when appropriate, discuss the calibration details with the user and reserves the

right to modify the testing procedures accordingly. All work is done on an "At Cost"

basis.

Reference

[1] A study of the National Force Measurement System, D. E. Marlowe, NBSIR 75-929, 40

pages (June 1975). Order from NTIS as PB261498.

B. Acoustic Measurements

NBS performs calibrations of special microphones and ultrasonic reference blocks

as described below.

1. Microphones

Pressure calibrations are performed on type-L microphones satisfying the

requirements of American National Standards Institute publication SI. 12. 1967 (R1977),

Specifications for Laboratory Standard Microphones. In addition, microphones must be

suitable for use with the calibrating couplers shown in figures 6 and 10 of publication

SI. 10- 1966 (R1976), Calibration of Microphones. Copies of these standards can be

purchased from the Institute at 1430 Broadway, New York, NY 10018. Pressure calibrations are reported in terms of open-circuit voltage per unit sound

pressure applied uniformly to the diaphragm. The open-circuit voltage at the electrical terminals of a microphone may be influenced by stray capacitances evident at these

terminals. These capacitances are defined by the geometrical configuration of the ground

shield shown in figures 6 and 13 of SI. 10 (R1976). If the ground shield dimensions are not adhered to in making use of the response levels reported by the Bureau, errors may result. Methods for the measurement of open-circuit voltage are described in Clause 2.1 and in figures 1 and 7 of SI. 10 (R1976).

The free-field response levels of Western Electric Company Type 640AA condenser

microphones, or equivalent, can be computed with good accuracy from the pressure response levels reported by the Bureau, using table Al of SI. 10 (R1976), provided the

microphone is mounted on a preamplifier of size and shape illustrated in figure 12 of SI. 10

(R1976). However, the use of manufacturers' response levels in conjunction with table Al

might result in errors unless the manufacturer has measured pressure response levels in accordance with SI. 10 (R1976).

Since American National Standards Institute publications SI. 10- 1966 (R1976) and

SI. 12-1967 (R1977) were issued, certain types of "half-inch" diameter precision

microphones have attained widespread use. Therefore, NBS has developed procedures for determining the pressure and free-field response of "half-inch" microphones. Since no

current standards are directly applicable, details of the ground-shield configuration,

coupler and other procedures will be incorporated in the test report.

Associated preamplifiers or power supplies should not be forwarded to the Bureau. The response levels reported are based on measurements of open-circuit voltage and are

essentially independent of the electronic equipment used in the test.

Reference

[1] Calibration of laboratory condenser microphones, V. Nedzelnitsky, E. Burnett, and W.

Penzes, Prdc. of Tenth Transducer Workshop, Transducer Committee, Telemetry

Group, Range Commanders Council, Colorado Springs, CO (June 1979).

22
2. Ultrasonic Transducer and System Calibration

Measurement services are offered for the determination of total forward radiated

ultrasonic power into a water load for transducers as an element by themselves and for ultrasonic systems. These transducers and systems are ordinarily of the type used for nondestructive testing or for medical therapeutic or diagnostic purposes. There are some

limitations on aperture size. Transducers are generally measured using a modulated radiation pressure system

which gives output power versus frequency over any part of a frequency range of about

• 1-20 MHz. Absolute power is also measured at specific frequencies to yield the

transducer's radiation conductance for continuous-wave excitation. Power levels from

microwatts up to cavitation in water can be accommodated.

Ultrasonic system time-averaged total power into a water load is measured using a

calorimeter. The power levels can be from tenths of a watt to cavitation in water and

frequencies from about 1-15 MHz. The systems can be either continuous-wave or pulsed

type since the system responds to a time average of the total power.

Some air-backed quartz transducers are also available on a loan-for-fee basis. These

transducers have accurately known total power output if the applied cw voltage input is known. They are used in providing a known source for the in-situ calibration of ultrasonic

power measurement systems.

References

[1] Ultrasonic transducer power output by modulated radiation pressure, M. Greenspan, F. R. Breckenridge, and C. E. Tschiegg, /. Acoust. Soc. Am. 63(4), (Apr. 1978).

[2] Ultrasonic transducer power output by modulated radiation pressure, (with details), M.

Greenspan, F. R. Breckenridge, and C. E. Tschiegg, NBSIR 78-1520, Report to the

Department of the Navy (Sept. 1978).

[3] Ultrasonic calorimeter for beam power measurements, T. L. Zapf, M. E. Harvey, N. T. Larsen, and R. E. Stoltenberg, Nat. Bur. Stand. (U.S.), Tech. Note 686 (Sept. 1976).

[4] Ultrasonic calorimeter for beam power measurements from 1 to 15 megahertz, T. L. Zapf, M. E. Harvey, N. T. Larsen, and R. E. Stoltenberg, 1976 Ultrasonics

Symposium Proceedings, IEEE Cat. #76 CH1120-5SU.

[5] Calibration of quartz transducers as ultrasonic power standards by an electrical method,

T. L. Zapf, 1974 Ultrasonics Symposium Proceedings, IEEE Cat. #74 CHO 896-ISU.

3. Acoustic Emission Sensors

Acoustic emission sensors are used as part of systems to passively determine or monitor the integrity of structures. The method relies on the detection of stress waves in a structure due to local changes associated with a defect. The sensors are generally used in the frequency range of from 0.1 to 1 MHz. Such sensors can presently be calibrated at NBS to obtain the amplitude and phase spectra using the surface wave technique. A pointforce step function is created as an input to a large polished steel block. A standard

transducer and a transducer under test are located on the same surface of the block as the

input. The output waveforms of these transducers are processed to determine the absolute

response of the transducer under test over the approximate range of 0.1 to 1 MHz. If the

transducer under test is to be used as a transfer device, it must have adequate sensitivity over the frequency range of interest.

References

[1] Characterization and calibration of acoustic emission sensors, N. N. Hsu and F. R. Breckenridge, to be published in Materials Evaluation.

23
[2] Surface-wave displacement: Absolute measurements using a capacitive transducer, F. R. Breckenridge and M. Greenspan, to be published in the Journal of the Acoustical

Society ofAmerica.

4. ASTM-Type Ultrasonic Reference Blocks

The ultrasonic response of 7075 aluminum alloy reference blocks is determined

relative to an NBS interim reference standard by immersion, pulse echo, longitudinal wave testing. An NBS developed procedure is used. The data is also compared to the data

base of all blocks measured by NBS.

Response following the ASTM Recommended Practice E- 127-75 can also be

determined. The response of steel reference blocks can also be obtained by special

arrangement.

References

[1] Procedures for the calibration of ASTM E127-type ultrasonic reference blocks, D. J. Chwirut, G. F. Sushinsky, and D. G. Eitzen, Nat. Bur. Stand. (U.S.), Tech. Note 924,

Final Report of the Air Force Materials Laboratory (Sept. 1976).

[2] Improved ultrasonic standard reference blocks, G. F. Sushinsky, D. G. Eitzen, D. J. Chwirut, C. J. Bechtoldt, and A. W. Ruff, NBSIR 76-984, Final Report for the Air

Force Materials Laboratory (Nov. 1976).

[3] Recent improvements to the ASTM-type ultrasonic reference block system, D. J. Chwirut, NBSIR 79-1742, Final Report for the Air Force Materials Laboratory

(Apr. 1979).

[4] A basis for traceable NDE measurements, D. G. Eitzen, H. Berger, and G. Birnbaum,

NBSIR 80-2109

C. Vibration Measurements

Calibrations of vibration exciters and accelerometers are performed by comparison

with the response characteristics of NBS standard vibration exciters or NBS standard

pickups, or by absolute displacement measurements obtained with interferometric

techniques. Details of these procedures are given in the cited references. Calibration

results are stated in tabular form with the pickup sensitivity given in terms of either

charge or voltage per "g" as a function of frequency. The customer may select a frequency range for calibration suitable to his needs from those listed in the current

edition of the "Appendix—Fees for Services." Phase angles may also be determined upon

request. Each piezoelectric acceleration pickup submitted for calibration which is used

with a cathode-follower or amplifier shall be accompanied by the cathode-follower or

amplifier and all necessary connecting cables. When instruments submitted are found to be

unsuitable for test or are unreliable, a charge will be made to cover the cost of the work

done.

References

[1] Accelerometer resonances affect vibration measurement, E. T. Pierce, O. W. Price, S. Edelman, and E. Jones, /. Environ. Sci. 10, No. 6, 17-21 (Dec. 1967).

[2] Piezoelectric shakers for wide-frequency calibration of vibration pickups, E. Jones, W.

B. Yelon, and S. Edelman, /. Acoust. Soc. Am. 45, No. 6, 1556-1559 (June 1969).

[3] An automated precision calibration system for accelerometers, B. F. Payne, Instrum.

Soc. Am. 17th National Aerospace Instrumentation Symposium (May 1971).

24
[3] A systematic study of vibration transfer standards—Mounting effects, R. S. Koyanagi,

J. D. Pollard, and J. D. Ramboz, NBSIR 73-291, 42 pages (Sept. 1973). Order from

NTIS as PB272376.

[S] Piezoelectric accelerometer low-frequency response by signal insertion methods, R. S. Koyanagi and J. Pollard, NBSIR 74-597. 33 pages (May 1975). Order from NTIS as COM 75-11069.

[6] Shock calibration of accelerometers, C. Federman, W. Walston, and J. Ramboz,

Minutes of the 8th Transducer Workshop, Telemetry Group, Inter-Range

Instrumentation Group, Range Commanders Council, Wright-Patterson AFB (Apr.

1975).

[7] Accelerometer calibration at the National Bureau of Standards, B. F. Payne, R. S. Koyanagi, C. Federman, and E. Jones, 21st Int. Instrumentation Symp. ASD/TMD,

Philadelphia, PA, May 19-21, 1975, pp. 1-17 (1975).

[8] Development of a low-firequency-vibration calibration system, R. S. Koyanagi, Exp.

Mech. 15, 443-448 (Nov. 1975).

D. Structural Engineering—High Capacity Testing Machine

The research and testing facilities for structural engineering include a 53-MN (12

million-lbO capacity universal testing machine believed to be the largest in the world. A

significant addition to the nation's facilities for research and testing in the field of large

structures, this unique machine is available to do work for the entire technological

community upon consideration of requests on a case-by-case basis. This hydraulically

operated machine is a vertical, four-screw type with the main fixed platen flush with the

floor. It is capable of applying 53 MN (12x 10* Ibf) in compression to test specimens up to 17 m (58 ft) in height and 27 MN (6x 10* Ibf) in tension to specimens up to 16 m (53 ft) in length. To extend the versatility of the machine, the reinforced concrete foundation

incorporates a floor tie-down system which can accommodate test specimens for transverse loading up to 27 m (90 ft) in length. Calibration of all load ranges indicates that they exhibit error generally no greater than 0.5 percent of the applied load. A more

detailed description of this facility is presented in NBS Special Publication 355.

E. Fluid Flow

Standards for flow measurement of fluids are maintained using water, air and a

stable hydrocarbon as test mediums. These facilities are used for extending the types of

measurement services, for research on how to characterize flow measurement systems and'

to evaluate and test transfer flow standards.

1. Fluid Quantity and Flowrate Meters

Flow meter systems, i.e., meter, pertinent adjacent tubing, and readout equipment

having precision demonstrated stability and precision commensurate with the quality of

the calibration are accepted for calibration over a wide range of flows in air, water, and

hydrocarbon fluids. Meter systems should not be sent to the Bureau until all arrangements

for the calibration have been completed. When submitted meter systems are found to be

unreliable or unsuitable, a calibration may be discontinued and a charge will be made to cover the cost of the work done.

Meter systems submitted should have connections for A/N flare fittings up to 2-in (5

cm) nominal diameters, or with connections for National Pipe Thread fittings up to 3-in (7.6 cm) nominal diameter and larger meters must terminate with ASA 150-lb steel flanges, or grooved-end steel pipe compatible with Victaulic couplings (for water meters), or with adapters thereto; for air, flanges must terminate with ASA 300-lb steel flanges

except for laminar element flowmeters operated near ambient pressure levels. Connections

25
other than these should not be submitted unless special arrangements have been made in advance.

Fees are based on the actual cost of calibration. Fees are based on calibrations of meters submitted with connections for A/N flange

fittings up to 3-in nominal diameters, or with connections for National Pipe Thread fittings

up to 3-in nominal diameter. For pipe sizes 4-in nominal diameter and larger, meters must

terminate with ASA 150-lb steel flanges, or grooved-end steel pipe compatible with

Victaulic couplings (for water meters), or with adapters thereto. Meters with connections

other than these should not be submitted unless special arrangements have been made in advance.

References

[1] Introduction to liquid flow metering and calibration of liquid flowmeters, L. O. Olsen,

Nat. Bur. Stand. (U.S.). Tech. Note 831, 60 pages (June 1974).

[2] Flow measurement: Procedures and facilities at the National Bureau of Standards, F. W. Ruegg and M. R. Shafer, (Proc. Symp. Flow Measurement, San Francisco, CA,

Jan. 19-22, 1970), Chapter in ASHRAE (Amer. Soc. Heat Refrig. Air-Cond. Eng.).

Bull. Flow Measurement Part 1, SF70-7, 1-8 (1972).

[3] Practical considerations for gas flow measurement, M. R. Shafer, Jr. and D. W. Baker,

Proc. 3d Annual Precision Measurement Association Metrology Conf.. National Bureau

of Standards. Gaithersburg. MD, June 17-18. 1970. 1, 187-227 (Precision

Measurements Association, Burbank, CA, 1970).

[4] Gas flow measurement by collection time and density in a constant volume, L. Olsen

and G. Baumgarten, Symposium on Flow. Its Measurement and Control in Science and

Industry, ISA, 1, Part 3, p. 1287 (1972).

[S] Evaluation of a low flow generator and calibrator as a flow measurement standard, G.

Baumgarten, NBS Technical Report 10921 (1972).

[6] The National Measurement System for fluid flow, W. C. Haight, P. S. Klebanoff, F. W. Ruegg, and G. Kulin, NBSIR 75-930, 66 pages (Aug. 1976). Order from NTIS as PB258250.

F. Hydraulics

Hydraulic flow measurement facilities are operated to evaluate measurement systems

and to provide information on relating laboratory practices to field requirements,

particularly for supply water and waste water systems.

1. Water Current Meters

Current meters will be calibrated in a water tunnel with rod mounting. When meters

submitted are found to be unsuitable for tests or unreliable, a charge will be made to cover

the cost of the work done.

References

[1] A guide to methods and standards for the measurement of water flow, G. Kulin and P. R. Compton, Nat. Bur. Stand. (U.S.), Spec. Publ 421, 97 pages (May 1975).

[2] Hydraulic research in the United States and Canada, 1976, P. H. Gurewitz, Ed., Nat.

Bur. Stand (U.S.), Spec. Publ. 497 (Apr. 1978).

[3] Some error sources in Price and Pygmy current meter traverses, G. Kulin, Nat. Bur. Stand (U.S.), Spec. Publ. 484 (Oct. 1977).

26
G. Aerodynamics

Calibrations of air speed measuring instruments are performed in high quality wind

tunnels covering the speed range from 0.1 mph to 150 mph using a laser velocimeter and a

laboratory standard Pitot-static tube as the primary velocity standards for the lower and

higher speed ranges, respectively. In addition, NBS also offers a calibration service for the

dynamic response of mechanical-rotary anemometers by which the "distance constant" for such instruments can be specified over a range of mean speeds from 10 fps to 45 fps. This

service is performed in an unsteady flow wind tunnel that has a 4.5 ft square test section, 16 ft in length, in which oscillatory flows over a frequency range from 0.1 Hz to 25 Hz

can be generated. The rate at which the amplitude of oscillation attenuates with increasing

frequency is determined by the inherent time constant of the tunnel which is approximately 0.25. The NBS air flow facilities, apart from providing a calibration capability for air speed measuring instruments, are extensively used for fundamental research on turbulence

and fluid dynamical problems of national interest that serve to extend measurement

competence.

References

[1] Mechanism by which a two-dimensional roughness element induces boundary-layer

transition, P. S. Klebanoff and K. D. Tidstrom, Phys. Fluids 15, No. 7, 1173-1188

(July 1972).

[2] Probability distributions and correlations in a turbulent boundary layer, F. N. Frenkiel

and P. S. Klebanoff, Phys. Fluids 16, No. 6, 725-735 (June 1973).

[3] On the lognormality of the small-scale structure of turbulence, F. N. Frenkiel and P. S. Klebanoff, Boundary-Layer MeteoroI. 8, No. 2, 173-200 (Mar. 1975).

[4] The dynamic response of helicoid anemometers, J. M. McMichael and P. S. Klebanoff,

NBSIR 75-772. 54 pages (Nov. 1975). Order from NTIS as PB246861.

[5] Characteristics of helicoid anemometers, J. M. McMichael and W. G. Cleveland,

NBSIR 78-1505, 12 pages (Aug. 1978). Order from NTIS.

[6] A low-velocity airflow calibration and research facility, L. P. Purtell and P. S. Klebanoff, Nat. Bur. Stand. (U.S.). Tech. Note 989, 18 pages, March 1979. Order

from NTIS. Also available from Supt. of Documents, Washington, DC, SD Stock

No. SN003-003-02038-9.

[7] Low velocity performance of anemometers, L. P. Purtell, NBSIR 79-1759, 168 pages

(May 1979). Request from Bureau of Mines.

27
I
CHAPTER IV

IV. Electrical Measurements—DC and Low Frequency

The National Bureau of Standards has the responsibility for the establishment and

maintenance of the legal electrical units and in addition for making them available for use by industry, science, and government at all levels. The electrical quantities program

provides for the dissemination of the electrical units and related electrical and magnetic

quantities in the frequency range from 0 (dc) to 1 MHz. Dissemination takes place in three

ways: via routine calibration of electrical, electronic, and standards and measurement

apparatus of the highest accuracy; by in-situ calibrations performed on equipment which

for technical reasons cannot be moved to or calibrated at NBS; and through Measurement

Assurance Programs. Educational, consultative, and metrology engineering services are

also available. The following sections contain more detailed descriptions of each type of

service.

The group of services offered has evolved over the years in response to needs

emanating from the measurement, technical, and scientific communities. Suggestions of

new, needed or useful, services are welcomed by NBS.

Note: Calibration and other metrology-related services to support measurements of

electromagnetic quantities in the frequencies above 1 MHz are discussed in the next

chapter.

Services

Educational: The Bureau sponsors low frequency electrical measurement seminars

generally held in the spring at the Gaithersburg site of NBS. The 4-day events cover the

methods used by the Bureau to establish and maintain the basic electrical units and to perform many of the dissemination services covered in this publication. Emphasis is on

techniques useful to standards and calibration laboratories and that are applicable to general quality assurance situations. Topics peripheral to electrical metrology such as

solid-state devices, statistical data analysis techniques, and laboratory automation are also generally covered. The course is generally geared to those working at a professional level and involved with using basic electrical standards for quality control purposes.

Consultations: Arrangements may be made for consultations with appropriate NBS

staff members either at NBS or in a client laboratory to solve critical measurement

problems. This is done on a cost-reimbursable basis. Because of the depth and variety of

NBS staff expertise, many such problems may be simply solved with a telephone call. Requests for limited assistance, such as can be handled with a telephone conversation or a

letter, are encouraged as they contribute to NBS awareness of problems facing the

metrology community.

Research and Development: R&D activities on new methods of measuring electrical quantities and the development, design, and construction of measurement apparatus are

natural outgrowths of the Bureau's unique responsibility. The special competence of

scientists in the electrical quantities program is reinforced by the ready accessibility of

experts in the many other technical areas of NBS. Expertise from diverse fields, such as cryogenics, solid state electronics, semiconductor physics, automatic data processing and

metallurgy may be quickly brought to bear on a problem. Special measurement problems

of concern to industrial or technical organizations, whether they arise in the course of

pure research or result from application of new technology—may be addressed by NBS

scientists and engineers on a contractual basis.

29
Publications: NBS publishes the results of its researches extensively to aid others

who may wish to use its developments or services. An up-to-date list of publications (NBS

LP 38 and LP 94) on electrical units, instruments and measurements is available upon

request. NBS has recently joined GIDEP, the Government-Industry Data Exchange

Program. All NBS publications will be included in the GIDEP data bank for ready

reference.

Calibrations: Routine calibrations of standards and precision electrical measurement

apparatus submitted to NBS are performed on a cost reimbursable basis using permanent

facilities at the Bureau. These services are intended to support primary standards

laboratories rather than to assign values to apparatus used by secondary laboratories.

Accordingly, NBS will calibrate only standards and apparatus of the highest quality

except under unusual circumstances such as to fulfill legal requirements or to resolve

certain technical disputes. Those requiring support for secondary activities are encouraged

to seek help from the numerous commercial calibration sources available. NBS can be of some assistance in locating a convenient source. NBS will, within the constraint of its resources, address critical measurement

problems not explicity covered in the fee schedule. Involvement in these problems may be

negotiated in certain measurement areas as indicated in the Appendix (Fee Schedule) to

this document by the heading "Special Measurement Services by Prearrangement."

Note: The Electrical Quantities Program does not provide tests for electrical devices or supplies not directly related to the field of measurement except occasionally for other

agencies of the Federal government. Tests of power transformers, motors, generators,

relays, wiring, appliances, etc., should not be requested. Tests are not generally performed on low accuracy electrical devices or components not intended for use as reference

standards.

Limited manpower precludes NBS from undertaking repair activities. Therefore all apparatus submitted for calibration should be free of defects and in proper working order.

Electrical contacting surfaces should be in proper condition both mechanically and

electrically. Minor repairs and necessary cleaning may be performed, workload

permitting, at the discretion of NBS personnel and on an at-cost basis. Electrical standards

and instruments, with the exception of unshippable saturated standards cells, may

generally be shipped. However, due to their delicate nature and to the limited NBS repair

capability, it is advisable to pack them extremely carefully. Special reusable shipping

containers customized for this purpose are a worthwhile consideration.

A report is issued upon the calibration of each item. This report contains the

measured values of each of the appropriate attributes of the device and their uncertainties

relative to the legal units. The measured values and the reported uncertainties describe the

results of the calibration process only. No allowance is made for the long-term drift of the

item, for its performance under conditions differing from those of the test, or for the

effects of transporting the item to and from the Bureau. Additional uncertainties

associated with these effects must be quantified by the user from additional measurements.

NBS personnel can assist in setting up the appropriate experiments.

NBS neither insists upon nor recommends intervals between NBS calibrations for

electrical standards except as explicitly stated in the following sections. These intervals

depend upon the performance of the individual standard and the accuracy requirements of

its application. These must both be determined by the user. A number of users make lists of intervals they have determined to be necessary for specific items available to anyone

interested. NBS can serve as a focal point for contacting those organizations.

Laboratory turnaround time for fixed-fee calibrations can be as long as 6 weeks,

depending upon workload, except in the case of standard cells, which require longer

periods to stabilize. An estimate of turnaround time will be given for at-cost items.

Technical details particular to each type of calibration are to be found in the

following appropriate sections of this chapter.

30
In situ Calibration: NBS performs calibrations in instances of critical need in which

the transportation of the apparatus to be calibrated to the Bureau is not possible or when

the magnitude of the stimuli required lies outside its in-house capability. For example,

NBS has developed a special portable, current-comparator bridge of high accuracy. This

bridge, together with low and intermediate voltage compressed gas dielectric capacitors, can readily be taken to field sites or high voltage laboratories. This system is then used by

NBS personnel to calibrate voltage transformers, high voltage capacitors, shunt reactors,

and other high voltage equipment for the power industry. This approach not only permits

equipment to be calibrated in its normal environment and location under normal

conditions of use but serves as well as a training experience for the calibration customer's

personnel.

Well-established Measurement Assurance Programs (MAP's) are available in the

areas of resistance, capacitance, voltage, voltage ratio, and electrical energy. Specifics of

each of these are discussed in the appropriate following technical section. Consideration will be given to establishing new Measurement Assurance Programs

in areas other than those described below where critical needs can be documented. For

those instances where the required program is not generally applicable to a broad user group, NBS may provide the requesting organization with guidance so that they may

develop the necessary techniques themselves.

The most recent information on dc and low frequency electrical measurements is frequently published in: "IEEE Transactions on Instrumentation and Measurement" [December issues are proceedings on the Conference on Precision Electromagnetic Measurements (CPEM) and

Electrical and Electronic Measurement and Test Equipment Conference (EEMTIC)

alternately].

IEEE

345 East 47 Street New York, NY 10017

"Metrologia"

Springer-Verlag New York, Inc. 175 Fifth Avenue

New York, NY 10010

References

[1] Basic electronic instrument handbook, C. F. Coombs, Jr., Ed., (McGraw-Hill, NY

1972).

[2] Electrical measurements, F. K. Harris (Wiley & Sons, NY, 1952).

[3] Basic electrical measurements, M. B. Stout (Prentice Hall, Inc., Englewood Cliffs, NJ,

1960).

[4] Precision measurement and calibrations, electricity-low frequency, F. L. Hermach and

R. F. Dziuba, Eds., Nat. Bur. Stand. (U.S.), Spec. Publ. 300, Vol. 3, U.S. Government Printing Office, Washington, DC

A. Resistance Measurements

Services covered in this section include the following types: Resistance

Measurement Assurance Programs, resistance standards with nominal values in the range

between 10"^ and 10* fi, high valued standards falling in the range between 10'' and 10''' fi, and standard resistors for the measurement of high currents. Resistors not intended for use

as primary standards, such as common decade resistance boxes, are not normally dealt with.

31
In addition to the routinely offered resistance measurement services described here

and in the Appendix to this document, NBS will provide such other special services as to aid in the solution of particular measurement problems deemed to be significant. For

example, the Bureau has provided a special valued resistor which when used at a production line, assisted a company in ensuring the quality of high precision manufactured

electronic components. Requests for assistance of this type will be given thorough

consideration. If undertaken as a development project, programs of this nature will generally lead to joint publications in the open literature describing the new measurement

techniques or approach. Such services are provided on the basis of operational costs being

defrayed by the user.

1. Resistance MAP Services

In the Resistance Measurement Assurance Program, the quality of the client's laboratory standards, maintenance program, and ability to disseminate the unit of

resistance is assessed and new values determined for his standards, if necessary. This is done by viewing the measurement process as a system and sampling the measurement

quality at a point in that system using NBS transportable standards with well understood

properties. As the objective is to determine the laboratory's primary capability, NBS

standard resistors at levels selected by the client are measured as unknowns in the client's standards laboratory or elsewhere as appropriate. In addition to using their normal

procedure to obtain assignments for the values of the transport group, the laboratory uses NBS suggested procedures which permit determination of the process precision as well as

offsets in equipment, etc. Data taken using the client's normal measurement process are combined with those taken before and after the transport process at NBS using least squares techniques to determine the precision and random error of the measurement

process as well as the offset of the calibrated values from those expected.

It should be understood that participation in this program is generally not advisable

unless one is required to support resistance measurements at or near the state-of-the-art in accuracy and is willing to adopt a system for the continuous surveillance of standards

during the intervals between NBS MAP transfers. A successful transfer requires a considerable amount of data collection and a willingness to become involved in the data

analysis process. Data supplied in the course of routine NBS calibrations suffice for normal measurement requirements of standards laboratories if proper methods are used by

the laboratory to quantify the additional uncertainties caused by transportation and their own measurement process. NBS stands ready to assist client laboratories in this regard.

Apparatus and procedures exist for routine performance of this service at the

following levels of resistance, listed in order of increasing uncertainty: one 10^ 10\ lO'*, 10^, 10*, lO**,^ and 10'° ft. Although the actual uncertainty of these transfers depend upon

the capability of the laboratory, they can be said to range from sub-tenths of a ppm in the case of the 1-ft level to the 20-30 ppm range for the highest valued resistors. Measurement

assurance programs to address the maintenance problem at other levels of resistance may

also be arranged by consultation.

2. Resistance Standards (IC^-IO* ft)

Resistance standards with nominal values in the range between lO * and 10^ ft are calibrated by comparison with NBS working standards of the same nominal value. This

service is for primary standards of resistance only. Because of this and to maximize their value as standards, resistors submitted for calibration should have the following attributes:

1. A drift rate of less than 20 ppm per year

2. A temperature coefficient of less than 30 ppm per °C at the temperature of use

3. A low thermoelectric coefficient against copper, especially for low-valued

resistors

32
In essence, the standard should be capable of performance at the 0.2 ppm level over a short period (1-3 days).

The standard resistors are generally calibrated in stirred oil at 25 °C. Normally a power level of 0.01 W is maintained in the resistor during the calibration as at that level neither the load coefficient of the standard nor the flow characteristics of the oil bath

have appreciable effect upon the outcome of the calibration. At the levels of accuracy

involved, four terminal measurements are required for resistors whose nominal value is 100 fl or less. Any resistor submitted should be designed accordingly. Precision standard

resistors of this type are commonly designed with amalgamated current terminals. These

should be clean and in generally good condition upon submission to NBS for test. High quality resistors suitable for use as standards, but not intended for oil immersion, may be accepted for calibration in air (22-44 °C) if their variation of resistance

with temperature does not exceed 2.5 ppm per K and if the design provides for accurate determination of the temperature of the resistor under conditions of test. Such standards

may have any nominal value. The uncertainty of calibration depends largely upon the

performance characteristics of the resistor itself. Acceptability for NBS calibration of

these standards remains at the discretion of the Bureau. A number of services may be performed by prearrangement as indicated in the

Appendix to this document. They are:

a. The determination of pressure coefficients for Thomas-type resistors and others

affected by variations in ambient pressure

b. The calibration of resistance standards at power levels exceeding 0.01 W

c. The determination of the load (power) coefficients of standard resistors for nominal values of one ohm and higher

3. Resistance Standards (10^-10'^ ft)

High-valued resistance standards in the range between 10^ and lO'^ ft are calibrated

at room temperature (22-24 °C) and at a relative humidity of less than 50 percent. This is done by one to one comparison with NBS standards at levels of 10'° ft and lower and via

capacitive discharge techniques for those resistors whose nominal value exceeds 10'° ft. Uncertainties depend upon the stability and performance of the specific resistor involved.

They can be as low as 10-20 ppm for 10^ ft resistors to as high as 0.3 percent for 10'^ ft. Because of the high resistances of these standards, their measured values can be

affected by leakage currents, which in turn are related to relative humidity and surface

cleanliness. Accordingly, they should be so constructed and treated that the effect of

humidity is minimized. As with other standards, these resistors should be made of suitable

materials and processed in such a manner that resistance values do not change appreciably

with time relative to the uncertainty required by their use. The resistance of such

standards is frequently highly voltage dependent. Hence, the magnitude of the appropriate

test voltage should be specified for each resistor submitted. Each resistor should also have

an identifying number engraved on or permanently attached to it. The Bureau does not have facilities for the determination of temperature or humidity coefficients of resistors whose nominal values exceed 10*" ft. Allowance for normal variations due to these effects is made in the uncertainty statement of the test.

4. Resistance Standards (High Current)

Standard resistors for use in precise measurement of high direct currents (shunts) are

also calibrated by NBS. Normally only those resistors of 0.04 percent accuracy or better are calibrated. Although the uncertainty of measurement as in other areas depends largely

upon the performance of the standard involved, it may be generally said that oil-immersed

shunts having a rated current capacity of 50 A or less can be calibrated with uncertainties

33
of the order of 50 ppm as can those designed for use in air at currents of 25 A or less. As

current ratings increase, caHbration uncertainties increase.

To be effective, standard resistors for current measurements must be of four-terminal

design, i.e., one for which the resistance is defined as the ratio of the open-circuit potential

difference between the potential terminals to the current flowing through the current

terminals. The resistance value will be definite and reproducible only if the current flow

pattern at the potential terminals is independent of the way in which current is introduced

at the current terminals, and if the voltage observed at the potential terminals is independent of the location of leads on the potential terminals. In some instances where

this has not been done, the type and location of connections to the current terminals can

be specified adequately to fix the flow pattern at the potential terminals.

Resistors for very high currents (above about 1000 A) require considerable power so

that their temperature rise between low and rated current, and the resulting change in resistance, will depend not only on their design, including means provided for dissipating

heat, but also on the connecting bus bars and their junctions to the resistor. Bus bars of

generous cross-section may carry away a significant part of the heat generated in the

resistor. In addition, contact resistance at the points of connection to the bus bars, unless

carefully minimized, may contribute appreciably to the heating. (Contact resistance of

bolted connections depends on area of contact, cleaniness of surfaces, and pressure.)

Resistance determinations made in the laboratory at rated current, therefore, may be of

little value because the working temperature conditions cannot be duplicated. The best experimental procedure to use in such cases is to place the standard in a temperaturecontrolled enclosure and measure its resistance with a comparatively low test current when it is heated uniformly to a temperature approximately that at which it will operate

in service. From data at two or more elevated temperatures, combined with that at room

temperature, a curve can be plotted from which the resistance at the operating

temperature can be read, provided this temperature is determined by the user with the

resistor under the actual operating conditions.

Changes in resistance may also result from strains in the resistance element produced

by mechanical forces incidental to clamping the resistor connections, as well as from

inherent internal expansion constraints on resistor parts, or forces from the magnetic field produced by the current.

References

[1] Methods, apparatus, and procedures for the comparison of precision standard resistors, F. Wenner, /. Res. Nat. Bur. Stand. (U.S.), 25, 229-294 (1940), RP 1323.

[2] Stability of double-walled manganin resistors, J. L. Thomas, /. Res. Nat. Bur. Stand.

(U.S.), 36, 107-110 (1946), RP 1692.

[3] Precision resistors and their measurement, J. L. Thomas, Nat. Bur. Stand. (U.S.), Circ. 470, 32 pages (Oct. 1948).

[4] Measurement of multimegohm resistors, A. H. Scott, /. Res. Nat. Bur. Stand. (U.S.), 50,

147-152 (1953), RP 2402.

[5] Calibration procedures for d-c resistance apparatus, P. B. Brooks, Nat. Bur. Stand.

(U.S.), Monogr. 39, 53 pages (Mar. 1962).

[6] Errors in the series-parallel buildup of four-terminal resistors, C. H. Page, /. Res. Nat.

Bur. Stand (U.S.), 69C (Eng. and Instr.), No. 3, 181-189(July-Sept. 1965).

[7] 10-kilohm standard resistors, E. F. Gard and T. E. Wells, Nat. Bur. Stand. (U.S.), Tech. News Bull. 52, No. 11, 250-251 (Nov. 1968).

34
B. Precision Apparatus

Ratio, transfer, and scaling devices of high accuracy are covered in this category.

Included are Hamon and other transfer devices, and inductive voltage dividers, as well as volt ratio MAP services and Silsbee dividers. Specific details for each type of calibration not following or contained in the Appendix to this document may be obtained from the

office designated in the Appendix.

In general, precision apparatus should be packed carefully to avoid damage during

shipment. Instrument lids of light metal or wood associated with heavy items of

equipment should be protected from damage by the weight of the instrument itself such as might occur if the shipping container were inverted during transportation.

Calibrations of such apparatus are carried out at room temperature (22-24 °C) with

the exception of Hamon transfer units designed for oil immersion at 25 °C. These devices

exhibit a certain amount of after effect due to the abrupt change in the temperature of

resistors. Accordingly, they are allowed to "soak" for a period not shorter than 1 week at their normal operating temperature before calibration. This same precaution should be

observed in use.

1. Inductive Voltage Dividers

Inductive voltage dividers (decade transformer dividers) are accepted for calibration

only at frequencies near optimum design frequencies. The largest contribution of

instability in inductive voltage dividers often arises in the decade switches. Variable

contact resistance in these switches sometimes affects the stability of voltage-ratio

measurements to a significant extent but is most evident by its effect on the phase angle.

When a decade inductive voltage divider exhibits large changes in phase angle for repeated measurements after the switches have been disturbed, the divider should no

longer be considered satisfactory for use as a reference standard of voltage ratio. Inductive voltage dividers which use pushbutton switching or which incorporate a

resistive divider as a fine adjustment usually are not accepted for calibration. Corrections to the separate decades of an inductive divider, in general, cannot be

simply combined; however, the correction to a step setting of one of the higher decades

usually is independent of the setting of the lower decades. The effects of stray impedances

must be corrected by connecting the case to the divider at one point, and unless otherwise

specified, the case will be connected to one of the common terminals.

Decade inductive voltage dividers are calibrated at the Bureau by a comparison

method, using as a working standard a well-constructed inductive divider which has been

calibrated previously by capacitance-ratio or other suitable methods. The comparison

method is simple and convenient and can be used in other laboratories for the rapid

calibration of other voltage dividers. Also, it can be extended readily to permit calibration

of the lower decades of a divider in the user's laboratory. (See NBS Technical News

Bulletin 49, 1, Jan. 1966.) Accordingly, it is recommended that, in general, only one

divider from a laboratory be submitted to the Bureau for calibration and that other

reference dividers be calibrated by a comparison method using it as a standard.

2. Volt Ratio Standards

Resistive volt ratio standards (volt boxes) are used to extend the range of the voltage

measured by a potentiometer. The ratio for any range is obtained by dividing the voltage

across the input terminals by the open-circuit voltage across the section to be connected to the potentiometer. Normally, NBS calibrates only Silsbee dividers. These dividers of

design similar to that described in NBS RP 1419 are calibrated by the Dunfee method

(NBS Paper 67C1-114). Values of ratio normally are reported with an uncertainty of 10 ppm, but higher accuracies could be attained if the apparatus were to warrant it.

35
The following should be considered in the calibration and use of such dividers:

(1) The insulating structure of a divider is equivalent to a network of high

resistances in parallel with one or more of its wire-wound precision resistance elements.

Thus, changes in insulation resistance as a result of variations in surface or volume

moisture may affect the ratios. Such ratio changes normally are less on low than on high

ratios. This effect can be reduced or eliminated by constructions which provide built-in guard electrodes, maintained at appropriate potentials. Another effect of humidity is to produce changes in the values of the precision wire-wound resistors. The magnitude of

this effect varies with coil construction and with wire size and coating. Because humidity

effects may reach equilibrium only after days (or even weeks), it is recommended that laboratory humidity be held continuously at or below 50 percent.

(2) Changes in ambient temperature should have little effect on the ratio if all the

resistance elements have small and nearly equal temperature coefficients.

(3) Internal heating, which includes both the self-heating of the high-resistance coils and the transfer of heat to other coils (proximity heating), may significantly change the

ratio of a divider.

(4) The magnitude of this change and the time required for a state of "ratio

equilibrium" to be established varies with divider construction and with operating and

ambient conditions. The Silsbee-type and special dividers that are constructed of resistance

elements having small and nearly equal temperature coefficients and designed to have low

power dissipation per element usually have ratio changes less than 10 ppm due to internal

heating.

(5) Corona and other high voltage effects usually are negligible for voltages up to 1500 V.

Dividers are calibrated on each range at rated voltage. However, the first calibration

test should also include a complete test at reduced voltage (20% rated voltage or less) to provide significant information on heating effects.

3. Volt Ratio MAP Services

Specially designed NBS transportable volt ratio standards provide the means for a Measurement Assurance Program in dc voltage ratio. This program can be used in two

ways. It can be used to evaluate the ability to maintain a high precision volt box or ratio standard as a basis for the calibration of similar devices. A variation in approach,

however, permits the evaluation of the ability of the laboratory to produce accurate dc

voltages up to 1000 V for the calibration of precision sources and voltmeters. This

program is unique among MAP's for electrical parameters in that portable NBS

measurement instrumentation is also used to aid in the identification and solution of

problems limiting laboratory performance at high levels of accuracy.

Data handling is very much like that in other MAP's. Before and after data are coupled with the client's calibration results to determine the offsets of his ratio assignments from those of NBS and the uncertainty of those offsets. Redundancy of

measurement is achieved through the use of interchangeable connectors which permit

slight but well-characterized alterations of the ratios of the transport standard. Their use is effectively identical to using a number of volt-ratio standards. Uncertainties less than 1 part in 10^ can readily be achieved using this program.

C. Impedance Measurements

1. Impedance MAP Services

The Measurement Assurance Program in capacitance is routinely carried out at the

1000 pF level. An NBS designed transportable capacitance standard forms the basis for

this service. It consists of four specially-selected commercial gas dielectric 1000 pF

36
capacitors housed in a temperature-controlled oven capable of sustained operation via a battery pack. Data handling in general is similar to other MAP's. Redundant measurement

designs provide data for estimation of process precision and of the magnitude of some

systematic effects such as ground loops. Before and after data are combined with client data in a linear regression analysis used to determine the difference between the client's unit of capacitance and the legal unit. Upon completion of a number of transfers, this difference may be characterized as a function of time and that characterization used in conjunction with the results of a continuing internal surveillance program to ensure the

quality of capacitance measurements at the client facility. The uncertainty of the process

depends very much on the client's laboratory capability and could be as low as one in 10^

Similar programs at other levels of capacitance will be initiated as needs and

resources dictate. NBS personnel can assist in the design of measurement assurance programs using client equipment and normal routine calibration data if circumstances

permit.

2. Standard Capacitors

The following apply to the calibration of standard capacitors at NBS:

(a) Calibrations are ordinarily performed at an ambient temperature of 23±1°C

except for high stability gas dielectric capacitors. These are placed in a highly insulated

chamber for 48 hrs. to achieve temperature stability during calibration. Calibration

temperature is reported to within ±0.1 °C. Relative humidity is maintained at 50 percent or less in all cases.

(b) Precision three terminal air capacitors, such as ESI Model SCI 000 and

GENRAD Model 1404 have been found to be variously affected by mechanical shock.

Accordingly, two types of calibrations are offered. The higher accuracy calibration

requires a qualification test to determine the effects on capacitance of various impacts and

changes in orientation. Results of this test are coupled with the random error of the

precision calibration which follows to provide a definitive accuracy for the process. For

the lower accuracy test, a similar calibration, albeit with reduced resolution, is performed.

The assigned uncertainty is fixed and has been deduced from an analysis of data taken

from tests on a large population of standard capacitors.

(c) The frequencies available for these calibrations depends upon the type of

capacitor and its connectors. In general, capacitors with coaxial connectors can be

calibrated at 100, 400, and 1000 Hz. Capacitors with binding posts, banana plugs, etc., can be calibrated at 66-2/3, 100, 400, 1000 and 10,000 Hz.

(d) The capacitance value given is the equivalent parallel capacitance. In general a' determination of the equivalent parallel conductance with high accuracy is not feasible; however, for solid dielectric capacitors an approximate value is given without charge.

(e) The uncertainty stated in the report of calibration is determined in part by the

accuracy of the Bureau's measurements and in part by the characteristics of the capacitor

itself, and is sufficiently broad to allow for variations in the stray capacitance at the

connectors, variations in temperature of a few degrees Celsius, considerable variation in relative humidity and atmospheric pressure, and frequency deviations of a few percent

from the stated test conditions. Over the above frequency range, and in the capacitance

range from 0.001 to 100 \x¥, the uncertainty usually lies in the range 0.002 to 0.5 percent.

(f) When capacitors requiring terminal plugs (banana plugs) for parallel connection

are sent to the Bureau, the plugs which will be used with the capacitor after calibration

should be sent also. If such a capacitor arrives without plugs, the Bureau must attach

plugs temporarily in order to calibrate the capacitor. The plugs used by the Bureau are GENRAD Type 274-P. If after calibration with these plugs, the capacitor is used with

plugs of even slightly different length and base, the value of capacitance can differ significantly from the value reported. Unless otherwise requested, the measured value

reported by NBS is the capacitance added when the standard is plugged directly into the

37
binding posts of the Bureau's bridge. For two-terminal GENRAD capacitors Type 1401,

Type 509, and Type 1409 (used as two-terminal capacitors) it has been found that plugs

which are different from the Type 274-P can cause a capacitance increase ranging from

0.04 to 0.14 pF. For three-terminal use of GENRAD Type 1409 capacitors it has been

found that different plugs can cause a capacitance increase ranging from 0.01 to 0.04 pF.

No significant change in conductance has been found in either the two-terminal or three- terminal value. The importance of terminal connection methods becomes extremely

critical when capacitance values of 0.01 /xF or less are being measured. Improved

accuracy in two-terminal measurement can be realized if standards are provided with

precision coaxial connectors.

(g) In the case of direct capacitance standards, it is assumed that the connectors are

coaxial. While the connectors available for this purpose are adequate, it should be noted

that changes or instabilities in the impedance of the shield or guard connection of a three- terminal capacitor can change the capacitance significantly.

(h) The following capacitors are no longer accepted for calibration by the Bureau: Two-terminal air capacitors with exposed terminals, nominal value of

capacitance 1000 pF or less. (For example, GENRAD Type 1401 capacitors with either

old or new style terminals.) Two-terminal air capacitors with the "low" terminal (ground

terminal) extended and surrounding the "high" terminal are acceptable. Refer questions

regarding acceptable types to the information source given in the Appendix.

(i) In applying the fee schedule in the appendix to decade capacitance boxes and

variable air capacitors, the first entry applies to a determination of the zero capacitance

and conductance of the box (all dials set at zero). The second entry applies to the

determination of the capacitance and conductance added to the circuit when any one dial

is advanced from zero to a specified setting, and at the frequency used in determining zero

capacitance. For measurements at additonal frequencies the schedule is applied in the same manner, i.e., the higher fee is used for the first point (zero calibration) at the new

frequency and the lower fee applied to additional points at that frequency.

(j) Unless otherwise specified in the client's purchase order, capacitors with solid dielectric will be calibrated as two-terminal capacitors (measurement of "grounded"

capacitance, case connected to low terminal).

(k) If a capacitor arrives in a condition such that minor repairs are required, the owner will be notified and requested to supply a letter granting permission for NBS to perform the repairs.

3. Standard Inductors

Standard inductors for use in ac bridges are tested at 100, 400, 1000, or 10,000 Hz at a room temperature of 23 °C and a relative humidity of 50 percent or less. Measurements

at 10,000 Hz are limited to standard inductors of 0.1 H or less. Most inductors used at 60

Hz can be tested at 100 Hz since the variation of inductance with frequency in this range

is usually negligible. Purchase orders should state which frequency or frequencies are to be used for calibration purposes. If no test frequency is specified in the order,

measurements will be made at 1000 Hz. A metal encased standard is calibrated with the case connected to the "low" terminal of the inductor unless other conditions are specified.

Variable inductors used as circuit elements in laboratory setups are low accuracy devices

which do not come within the purview of this schedule and should not be submitted for

calibration. Q values are not supplied for inductors calibrated in this section. Inductors

intended for use as Q standards at radio frequencies are covered in the next chapter.

Accuracy: The effective series inductance value is given to as many significant

figures as are justified at the time of measurement. The uncertainty figure given in the

report of calibration will vary from 0.02 to 0.2 percent depending on the nominal value of

the inductor and the frequency of the test current employed.

38
Usually inductors can be shipped safely by express but they should be packed

carefully to avoid damage to the coil fastenings and terminals.

References

[1] New measurements of the absolute farad and ohm, R. D. Cutkosky, IEEE Trans. Instr. Meas. IM-23, No. 4, 305-309 (Dec. 1974).

[2] Measurement of four-pair admittances with two-pair bridges, J. Q. Shields, IEEE

Trans. Instr. Meas. IM-23, No. 4, 345-352 (Dec. 1974).

[3] Improved ten-picofarad fused silica dielectric capacitor, R. D. Cutkosky and H. L. Lee, / Res. Nat. Bur. Stand. (U.S.), 69C (Eng. and Instr.), No. 3, 173-179 (July-Sept.

1965).

[4] Voltage dependence of precision air capacitors, J. Q. Shields, /. Res. Nat. Bur Stand.

(U.S.), 69C (Eng. and Instr.), No. 4, 265-274 (Oct.-Dec. 1965).

[5] Variable capacitor calibration with an inductive voltage divider bridge, T. L. Zapf,

Nat. Bur. Stand (U.S.). Tech. Note 57. Order from NTIS as PB161558.

[6] Capacitor calibration by step-up methods, T. L. Zapf, /. Res. Nat. Bur. Stand. (U.S.),

64C (Eng. and Instr.), No. 1, 75-79 (Jan.-Mar. 1960).

[7] Calibration of inductance standards in the Maxwell-Wien bridge circuit, T. L. Zapf, /. Res. Nat. Bur Stand (U.S.), 65C (Eng. and Instr.), No. 3, 183-188 (July-Sept. 1961).

[8] Some techniques for measuring small mutual inductances, D. N. Homan, /. Res. Nat.

Bur. Stand (U.S.), 70C (Eng. and Instr.), No. 4, 221-226 (Oct.-Dec. 1966).

[9] Techniques for comparing four-terminal-pair admittance standards, R. D. Cutkosky, /

Res. Nat. Bur. Stand. (U.S.), 74C (Eng. and Instr.), Nos. 3 and 4, 63-78 (July-Dec.

1970).

[10] The accurate measurement of voltage ratios of inductive voltage dividers, T. L. Zapf,

ACTA IMEKO 3, 317 (1964).

[11] An injection method for self-calibration of inductive voltage dividers, W. C. Sze, J. Res. Nat. Bur. Stand (U.S.), 72C (Eng. and Instr.), No. 1, 49-59 (Jan.-Mar. 1968).

D. Voltage Measurements

/. Voltage MAP Services

The Volt Transfer Program, VTP (the MAP for standard cells), is designed to reduce the uncertainty of the assignment of the unit of voltage at the participating

laboratory, and to provide the laboratory with quantitative information about its own

measuring process. In the VTP, rather than have standard cells sent to NBS for calibration as is usually

done, the Bureau will provide transport standards of voltage, appropriate measuring

techniques, and overall supervision of the experiment. As far as is practical, the service

will be tailored to meet the needs of each participating laboratory.

The experiment is carried out in three phases: First, NBS analyzes the participant's

procedures and measurement setup. From the analysis, NBS proposes certain internal

experiments, determines various operating parameters, and establishes that the local measurement process is in control. Where measurement problems are encountered, NBS

will assist in solving these problems even to the extent of sending personnel to the

laboratory, if necessary. Second, NBS provides the laboratory with a suitable transport standard (saturated

cell group) and the procedures for intercomparing it with its reference group of standard

cells. Usually, the transport standard will be shipped via air freight under carefully

controlled conditions.

Finally, upon return of the transport standard, NBS analyzes the results and suggests

any change in the laboratory's unit of voltage that might be in order.

39
The level of participation by each laboratory will depend on its own internal

requirements. It may vary from four such experiments per year to one every 2 years. This

type of approach eliminates several sources of uncertainty that are inherent in the regular

procedure in which customers send or bring their cells to NBS. This is because the Volt

Transfer Program calibrates the whole process and not just a portion of it. At the present

time, NBS is quoting as the standard deviation of a single experiment 0.14 ppm or a three

standard deviation uncertainty for the complete calibration of 0.42 ppm. It should be again

emphasized, however, that this service is not meant to replace the regular calibration

procedure. Rather, it is designed for those laboratories that need and can utilize the most

precise unit of voltage obtainable.

To keep the data analysis activity at a manageable level, the transport standard

should be compared with a single laboratory reference group of cells containing a number

of cells such that a single measurement design may be used for a total comparison. The

use of cells configured so as to require more than a single, statistically optimized

measurement design to assign values to the transport standard in terms of the client's unit of voltage will result in extra charges. Large designs, i.e., those involving more than 12

cells, are undesirable as individual cell drifts can tend to mask the estimates of process

precision levels. NBS can provide computer programs and design information to permit

the client to perform his own data analysis where large numbers of cells are involved.

Since the amount of equipment available for this program is limited, it is vital that interested parties advise us of their intent to use the program in any calendar year by the

end of the previous year.

2. Dc Voltage Standards

Routine calibrations of voltage standards involve the following considerations:

(a) Unsaturated cells require approximately 3 weeks for a complete calibration. Such cells are placed in a thermally lagged enclosure and their emfs are read daily for a period of 10 days. If the measured emf fluctuates unduly or is unusually low, or if the cell shows abnormal indications, the report of calibration will reflect these circumstances.

Unsaturated cells are not likely to be injured by normal transportation (mail or express) if they are packed carefully. Because of the possible hazard from freezing, shipment during

very cold weather should be avoided.

(b) Saturated standard cells of the unshippable type should always be transported by

messenger because such cells should never be tipped from an upright position by more

than 45° in any direction. Unshippable saturated cells contained in portable, temperatureregulated enclosures should also be transported by messenger and with the enclosure

activated or under power, if possible.

(c) Saturated standard cells of the shippable type housed in portable

thermoregulated enclosures should be packed carefully and shipped under power if possible. Liquid-in-glass thermometers normally mounted in such devices should be

removed and provided with additional rigid packing for protection against breakage.

Enclosures having a nominal cell temperature of 28 °C or lower should not be transported

during the summer due to the danger of over heating. Enclosures should not be energized

by using the ac power mains while they are packed in shipping containers.

(d) Saturated standard cells which arrive having been maintained continuously at their nominal temperature of use will, workload permitting, undergo test immediately

upon receipt for a period not to exceed 4 weeks, unless other arrangements are made. If such cells perform abnormally with respect to the typical performance of like cells in similar environments, the owner will be notified. Arrangements for further testing may be

made at that time if desired. Cells will be returned as soon as possible after calibration.

(e) Saturated cells arriving at a temperature other than their nominal temperature of

use will be brought to their use temperature as soon as possible after receipt. Starting the

month after they are initially brought , to temperature, weekly readings will be taken to

40
observe the stability of the cells. When the cells stabilize, 10 daily readings will be taken

and used to assign values to them. This process will not exceed 90 days without special

arrangements being made.

(0 Solid-state voltage reference devices for test must have output voltages in the

range between 1.015 and 1.020 V and/or 10 V and a rated accuracy of 0.005 percent or

better. Such devices will be tested under continuous power.

References

[1] Volt maintenance at NBS via 2e/h: A new definition of the NBS volt, B. F. Field, T. F. Finnegan, and J. Toots, Metrologia 9, 155-166 (1973).

[2] A high-resolution prototype system for automatic measurement of standard cell voltage,

D. W. Braudaway and R. E. Kleimann, IEEE Trans. Instr. Meas., IM-23, No. 4, 282- 286 (Dec. 1974).

[3] Regional maintenance of the volt using NBS volt transfer techniques, W. G. Eicke and

L. M. Auxier, IEEE Trans. Instr. Meas. IM-23, No. 4, 290-294 (Dec. 1974).

[4] Standard cell enclosure with 20-/iK stability, R. D. Cutkosky and B. F. Field, IEEE

Trans. Instr. Meas. IM-23, No. 4, 295-298

[S] Effect of vibration and shock on unsaturated standard cells, R. J. Brodd and W. G.

Eicke, Res. Nat. Bur. Stand. (U.S.). 66C (Eng. and Instr.), No. 2, 85-97 (Apr.-June

1962).

[6] Comments on Zener diodes as voltage standards, W. G. Eicke, Proc. 10th Session

Comite Consultatif dElectricite du Comite International des Poids et Measures (1963).

[7] Making precision voltage measurements on Zener diodes, W. G. Eicke, IEEE Trans. Paper CP 63-416 {my).

[8] Standard cells, their construction, maintenance, and characteristics, W. J. Hamer, Nat.

Bur. Stand. (U.S.). Monogr. 84. 38 pages (Jan. 1965).

[9] Designs for surveillance of the volt maintained by a small group of saturated standard

cells, W. G. Eicke and J. M. Cameron, Nat. Bur. Stand. (U.S.). Tech. Note 430. 19 pages (Oct. 1967).

[10] Transfer of the unit of voltage, N. B. Belecki, Proc. ISA Ann. Conf. 5, 608 (1968).

[11] Method for calibrating a standard volt box, B. L. Dunfee, /. Res. Nat. Bur Stand.

(U.S.), 67C (Eng. and Instr.), No. 1, 1-13 (Jan.-Mar. 1963).

[12] Practical methods for calibration of potentiometers, D. Ramaley, Nat. Bur. Stand.

(U.S.), Tech. Note 172, 44 pages (Mar. 1963).

[13] A resistive voltage-ratio standard and measuring circuit, R. F. Dziuba and B. L. Dunfee, IEEE Trans. Instr. and Meas. IM-19, No. 4, 266-277 (Nov. 1970).

3. Ac Voltage Calibrations in Range of 0.1 Hz-10 Hz

NBS offers a calibration service for ac voltage standards and rms voltmeters in the

0.1 Hz to 10 Hz range. The means for this service is an "AC Voltmeter/Calibrator," an NBS-developed instrument containing a high resolution rms digital voltmeter and both ac and dc voltage calibrators. Ac voltmeters can be calibrated at 0.1, 0.2, 0.5, 1, 2, 5, and 10 Hz for voltage levels ranging from 0.2 mV to 7 V rms, using the ac voltage calibrator. Ac voltage standards are calibrated by comparing their voltage levels with ac calibrator voltages of approximately

the same frequency, using the rms voltmeter (i.e., by performing ac-ac transfer measurements). The ac calibrator is accurate to within ±0.02 percent at 23±1 °C for the

voltage range of 2 mV to 7 V. For calibrator voltages below 2 mV, the accuracy

decreases with decreasing voltage to within ±0.2 percent at 0.2 mV.

For calibration of a voltmeter, the customer should specify the voltage levels, ranges, and frequencies. The uncertainty of a given calibration point is: ac calibrator

41
accuracy + resolution of voltmeter being calibrated + Mcr^, where cr^ is the standard

error (i.e., ar^ is the standard deviation of the reported value). A multiplying factor of 3 is usually used for M. If the voltmeter being calibrated has an analog readout, the readout

error caused by lack of resolution is included in the standard error. For calibration of an ac voltage standard, two options are available: (1) measurement

of rms voltage level with frequency on one or more voltage ranges, and (2) absolute

voltage calibrations at specific voltage levels and frequencies. If the voltage standard is of

a design which has inherently stable voltage with respect to frequency, the first approach

is recommended. The user then establishes the absolute voltage levels by calibrating the

standard at one frequency, e.g., 10 Hz. For type (1) calibrations, the uncertainty is 0.006% + 2x (resolution of NBS DVM) + Mcr3. For type (2) calibrations, the uncertainty

is ac calibrator accuracy + 2x (resolution of NBS DVM) -|- Mcr^.

References

[1] An RMS digital voltmeter/calibrator for very-low frequencies, H. F. Schoenwetter,

IEEE Trans. Instr. Meas. IM-27, No. 3, 259-267 (Sept. 1978).

[2] NBS provides voltage calibration service in 0.1-10 Hz range using AC

voltmeter/calibrator, H. K. Schoenwetter, IEEE Trans. Instr. Meas. IM-28, No. 4, 327-331 (Dec. 1979).

E. Electrical Instruments (ac-dc)

RMS ac-dc transfer standards [thermal voltage converters (TVC's) and thermal

current converters (TCC's), covering the ranges 2 Hz to 1 MHz, 1 mA to 20 A, and 0.5 to 1000 V] meeting certain requirements, are accepted for calibration. Accuracies and

limitations are listed in the table at the end of this section.

(a) Ordinarily only ac-dc transfer standards and thermal converters of 0.05 percent

rated accuracy or better are accepted for test, which consists of ac-dc difference

determinations as described below.

(b) Ac-dc difference tests consist of determinations of the differences between the

quantities (current, voltage, or power) required to give the same response (output) of the

transfer standard on alternating current and on reversed direct current (the average of the

two directions of direct current). The alternating quantity, Q^, required for a given

response of the instrument or converter is then Qa = QX^-^^) where is the average

quantity required for this response on reversed direct current, as determined by dc

standards, and s is the small fractional ac-dc difference.

(c) Tests are recommended at rated voltage or current on each range at 20 kHz (the

upper limit for NBS' best accuracy) or at the highest frequency of interest. Additional

tests are recommended at lower frequencies only if the ac-dc differences are large at the

initial frequency. Since the need cannot be predicted, it is recommended that the purchase

order include an allowance for a few such tests, perhaps by stating an upper limit of cost. A second test at 600 V is recommended for 1000 V ranges, for these ranges may be

affected by self heating. Tests are made from 30 kHz to 100 MHz or more at NBS

Boulder, Colorado (see ch. V).

(d) In addition to the high frequency tests, an ac-dc difference test (ordinarily at 20

Hz) is recommended for one range, to verify the low frequency accuracy.

Thermoelements have a low frequency limit, below which they fail to integrate properly.

The ac-dc difference may approach 0.02 percent at frequencies ranging from about 5 Hz

for most low range thermoelements to about 60 Hz for some thermoelements with ratings

above 1 A. This low frequency ac-dc difference is the same for all ranges of a multirange

converter in which a single thermoelement is used with shunts or multipliers. For

convenience, usually a low voltage or current range is chosen for the test.

42
(e) At special request, high-grade thermal voltage converters of the coaxial type,

having plug-in series resistors for one or more thermoelements to form ranges from 1 to 1000 V, can be evaluated to 20 ppm at a higher cost. The dc reversal differences of the converter must be less than 200 ppm and the ac-dc differences less than 100 ppm. The

design should permit intercomparisons between ranges by the user. If such

intercomparisons are made, initial tests at NBS are recommended only for the lowest,

middle, and highest ranges, to provide tie points and to verify the accuracy of the user's intercomparison (step-up) procedures. Any relative changes in the converters can be

detected by subsequent periodic intercomparisons in the user's laboratories. If no changes

are observed, retests at NBS should not be necessary.

(f) At special request, the ac-dc differences of high-grade thermal converters (thermoelements) for current measurements from 5 to 50 mA can be determined by direct

comparison with the basic NBS ac-dc transfer standards, to 5 ppm from 20 Hz to 20 kHz

and 10 ppm up to 50 kHz, at a higher cost. The dc reversal differences of the converters must be less than 100 ppm and the ac-dc differences less than 20 ppm.

(g) The ac-dc differences are small and very stable in well-designed, rigidly

constructed ac-dc transfer standards. For such a standard, a recalibration interval of not

less than 5 years is recommended if checks are made periodically by the user (by

comparing it with another ac-dc transfer standard or by measuring a stable ac voltage

standard with adjacent ranges of the transfer standard).

Range and uncertainties of ac-dc difference calibration

Frequency 2-5 5-20 20-Hz 20-50 50-100 0.1-0.5 0.5-1 Hz Hz 20-kHz kHz kHz MHz MHz

Voltage limits (V) 50 100 1000 1000' 600 100 100 Current limits (A) 0.05 0.05 20 16^ Uncertainty (%)'

Multi-range TVC's 0.02 0.01 0,005 0.01 0.01 0.02 0.03 Coax single range TVC's .02 .01 .002' .003" .005'' ,02 .03 TE (5 to 50 mA) .02 .01 .0005' .001" TCC (0.005 to 5A) .02 .01 .005 .01 TCC (5 to 20 A) .01 .01

'200 V at 20 Hz

^5 A at 20 Hz

^The lowest uncertainty applies at the crossover frequencies. Uncertainties may be increased if the ac,dc differences are

large or change with self-heating. 'See appropriate paragraph (e or f above)

A calibration service for ac-dc wattmeters will be provided on a special test basis. Direct inquiries to address given in the Appendix.

AC Resistors (1 to 0.001 O, 50 Hz to 10 kHz). Properly designed four-terminal ac

resistors (having small phase angles) can be measured at current ratings not to exceed 50

A. The values for the in phase and quadrature component can be reported for frequencies

up to and including 10 kHz.

References

[1] An investigation of multijunction thermal converters, F. L. Hermach and D. R. Flach,

IEEE Trans. Instr. Meas. IM-25, No. 4, 524-528 (Dec. 1976).

[2] Ac-dc comparators for audio-frequency current and voltage measurements of high accuracy, F. L. Hermach, IEEE Trans. Instr. Meas. IM-25, No. 4, 489-494 (Dec.

1976).

43
[3] Thermal current converters for accurate ac current measurements, E. S. Williams,

IEEE Trans. Instr. Meas. IM-25, No. 4, 519-523 (Dec. 1976).

[4] Ac-dc transfer instruments for current and voltage measurements, F. L. Hermach, IRE

Trans. Instr. 1-8, 235 (1958).

[5] Thermal converters for audio frequency voltage measurements, F. L. Hermach and E.

S. Williams, IEEE Trans. Instr. and Meas. IM-15, 260 (1966).

F. Instrument Transformers and Comparators

7. Voltage Transformers

NBS provides routine services for measurement of complex voltage ratios (magnitude and phase angle) of transformers for primary voltages up to 50 kV at 60 Hz.

The estimated limit of measurement uncertainty is 0.01 percent for ratio and 0.1 mrad (1 mrad = 3.438 min) for phase angle for stable transformers tested with low burdens.

Inquiries are invited concerning related measurements at higher voltages, and at frequencies other than 60 Hz, not listed in the present appendix.

The following test information must be furnished for each transformer or for each

range on a multirange transformer.

1. Frequency

2. Secondary voltages

3. Secondary burdens

Ambiguity of test burdens can be avoided if the impedance and power factor or the

resistance and reactance, rather than volt-ampere rating, of each burden is specified.

Measurements are made with one side of both the primary and secondary windings

connected to ground.

Reference

[1] A wide range high-voltage capacitance bridge with one ppm accuracy, O. Petersons and

W. E. Anderson, IEEE Trans. Instr. Meas., IM-24, No. 4, 336-344 (Dec. 1975).

(Calibrations of voltage transformers at NBS are performed with the bridge

described in this paper.)

2. Current transformers

Normally the Bureau calibrates only current transformers of high quality for use as reference standards. The Bureau may decline requests for tests which are not to be used

for establishing or checking a reference standard. If the transformer quality is stated in terms of ANSI accuracy classes, calibration will be limited to transformers stated to be in the 0.3 percent class for one or more ANSI burdens. Bureau equipment is designed

primarily for testing current transformers whose rated secondary current is 5 A. Results

obtained at frequencies near 60 Hz normally will be reported to an accuracy of 0.01 percent in ratio and 100 /xrad (approximately 0.3 min) in phase angle. Tests cannot be started until information is furnished concerning the following

conditions: (1) test frequency, (2) secondary test currents, (3) secondary burdens, (4)

ranges to be tested. It is customary to make tests at secondary currents of 0.5, 1, 2, 3, 4, and 5 A.

Transformer Burden: Current transformers should be tested with burdens equivalent

to the impedance imposed when the transformer is used as a reference standard. Inclusion

of tests at ANSI burdens is not recommended. The burdens listed in the Standard for Instrument Transformers, C-57.13, are for rating purposes only and differ from the

instrument burdens imposed on a reference standard. Large errors in measurement can

result if the values of ratio and phase angle obtained with an ANSI burden are used for the transformer when it supplies only an instrument burden.

44
Preferably the burden should be specified in terms of the measured resistance and

inductance, including the leads to connect the instruments to the secondary of the

transformer. If this measurement cannot be made conveniently, it will suffice in most cases

to state the name of the maker, the type, range, and serial number of each instrument used

in the burden, and the length and size of the wire of the leads used in the secondary

circuit. Alternatively, the burden may be stated in terms of the volt-amperes and power

factor of the secondary circuit at the test frequency.

The test equipment regularly used at the Bureau imposes a minimum test burden of

about 0.03 ft with a minimum inductance of about 10 jnH.

Demagnetization: Unless otherwise specified, current transformers will be

demagnetized before being tested. If it is desired to have a transformer tested as submitted

(without demagnetization), this fact should be stated specifically. Test limitation at frequencies greater than 60 Hz: At 400 Hz, the maximum current range for which tests are made is about 200 A and normally the values are reported to an accuracy of 0.02 percent for ratio and 200 jitrad for phase angle. At 800 Hz there is a

further reduction in the current range and accuracy.

Recalibration: At room temperature the ratio and phase angle under a specified test condition should be repeatable unless the core is magnetized. Once stability has been

demonstrated, a current transformer should not require recalibration at intervals less than

5 years.Contact resistance: Loose or dirty primary and secondary terminations may

contribute appreciably to the calibration values obtained. These surfaces should be

cleaned thoroughly prior to shipment for test to avoid additional errors.

References

[1] An electronic ratio error set for current transformer calibrations, R. L. Kahler, IEEE

Trans. Instr. Meas., IM-28, No. 2, 162-164 (June 1979).

[2] A wide range current comparator system for calibrating current transformers, T. M.

Souders, Trans. App. Syst.. PAS-90, No. 1, 318-324 (Jan./Feb. 1971).

[3] Wide-band two-stage current transformers of high accuracy, T. M. Souders, IEEE

Trans. Instr. Meas., IM-21, No. 4, 340-345 (Nov. 1972).

G. High Voltage and Energy Measurements

1. Voltage Dividers and High Voltage Resistors

The calibration of ratio devices such as voltage dividers need not be referred to the

national standards of inductance or resistance or to any other national standard. However,

methods and equipment are available at the Bureau for the measurement of voltage ratios with high accuracy, and a regular calibration service is provided for certain types of

voltage dividers which are sufficiently stable for use as reference standards.

Resistive Voltage Dividers and High Voltage Resistors

Resistive dividers and resistors designed for use at high voltage levels are accepted

for calibration only if they are nearly corona-free at the rated operating voltage and are designed to have small temperature and voltage coefficients.

High Voltage Pulse Dividers

Ratios of resistive, capacitive or mixed voltage dividers are determined under pulsed

high voltage conditions. Determinations employ specially designed pulse dividers and

calibrated Kerr cells as reference standards. Pulses applied during calibration are intended

to simulate the divider's routine operation. Calibrations are made at selected voltage

intervals up to 300 kV as requested.

45
Dividers Used to Measure High Voltage in Diagnostic X-Ray Units

The calibration of dividers used to measure the high voltage in diagnostic x-ray units

consists of three measurements. These are the measurements of the variation of the ratio with a change in the frequency of the applied voltage in the frequency range of dc up to 10,000 Hz; a determination of the ratio under direct voltage at 25 kV; a measurement of

any voltage induced variation of the ratio in the voltage range from 20 to 70 kVp.

References

[1] X-CAL—A calibration system for electrical measurement devices used with diagnostic x-ray units, R. H. McKnight and R. E. Hebner, NBSIR 79-2072, 74 pages (June

1980).

[2] Evaluation of a multimegavolt impulse measurement system, R. E. Hebner, D. L. Hillhouse, and R. A. Bullock, NBSIR 79-1933. 97 pages (Nov. 1979).

[3] Calibration of high-voltage pulse measurement systems based on the Kerr effect, NBSIR

77-1317, 33 pages (Sept. 1977).

[4] Special shielded resistor for high-voltage measurements, J. H. Park, J. Res. Nat. Bur.

Stand. 66C, No. 1, 19-24 (Jan.-Mar. 1962).

2. High Voltage Capacitors and Capacitance Bridges

Calibration services are provided for capacitors and capacitance bridges having

voltage and/or current ratings beyond the capability of the facilities used in providing the

services described above (i.e., voltages > 100 V at 60 Hz). Gas-dielectric capacitors

(values to 1000 pF), and high voltage capacitance bridges can be calibrated at 60 Hz.

Other frequencies are also available at reduced voltages. Power factor correction

capacitors rated up to 1000 jliF and 10 kVA can also be calibrated at 60 Hz. The above

calibrations are special, and advance arrangements must be made. For devices exceeding

the above voltage, current, or kVA ratings, see "High Voltage Field Calibrations" below.

References

[1] A wide-range high-voltage capacitance bridge with one ppm accuracy, O. Petersons and

W. E. Anderson, IEEE Trans. Instr Meas., IM-24, No. 4, 336-344 (Dec. 1975).

[2] An international comparison of high voltage capacitor calibrations, W. E. Anderson, R.

S. Davis, O. Petersons, and W. J. M. Moore, IEEE Trans. Power App. Syst., PAS-97,

No. 4, 1217-1223 (July/Aug. 1978).

3. Kerr Electro-Optical Pulse- Voltage-Measuring Systems

Calibration services are provided for Kerr cells used for electro-optical measurement

of high voltage pulses. The Kerr cell constant is determined by reference to calibrated

pulse-voltage measurement systems. Calibrations can be performed for Kerr cells designed

for measurement of pulse voltages peaking as high as 300 kV. Service is provided on a

special test basis. (Listed in the Appendix under "High Voltage and Energy

Measurements.")

References

[1] Recent refinements and developments in Kerr system electrical measurement techniques, E. C. Cassidy, W. E. Anderson, and S. R. Booker, IEEE Trans. Instr. Meas., IM-21,

No. 4, 504-510 (Nov. 1972).

46
[2] Calibration of high-voltage pulse measurement systems based on the Kerr effect, R.

E. Hebner and M. Misakian, NBSIR 77-1317, 33 pages (Sept. 1977).

4. High Voltage Field Calibrations

Calibration services are provided for devices such as standard capacitors, dividers,

inductive and capacitive voltage transformers, bridges, and power factor correction

capacitors whose physical size and/or voltage and power ratings exceed the capabilities of

NBS in-house facilities or preclude shipment to NBS. Calibration can be carried out in the

client's laboratory or plant, in a mutually agreed-upon commercial, governmental, or

university laboratory, or in special cases, in the field. Charges for the service will be

actual expenses. These include preparation and shipping of test gear, travel and living

expenses for test personnel, data analysis and report writing, and overhead. Since this is a

special service, prior consultation and arrangement is essential. However, use of this service is strongly encouraged.

NBS has also developed a mobile system for the precise calibration of CCVT's,

coupling-capacitor voltage transformers, used by the electric power industry for metering.

This precision system can be transported via van to the power station site of CCVT's. The

calibration unit is then assembled and placed in parallel with the transformer to be

calibrated. The outputs of the two devices are compared to obtain the precise ratio of the

device under test.

References

[1] A prototype system for on-site calibration of coupling capacitor voltage transformers

(CCVTS), D. L. Hillhouse, O. Petersons, and W. C. Sze, IEEE Trans, on Power App.

Syst., PAS-98, No. 3, 1026-1036 (May/June 1979).

[2] A prototype field calibration system for coupling capacitor voltage transformers

(CCVTS), D. L. Hillhouse, O. Petersons, and W. C. Sze, EL-690 Final Report

Project 134-1, Electric Power Research Institute, Palo Alto, CA, 193 pages (Apr.

1978).

5. Watthour Meters

Only portable standard watthour meters (rotating standards) will be accepted for

test, which consists of determinations of the percentage registration of the meter "as received." If meters are to be cleaned and adjusted this must be done before they are submitted for test. The Bureau does not undertake the cleaning and adjustment of meters and does not knowingly begin tests on faulty meters. Before tests can be started the test conditions must be completely specified by the user as to current and voltage ranges to be

tested, frequency, applied voltage and current, and power factor. A guide listing a limited

yet adequate schedule of tests is available at no charge. Test voltages should be chosen

from the following values: 1, 2, or 4 times 110, 115, 120, 125, and 130 V (but not to exceed 480 V). Test currents should be chosen from the following values: 1, 10, or 100

times 0.5, 0.75, 1, 1.25, 1.5, 2, 2.5, 3, 3.75, 4, 5, 7.5 A (but not to exceed 100 A). Unless

otherwise specified, test runs on portable standard watthour meters (rotating standards)

are of approximately 100 s duration. The meters are energized for at least 1 h at rated

voltage and current on one range before starting the test. Normally values are reported

with an uncertainty of ±0.05 percent.

The NBS Measurement Assurance Program (MAP) for electric energy (NBS

Technical Note 930) is designed to evaluate energy measuring equipment. An NBS-owned

transport standard watthour meter (WHM) is shipped to a customer, and a tie to the U.S.

national energy unit is made without the down-time encountered when WHM's are

calibrated at NBS. In addition, and more important, for those who calibrate reference

47
standard WHM's, a MAP standard can be used to evaluate an entire measuring system. By

request, for smallest uncertainties, the MAP transfer standard WHM's can be calibrated

by means of the current comparator system used to establish the unit of energy. Tests are made at 5 A, 120 V, unity and 0.5 pF, current lagging voltage.

References

[1] A measurement assurance program for electric energy, N. M. Oldham, Nat. Bur. Stand.

(U.S.) Tech. Note 930, 17 pages (Sept. 1976).

[2] Sampling techniques for electric power measurement, R. S. Turgel, Nat. Bur. Stand.

(U.S.) Tech. Note 870, 31 pages (June 1975).

[3] Transfer of the kilowatthour, S. R. Houghton, IEEE Trans. Power App. Syst., PAS-94,

No. 4, 1232-1240 (July/Aug. 1975).

[4] A current comparator system to establish the unit of electrical energy at 60 Hz, K. J. Lentner, IEEE Trans. Instr. Meas., IM-23, No. 4, 334-336 (Dec. 1974).

48
CHAPTER V

V. Electromagnetic Measurements at Radio, Microwave,

Millimeter Wave and Laser Frequencies

A. Introduction

The National Bureau of Standards provides methods and standards of measurement

for electromagnetic quantities used in devices and systems in the electronics and laser industries and in related fields. This support is provided as a function of frequency ranging

from 30 kHz to optical frequencies. However, specific electromagnetic characteristics are usually not available as a continuous function of frequency except for limited portions of

this frequency spectrum. Besides measurements requiring coherent frequency sources,

pulse, noise and electromagnetic interference measurements are provided. As a result calibration and consultation services are available for voltage, power, attentuation,

impedance, noise, fields and other electromagnetic quantities. Details on ranges and

magnitudes for specific quantities are itemized in the service listings to follow below.

Special Instrumentation and Scheduling Requirements

In order to provide meaningful and reproducible (uncertainties minimized)

electromagnetic measurements or calibrations the terminations and interfaces must be well

characterized. Therefore standards, instruments, and devices submitted for calibration or

evaluation, as a general rule, must be equipped with precision coaxial connectors or

Electronic Industries Association standard rectangular waveguide sizes terminated with

appropriate flanges. Terminations, frequency ranges, magnitudes and other details for a given type of service are stipulated in the Appendix and in service descriptions which will follow below. To improve service, reduce cost and turnaround time and facilitate planning, some of the electromagnetic services are available only on a scheduling basis. If services are available on a scheduled basis the Appendix will so indicate for the quantity

listed. Further considerations on terminations are as follows:

(a) Coaxial Connectors

In coaxial systems, the use of precision coaxial connectors is strongly advocated for calibrations involving immittance, attenuation, voltage, and other quantities. Precision

coaxial connectors are those which meet or exceed the electrical and mechanical

specifications set forth by the Institute of Electrical and Electronic Engineers [1,2]. As a general rule only those standards and instruments so equipped can be calibrated to the

highest accuracies.

For immittance, the difference in calibration uncertainties may vary by as much as a

factor of 10 depending upon whether the connectors on an item are of the precision or

nonprecision type.

Similar advantages are realized in attenuation measurements where the voltage

standing wave ratio (VSWR) of a mated pair of connectors is highly important. A typical

measurement at 4 GHz might yield the following results. With precision sexless coaxial

connectors, mismatch errors, due to mated connector pairs, are of the order of 0.01 dB as compared to 0.02 dB for the improved Type N and 0.05 dB for the ordinary Type N

connectors. Systematic errors in the measurement system are about 0.03 dB. Therefore the

precision connector is a practical necessity for utilizing the full capabilities of the

measurement system.

Calibrations involving power are not as critically dependent upon connector

uncertainties because the VSWR of a connector pair need only be 1.05 or better to avoid

49
significant uncertainties at the present state of the art. However, the use of precision

connectors in power instrumentation provides assurance that connector VSWR's greater

than 1.05 are not present to hmit the best available performance.

References

[1] Precision coaxial connectors (IEEE Standard 287-1968) (ANSI C16.43-1972), (Institute

of Electrical and Electronic Engineers, Inc., New York, NY).

[2] B. O. Weinschel standardization of precision coaxial connectors, Proc. IEEE 55, 923

(June 1967).

(b) Rectangular Waveguides (Flange Terminations)

Each EIA designated waveguide size covers a range of frequencies. In general, the

measurement systems provide complete and continuous coverage as appropriate for the

various waveguide sizes. However, for some electromagnetic quantities an NBS automatic

network analyzer is used. Details are provided in the sections to follow, describing the

quantities for which services are available. Information concerning connectors,

frequencies, and magnitudes also appears in the Appendix.

A common metrology practice employs the echelon or chain system of calibration

with a degradation in accuracy in each step of the chain. Each succeeding laboratory is less accurate than the one from which it received its calibration. This degradation can be

minimized if an operating laboratory maintains adequate control procedures and utilizes measurement assurance programs (MAPs) available from NBS. It is intended that ultimately measurement assurance programs will be available for critical electromagnetic

quantities to allow laboratories to maintain these units with state-of-the-art uncertainty.

The rate at which such programs are established is contingent upon the demand and

availability of NBS staff. As measurement assurance programs become available they will be listed in the Appendix.

B. Attenuation Measurements

1. Special Attenuation Measurement Services and Consultation

The specific attenuation services listed below are available on a limited basis depending on other demands and staff availability. Measurements not listed can be

provided if sufficient advance notice is given and resources permit. The cost of such

services must be negotiated and will, in general, be higher than other established services. Consultation by telephone or written correspondence as indicated in the Appendix is suggested. Often a measurement technique can be suggested that will permit the customer

to perform his calibrations in-house with appropriate reference to other NBS supported

standards. (Note the definitions given in sec. V.B.5.)

2. Attenuation Measurement of Coaxial Attenuators

Coaxial fixed and variable attenuators are measured on the NBS modified automatic

network analyzer (ANA) over the frequency range indicated in the Appendix (see

Attenuation in the Appendix).

All measurements are made by the substitution method, which requires that the

connectors used be asexual or that the attenuator have a male connector at one port and a female at the other. If an adapter is required to comply with the foregoing, it must be

supplied with the attenuator. The combination will be calibrated as one unit. In addition to measurements performed on the ANA, fixed frequency of 30 MHz is available referenced to the NBS waveguide below cutoff standard at this frequency.

50
Limits of Uncertainty

Coaxial attenuators are normally measured in a system having a characteristic

impedance of 50 il. Because measurement limits of uncertainty are degraded by any

deviation from this characteristic impedance, the types of allowable connectors are

limited. Connectors having a known plane of reference, such as the sexless precision

connectors or Type N connectors meeting Mil C 39012, are acceptable. Limits of

uncertainty also depend upon the VSWR of the individual attenuator, quality of the attenuator and connectors, and the magnitude of the attenuation [IJ. Typical systematic

uncertainties range from 0.03 to 0.05 dB/10 dB.

3. Attenuation Calibrations of Variable Rectangular Waveguide Attenuators

Variable waveguide (usually rotary vane) attenuators are calibrated in the frequency

bands indicated in the Appendix by the IF-Substitution technique referenced to 30 MHz,

direct RF substitution or on the NBS modified ANA as appropriate.

It is suggested that measurements requested be held to a minimum number of

settings at a single band-center frequency which should be sufficient to determine the

characteristics of the device. It is further recommended that previously calibrated units not be resubmitted unless tests performed by the user indicate a shift in values.

Limits of Uncertainty

This is a function of resettability and input VSWR of the waveguide ports as well as internal leakage and quality of flanges. Devices submitted should be in the best possible

condition to justify calibration and insure stability of assigned values. Typical systematic

uncertainties range from 0.03 to 0.05 dB/10 dB.

4. Attenuation Measurements of Waveguide Below Cutoff (Piston) Attenuators With Coaxial Connectors

Measurements on piston (WBCO) attenuators are performed at 30 MHz as indicated

in the Appendix (see Attenuation). These attenuators are normally quite stable and seldom

need recalibration unless damaged or mechanically worn. Since any laboratory can perform independent checks to determine continuing repeatability and linearity of

attenuation we do not recommend periodic NBS recalibrations. This recommendation, in part, is also because more damage is suffered in transit than in daily use. In any

measurement, the maximum power delivered to the test attenuator will not exceed 400

mW. If the attenuator cannot tolerate this power level, some reduction of measurement

range indicated in the Appendix will be required.

Limits of Uncertainty

These attenuators are normally calibrated in a system having a characteristic

impedance of 50 ft. Since only measurements of incremental attenuation are made on this type of attenuator. Type BNC, C, TNC, and similar connectors are acceptable, but

precision connectors* are preferred to reduce leakage [1]. Limits of uncertainty depend

upon the quality of the attenuator and connectors, as well as upon the VSWR of the

attenuator, and the magnitude of attenuation. Typical systematic uncertainties range from

0.003 to 0.005 dB/10 dB.

•Standard connector pair or waveguide joint. A "standard connector" is one which is made precisely to standard specifications for the particular type of connector under consideration. Standard connector pairs usually have low but measurable loss and reflections [1,3,4].

51
5. Definitions

(1) Insertion loss—The 1959 IRE standards [2] give two definitions for insertion

loss, one in which system mismatch is not specified, the other in which the system is nonreflecting. The definitions contradict each other since the insertion loss of an

attenuator will be different for each case. The measurement procedure for both definitions

is to open the system, insert the attenuator, and note the relative power absorbed by the

load (or detecting device) before and after insertion. The insertion loss in decibels is computed from these two values. If the attenuator is variable and remains in the system,

the initial and final powers absorbed by the load for two settings are used. This

determination is more properly called "Change in Insertion Loss." Either definition is entirely adequate for a single, unique system, but if the loss (or

gain) measurement is to be transferred from one laboratory to another, more must be

specified about system conditions.

(2) Attenuation—This is defined as the insertion loss in a nonreflecting system (Fq =

rL=0) where Fq and Fl are defined as the reflection coefficients of the generator and

load, respectively. In actual measurement these initial conditions cannot be achieved

because of imperfections in connectors or adapters and the uncertainties in reducing

system reflections to zero. Since attenuation cannot be measured exactly, the more

practical term "standard attenuation" is coming into more general use.

(3) Standard attenuation—This is defined as the insertion loss of a linear two-port

device in a nonreflecting system which is initially connected together at the insertion

point by a standard connector pair (as defined earlier) or waveguide joint, the

nonreflecting condition being obtained in the standard waveguide sections to which the

standard connectors or waveguide joints are attached. The standard attenuation is the

ratio expressed in decibels of the powers absorbed by the load before and after insertion of

the two-port device being calibrated.

(4) Incremental attenuation—Incremental attenuation is the change in attenuation of an adjustable attenuator between a reference setting (usually zero) and any other setting.

The same restraints on system conditions apply as for attenuation and standard

attenuation. The term "differential attenuation" is sometimes applied to this case and

usually refers to two non-zero settings.

References

[1] Microwave attenuation measurements and standards, R. W. Beatty, Nat. Bur. Stand.

(U.S.). Monogr. 97, 50 pages (Apr. 3, 1967).

[2] IRE standards on antenna and waveguides: Definitions of terms, Proc. IRE 47, No. 4, 568-582 (1959 Standards 59 IRE 2.S1).

[3] Effects of connectors and adapters on accurate attenuation measurements at microwave

frequencies, R. W. Beatty, IEEE Trans. Instr. 13, 272-284 (Dec. 1964).

[4] Insertion loss concepts, R. W. Beatty, Proc. IEEE 52, No. 6, 663-671 (June 1964).

6. Note:

Also see section J. 3 for wideband attenuation or gain.

C. Electromagnetic Fields and Microwave Antenna Measurements

Accurate measurement of antenna gain, pattern, and polarization are generally

available from about 750 MHz to about 75 GHz. However, measurements of all three

characteristics may not be practical for a given antenna because the measurement

accuracy, capability, and cost depend on the frequency, type and size of antenna, and the

parameters to be measured. Therefore, a particular measurement must be negotiated in advance. The following methods and facilities are used for these measurements.

52
1. Planar Near-Field Scanning Method

With this technique, gain, pattern and polarization parameters are calculated from

near-field amplitude and phase measurements taken over a plane area close to the test antenna. The absolute gain can be determined to within about ±0.15 dB, the polarization

axial ratio to within about ±0.10 dB/dB and side lobe levels can be obtained down to -50

or -60 dB. (The exact uncertainties will depend on the frequency, type, and size of

antenna, etc.) Antennas with apertures up to about 3.5 m in diameter can be managed.

Measurements can be made from 750 MHz up to 75 GHz.

References

[1] Correction of near-Held antenna measurements made with an arbitrary but known

measuring antenna, D. M. Kerns, Electronics Letters 6, No. 11, 346-347 (May 28,

1970).

[2] New method of gain measurement using two identical antennas, D. M. Kerns, Electronics

Letters 6, No. 11, 348-349 (May 28, 1970).

[3] Recent experimental results in near-field antenna measurements, R. C. Baird, A. C. Newell, P. F. Wacker, and D. M. Kerns, Electronics Letters 6, No. 11, 349-351 (May

28, 1970).

[4] Plane-wave scattering-matrix theory of antennas and antenna-antenna interactions: Formulation and applications, D. M. Kerns, Res. Nat. Bur. Stand. (U.S.), SOB

(Math. Sci.), No. 1, 5-51 (Jan.-Mar. 1976).

. 2. Extrapolation Range Measurements

In this method, the received signal transmitted between a pair of antennas is measured as a function of the separation distance between the antennas. The antennas need not be identical, and no assumptions concerning the polarization are required. The

method is not well suited for pattern measurements, but it is the most accurate technique

known for absolute gain and polarization measurements. Above 1 GHz, the accuracies are

typically ±0.10 dB for gain measurements, and ±0.05 dB/dB for polarization axial ratio measurements. There are upper size limitations associated with existing NBS extrapolation

ranges. These limitations depend on the type of antenna, the frequency, and the desired

measurements and accuracies. Therefore, negotiations must be conducted prior to submitting antennas for calibration to ascertain if all requirements can be met.

Reference

[1] Accurate measurement of antenna gain and polarization at reduced distances by an

extrapolation technique, A. C. Newell, R. C. Baird, and P. F. Wacker, IEEE Trans. Antennas Propagat. AP-21, No. 4, 418-431 (July 1973).

D. Impedance and/or Reflection Coefficient

Services provided in this category are for passive devices over the frequency range

from 30 kHz to 65 GHz. Specific frequencies where calibrations are available are listed in the Appendix. Highest accuracy is guaranteed only for standards equipped with precision

coaxial connectors or waveguide flanges. Standards submitted for calibration should be in good repair and except for very minor cleaning of connector surfaces, should require no

precalibration maintenance. NBS does not provide repair services so that items received

which require maintenance will be returned to the sender and a handling fee will be

charged.

53
Calibration service for measuring instruments such as bridges or meters is not

provided. It is recommended that the accuracy of these instruments be verified by the owner through the use of stable standards especially selected for particular values and

frequencies appropriate to the instrument in question.

Measurement Conditions

All calibrations are performed under typical ambient laboratory conditions of 23 "C,

and an atmospheric pressure of approximately (8.4±0.2)X 10* Pa. Services at ambient

conditions outside these limits is not provided. Also the power applied to any device

being calibrated does not exceed 1 W. Additional information pertaining to immittance

(impedance and admittance) measurement and standards is contained in the following

references.

References

Lumped Parameter

[1] The measurement of lumped parameter impedance: A metrology guide, R. N. Jones, Nat.

Bur. Stand. (U.S.), Monogr. 141, 211 pages (June 1974).

[2] Impedance of lumped circuits, L. E. Huntley and R. N. Jones, Proc. IEEE 55, No. 6, 900-911 (June 1967).

[3] A technique for extrapolating the 1 kc values of secondary capacitance standards to higher frequencies, R. N. Jones, Nat. Bur. Stand. (U.S.), Tech. Note 201, 15 pages

(Nov. 1963).

[4] A precision, high frequency calibration facility for coaxial capacitance standards, R. N.

Jones and J. E. Huntley, Nat. Bur. Stand. (U.S.), Tech. Note 386, 27 pages (Mar.

1970).

[5] Standards for the calibration of Q-meters, 50 kHz to 45 MHz, R. N. Jones, J. Res. Nat. Bur. Stand (US), 58C (Eng. and Instr.), No. 4, 243-248 (Oct.-Dec. 1964).

[6] Evaluation of three-terminal and four-terminal pair capacitors at high frequencies, R. N.

Jones, Nat. Bur. Stand. (U.S.). Tech. Note 1024, 15 pages (Sept. 1980).

Coaxial

[1] Impedance measurements in coaxial waveguide systems, R. L. Jesch add R. M. Jickling,

Proc. IEEE. 55, No. 6, 912-923 (June 1967).

/. Special Measurement Services arid Consultation on Measurement Problems

Regular established calibration services for impedance standards are generally

described in succeeding paragraphs and specific information is provided in the Appendix

(see Impedance).

2. Capacitance, Two-Terminal, Low-Loss

In the frequency range from 30 kHz to 250 MHz capacitance calibrations to a minimum uncertainty of ±0.1 percent are available from 1 pF to 1 /liF depending upon

frequency. The upper capacitance limit for calibration decreases as the frequency

increases and is 50 pF at 5 MHz and above. (See refs. [1], [2], and [3] above.)

At 1 MHz a special high accuracy service is available for capacitors with nominal

values of 50, 100, 200, 500 and 1000 pF provided they are equipped with 14 mm coaxial connectors. See Appendix (Impedance) for additional requirements. (See ref. [4]).

54
Reports of calibration for capacitors normally do not give conductance values. This

is because capacitors of standard quality, especially those with air-dielectric, have

conductance values too small to be measured accurately at the present state-of-the-art.

Reference

A technique for extrapolating the 1 kHz values of capacitance standards to higher

frequencies is described by R. N. Jones in NBS Technical Note 201 (Nov. 1963). This

reference describes a technique for obtaining a high frequency value of a capacitor

equipped with an unshielded (banana plug) connector. The measurement technique yields

effective capacitance values at high frequencies using the capacitance value at 1 kHz and

the residual series inductance.

3. Capacitance Three-Terminal Low-Loss

Services are available at 100 kHz, 465 kHz and 1 MHz for capacitors having values

of 10, 100, and 1(XX) pF. Calibration uncertainty is typically ±0.06 percent for all frequencies and values except for 1000 pF at 1 MHz where the uncertainty is nominally

±0.1 percent.

Fixed value reference standards are maintained by NBS for values of 10, 100, and

1000 pF. High quality three-terminal capacitance standards should have low residual series inductance (<0. 1 /aH). This being the case, it may be assumed that to an accuracy of

±0.10 percent, the capacitances of standards of 1 pF or less is the same at 1 MHz as it is at 1 kHz. Thus, it is unnecessary to have capacitors smaller than 10 pF calibrated at 1 MHz.

4. Inductors, Two-Terminal, High-Q

In the frequency range from 10 kHz to 250 MHz inductance calibrations to a minimum uncertainty of ±0.1 percent are available from 0.01 \iYi. to 1 H. The upper

inductance limit for calibration decreases as the frequency increases and is 1 jLiH at 250

MHz. In the Report of Calibration, the resistance of the inductor is also given.

5. Resistors, Two-Terminal, Low-Q

In the frequency range from 30 kHz to 250 MHz resistance calibrations to a minimum uncertainty of ±0.1 percent are available from 0.1 O to 10 Mft. At higher

frequencies the upper limit for resistance decreases and is 20 kft at 250 MHz. Calibration

services for resistors less than 20 kfi are not available at frequencies above 2 MHz.

Reports of Calibration for resistors will include the inductance or capacitance

associated with the resistor. Equivalent series values are normally given for inductive

resistors and equivalent parallel values for capacitive resistors.

6. Q-Standards

Standards for Q-measurements are maintained at NBS. These are high Q inductors

equipped with banana plug connectors at a spacing of 1 inch on centers. These standards

have inductance values of 0.25, 2.5, 25, 250, 25(X) and 25,(XX) /itH, and effective Q values

from 100 to approximately 600. These serve as working standards for calibration of

Q-standards of a similar type. Calibration frequencies range from 50 kHz to 45 MHz. The

calibration report includes effective resonating capacitance and effective Q. Uncertainties

are of the order of ±0.2 percent for capacitance and 2 percent for Q. Provisions are made

for calibrating each Q-standard at three frequencies, however, adequate assurance of

stability is usually provided by recalibrating only at the center frequency.

55
Limits of Uncertainty

Estimated limits of uncertainty are based upon a statistical analysis of previously

obtained calibration data. These uncertainties are believed to result solely from sources of

random error as opposed to known systematic errors.

Reference

[1] Standards for the calibration of Q-meters, 50 kHz to 45 MHz, R. N. Jones, / Res. Nat.

Bur. Stand. (U.S.), 58C (Eng. and Instr.), No. 4, 243-248 (Oct.-Dec. 1964).

7. Standards for Distributed Parameter Measurement

Depending upon the application, lumped parameter impedance standards such as capacitors, inductors and resistors are replaced by distributed parameter devices in coaxial

systems in the region of 100 to 300 MHz. This brings about the need for standards of

impedance magnitude and phase angle, voltage standing wave ratio (VSWR), phase shift and length of equivalent air-dielectric transmission line. Services of this type extend from

0.1 to 8 GHz.

Coaxial Impedance Measurements

Services are available for complex impedance and reflection coefficient, voltage

standing wave ratio (VSWR), insertion phase and length of equivalent air-dielectric transmission line. The length of equivalent air line is defined as the length of a section of

lossless reference coaxial air line required to produce the same total insertion phase as the

item being measured.

General

A short or open-circuit termination may be furnished with calibration items

requiring measurement of impedance, phase shift, and length of equivalent airline in order

to establish a reference plane, reproducible by the customer. VSWR measurements are normally referenced to 50 fl. The calibration services usually apply to determining the impedances or VSWR of

standard terminations and mismatches, or to determining the length of sections of

precision coaxial air-dielectric transmission line.

Reference

[1] Impedance measurements in coaxial waveguide systems, R. L. Jesch and R. M. Jickling,

Proc. IEEE 55, No. 6, 912-923 (June 1967).

Reflection Coefficient Magnitude Measurements

of Reflecting and Nonreflecting Waveguide Ports

Waveguide ports are measured in a reflectometer system relative to a sliding short or sliding load in a precision section of waveguide.

Some measurements in waveguide bands below 18 GHz are performed on the NBS

modified automatic network analyzer (ANA) while all of those above 18 GHz are performed on manual fixed-frequency systems. (See Appendix.)

It is recommended that previously calibrated units not be resubmitted unless tests performed by the user indicate a shift in values. The reflectors must be fitted with standard waveguide flange-type connectors. The

faces of these flanges should be machined flat and smooth and should not contain

56
protrusions or indentations. Considerable care must be exercised in keeping the mating

connector flange surfaces smooth and clean. Accurate alignment of the interior surfaces of

the joining waveguides at the flange junction also is very important. The back of the

flange which makes contact with the connecting bolts should be nominally flat and free of

soft materials including paint. The connecting holes of the flange should be symmetrically

and accurately aligned to the rectangular waveguide opening. These precautions must be

observed when using a waveguide port in a precision measurement system.

The term nonreflecting as used here indicates that the waveguide port has been

designed or adjusted with the intent to produce a reflection coefficient magnitude, | F |

,

equal to zero. Although most waveguide ports for such applications cannot produce a

reflection coefficient magnitude identically equal to zero, their reflection coefficient

magnitudes often approach zero very closely.

Limits of Uncertainty

Assigned limits as noted in the Appendix depend upon the quality of the flanges as well as the numerical value of reflection coefficient magnitude. Systematic errors assigned

by NBS vary with waveguide size and relate to absolute dimensions of the precision

waveguide sections and internal surface finish. Reference

[1] A guide to the use of the modified reflectometer technique of VSWR measurement, W. J. Anson, /. Res. Nat. Bur. Stand. (U.S.), 65C (Eng. and Instr.), No. 4, 217-223 (Oct.-

Dec. 1961). (The measurement technique utilized in reflection measurements is described in this paper.)

E. Laser Power and Energy

The National Bureau of Standards develops and maintains the U.S. National

Standards for measurement of laser power and energy. These standards are isoperibol type

calorimeters which compare the absorbed laser radiation to an equivalent quantity of

electrical energy. Limited calibration services and Measurement Assurance Programs

(MAP's) are available for laser power and energy measurements.

7. NBS Laser Standards

The national standards for laser power and energy are three types of isoperibol type

calorimeters. These calorimeters compare the absorbed laser radiation to an equivalent

quantity of electrical energy. These calorimeters are maintained in a measurement system

such that other laser power or energy meters can be calibrated against the national

standards. The uncertainty of these calibrations is about 1 to 5 percent at the 99 percent

level of confidence depending on the power (or energy) and wavelength at which the

calibration is performed.

2. NBS Laser Power and Energy Dissemination Services

The laser power and energy measurement services consist of limited calibration of

power or energy meters and Measurement Assurance Programs (MAP's). The MAP's are implemented by means of transfer standards which have been evaluated and characterized

relative to the national standards. The characteristics of these transfer standards are well

understood, and their associated accuracies are not significantly different from the

accuracies associated with direct comparisons to national standards. '

57
Reference

[1] Quality assurance program for the NBS, C, K, and Q laser calibration systems, W. E. Case, NBSIR 79-1619, 96 pages (Aug. 1979).

F. Noise Temperature Measurements

DeHnition

The Effective Noise Temperature, T„^, is proportional to the power emerging from

the output port of the coaxial or waveguide noise source when it is connected to a nonreflecting load. The noise temperature, T, of the noise source is analogous to the

available power [1,2] from a source and is obtained when corresponding reflection

coefficients for source and load are complex conjugates of each other (characteristic

impedances being chosen real). The relationship between the noise temperature and

effective noise temperature is

r„,= 7Ti-|r|^]

where | F | is the reflection coefficient magnitude of the coaxial or waveguide noise

source, and and T are in kelvins.

General

Coaxial Noise Sources:

(1) Effective noise temperature measurements are made on one-port devices that serve as coaxial noise sources under conditions of continuous, unmodulated operation.

(2) In noise sources utilizing a gas-discharge tube, the tube should be securely fitted into a mount terminated at the cathode end with a suitable matched load. Direct current required for the tube should not exceed 300 mA but should be sufficient to prevent

excessive plasma oscillations. Complete information on the operating current of the tube

and a wiring diagram of the noise source must be supplied.

(3) In noise sources utilizing a temperature-limited diode, the diode should be

securely fitted into a mount terminated at the cathode end with a suitable matched load. Complete information on the operating current of the tube and a wiring diagram of the

noise source must be supplied.

(4) Each type of noise source must be fitted with a 14-mm precision output

connector. A 14-mm adapter is acceptable if it is securely attached to the existing coaxial

connector on the mount.

Rectangular Waveguide Noise Sources:

(1) Effective noise temperature measurements are made on waveguide noise sources

(usually a gas-discharge tube) under conditions of continuous, unmodulated operation.

(2) The direct current required for normal operation of the gas discharge tube

should not exceed 200 mA but should be sufficient to prevent excessive plasma oscillation.

(3) Complete information on the operating current of the tube and a wiring diagram

of the noise source must be supplied. In some cases it is necessary to request appropriate

electrical connectors to be supplied with the unit for use during calibration.

(4) The gas-discharge tube should be secured in a terminated waveguide noise-tube

mount.

References

[1] Basic theory of waveguide junctions and introductory microwave network analysis, D.

M. Kerns and R. W. Beatty, Chapter in International Series of Monographs on

Electromagnetic Waves 13, 150 pages (Pergamon Press, Inc., New York, NY, 1967).

58
[2] Measurement of effective temperatures in microwave noise sources, J. S. Wells, W. C. Daywitt, and C. K. S. Miller, IEEE Trans. Instr. Meas. IX-13, No. 1, 17-28 (Mar.

1964). (This reference describes the method of measurement and error analysis.)

[3] Some applications of the Josephson effect, R. A. Kamper, Nat. Bur. Stand. (U.S.), Tech. Note 381, 63 pages (Oct. 1969). (This technical note contains a discussion of noise

thermometry at very low temperatures.)

G. Phase Shift

1. Special Phase Shift Measurement Services and Consultation

The specific phase shift services listed below are available on a limited basis depending on other demands and staff availability. Measurements not listed can be

provided if sufficient advance notice is given. The cost of such services must be

negotiated and will, in general, be higher than the established phase shift services. Consultation by telephone or written correspondence is suggested as indicated in the

Appendix. Often a measurement technique can be suggested that will permit the customer

to perform calibrations in-house with appropriate reference to other NBS-supported

standards.

2. Coaxial Devices

Fixed and variable coaxial two ports are measured on the NBS modified automatic

network analyzer (ANA) over the frequency range indicated in the Appendix. In addition,

measurements can be performed with reference to a precision variable air line at 30 MHz.

Because of the specialized nature of coaxial phase shifting components, it is requested that prior discussions be held before submission of any devices to NBS.

Limits of Uncertainty

The limits of uncertainty stated are the sums of systematic, mismatch, and random

errors. Their relative values are dependent upon the particular standard under calibration. The VSWR of the device and the quality of the connectors will contribute to the

uncertainties of calibration. Items to be calibrated must be fitted with connectors having a known plane of

reference such as the sexless precision connectors, or Type N connectors meeting Mil C

39012.

General

The phase angle measured is i|/+360 «, where n is an integer. The value of n is not

determined.

Definitions

(1) Characteristic insertion phase shift (phase change)—is the phase change of a wave

incident upon the load before and after insertion of a two-port device between the

generator and load of a stable nonreflecting system.

(2) Characteristic phase shift difference (phase change)—is the phase change of a wave incident upon the load from an initial to a final condition (setting) of a two-port

device between the generator and load of a stable nonreflecting system.

Note: The following conditions apply: (1) The frequency, the load impedance, and

the generator characteristics (internal impedance and available power) have the same

values before and after the device is inserted or changed; (2) the joining devices

(connectors or adapters) belonging to the system all conform to a given set of standard

specifications (the same specifications must be used by different laboratories if

59
measurements are to agree precisely); (3) the nonreflecting conditions are to be obtained

in uniform, standard sections of transmission line on the system sides of the connectors at the place of insertion; (4) this definition is not for phase shift in general, but for a

particular phase shift which is characteristic of the device under measurement.

3. Waveguide Devices

In a rectangular waveguide the measurement services are limited to phase shift difference. Measurements are made on continuously variable waveguide phase shifters with the zero value of the scale as the normal reference position. Since 360 mechanical

degrees of rotation represent 720 electrical degrees, attention should be given to the

relationship between dial indication and actual mechanical position of the rotating vane

assembly.

Measurements are performed for phase angle values from 0° to 720°. Variable phase shifters should have a repeatability of dial setting better than ±0.5°

and an input VSWR less than 1.4 at each waveguide port.

It is suggested that measurements requested be held to a minimum number of

settings at a single band center frequency which should be sufficient to determine the

characteristics of the device. We further recommend that previously calibrated units not be resubmitted unless tests performed by the user indicate a shift in values.

Limits of Uncertainty

The estimated Hmits of uncertainty range from ±0.1° to ±1.0° for input VSWR

values of the phase shifter waveguide ports in the range 1.1 to 1.4.

Reference

[1] Evaluation of a microwave phase measurement system, D. A. Ellerbruch, /. Res. Nat.

Bur. Stand. (U.S.), 69C (Eng. and Instr.), No. 1, 55-65 (Jan.-Mar. 1965).

H. Power Measurements

Regular calibration services include thermistor-type bolometer units having a nominal resistance of either 100 or 200 Cl at a bias current between 3.5 and 15 mA and

thermoelectric (TE) power sensor-power meter units. Thermistor-type bolometer units have shown adequate stability over long periods of

time (approx. 10 yr) and warrant long recalibration intervals. Two- or three-year

recalibration intervals are recommended once the stability of a bolometer unit has been

verified. Thermoelectric power sensor-power meter units must be linear with power level to within 0.02 dB/10 dB on 10 mW range. Measurement of the output of the internal

power reference prior to calibration is required.

Assistance is available for applying published, technically valid measurement

techniques in lieu of previously available calibration services for coaxial and waveguide

calorimeters, power meters, and bolometer coupler units. The attainable limits of

measurement uncertainty using these techniques are comparable to those of the previously

available calibration services for these devices.

Reference

[1] Accurate microwave high power measurements in using a cascaded coupler method, K.

E. Bramall, / Res. Nat. Bur. Stand. (U.S.). 75C (Eng. and Instr.), Nos. 3 and 4, 181- 186 (July-Dec. 1971).

60
Services Available

Effective Efficiency

The effective efficiency is the ratio of the bolometrically substituted dc power in the

bolometer unit to the cw rf/microwave power absorbed by the bolometer unit. The

principal emphasis is on those calibrations and other tests requiring such accuracy as can be obtained only by direct comparison with NBS standards. However, in order to maintain efficient utilization of specialized equipment and skilled personnel, when

workload permits NBS may calibrate devices requiring lesser accuracy but suitable for working standards in plant or laboratory. Also, upon request, special measurements may

be made. Inquiries should describe clearly the measurement desired and indicate the

scientific or economic basis for the requirement.

Calibration Factor—Bolometer Units

The calibration factor is the ratio of the bolometrically substituted dc power in the

bolometer unit to the cw rf/microwave power incident upon the bolometer unit. Amplitude Reflection Coefficient

Amplitude reflection coefficient is the ratio of the reflected wave amplitude to the

incident wave amplitude.

Effective efficiency and reflection coefficient are included in the Report of

Calibration for all single frequency measurements. (For details see Appendix under Power

Measurements, Electromagnetic, Single Frequency.)

Effective efficiency and reflection coefficient are included in the Report of

Calibration for all multiple frequency broadband measurements. (For details, see Appendix under Power Measurements, Electromagnetic Multiple Frequency Broadband.)

Efficiency Factor—TE sensor-power meter units: Efficiency factor (mW/V) is the

ratio of the power absorbed by the sensor head to the dc voltage at the recorder output of

the meter on the 10 mW range. Reflection coefficient magnitudes at measured frequencies

are included in the report of calibration. The above services are restricted to the following conditions and equipment (except

for special calibrations).

Power Level: 10 mW (nominal)

Bolometer Units and TE Power Sensor—Power Meter Units

Connector Types: Coaxial—type N to 18 GHz

7 mm precision to 1 8 GHz

(Measurements will be performed only in frequency bands appropriate to connector

type.)

Waveguide:—WR15, and WR28 to WR284

(1) Single Frequency Measurements

Specify frequency for waveguide WR 15 (50.0-75.0 GHz) and WR28 to WR42

(2) Multiple Frequency Broadband Measurements

Schedule of Services

To improve service, reduce cost and "turnaround" time and facilitate planning,

measurements will be conducted according to the following schedule. Purchase orders

and devices should arrive at NBS/Boulder prior to the first day of the month in which

calibration is desired.

61
Type N Jan., Apr., June, Aug., Nov.

7 mm precision Mar., Sept.

Waveguide Feb., May, July, Oct., Dec.

Coaxial

100-100 MHz 10, 50, & 100 MHz

100-1000 MHz 100, 500, & 1000 MHz

10-1000 MHz 10, 50, 100, 500, & 1000 MHz

Note: Only bolometer units designed for low frequency operation

will be calibrated below 100 MHz.

Coaxial

1-2 GHz 50 MHz intervals

2-4 GHz 100 MHz intervals

4-8 GHz 200 MHz intervals 8-12.4 GHz 200 MHz intervals

12.4, 12.75-18[ GHz 250 MHz intervals

Waveguide

WR284 2.6-4.0 GHz 6 frequencies

WR187 4.0-5.8 GHz 100 MHz intervals

WR137 5.8-8.0 GHz 100 MHz intervals

WR112 7.0-10.0 GHz 200 MHz intervals

WR90 8.2-12.4 GHz 200 MHz intervals

WR75 10.0-15.0 GHz 250 MHz intervals

WR62 12.4-18.0 GHz 250 MHz intervals

Limits of Uncertainty

Effective efficiency and calibration factor: The estimated limits of uncertainty will vary from approximately ±0.5 to 2 percent depending on the frequency and the

characteristics of the unit being calibrated such as connector type, reflection coefficient,

and repeatability.

Reflection coefficient: The estimated limits of uncertainty are ±0.005Vf(GHz).

(3) Rf and Microwave Coaxial Peak Pulse Power Meters and Power Meter-Directional

Coupler Combinations

Services Available

(1) Input peak pulse power versus scale reading of terminating-type instruments.

(2) Input and/or output peak pulse power versus scale reading of feed-thru

instruments.

Frequency range Peak power range (GHz) (W)

0.3 to 0.5 0.001 to 2500

0.95 to 1.25 .001 to 5000

4.0 to 4.4 .001 to 2000

The uncertainties of these measurements are typically ±3 percent. The ±3 percent

uncertainty is based on:

(a) ± 1 percent on the cw power measurement

(b) ± 1 percent on cw-pulse power comparison circuits

(c) ± 1 percent on the calibration of range-extending directional couplers

These errors are independent, and should be treated as such.

62
General

Calibrations are made with pulsed rf waves modulated by a baseband trapezoidal

pulse. Limits of other basic parameters of the pulses are as follows:

Instruments submitted for calibration should have a nominal impedance of 50 ft, and

be fitted with Type N, BNC, HN, or 7 or 14-mm precision connectors. If other

connectors are used, degradation of the above mentioned uncertainty limits could result.

I. Voltage Measurements

Services are available for two types of electromagnetic voltage measuring devices as follows:

1. Voltage Measurements of Thermal Voltage Converters (TVC's)

The TVC category also includes other devices using thermal detectors such as Rawson RF Voltmeters, Thermal Transfer Standards, RF Voltage Standards and AC-DC

Transfer Standards.

The rf-dc difference is defined as the percentage difference between the rf and dc

output voltages required to produce the same thermocouple output, i.e.,

Pulse duration range

Pulse repetition rate range

Maximum duty factor

0.5 to 10 ^is 100 to 1600 pps

0.0033

Services Available: rf-dc Difference

rf-dc difference

Frequency

(MHz)

rf voltage range (V)

Estimated

limits of uncertainty*

(±%)

131030

100

0.03

.1

.3

0. 1 to 200

1 to 200

1 to 200

1 to 200

1 to 200

1 to 200

1 to 200

1 to 200

0.05

.05

.05

.05

.1

.1.2 1.0 No rf-dc differences greater than ±20 percent will be reported. This normally limits the calibrations to 100 MHz and below.

63
For high frequency TVC's with a built-in "T" connector, the services available are:

rf voltEgc Estimated

Frecjucncy r&ngc Illlllla Ul UIICCI Lallliy

10, 30, 100 0.1 to 7.5 1 200, 300, 400

500, 600, 700

800, 900, 1000

Calibrations having ±1 percent accuracy are performed only on the new high

frequency thermal voltage converters with a "T" connector incorporated in the converter

housing. The measurement reference plane is at the Type "N" male output connector. Other types of TVC's calibrated above 100 MHz will have accuracies of 3 to 5 percent.

General

Most converters have rf-dc differences within ±0.01 percent of zero at 1 MHz and

below. All converters having previous calibration history that are submitted for recalibration should be evaluated at 1 MHz and results compared to prior data. If the

difference is negligible, no further calibrations are usually necessary below 1 MHz.

Many years of experience in calibration of micropots and TVC's have shown that these are very stable devices even for periods of up to 10 years. Thus, in most cases, a 2-year or longer recalibration cycle is recommended. An exception might be the

micropots which operate below 100 ^V.

Assurance of device stability can be obtained by intercomparison of micropots or TVC's with others which are adjacent in voltage. For example, a 0.3 to 1 V TVC can be

compared with a 1 to 3 V TVC at 1 V, etc.

2. Voltage Measurements of rfMicropotentiometers

Service Available: rf-dc Difference

The rf-dc difference is defined as the percentage difference between the rf and dc

output voltages required to produce the same thermocouple output, with the resistive element terminated in 50 fi.

Any frequency rf voltage Estimated*

within band range limits of uncertainty

(MHz) (/xV) (±%)

0.05 to 100 1 to 100,000 2

100 to 500 1 to 100,000 3 500 to 900 1 to 100,000 5

*For rf-dc differences greater than ±20 percent, estimated limits of uncertainty

are larger than those listed.

64
General

Rf micropotentiometers are usually calibrated at their nominal rated output voltages.

Frequencies suggested for a normal calibration are 5, 100, 300, 400, 500, 700, and 900. For

special calibrations to 1000 MHz see Appendix 4.9E.

Rf micropotentiometers having resistive elements greater than 10 mCl, in combination with thermoelement housings between 5 and 100 mA, usually have rf-dc differences within ±1 percent of zero at 5 MHz. Since the rf-dc difference approaches

zero below 5 MHz, calibrations at 50 kHz and 5 MHz would suffice to determine

interpolated points of interest between 50 kHz and 5 MHz, with no appreciable loss of

accuracy.

An rf-dc difference of about ±5 percent at 1 MHz usually results from a combination using a 1 mfl element. Interpolation below 1 MHz is not recommended in this case.

J. Baseband Pulse Parameters

Pulse characteristics are important in a variety of applications. As a result NBS

offers the following pulse measurement services.

/. Impulse Generator Spectrum Amplitude

In response to calibration needs from the electromagnetic interference (EMI)

community, NBS has developed a measurement service to calibrate the broadband

spectrum amplitude output from impulse generators. Such a generator can then be used as a transfer standard of broadband impulsive noise for field calibration of spectrum

analyzers and field intensity meters. The NBS calibration service uses the time domain

measurement/Fourier transformation computation (TD/FFT) method for calibration of

impulse generators. A wideband (dc-18 GHz) sampling oscilloscope is used to measure the

time domain waveform from the impulse generator. A dedicated minicomputer then

computes the spectrum amplitude, S(J), versus frequency using the fast Fourier transform

(FFT).

Many of the limitations concerning the frequency range and spacing of the

calibration service are related to the FFT. With an oscilloscope it is not possible to observe waveforms from t=-oo to +oo. One can only observe a waveform within a

limited time window. For this particular measurement service time windows of 10, 20, 50,

100, and 200 ns are available. Within the time window the sampling oscilloscope measures

1024 separate, uniformly spaced, values of the waveform. This sampled data is then

transformed to the frequency domain using the FFT. Due to the mathematics of the FFT,

the lowest frequency resulting from the computation is the reciprocal of the time window

(i.e., 50 ns ^20 MHz). The other frequency components are harmonics of the fundamental

(i.e., 20, 40, 60, 80 MHz . . .). The highest frequency component is 1/(2 Ar),where Ar is the

time domain sample spacing and At=T/N. T is the time window and is the number of

sampled data values in the time window. For the 50 ns window At is 48.8 ps and f„^^ is 10.24 GHz. Due to various accuracy considerations the time window is chosen such that

at least 10 data points are obtained on the major feature of the impulse waveform. Fewer

data points rapidly introduce significant errors in the computations. As an example an

impulse of 500 ps duration would be measured over a 50 ns time window with a At of 48.8 ps and a resulting spectrum amplitude data table starting at 20 MHz. Another significant

limitation on the choice of the time window is the requirement that no other spurious

pulses occur outside of the window and that the waveform be completely relaxed and

resting on the baseline at the beginning and end of the time window. If these criteria cannot be met then the generator is rejected for calibration. Another major requirement is the necessity to trigger the sampling oscilloscope in advance of the impulse to be measured. As noted in the first reference in the next 65
paragraph most impulse generators use a mercury switch and thus do not furnish a

suitable trigger signal. For these generators a delay Hne triggering arrangement is used. A

time window of 200 ns is the broadest that can be used with this arrangement. This sets the 5 MHz lower limit on the service capabilities given below. This limit can be extended

to broader time windows and lower frequencies if the generator is an electronic generator

with a suitable trigger pulse and adjustable delay.

Several techniques are available for measuring spectrum amplitude. For a summary

see: J. R. Andrews, "Impulse generator spectrum amplitude measurement techniques,"

IEEE Trans. Instr. & Meas., 25, No. 4, (Dec. 1976) p. 280 ff. For other details on spectrum

amplitude measurements, see J. R. Andrews, M. G. Arthur, Spectrum amplitude definition,

generation and measurement, Nat. Bur. Stand. (U.S.), Tech. Note 694. Eighty percent of the impulse generators calibrated at NBS are of the mercury

switch variety with an impulse duration of the order of 0.5 ns and adjustable amplitude.

NBS impulse generator spectrum amplitude measurement service capabilities are as follows:

Parameter Limits Notes

Maximum impulse amplitude without attenuators ±400 mV 1, 2, 3 Maximum impulse amplitude

Willi CAI-Clllal AllCllUaLwia ±1.2 mV 3,4 Spectrum amplitude -15 dBfiV/MHz<[5(/)-5o]< + 5 dB/xV/MHz

5, 6, 7

S(J) uncertainty Nominally /<1 GHz, ±0.6 dB

1 GHz</"<4GHz, ±1.2dB

4 GHz</<6 GHz, ±2.0 dB

5,

6, 7

8 & 9 Frequency range 5 MHz to 6 GHz 5, 6, 7 & 10 Frequency spacing A/= 5, 10,20,50, or 100 MHz 5, 10

Frequency uncertainty of the order of ±1% 7 Load impedance 50.0 n

Load impedance uncertainty Nominally ±0.1 fl at dc VSWR < 1.3 up to 6 GHz

8, 11

Trigger pulse magnitude >100 mV 12

Trigger pulse transition time <5 ns 12

Trigger to impulse delay 75 ns</,<100 ns 12

Trigger to impulse jitter <20 ps 12

Notes

(1) The impulse generator is characterized by its impulse output waveform into 50 fi of peak amplitude

(P'pk), 50 percent level duration (t), and low frequency spectrum amplitude (5o~2Kp^T).

(2) Impulse generator with an adjustable amplitude impulse output will be calibrated with the generator

adjusted to give a peak amplitude in the range of 200 to 400 mV.

(3) Impulse generators with fixed outputs greater than ±400 mV must have the impulse attenuated to the 200-400 mV level by 50 H wideband coaxial attenuators.

(4) Either customer supplied or NBS attenuators may be used.

(5) Depends upon actual generator characteristics.

(6) Data will not be given in the first spectrum null or at frequencies above. Typically 100 data points are

supplied.

(7) Subject to revision.

(8) Only for impulse amplitudes less than ±400 mV.

(9) If external attenuators and/or a 6 dB tee and delay line are used then the uncertainty associated with

the attenuator calibration is added to these values.

(10) Lower frequencies (<5 MHz) are available as a special test. (11) Depends upon input impedance of external attenuators when used.

(12) If the impulse generator does not supply a trigger output or if the trigger output does not have the proper characteristics then a 6 dB tee and a delay line will be used to provide a suitable trigger pulse. 66
2. Pulse Transition Duration

Pulse transition duration (rise time) measurements for step function pulse generators

and low pass filters are also available. Both measurements use an 18 GHz bandwidth, 20

ps transition time sampling oscilloscope to measure the pulse waveform. The observed

waveform is computer processed to determine the transition duration. In the case of filters a 1 5 ps tunnel diode pulse generator is used as the source. For details on specific services, see the Appendix under pulse generator transition duration and low pass filter transition duration respectively.

3. Wideband Attenuation or Gain

Wideband attenuation or gain measurements on coaxial networks, using pulse

techniques, provide data over a 100 kHz to 12.5 GHz frequency range, and 0 to 40 dB loss or gain range. This is accomplished by use of the NBS Time Domain Automatic Network

Analyzer. It consists of a 20 ps transition time pulse generator, a 20 ps sampling

oscilloscope and a minicomputer. Two waveforms are measured, one with the generator

connected to the oscilloscope and the other with the unknown network inserted between

the generator and the oscilloscope. The minicomputer provides the spectrum data for the

two waveforms using the fast Fourier transform. The ratio of the two spectra is the

attenuation or gain of the network. A wide variety of connectors can be accommodated.

4. Pulse Time Delay

Pulse time delay is also measured using the same generator and oscilloscope. A

coaxial trombone line stretcher is connected in the trigger circuit between the generator

and oscilloscope. The length of the line stretcher is adjusted to center on the 50 percent

point on the pulse leading edge. The change in length of the line stretcher for this adjustment with the unknown alternately inserted and removed from the signal channel

between the generator and oscilloscope is used to compute time delay. For details on

services available, see the Appendix under Attenuation, Phase Shift, and Power,

respectively. For information on Pulsed rf Power measurements, see section V.H.

K. Electromagnetic Interference Measurements

An electromagnetic interference (EMI) program in process is currently developing

measurement methods and instrumentation for the evaluation of susceptibility and

emission of devices, components, subsystems and small systems. Large subsystem and

system measurements will be addressed on an individual case basis. The objective of the

test methods under development is to achieve repeatable (to <0.5 dB) and accurate EMI

measurements, exceeding the accuracies called for in Mil. Standards 461 and 462. This program is also directed toward developing measurement methods and

instrumentation for the evaluation of the electromagnetic (EM) environment primarily to

assess the electric and magnetic fields in the near-field area of intentional or unintentional

emitters to establish the EM ambient environment. Where necessary methods and

instrumentation for far-field measurements will be developed. This work will also concern

itself with display methods and statistical measures for EM ambient environment

evaluation.

Another area this program is directed to is electromagnetic radiation hazards. The

measurements needed to support this area are very nearly the same as those for EMI;

therefore, only minor changes in instrumentation and measurement techniques are needed

for hazard measurements.

NBS does not provide routine calibrations for electromagnetic interference,

although a limited number of measurements are available by special arrangements. Instead,

consulting and advisory services are available.

67
€ii
CHAPTER VI

VI. Time and Frequency

Direct inquiries to the Time and Frequency Program in Boulder (address and phone

number as stated in the Appendix).

The National Bureau of Standards develops and maintains the U.S. National

standards of frequency and time. It disseminates frequency and time from these standards

via radio broadcasts from Colorado and Hawaii and experimentally from satellites. In

addition, frequency and time calibration services using network television and other non- NBS broadcasts are also available. A limited service of direct comparison of a customer's

signal source (precision oscillator) or clock with the NBS frequency and time standard is also provided as well as direct noise measurements of precision oscillators. The current

basic frequency standards operate at 5 MHz, however, NBS' frequency technology

permits comparisons ranging from several hertz to about 900 THz. Consultation on atomic

clocks and frequency standards, precision oscillators infrared and optical coherent

sources, stabilization and intercomparison is available. NBS conducts scientific comparisons with the frequency and time standards of other nations and NBS standards

provide data for the development of International Atomic Time (TAI).

A. NBS Frequency Standard

The General Conference of Weights and Measures, at its 13th General Meeting

(1967), adopted the following definition for the second in the International System (SI) of

Units.

"The second is the duration of 9,192,631,770 periods of the radiation corresponding

to the transition between the two hyperfine levels of the ground state of the atom of

cesium 133." NBS maintains equipment, known as the NBS Frequency and Time Standard (NBS FTS)

for realizing this definition of the second with an accuracy of 1 X 10"'\

B. NBS Time and Frequency Dissemination Services

Services available from radio broadcasting stations and time and frequency

calibration services using network television and the experimental GOES satellite time

code are detailed in NBS Special Publication 432. This publication is revised from time to time as changes are introduced.

For experimental services available from satellites see:

[1] Dissemination of time and frequency by satellite, R. L. Easton, L. C. Fisher, D. W.

Hanson, H. W. Hellwig, and L. J. Rueger, Proc. IEEE 64, No. 10, 1482-1493 (Oct.

1976).

[2] NBS time to the Western hemisphere, D. W. Hanson, D. D. Davis, and J. V. Cateora,

Radio Science 14, No. 4, 731-740 (July-Aug. 1974).

[3] Time recovery measurements using operational GOES and TRANSIT satellites, R. E. Beehler et al., Proc. 11th Annual Precise Time and Time Interval (PTTI)

Applications and Planning Meeting, 283-315 (Nov. 1979).

69
C. Direct Signal Source Calibration Services

NBS does not routinely calibrate signal sources or clocks, however, upon request

special measurements may be made. Special requests will be considered for oscillator frequency and noise measurements. Inquiries should describe clearly the measurement

desired and the importance or significance of making a direct comparison with the NBS

FTS. The signal source to be measured should have a power output of 20 mW (into a matched load) or greater. For details on nominal frequencies at which calibrations can be

readily done depending on workload, see Appendix under the heading "Frequency

Measurements."

For the current primary cesium standard NBS-6, the accuracy. A/// has been

determined to be ±8xlO"'\ which includes one sigma estimates of the possible random

errors in frequency associated with the measurement of various parameters of the

standard. The stability of the standard becomes a factor in measuring stability of other

precision oscillators. The contribution to the overall measurement uncertainty due to the

instability imposed by fluctuations of NBS-6 when paired with a similar oscillator depends on measurement averaging time and can be expressed as

o-/t)= IXlO-'^ t''\

where <t/t) is the two-sample deviation of the frequency fluctuations and T=the length

of the frequency comparison in seconds. The measurement limit is 10"''*. Noise

measurements are expressed in the time domain by measurements of the two-sample

deviation crjij) or in the frequency domain by measurement of phase noise S<j,(/). For

specific details on the noise services available see the Appendix under the heading

"Oscillator Noise Measurements."

In addition, time measurement services are also available for time pulses at the rate of one pulse per second. See the Appendix under the heading "Time Pulses."

References

[1] Clocks and measurements of time and frequency, H. Hellwig, Proc. 1976 WESCON,

Session 32, pp. 1-4 (Institute of Electrical and Electronics Engineers, New York,

NY, Sept. 1976).

[2] The National Bureau of Standards Atomic Time Scale: Generation, stability, accuracy

and accessibility, D. W. Allan, J. E. Gray, and H. E. Machlan, Nat. Bur. Stand.

(U.S.). Monogr. 140, Chap. 9, 205-231 (May 1974).

[3] Design principles and characteristics of frequency and time standards, H. Hellwig,

IEEE Trans. Nucl. Sci. NS-23, No. 6, 1629-1635 (Dec. 1976).

[4] Results on limitations in primary cesium standard operation, D. J. Wineland, D. W.

Allan, D. J. Glaze, H. W. Hellwig, and S. Jarvis, Jr., IEEE Trans. Instrum. Meas. IM-25, No. 4, 453-458 (Dec. 1976).

70
CHAPTER VII

VII. Thermodynamic Quantities

The Thermodynamic Quantities program provides measurement services in the areas of thermometry, pressure and vacuum, humidity, and cryogenic measurements as described in the sections which follow.

A. Thermometry

The National Bureau of Standards employs the International Practical Temperature

Scale as the basis for its calibration activities in thermometry. However, the NBS

temperature calibrations are not limited to those which are specified in the IPTS; non- "standard" sensors may be calibrated, and calibrations beyond the limits of the IPTS may

also be provided. In all cases, however, great care is taken to make such calibrations

consistent with the IPTS.

The platinum resistance thermometer and the platinum- 10 percent rhodium versus platinum thermocouple are the specified interpolating instruments in the IPTS for the

range 13.81 K to 630.74 °C and 630.74 to 1064.43 °C, respectively. NBS offers calibrations

for both of these instruments throughout their respective ranges. In addition to these

calibrations, NBS routinely provides many other calibration services. These include

calibrations of:

° many liquid-in-glass laboratory thermometers

° thermocouple thermometers of many types, from cryogenic temperatures to 1750 °C

" germanium resistance thermometers and other thermometers used in cryogenics to temperatures below 1 K.

Moreover, in collaboration with its thermometry research and development staff, NBS provides special at-cost calibrations of thermistor thermometers, of certain industrial types of resistance thermometers, and of a variety of thermometry systems.

Whenever there exists a desire to calibrate contact thermometry equipment over the

temperature range 0. 1 to 2000 K, the user is invited to contact the temperature calibration

staff as indicated in the Appendix.

/. Laboratory Thermometers

Kinds of Thermometers Accepted for Calibration

Thermometers belonging to the large and varied group which may be classed as laboratory or "chemical" thermometers are regularly accepted. Many of these are of the

liquid-in-glass type with either solid-stem or enclosed scale. Other acceptable types include

such special-purpose thermometers as Beckmann and calorimeter thermometers.

Ordinary household or meteorological thermometers will not, in general, be

accepted unless the scale is graduated on the glass stem itself and the thermometer can be

readily detached from its mounting for insertion in a testing bath. Every thermometer submitted must be uniquely identified by a serial number and

must pass a preliminary examination for fineness and uniformity of graduation; for cleanliness of the mercury and capillary bore; for freedom from moisture, gas bubbles, and

cracks in the glass; for adequacy or omission of gas filling where needed; for insufficient

71
annealing; and for misnumbered graduations. When these or other serious defects are found, the thermometer is returned untested.

Shipping Instructions

Shipping charges, both to and from the Bureau, must be assumed by the appHcant.

Return shipments are made by the Bureau in accordance with its judgment of the best method of shipping unless specific instructions are received. Such instructions should be

supplied at the time that arrangements are being made for the test. If a test number has

been assigned prior to the shipment, this number should appear on the shipping container.

If a test number has not been assigned at this time, a purchase order, or letter should be

sent under separate cover. In either case, the shipment should include a packing list. All possible care will be taken in handling thermometers at the Bureau, but the risk of damage either in shipment or in testing must be assumed by the applicant. The

applicant should consider the nature of the equipment shipped and pack it accordingly,

with appropriate labeling. Attention is called to the availability of security express in shipping thermometers.

Details of the procedures used in calibrating laboratory thermometers may be found

in NBS Monograph 150, "Liquid-in-Glass Thermometry," U.S. Government Printing

Office, Washington, DC 20402, 1976.

2. Thermocouples, Thermocouple Materials and Pyrometer Indicators

Methods of Calibration

In order to calibrate thermocouples to yield temperature versus emf relationship on

the International Practical Temperature Scale, they must be so calibrated that their indications agree with those of the standard platinum resistance thermometer in the range

-259.34 to 630.74 °C, the standard platinum-10 percent rhodium versus platinum

thermocouple in the range 630.74 to 1064.43 °C, and the optical pyrometer above

1064.43 °C. The temperature-emf relationship of a homogeneous thermocouple is a definite physical property and, therefore, does not depend upon the details of the apparatus or method employed in determining this relation. Consequently, there are numerous methods

of calibrating thermocouples, the choice of which depends upon the type of

thermocouple, temperature range, accuracy required, size of wires, apparatus available,

and personal preference.

Services Offered

A listing of the thermocouple calibration services is given in the Appendix under the

title "Thermocouples and Thermocouple Materials."

Only the bare wires are required to perform the thermocouple calibrations.

It is preferable not to send insulating and protecting tubes as the rate of

breakage of these in shipment is high.

If the thermocouple is furnished mounted (as in a protection tube assembly)

a nominal charge will be made for dismantling the mounting and the various

parts will be returned to the sender without reassembling them.

Thermocouple length requirements listed in the Appendix are exclusive of

lead wire. Lead wire need not be sent with thermocouples.

All thermocouple calibration data furnished in reports will be on the basis of a reference junction temperature of 0 °C or 32 °F. The calibration results will be given in degrees C or F, as requested by the customer.

72
° The calibration or test of a thermocouple will not be undertaken if, in our

opinion, it will not yield the specified accuracy or if it possesses such unusual

characteristics as to prevent the carrying out of the calibration or test at a reasonable cost. Only unused base-metal thermocouples and thermocouple

materials will be accepted for test.

° Inquiries concerning other types of thermocouples, e.g., tungsten-rhenium

types, are welcome. NBS will attempt, whenever possible, to meet reasonable

calibration requests. The fees for the work accepted will be based on the

actual costs incurred. The technical staff should make the inquiry directly

with information on the nature of the' thermocouple and the type of

calibration required so that services that are needed can be determined

expeditiously.

3. Resistance Thermometers

The National Bureau of Standards offers calibrations of the standard platinum

resistance thermometers which are specified for use as interpolation instruments in the text of the International Practical Temperature Scale of 1968 as amended 1975 (see the journal

Metrologia, Vol. 12, pp. 7-17, 1976). In addition, less-precise resistance thermometers and

thermistor thermometers are calibrated on the International Practical Temperature Scale,

but with correspondingly reduced precision limits.

Standard Platinum Resistance Thermometers

To qualify for testing, either long-stem or capsule platinum resistance thermometers

must meet several conditions:

° They must reasonably be expected to meet the requirements of the IPTS-68

for a standard interpolating instrument (i.e., a four lead resistor of high-purity

platinum hermetically sealed in a protecting tube).

° They must be compatible with the NBS highest-precision calibration

equipment (see the Appendix for further information and contact point).

The user may choose any of several types of calibration, as listed in the Appendix.

He may also specify the form of the resulting calibration, R(t), [R{t)/R{0)], or [R{t) - R(0)] versus temperatures based on the IPTS-68.

A minimum charge (see Appendix) is made on all standard platinum thermometers

received. This charge will cover, in part, the cost of receiving and returning

thermometers which, for some reason, are found to be unsuitable for test.

It is very important that, insofar as possible, resistance thermometers be protected

from any mechanical shock which will alter their calibration. To be shipped, the

thermometer must be softly supported within a case but not be free to rattle. This

necessitates the use of packing material that does not become compacted. The

thermometer case should in turn be softly packed inside a shipping container. The outside

shipping container must be sufficiently rigid and strong not to appreciably deform under

the treatment usually given by shippers. Styrofoam is not sufficiently rigid to be used as an

outside container. Thermometers will not be returned in containers which are obviously

unsuitable, such as those closed by nailing. Suitable containers will be provided, for a fee, when thermometer shipping container is not satisfactory for re-use.

"Non-Standard" Resistance Thermometers, Thermistors,

and Other Thermometers

Industrial and laboratory-grade resistance thermometers which are not suitable for work at the highest precision may be calibrated also. In this case, the calibration will be

73
referred to the IPTS-68, but the measurement precision and the equipment to be used will be appropriate to the quality of the sensor. Some thermometers may require a stability test before calibration.

These comments apply, as well, to thermistor thermometers, quartz thermometers,

and other temperature sensors. These instruments may be offered for calibration on an "At Cost" basis. Special requirements for testing should be discussed with the NBS staff indicated in the Appendix.

4. Thermometer Systems

The National Bureau of Standards offers a service of analysis and calibration of

thermometry systems, including sensors, reference baths, data loggers, and the like on an "At Cost" basis. This testing should be discussed with the appropriate NBS staff member

well in advance of need, since it generally involves a complex relationship of the

laboratory environment to the several component parts of the measurement apparatus.

5. Training

Twice a year, in spring and fall. Precision Thermometry Seminars are held at NBS.

These seminars include 2 days of instruction and laboratory practice in platinum resistance

thermometry, and 1 day each for thermocouple thermometry and liquid-in-glass

thermometry. A fifth day is utilized for general tours and further laboratory consultation.

The instruction is given by the NBS temperature calibration staff, and hands-on laboratory

experience is included.

6. Radiation Thermometry

The calibration services available for optical pyrometers and ribbon filament lamps

are listed in the Appendix under the title "Radiation Thermometry."

B. Pressure and Vacuum Measurements

The National Bureau of Standards develops and maintains primary and secondary

standards of pressure and provides calibration services for a wide variety of instruments

against these standards. These services are supplemented by extensive consultations,

training of industrial personnel, measurement assurance programs, evaluation of the

performance of transfer standards, special tests and services, and research and

development sponsored by industrial organizations or government agencies.

7. Piston Gages

The effective area and the pressure coefficient of piston gages are determined by

comparison with NBS standards. A detailed report containing all the data taken and

giving a full account of the numerical evaluation of the data is issued for each gage

calibrated. If customer weights are not available, NBS owned weights can be used. The

mass of small parts, such as pistons and small weight tables, can be determined as part of

the calibration service. The mass of larger weights must be known before they are used

for the piston gage calibration. Bases for some types of piston gages are available at NBS.

In these cases only the piston cylinder assembly needs to be shipped to NBS.

2. Controlled Clearance Piston Gages

The effective area, the pressure coefficient of the area, and the jacket pressure

coefficients are determined. A detailed report containing all the data taken and giving a

full account of the numerical evaluation of the data is issued for each controlled clearance

74
piston gage calibrated. Since these calibrations are very time-consuming, NBS should be

contacted in advance for scheduling.

3. Barometers

Mercurial barometers in which both menisci are observable are calibrated by

comparison with a standard mercury manometer or a suitable transfer standard. A table of

corrections to be applied to the barometer readings is issued.

4. Manometers

Mercury manometers in which both menisci are observable are calibrated by

comparison with a standard manometer or a piston gage transfer standard. A table of

corrections to be applied to the manometer readings is issued.

5. Pressure Gages

Indicating pressure gages with aneroids, bourdon tubes or other transducing

elements are calibrated against suitable standards. A variety of gases or liquids can be used

as pressure transmitting fluids. Gage, absolute or differential pressures can be applied. A

table of corrections to be applied to the indicated pressure is issued.

6. Pressure Transducers

Absolute, differential, or gage pressure transducers are calibrated against suitable

standards using a variety of liquids or gases as pressure transmitting fluids. Either the

transducer only or the transducer with the associated instrumentation can be calibrated. The results are generally expressed as a polynomial in the indicated pressure. A detailed

report containing the data taken and giving a full account of the numerical evaluation of

the data is issued.

7. Vacuum Gages

Low and medium vacuum gages are calibrated by comparison with mercury or oil manometers, or with suitable transfer standards. Various gases such as N2, Hj, He, and Ar

can be used as the pressure transmitting medium. The results are generally expressed as a polynomial in the indicated pressure. A detailed report containing the data taken and

giving a full account of the numerical evaluation of the data is issued.

8. Training

Training courses on pressure measurements with piston gages are held several times

a year at NBS. These and other courses are also held upon request at other locations.

These courses help to acquaint users of pressure standards and transfer standards with

methods used at NBS and with the interpretation of calibration reports. Particular

attention is paid to the assessment of uncertainties and the propagation of errors in the

calibration chain.

9. Miscellaneous Tests and Services

Research and development projects in pressure and vacuum are sometimes carried out for industrial associations and government agencies. Upon request, subject to available resource constraints, NBS may provide measurement assurance services, assist in the

investigation of accidents, assess the measurement capabilities of other laboratories, or provide consultation on pressure and vacuum measurements.

75
References

[1] Pressure measurements and services at NBS, P. L. M. Heydemann, presented at the

Fluid Power Testing Symposium, Milwaukee, WI (1976). Available from NBS at the address listed in the Appendix.

[2] Piston gages, P. L. M. Heydemann and B. E. Welch, Chapter 4 in Experimental

Thermodynamics, Vol. II. Experimental Thermodynamics of Non-Reacting Fluids, B. Le Neindre and B. Vodar, Eds., Part 3, pp. 147-202 (Butterworth and Co., London,

England, 1975).

[3] Ultrasonic manometers for low and medium vacua under development at NBS, P. L. M.

Heydemann, C. R. Tilford, and R. W. Hyland, J. Vac. Sci. Technol. 14, 597 (1977).

[4] Ultrasonic and dilatometric measurement at very high pressures, P. L. M. Heydemann

and J. C. Houck, Nat. Bur. Stand (U.S.), Spec. Publ 326, pp. 11-22 (Mar. 1971).

[5] NBS pressure transducer characterization service, V. E. Bean. Available from NBS at the address listed in the Appendix.

[6] The National Measurement System for pressure, P. L. M. Heydemann, NBSIR 75-931,

42 pages (Sept. 1976). Order from NTIS as PB261030.

C. Humidity Measurements

The National Bureau of Standards provides calibration services for a wide variety of

humidity-measuring instruments. Calibrations are performed by subjecting the instrument

under test to atmospheres of known moisture content produced by the NBS two-pressure

humidity generator.

1. Dew-Point Hygrometers

Dew-point hygrometers can be calibrated over the dew/frost-point range of 80 to -80 °C.

2. Electric Hygrometers

Hygrometers classified under this category are sensors which sorb water vapor as a function of relative humidity and associated with this sorption is a corresponding change

in an electrical parameter (i.e., resistance, capacitance). The range of calibration is 3 to 98

percent RH over the temperature range -55 to 80 °C.

3. Psychrometers

A limited number of types of wet-dry bulb hygrometers (aspirated psychrometers) can be calibrated at the National Bureau of Standards. The staff of the Humidity Group

should be consulted for the special features of the psychrometer which are necessary

before the instrument can be calibrated at NBS.

4. Coulometric Hygrometers

Coulometric hygrometers are devices which electrolyze the water into gaseous

oxygen and hydrogen by the application of a voltage in excess of the thermodynamic

decomposition voltage and measure this electrolysis current. The range of calibration is 1 to 31,000 ppm by volume.

5. Pneumatic Bridge Hygrometer

Pneumatic bridge hygrometers are instruments which measure the variation of

pressure drop across two combinations of nozzles, operating at critical flow, with a

desiccant between one pair of nozzles. The range of calibration in mixing ratios, (gram

water vapor/gram dry air), is 0.0005 to 0.015.76
References

The following references are available from NBS at the address shown in the

Appendix under "Humidity Measurements."

[1] The NBS standard hygrometer, A. Wexler and R. W. Hyland, Nat. Bur. Stand. (U.S.). Monogr. 73, 35 pages (May 1964).

[2] The NBS two-pressure humidity generator, Mark 2, S. Hasegawa and J. W. Little, /

Res. Nat. Bur. Stand. (U.S.), 81A (Phys. and Chem.), No. 1, 81-88 (Jan.-Feb. 1977).

[3] Vapor pressure formulation for water in range 0 to 100 *C. A revision, A. Wexler, J. Res. Nat. Bur. Stand. (U.S.), 80A (Phys. and Chem.), Nos. 5 and 6, 775-785 (Sept.-

Dec. 1976).

[4] Vapor pressure formulation for ice, A. Wexler, /. Res. Nat. Bur Stand. (U.S.), 81A

(Phys. and Chem.), No. 1, 5-20 (Jan.-Feb. 1977).

[5] A correlation for the second interaction virial coefficients and enhancement factors for moist air, R. W. Hyland, /. Res. Nat. Bur Stand. (U.S.), 79A (Phys. and Chem.), No.

4, 551-560 (July-Aug. 1975).

D. Cryogenic Measurements

The National Bureau of Standards provides consulting and advisory services in selected areas of cryogenics including properties of solids and fluids, cryogenic flow

measurement^, density measurements, and liquefied natural gas. Limited calibration and

test services are provided for cryogenic liquid flow and a static liquid density reference

system is maintained. The primary aim of both systems is the establishment and calibration

of transfer standards.

/. Cryogenic Flow Measurements

Mass flow measurements are performed with a gravimetric system using liquid

nitrogen or liquid argon with flow rates of 76 to 757 liters/minute.

2. Density Reference System

A static density reference system is maintained which can compare liquid density

transducers against a gravimetric balance. Measurements are performed in liquid methane

and liquefied natural gas.

3. Liquefied Natural Gas

A broad program in measurement services to the LNG industry has been pursued by

NBS since 1969. Included are measurements of the properties of LNG and its constituents,

properties of materials of construction including thermal insulation and measurements on

flowing and static LNG. A handbook of property data has been prepared.

References

[1] NBS-CGA cryogenic flow measurement program, J. A. Brennan, R. W. Stokes, C. H.

Kneebone, and D. B. Mann, (Proc. ISA International Instrumentation Automation

Conf and Exhibit, New York, NY, Oct. 28-31, 1974), Paper in Advances in Instrumentation 29, 612-1/612-13 (Instrument Society of America, Pittsburgh, PA,

1974).

[2] Progress report on cryogenic flowmetering at the National Bureau of Standards, J. A.

Brennan, J. F. LaBrecque, and C. H. Kneebone, Proc. 1st Biennial Symp.

Instrumentation in the Cryogenic Industry, Houston, TX, Oct. 11-14, 1976, 1, 621- 1/621-16 (Instrument Society of America, Pittsburgh, PA, 1976).

77
[3] Cryogenic liquid-measuring devices, National Code on SpeciHcations, Tolerances, and

other technical requirements, Nat. Bur. Stand. (U.S.), Handbook 44.

[4] An evaluation of commercial densimeters for use in LNG, J. D. Siegwarth, J. F. LaBrecque, and B. A. Younglove, Nat. Bur. Stand. (U.S.), Tech. Note 698 (1977).

[5] LNG density determination D. E. Diller, Hydrocarbon Process 56, No. 4, 142-144 (Apr.

1977).

[6] LNG materials and fluids—A user's manual of property data in graphic format, 1st Edition, D. B. Mann, Editor, CDC $35.00; Suppl. 1, D. E. Diller, Editor, CDC

$15.00; Suppl. 2, N. A. Olien, Editor (1980) CDC $15.00.

78
CHAPTER VIII

VIII. Optical Measurements

A. Radiometry and Photometry

The National Bureau of Standards provides radiometric and photometric

calibrations of three classes: Basic, gage, and special. Basic calibrations are those

considered fundamental to all work in radiometry and photometry, and for which

documentation of uncertainties relative to the International System of Measurements

exists. Gage calibrations are those routinely available calibrations for which uncertainties

have only been documented relative to NBS standards. Basic and gage calibrations are offered as fixed fee items, listed in the Appendix, and require only a purchase order

identifying the item number. Details of the calibration procedure and a summary of the

documentation of the uncertainties are provided in each calibration report. The listed Basic and Gage calibrations are performed under rigid, pre-determined conditions and are, therefore, restricted as to lamp type, measuring geometry, wavelength points, etc., and

requests for departure from these conditions will be considered as special calibrations. Special calibrations are those having unique requirements not satisfied by the listed basic and gage items, and are considered as small research efforts. Agreement to accept

such requests are handled on a case-by-case basis. Fees are charged on an actual cost basis,

with an estimate of cost, delivery time, and uncertainty being provided after receipt of a description of the desired test, and before actual work commences. The request for a

special calibration should include the following information:

(1) Detailed description of desired calibration

(2) Uncertainty required (SI units, NBS standards)

(3) Manner in which the calibrated device will be used

(4) The consequences of this calibration not being provided by NBS.

1. Basic Radiometric Calibrations

(a) Special radiance standard, ribbon filament lamp (30A/T24/13).

Lamps of this type are calibrated at 33 wavelengths from 225 to 2400 nm, for a

target area 0.6 mm wide by 0.8 mm high, and at a radiance temperature of about 2675 K

at 225 nm, 2495 K at 650 nm, 2415 K at 800 nm and 1620 K at 2400 nm, with approximate

uncertainties relative to SI units of spectral radiance of 4.5 percent at 225 nm, 1 percent at 650 and 800 nm, and 1.5 percent at 2400 nm. The lamp requires about 40 A dc at 12 V. An

interpolation formula allows calculation at all wavelengths except in regions of absorption

bands. The lamp is normally provided by NBS.

(b) Spectral radiance standard for vacuum ultraviolet, argon mini-arc.

The spectral radiance of argon mini-arc radiation sources is determined to within an

uncertainty of less than 7 percent over the wavelength range 140-330 nm and 20 percent over the wavelength range 115-140 nm. The calibrated area of the 4 mm diameter

radiation source is the central 0.3 mm diameter region. Typical values of the spectral

radiance are: at 250 nm, L^ = 30 mW cm'^nm 'sr"'; at 150 nm, L>^ = 3 mW cm^nm 'sr"'. The

transmission of the demountable lamp window and that of an additional MgF, window are determined individually so that the user may check periodically for possible long term

variations.

79
(c) Spectral radiance standard for vacuum ultraviolet, deuterium lamps.

The spectral radiance of low pressure deuterium arc lamps is determined to within an uncertainty of less than 7 percent over the wavelength range 165-350 nm in 10 nm

intervals. The calibrated area of the radiation source is the central 0.3 mm diameter

region. The lamp is normally supplied by the customer.

(d) Spectral irradiance standard, quartz-halogen 1000 watt type FEL lamp.

The lamp is calibrated at 26 wavelengths from 250 to 1600 nm, at a distance of 50

cm, and at a spectral irradiance of about 0.2 W/cm' at 250 nm, 220 W/cm^ at 900 nm and

115 W/cm^ at 1600 nm, with approximate uncertainties relative to SI units of 2.5 percent

at 250 nm, 1 percent at 650 nm, and 1 percent at 1600 nm. The lamp is normally supplied

by NBS, and requires about 8 A dc at 110 V. An interpolation formula allows calculation

of the value at any wavelength except in regions of absorption bands.

(e) Spectral irradiance standard, deuterium lamp.

The lamp is calibrated at 16 wavelengths from 200 to 350 nm, at a distance of 50 cm,

at a spectral irradiance of about 0.5 W/cm^ at 200 nm, 0.2 W/cm^ at 250 nm and 0.06

W/cm^ at 350 nm. The approximate uncertainty relative to SI units is 6 percent, the

approximate uncertainty in relative spectral distribution is 2.5 percent. The lamp is normally supplied by NBS and requires 300 ma at about 100 V.

(f) Spectral irradiance standard for the vacuum ultraviolet deuterium lamp.

The lamp is calibrated at 10 wavelengths from 165 to 200 nm, at a distance of 50 cm,

at a spectral irradiance of about 0.5 W/cm' at 165 nm, 0.3 W/cm^ at 170 nm, and 0.5 W/cm' at 200 nm. The approximate uncertainty relative to SI units is estimated to be less than 10 percent. The lamp is normally supplied by NBS and requires 300 mA at about 100

V.

(g) Spectral irradiance standard argon mini-arc.

An argon mini-arc lamp supplied by the customer is calibrated for spectral

irradiance at 10 nm intervals in the wavelength region 140 nm-300 nm. Absolute values

are obtained by comparison of the radiative output with laboratory standards of both

spectral irradiance and spectral radiance. The spectral irradiance measurement is made at a distance of 50 cm from the field stop. Uncertainties are estimated to be less than ±10

percent in the wavelength region 140-200 nm and within ±5 percent in the wavelength

region 200-330 nm. A measurement of the spectral transmission of the lamp window is included in order that the calibration be independent of possible window deterioration or damage.

References

[1] NBS ultraviolet radiometric standards, W. R. Ott, Symposium on Measurements for the Safe Use of Radiation, Nat. Bur. Stand. (U.S.), Spec. Publ. 456, 107-110 (Mar.

1976).

[2] Far UV radiometry—Survey of the NMS, W. R. Ott, NBSIR 75-941, 69 pages (June

1977).

[3] Vacuum ultraviolet radiometry. 3: The argon mini-arc as a new secondary standard of

spectral radiance, J. M. Bridges and W. R. Ott, Appl. Opt. 16, 367-375 (1977).

[4] Spectral radiance calibrations between 165-300 nm: An interlaboratory comparison, J. M. Bridges, W. R. Ott, E. Pitz, A. Schulz, D. Einfeld, and D. Stuck, Appl. Opt. 16,

1788-1790 (1977).

[5] Spectral irradiance standard for the ultraviolet: The deuterium lamp, R. D. Saunders,

W. R. Ott, and J. M. Bridges, Appl. Opt. 17, 593-600 (1978).

[6] Vacuum ultraviolet spectral irradiance calibrations: Method and applications, W. R. Ott,

J. M. Bridges, and J. Z. Klose, Optics Letters 5, 225-227 (1980).

80
2. Basic Photometric Calibrations

(a) Luminous intensity standards.

100-W (90-140 cd), 500-W (approximately 700 cd) and 1000-W (approximately 1400

cd) tungsten filament lamps with C-13B filaments in inside-frosted bulbs and medium

bipost bases are supplied by NBS calibrated at either a set current or a specified color

temperature in the range 2700-3000 K. Approximate uncertainties are 2 percent relative to the SI unit of luminous intensity and 1.5 percent relative to NBS standards.

(b) Luminous flux standards (geometrically total). 25-W vacuum tungsten lamps and 60-, 100-, 200-, and 500-W gas filled tungsten

lamps submitted by customers are calibrated. Lamps must be base-up burning and rated at 120 V. Approximate uncertainties are 2.5 percent relative to SI units and 1.5 percent

relative to NBS standards.

3. Radiometric and Photometric Gage Calibrations

The currently available calibration services are listed in the Appendix under the title "Radiometric and Photometric Gage Calibrations."

4. Special Calibrations

Electrical Substitution Radiometry

NBS provides a service for transferring photodetector absolute response

measurements at several discrete wavelengths in the 250 to 1150 nm wavelength range.

B. Spectrophotometric Standards

1. Standards ofSpectral Transmittance For Checking the Photometric

Scale ofSpectrophotometers

These are either 30 mm polished glass disks or 51 mm polished glass squares, 2 to 3

• mm thick, designated as cobalt blue, copper green, carbon yellow, and selenium orange

(disks will be supplied unless otherwise specified). The report includes: (1) values of

transmittance at 25 °C at certain wavelengths from 390 to 750 nm, (2) the estimated

uncertainty of each value, and (3) the effect of temperature change on transmittance at each wavelength.

2. Holmium Oxide Glass Standards

Holmium oxide glass standards are available for checking the ultraviolet and visible wavelength calibrations of recording spectrophotometers having a bandpass less than 2 nm. These are made of polished Corning 3130 glass, 51x51 mm, 2.5 mm thick. The report

includes a table of wavelengths of minimum transmittance.

3. Working Standards ofSpectral Reflectance Factor

Working standards of spectral reflectance factor are available for use on

spectrophotometers with 6° from perpendicular irradiation and diffuse reception. The

standards consist of white structural Vitrolite glass, 102 X 102 mm, 8 mm thick or 38x51

mm, 11 mm thick. The report includes a table of spectral reflectance factors relative to perfect diffuser at every 10 nm.

4. Spectrophotometric Measurements

The measurements of spectral transmittance, spectral reflectance, or spectral

reflectance factor, covered under this item are intended primarily for information

purposes and specimens so tested should not be accepted as "standards."

81
Measurements of spectral transmittance can be made for the wavelength region 0.19

to 2.5 jLim. One or more of the following spectrophotometers will be used, depending

upon the wavelength region to be covered: NBS Reference Spectrophotometer for Transmittance; Cary Model 14; General Electric. All measurements are made at room

temperature for certain samples.

Measurements of spectral reflectance factor, relative to some accepted reference

surface, can be made for the wavelength region 0.25 to 2.5 ju.m. One or more of the

following spectrophotometers will be used, depending upon the wavelength region to be

covered: NBS reference spectrophotometer for Diffuse Transmittance and Reflectance;

Cary Model 14 or General Electric. All of these instruments permit the measurement of

directional-hemispherical reflectance factor, while the first instrument also permits the

measurement of 0°-45° reflectance factor. All measurements are made at room

temperature.

Accuracy and precision estimates will be given, dependent upon the optical

characteristics of the submitted specimens.

Measurements will be made on an actual cost basis, subject to a nominal minimum

charge. Arrangements for measurements must be completed before shipment of specimens.

The decision to perform the measurements and the instruments to be used will rest with

NBS and refusal may be made after inspection of the specimens. Specimens not accepted

for measurement will be returned, the cost of examination or the minimum charge will be

applicable.

C. Image Optics and Photography

1. Refractive Index

(A) The index of refraction of a solid, transparent optical material is measured at visible wavelengths of calibrated emission lines of cadmium, helium, and mercury.

Ordinarily measurements are made at room temperature, near 20 °C. Specimens submitted

for test must 6e in one of the geometrical forms described below.

(1) Test specimens may be in the form of a prism, with two rectangular surfaces

polished optically flat within one-eighth wavelength. The minimum linear requirements

for these surfaces are approximately 25 mm (length) X 13 mm (height). The optimum

refracting angle varies from 70° for index «=1.4 to 50° for index n = 1.9. Before

submitting test materials, NBS personnel should be contacted for information concerning

the optimum prism angle.

(2) Test specimen may be in the form of a rectangular slab approximately 25

mm X 1 3 mm X 5 mm, such as is commonly used for calibrating an Abbe refractometer.

The two surfaces must be polished optically flat within approximately one wavelength

and intersect at 90° to form an unbeveled edge. The index of the specimen should not exceed n= 1.8. (B) The index of prismatic samples of refracting materials can be measured at wavelengths from 0.2 /xm to 20.0 \xm. The experimental data is fitted to a Sellmeier-type

dispersion equation which permits interpolation of the refractive index at any wavelength

within the wavelength range of the measured data. Special scheduling of this type of test would be necessary because of the time involved in making the measurements.

(C) The refractive index of non-corrosive, non-toxic, and transparent liquid

samples can be measured over the same wavelength range as described for the solid samples in (A) and (B) above. Liquids can be measured at temperatures from 10 °C to 80 °C. At least 200 cc of the sample should be submitted for the test. The accuracy of all refractive index determinations will be dependent on the optical

quality and form (prism, slab, or liquid) of the test specimen, and the wavelengths at which the measurements will be made.

82
Generally tests are based on correspondence regarding the exact nature of the

requirements. Do not ship materials to the Bureau until arrangements for test have been

completed. (See Appendix for NBS point of contact.)

2. Photographic Calibrations (e.g.. Step Tablet Transmission

Visual Density on Photographic and Radiographic Materials)

Diffuse transmission density is measured in accordance with ANSI Standard

PH2. 19-1959. Calibrated step tablets, both photographic and radiographic, are available

through the Office of Standard Reference Material.

In special cases, measurements may be made by arrangement. ANSI Standard

Diffuse Visual Transmission Density, Type Vl-b, is measured on step tablets of 2 1 steps or

less. Tablets submitted for calibration must be free of scratches, fingerprints, abrasions,

and foreign matter and must have steps of uniform density. Tablets not suitable as standards are not accepted for calibration.

References

[1] Conditions for microdensitometer linearity, R. E. Swing, / Opt. Soc. Am. 62, No. 2, 199-207 (Feb. 1972).

[2] A wide-angle retroflector, A. W. Hartman, NBS Report 10832 (July 1972).

]3] The measurement of the thicknesses of thin films: The Dyson interferometer, D. A.

Swyt, NBS Report 10918 (Sept. 1972).

[4] Basic considerations of densitometer adjustment and calibration, R. E. Swing, NBS

Report 10970 (Dec. 1972).

[5] Lens testing with a simple wavefront shearing interferometer, D. Nyyssonen and J. M.

Jerke, Appl. Optics 12, 2061-2070 (Sept. 1973).

[6] The optics of densitometry, R. E. Swing, Opt. Eng 12, No. 6, 185-198 (Nov./Dec.

1973).

[7] Basic considerations of densitometer adjustment and calibration, R. E. Swing, NBSIR

75-682. 18 pages (Feb. 3, 1975). Order from NTIS as COM 75-10524.

[8] The calibration of photographic edges at NBS, R. E. Swing, NBSIR 75-699, 37 pages

(Apr. 22, 1975). Order from NTIS as COM 75-11016.

[9] Semiconductor measurement technology: Optical and dimensional measurement

problems with photomasking in microelectronics, J. M. Jerke, Nat. Bur. Stand. (U.S.),

Spec. Publ 400-20, 42 pages (Oct. 1975).

[10] Interferometric lens testing, J. M. Jerke and D. Nyyssonen, Air Force Avionics

Laboratory (see [5]). [11] Development of dimensional measurement techniques from 1 to 10 micrometers and

application to optical microscope measurements, J. M. Jerke, Meeting of Society of

Photographic Scientists and Engineers, Proceedings (Feb. 1975).

[12] An improved photographic edge-artifact, W. R. Smallwood and R. E. Swing, NBSIR

76- 1129, 49 pages (Aug. 1976). Order from NTIS as PB274712.

4. Microcopy Resolution Test Charts

Microcopy resolution test charts conforming to ISO Standard 3334, NMA Standard

MS104- 1972, and MIL-M-9868D, are available through the Office of Standard Reference

Materials (SRM 1010a).

83
€Ii
CHAPTER IX

IX. Ionizing Radiation

The National Bureau of Standards carries out an extensive program of research

directed towards state-of-the-art measurement of ionizing radiation. Measurement services are available as described in the sections which follow for neutron sources and dosimetry,

for radioactivity, and for dosimetry of x-rays, gamma-rays, and electrons. Requests for additional information concerning NBS capabilities and services related to ionizing

radiation should be directed to the points of contact listed in the Appendix.

A. Neutron Sources and Dosimetry

The National Bureau of Standards provides neutron source calibrations as well as thermal neutron irradiations of foils and simple dosimeters in its thermal neutron density

of 4000 neutrons/cmVs. Due to an expansion in our neutron dosimetry and neutron

reaction rate standardization programs, we are now able to offer sample irradiations in fast neutron spectra as well as in thermal-neutron fields of much greater intensity and purity.

As a special measurement service, the following neutron fields are available for

irradiations:

(1) "^Cf spontaneous fission neutrons to a free-field fluence up to 10'^ neutrons/cm^

specified to an accuracy of ±2 percent (Icr) or better depending upon sample size and

arrangement.

(2) "'U cavity fission neutron field to a fluence of up to 2 X lO" neutrons specified to an accuracy of approximately ±5 percent (Icr). (3) A 24 keV Iron-Aluminum filtered beam to a fluence of 5x10" neutrons to an accuracy of about 10 percent.

(4) A 2 keV Scandium filtered beam to a fluence of 2 X 10" neutrons to an accuracy

of about 5 percent.

(5) A 144 keV Silicon filtered beam to a fluence of 10'^ neutrons to an accuracy of

10 percent.

(6) An external thermal neutron beam at the NBS reactor with free-field fluxes of up

to 2 X 10' neutrons/cmVs specified to an accuracy of ±3 percent (Icr) or better depending

upon sample size. (7) A cavity thermal neutron field with a near-isotropic neutron flux of about 10"

neutrons/cmVs specified to an accuracy of ± 5 percent ( 1 cr) or better.

/. Laboratory Standard Neutron Sources

The National Bureau of Standards provides a calibration service for laboratory

standard neutron sources. By calibration and certification of such standards, accuracy and

intercomparability are promoted in the measurements of neutron flux which play an

important part in current research. The service should be of particular assistance to those

concerned with fundamental nuclear experiments, design and control of nuclear reactors,

problems of protection from neutron radiation, and industrial applications of neutron

sources.The emission rate of neutron sources of 10^ to a few times 10* neutrons/s are

calibrated against the NBS primary Ra-Be photoneutron standard source by the

manganous sulfate bath technique. The neutron emission rate for more intense sources up

to about 10'° neutrons/s are calibrated by comparing their strength indirectly to the NBS

85
standard source. The total uncertainty of the cahbration is usually 1.2 percent to 1.7 percent depending upon time and physical size of the neutron source. The induced saturated manganese-56 activity of the bath is counted with two

scintillation counters at a well-shielded remote location by continuously pumping a

fraction of the bath volume to them. One scintillation counter samples the activity at the center of the volume and the second views a fraction of the same volume through a

collimator. The first counter operates at about 100 counts/s for the standard source of

about 10* neutrons/s and is paralyzed for the most intense sources. The second counter

operates at about 4000 counts/s for sources of about 10'° neutrons/s but close to background for the standard source. Relative counter- efficiencies are obtained with an

intermediate source of 10* neutrons/s.

About 1 month should be allowed for the calibration of such a laboratory standard.

Advance arrangements must be made for the calibration of laboratory standards.

The following information must be furnished:

(1) A diagram showing the source location in the shipping container and

instructions for removal of the source, if necessary.

(2) A description of any special markings on the source.

(3) The dimensions of the source, including the relative internal location of the

active ingredients.

(4) The nature and amount of radioactive materials and the ratio of neutron

producing ingredients.

(5) The kind of metal enclosing source and, if possible, the number of grams of each

element.

(6) The date the source was sealed. Sources accepted for calibration must be sealed, packed, and shipped in such a way

that leakage and contamination will be avoided. From 1 to 2 months should be allowed

for completion of the calibration.

References

[1] The correction factor for fast neutron reactions on sulfur and oxygen in the manganoussulfate-bath calibration of neutron sources, W. M. Murphey, Nucl. Instr. Methods 37,

No. 1, 13-21 (1965).

[2] Calculation of thermal neutron absorption in cylindrical and spherical neutron sources, V. Spiegel, Jr., and W. M. Murphey, Metrologia 7, No. 1, 34-38 (Jan. 1971).

[3] Absolute calibration of the National Bureau of Standards photoneutron source: III. Absorption in a heavy water solution of manganous sulphate, R. H. Noyce, E. R. Mosburg, Jr., S. B. Garfinkel, and R. S. Caswell, /. Nucl. Eng. 17, No. 7, 313-319

(1963).

[4] Absolute calibration of the National Bureau of Standards photoneutron standard: I., J. A. DeJuren, D. W. Padgett, and L. F. Curtiss, J. Res. Nat. Bur. Stand. (U.S.), 55, p. 63 (Aug. 1955).

[5] Absolute calibration of the National Bureau of Standards photoneutron standard: II. Absorption in manganese sulfate, J. A. DeJuren and J. Chin, J. Res. Nat. Bur. Stand.

55, p. 311 (Dec. 1955).

2. Thermal Neutron Dosimeters

NBS can provide calibration of a set (one to three) of thermal neutron dosimeters in the standard thermal neutron flux with and without cadmium covers. The response to gamma rays in the thermal flux is not provided.

86
3. Neutron Irradiation ofFoils

The NBS neutron physics laboratory also maintains a standard moderating geometry

supplying a uniform thermal neutron density of about 4000 thermal neutrons/cmVs for indirect calibration of unknown thermal neutron densities by irradiation of foils. The

standard thermal neutron density is produced by two 1 g Ra-Be (a,n) sources permanently

fixed in a moderation geometry of paraffin and graphite. The exposure cavity is in graphite.

Four foils, no larger than 2x3.4 cm each, can be irradiated at one time. Information on the cadmium ratio of gold foils of various thicknesses in this density is provided with

the calibration. Therefore, a separate cadmium-covered foil irradiation usually is not necessary but will be provided if requested. Foils usually are placed in the flux on a Monday and removed the following Monday and shipped by air mail to the laboratory

requesting the calibration.

If a specific date for the calibration is required, submit an advance request to NBS as indicated in the Appendix. Otherwise, foils will be irradiated as time is available in the

standard flux.

References

[1] Results of the intercomparisons of the thermal neutron flux density unit, (1966-1968), E.

J. Axron, Metrologia 6, No. 1, 25-32 (1970).

[2] Analysis of results of the Bureau International des Poids et Mesures thermal neutron

flux density intercomparison, W. M. Murphey and R. S. Caswell, Metrologia 6, No.

4, 111-115 (Oct. 1970).

[3] Absolute calibration of the NBS standard thermal neutron density, J. A. DeJuren and

H. Rosenwasser, J. Res. Nat. Bur. Stand. (U.S.), 52, p. 93 (Feb. 1954).

[4] A recalibration of the NBS standard thermal neutron flux, E. R. Mosburg, Jr., and W.

M. Murphey, Reactor Sci. and Tech. J. Eng. 14, Pt. A/B, 25-30 (1961).

B. Radioactivity

1. Calibration of Gamma-Ray-Emitting Radioactive Samples

Gamma-ray-emitting radioactive samples which conform to the physical, chemical

and activity level specifications for measurement at the National Bureau of Standards are

calibrated in a 47r ionization chamber. Solutions should be 5 ml in volume and flamesealed in glass vials or ampoules (free of high Z elements) of outer diameter 16.0±0.5 mm,

wall thickness approximately 0.5 mm. Ampoules, for this purpose, may be obtained from

NBS.

Gamma-ray-emitting radionuclides for which calibrations can be obtained, and their activity ranges are:

10 to 200 /LtCi, Na-22, Na-24, Sc-46, Co-60, Ga-67, Y-88, Ag-1 lOm-Ag-110

30 to 800 jLiCi, Mn-54, Fe-59, Zn-65, Se-75, Sr-85, Nb-95,

Sn-113-In-113m, 1-131, Ba-133, Cs-134,

Cs-137-Ba-137m, Ta-182, Au-198

60 to 1500 \xC\, K-42, Co-57, Tc-99m, Mo-99, Cd-109, 1-123,

Ce-139, Ce-141, Hg-197, Tl-201, Hg-203

200 to 1 500 fiCi, Cr-5 1 , Ce- 1 44

2. Calibration ofAlpha-Particle-Emitting Samples

THESE SAMPLES MUST BE SENT IN SUCH A WAY THAT THE ACTIVE

AREA IS NOT TOUCHED BY ANY MATERIAL.

87
Alpha-particle-emitting samples that conform to the physical activity spectifications for measurement are calibrated in the National Bureau of Standards 27r-proportional or

lir-scintillation counters. The preparation and calibration of polonium-210 sources, the

calibration of alpha-particle samples in both the 27r-proportional and the Itt -scintillation counters, for In count-rate or activity, respectively, are covered under this service. The

list of calibrations currently available is given in the Appendix under the title "Radioactivity."

Chemically and physically stable samples that conform to the specifications listed below can be measured for total particle count rate into a 2tt geometry using the NBS 2it proportional counter.

(1) Source diameter should not exceed 10 cm.

(2) Source thickness should be such that more than 99.5 percent of the emitted

alpha particles have an energy greater than 400 keV.

(3) Activity should not exceed 0.3 ju.Ci. (4) Activity should not be less than 20 pCi.

Samples that are chemically and physically stable can be measured for activity in the

NBS Itt counter if they meet the specifications outlined in (2) and (3) above. Activity

should not be less than 3 nCi. Source diameter should not exceed 1.6 cm. Samples that are chemically and physically stable and meet the specifications

outlined above can be measured in both the 27r-proportional and the iTr-scintillation counters.

3. Calibration of Beta-Ray-Emitting Radioactive Samples

Beta-ray-emitting radioactive samples that conform to the physical, chemical, and

activity level specifications for measurement are calibrated in either the National Bureau

of Standards Arr/i proportional counter, or by liquid-scintillation counting. See the

Appendix under "Radioactivity" for the current list of services. With the exception of

phosphorus-32, the solutions are not examined for beta-ray-emitting impurities.

Chemically stable solutions of the following radionuclides, having a radioactivity

concentration between 0.5 and 50 /nCi/ml, can be calibrated by liquid-scintillation

counting.

(1) hydrogen-3

(2) carbon- 14

(3) chlorine-36

(4) strontium-90-yttrium-90

Solutions should be approximately 5 ml in volume and flame-sealed in glass vials or ampoules.

Phosphorus-32 solutions, of radioactivity concentration between 2 and 50 /xCi/ml, can be calibrated using the NBS 47r^ proportional counter. Calibration includes a halflife fit to determine the phosphorus-33 impurity. Solutions should be neutral phospate or weak phosphoric acid. Chemically stable solutions of the following radionuclides, having a radioactivity

concentration between 0.5 and 50 fiCi/ml, can be calibrated using the Att^ proportional

counter.

(1) strontium-89

(2) promethium-147

(3) thallium-204

C. Dosimetry of X-Rays, Gamma-Rays, and Electrons

1. X-Ray and Gamma-Ray Measuring Instruments

X-ray measuring instruments are calibrated in terms of exposure by a substitution

method in an x-ray beam at a point where the exposure rate has been determined by means

88
of a standard free-air ionization chamber. In order to provide instrument calibrations over

a wide range of x-ray energies, many combinations of generating potential and filtration are available. These are listed in the Appendix under "Lightly Filtered X-Rays,"

"Moderately Filtered X-Rays," or "Heavily Filtered X-Rays." The calibration techniques

requested should be appropriate to the instrument submitted.

Gamma-ray measuring instruments are calibrated in terms of exposure or absorbed

dose at points in the collimated cobalt-60 and cesium- 137 gamma-ray beams that have been

standardized by means of graphite cavity chambers or a graphite calorimeter. Exposure

rates and absorbed dose rates at the time of calibration are computed from the original

beam standardization data and appropriate decay "corrections. Ionization chambers

submitted for an exposure calibration should have sufficient wall thickness to provide

electron equilibrium for the gamma-ray energy selected. Ionization chambers submitted

for an absorbed-dose calibration must be suitable for calibration in a phantom.

An ionization chamber and electrometer combination, with the electrometer marked

in terms of exposure or absorbed dose, is calibrated by providing a dimensionless

correction factor for the electrometer scale; special arrangements should be made for the

calibration of an ionization chamber to be submitted with an electrometer not marked in terms of exposure or absorbed dose. An ionization chamber submitted without an

electrometer is calibrated in terms of exposure, or absorbed dose, per unit charge.

Calibration can be based on measurements for positive or negative polarizing potential, or on the mean of measurements for both positive and negative potentials, as requested. The

ratio of ionization currents for full and half polarizing potentials and the corresponding

ionization current, will be stated in the calibration certificate. Ionization chambers are tested, prior to calibration, for leakage, radiation-induced

leakage, stabilization time, short-term stability, recombination loss, connection to the

atmosphere, and guard-electrode insulation (if applicable). Chambers found unsuitable for calibration will be returned with a statement of the reason for rejection. A charge may be

made for time i^icurred in the tests. A statement of the performance specifications that the

chambers are required to meet will be furnished on request to the address given in the

Appendix.

Irradiation of passive dosimeters, for readout by the user, is available for the

techniques listed in the Appendix. These irradiations are generally in terms of exposure;

for passive dosimeters suitable for insertion in a phantom, irradiation in terms of absorbed

dose can be provided by in-phantom irradiation using cobalt-60 gamma rays. X-ray penetrameters, of the Ardran-Crookes type, can be calibrated using constant

x-ray generating potentials up to 250 kV. These penetrameters are used for measurement

of the generating potential of diagnostic x-ray units. For details of the currently available services, see the Appendix under the title "XRay and Gamma-Ray Measuring Instruments."

2. Gamma-Ray and Beta-Particle Sources

Sources submitted to the Bureau for dosimetry calibration are subject to the

following conditions:

(a) Financial responsibility: Except for negligence by Bureau personnel, the Bureau

assumes no responsibility for loss or damage to the sources while in its possession. The

risk should be covered by insurance.

(b) Period of measurement: Inquiry should be made as to scheduling and turn- around time before the source is submitted.

(c) Preparation: Sources submitted for calibration must be sealed so that there can be no escape of any radioactive material, including any gaseous decay products. The

sources, shielding, and packaging must be free of contamination. Contaminated or leaking sources cannot be measured and may cause considerable loss of time and damage to laboratory facilities. Sources must have been sealed for a sufficient time to be substantially

89
in radioactive equilibrium with their decay products when these contribute to the emitted

radiation.

(d) Packaging for shipment: Packages must be in compHance with the regulations of

the Department of Transportation as detailed in CFR Title 49 and the regulations of the

Nuclear Regulatory Commission as detailed in CFR, Title 10, Part 71. Copies of the

Codes are available at the Government Printing Office, Washington, DC 20402.

A drawing showing the source container and a description of the method of source removal must be provided before shipment of the source for calibration.

(e) Possession of licensed materials: In submitting a source for calibration, it is necessary for the submitter to certify that he is duly authorized to possess the source under license by the applicable authority. In the case of individuals residing in a State that has entered into agreement with the Nuclear Regulatory Commission, State regulations are applicable to all sources including radium. In the case of other individuals, NRC

regulations are applicable (radium is not licensed by the NRC). This certification may be

by letter, by a suitable statement on the purchase order covering the calibration fee, or by

a clear copy of the submitter's Possession License for the source. Calibration in terms of exposure rate at 1 m is provided for gamma-ray sources of

cobalt-60, cesium- 137, and iridum-192; a similar service is planned for radium and iodine125. Radium is calibrated in terms of mass of equivalent radium content measured relative to the National Radium Standard through comparison of the gamma radiation from the

specimen and the standard; where the details of encapsulation of the specimen are known,

corrections can be made to obtain milligrams of radium content. Calibration in terms of

absorbed dose rate is provided for suitable encapsulated beta-particle sources; the dose

rate to a low-atomic-number material (graphite or plastic) is determined by measurement

with an extrapolation chamber. For details of the currently available services, see the

Appendix under the title "Gamma-Ray and Beta-Particle Sources."

3. Dosimetry of High-Energy Electron Beams

Dosimeters are provided twice a year to users requesting assistance with absorbeddose measurements in high-energy electron beams. The dosimeters employed will consist

of ferrous sulfate (Fricke) solution in radiation-resistant silica-glass spectrophotometer

cells. The user will irradiate all but one of the three furnished dosimeters to between 50

and 80 Gy (5000 and 8000 rad) to water at electron energies between 5 and 50 MeV,

employing the irradiation geometry (field size, phantom, position of dosimeter in phantom) given in the "Protocol for Dosimetry of High-Energy Electrons," Physics in Medicine and Biology 11, 505 (1966).

After irradiation, the dosimeters are returned to the Bureau for spectrophotometric

evaluation of the ferric-ion concentration in terms of absorbed dose in the phantom, using

the G-value given in the Protocol. For details, see the Appendix under the title "Dosimetry of High-Energy Electron Beams."

D. Radiochromic Dosimetry (High-Dose Gamma-Ray Calibrations)

Dosimeters of maximum size 2x 10 cm are irradiated with cobalt-60 gamma rays in the range 1 to 600 kGy (0. 1 to 60 Mrad) to water. The optical density can be determined

by a spectrum scan in both the visible and ultraviolet regions. If desired the temperature

dependence of gamma-ray response can be determined. For details see the Appendix

under the title "Radiochromic Dosimetry (High-Dose Gamma-Ray Calibrations)."

90
CHAPTER X

X. Computer Science—Magnetic Disk Calibration

The National Bureau of Standards conducts research and provides technical services

designed to aid Government agencies in improving cost effectiveness in the conduct of

their programs through the selection, acquisition, and effective utilization of automatic

data processing equipment; and serves as the principal focus within the executive branch

for the development of Federal standards for automatic data processing equipment,

techniques, and computer languages. For information on this program, either write or call NBS at the address listed in the Appendix.

Magnetic Disk Calibration

Magnetic disk calibration services in three density levels, are offered in support of

related Federal Information Processing Standards and American national and international

standardization activities. They include calibrating signal amplitude at the three recording

densities described below:

Low Density: 1.100 bpi at track 200, 100 tpi

Low density magnetic disk surfaces of the type used in the six-high disk pack (ANSI

Standard X3.46- 1974) will be calibrated relative to the Primary Standard Amplitude

Reference Surface held in repository at NBS. Submissions will be calibrated at 0.625 MHz

on track 000 and at 0.625 MHz and 1.25 MHz on track 200 along specified areas located

by scribe marks on these tracks. The customer can elect to have NBS place the necessary

scribe marks on a submitted surface or provide NBS with an already inscribed surface.

Medium Density: 2,200 bpi at track 200, 100 tpi

Medium density magnetic disk surfaces of the type used in the eleven-high disk pack

(ANSI Standard X3. 58-1977) and the one-high single disk cartridge (ANSI Standard

X3. 52-1976) are calibrated relative to the medium density Primary Standard Amplitude

and Data Reference Surfaces held in repository at NBS. Submissions are calibrated at specified areas located by scribe marks on track 000 and track 200. The customer can elect to have NBS place the necessary scribe marks on a submitted surface or provide NBS

with an already inscribed surface. A 2311 -type amplitude test head will be used to

calibrate a submitted surface at 0.625 MHz on both of these tracks. A 2314-type data test head is used to calibrate a submitted surface on track 200 at both 1.25 and 2.5 MHz.

High Density: 4,040 bpi at track 410, 192 tpi

High density magnetic disk data surfaces of the type used in the twelve-high disk

pack (ANSI Standard BSR X3.63) are calibrated with respect to the Primary Standard

Reference Data Surface held in repository at NBS. Submissions are calibrated using a data

test head at both 1.6 and 3.2 MHz along the entire area of tracks 000 and 410. Fees for these services are listed in the Appendix in the section on "Computer

Science."

91
4
CHAPTER XI

XI. Miscellaneous Services

The National Bureau of Standards provides many other services in addition to the

caUbration and testing programs to which this pubUcation is primarily addressed.

A. Standard Reference Materials

The Standard Reference Materials Program of the National Bureau of Standards

provides science, industry, and government with a central source of well-characterized

materials certified for chemical composition, or for some cjhemical or physical property.

These materials are designated Standard Reference Materials (SRM's) and are used to

calibrate and evaluate measuring instruments, methods, and systems or to produce

scientific data that can be referred readily to a common base. Approximately 900 SRM's

currently available from NBS are described in the Catalog of NBS Standard Reference

Materials, NBS Special Publication 260, and its supplement.

All of the Standard Reference Materials (SRM's), Research Materials (RM's), and

Special Reference Materials (GM's) listed in SP260 bear distinguishing names and

numbers by which they are permanently identified. Each SRM, RM, or GM bearing a given designation is of identical characterization with every other sample bearing the same designation, within the limits required by the use for which it is intended; or if necessary, it is given a serial number and an individual calibration. New SRM's are prepared each year and are announced through supplements to SP260 as well as directly to prospective users who have requested such notification. Price

lists are issued periodically and provide a complete list of the available SRM's, and their

prices.

Areas in which materials are available are listed below. Request for additional

information or copies of SP260 should be addressed to the Office of Standard Reference

Materials, at the address shown in the Appendix.

Standard Reference Materials (SRM's)

Chemical Composition Standards

Steels (chip form): Plain carbon, low and high alloys, stainless and tool.

Steels (granular form): Special ingot irons, low alloy, stainless, speciality, hightemperature alloys, and tool. Steelmaking Alloys

Cast Irons (chip form)

Cast Steels, White Cast Irons, Ductile Irons, and Blast Furnace Irons (solid form)

Nonferrous Alloys (chip form): Aluminum, cobalt, copper, lead, magnesium, nickel,

nickel oxide, selenium, tin, titanium, zinc, and zirconium

Nonferrous Alloys (solid form): Copper, lead, nickel, tin, titanium, zinc, and

zirconium

Gases in Metals

High-Purity Metals

Electron Probe Microanalytical Standards

93
Primary, Working, and Secondary Standard Chemicals

Microchemical Standards

Clinical Laboratory Standards

Biological Standards

Environmental Standards: Gases, liquids, solids, permeation tubes, and trace elements in fossil fuels

Industrial Hygiene Standards

Forensic Standards

Metallo-Organic Compounds

Fertilizers Ores

Cements

Minerals, Refractories, Carbides, and Glasses

Trace Element Standards

Nuclear Materials: Plutonium and uranium assay and isotopic standards, neutron

density standards, and fission track glass standards

Isotopic Reference Standards

Physical Property Standards

Ion Activity Standards: pH and pD standards, and ion selective electrodes

Mechanical and Metrology Standards: Coating thickness, glass, elasticity, density,

and polymer

Heat Standards: Superconductive thermometric fixed point devices, freezing points,

melting points, calorimetry, differential thermal analysis, vapor pressure, thermal

conductivity, thermal expansion, thermocouple materials

Magnetic Standards: Magnetic susceptibility

Optical Standards: Spectrophotometry, reflectance, refractive index

Radioactivity Standards: Alpha-particle, beta-particle, gamma-ray, and electron

capture standards, contemporary standard for carbon 14 dating laboratories,

environmental standards, low energy photon sources, and solution standards

Metallurgical

Mossbauer

X-Ray Diffraction

Permittivity

Reference Fuels

Resistivity

Engineering Type Standards

Standard Rubber and Rubber-Compounding Materials

Reference Magnetic Computer Tapes

Sizing Standards: Glass spheres for particle size, turbidimetric and fineness (cement)

Color Standards: The ISCC-NBS centroid color charts, light-sensitive papers, lightsensitive plastic chips

94
X-Ray and Photographic Standards

Surface Flammabihty Standards

Smoke Density Chamber Standards

Water Vapor Permeance

Tape Adhesion Testing Standard

Research Materials (RM's)

High-Purity Materials, Phosphors, Glass Fibers for Miro-Chemical Analysis

Special Reference (GM's)

Hydrogen in Steel, Cellular Plastics, and DTA Temperature Standards

B. Proficiency Sample Programs

In 1936, the first reference sample program was initiated for cement as part of the

NBS Research Associate Program of the Cement Reference Laboratory of the American

Society for Testing and Materials (ASTM) Committee C-1. In 1966, this program was

revised to essentially its present form wherein two pairs of samples for physical tests and

two pairs of samples for chemical analyses are distributed each year. Similar programs for masonry cement, blended cement, bituminous, bituminous concrete, soils, aggregates, and

Portland cement concrete were then established in rapid succession, the last in 1978. These

programs are under the sponsorships of ASTM and the American Association of State Highway and Transportation Officials. In 1969 a bimonthly collaborative reference program for paper and board testing was sponsored by the Technical Association of the Pulp and Paper Industry and a program for control of the quality of shipping container components was established for the Fourdrinier Kraftboard Institute. This latter program involves monthly reports but

weekly testing by the participants. A quarterly program for the rubber industry was

developed in 1970 with the help of ASTM Committee DOll, and a color and appearance

program, of interest to many industries and users, was sponsored by the Manufacturers

Council on Color and Appearance. This quarterly program presently includes tests for

gloss, color and color difference.

Each field of testing has its special problems and each sponsoring organization its own goals. Hence, no two of the existing programs are identical, but each has been

designed to meet the specific needs. New programs are created as the need is determined

and industry sponsors can be found. Inquiries concerning the establishment of new

programs are invited. NBS-Technical Association of the Pulp and Paper Industry Collaborative Reference Program

for Paper

(6 times per year)

This program provides approximately 40 tests in the areas of physical and optical

testing of paper and board.

NBS-Fourdrinier Kraft Board Group of the American Paper Institute Collaborative Reference

Program for Containerboard

(48 times per year)

Mullen burst of linerboard, concora test of medium

NBS-Manufacturers Council on Color and Appearance Collaborative Reference Program for

Color and Appearance

(4 times per year)

Gloss at 60°, color and color difference

95
NBS-Collaborative Testing Services Collaborative Reference Program for Rubber

(4 times per year)

Tensile strength, ultimate elongation and tensile stress, hardness, mooney viscosity,

vulcanization properties

ASTM Portland Cement (2 times per year)

ASTM Blended Cement (once a year)

ASTM Masonry Cement (once a year)

ASTM Concrete (2 times a year)

'AASHTO Aggregates (2 times per year)

Coarse (once a year); Fine (once a year)

AASHTO Soils (2 times per year)

AASHTO Bituminous

Asphalt cement (2 times per year), cutbacks (once a year)

AASHTO Bituminous Concrete (once a year)

C. National Voluntary Laboratory Accreditation Program

In a notice published in the Federal Register on February 25, 1976, (41 FR 8163-

8168), the Secretary of Commerce promulgated procedures for the operation of a National Voluntary Laboratory Accreditation Program (NVLAP). As announced in that

notice, the goal of this program is to provide in cooperation with the private sector a

national voluntary system to examine upon request the professional and technical

competence of private and public testing laboratories that serve regulatory and

nonregulatory product evaluation and certification needs. The program will accredit those

laboratories that meet the qualifications established under the mentioned procedures.

Section 7.4(b) of those procedures sets out the requirements to be met by those persons

who seek to have the Secretary find that there is a need to accredit testing laboratories

which render services regarding a specific product so that it may be ascertained whether

such product meets the requirements of applicable standards. The laboratory evaluation

process leading to accreditation is conducted by NBS and generally includes (1) questionnaires, (2) an on-site inspection, and (3) proficiency sample testing. Accreditation

programs for laboratories that test thermal insulation materials, carpet, and freshly mixed

field concrete are now in operation.

For additional information write to National Voluntary Laboratory Accreditation

Program at the address given in the Appendix.

D. Standards Information Services

The Standards Information Services (SIS) maintains a reference collection of some

260,000 engineering standards issued by U.S. technical societies, professional

organizations, and trade associations; State purchasing offices; U.S. civilian government

agencies; and the major foreign national and international standardizing bodies. The

collection is open to the public Monday through Friday from 8:30 a.m. to 5:00 p.m. SIS publishes general and special indices of standards. Information services consists

of responding to inquiries by searching Key-Word-In-Context (KWIC) Indices to determine whether there are any published standards, specifications, test methods, or recommended practices for a given item, product, or material. Inquirers are referred to the appropriate source to obtain copies of standards. SIS neither sells nor distributes standards.

Inquiries or requests for additional information should be directed to the address

listed in the Appendix.

96
E. Standard Reference Data

The National Standard Reference Data System (NSRDS) is a nationwide program

established to make critically evaluated data in the physical sciences available to the

technical community. It publishes compilations of critically evaluated data, critical reviews and bibliographies. A complete listing of the publications of the NSRDS is available from the Office of Standard Reference Data (OSRD). The OSRD responds to queries within the scope of the program by providing references, referrals, documentation,

or data, as available. The program's newsletter is available on request. Inquiries or

requests for further information should be directed to the address listed in the Appendix.

F. Technical Information and Publications

The Technical Information and Publications Division maintains a general

correspondence and inquiry service on the technical activities of the National Bureau of

Standards. Inquiries of a general nature and not covered by the services listed above

should be directed to Technical Information and Publications Division, National Bureau

of Standards, Washington, DC 20234.

G. NBS Measurement Seminars

Each year NBS holds several measurement seminars and workshops. Participation is open to a limited number of persons from measurement and standards laboratories who

meet appropriate prerequisites relating to education, work experience, and current

professional activity. Seminars last from 1 to 5 days and comprise lectures, group

discussions, and/or laboratory demonstrations. Names may be added to the mailing list for the seminar brochures by writing or telephoning the Office of Measurement Services at the address listed in the Appendix.

97
ii
INDEX

AC voltage calibrations IV.D.3

Accreditation Program (see Laboratory

Accreditation Program) XI.

Acoustic emission sensors III.B.3 Acoustic measurements III.B Aerodynamics III.G Air speed instruments III.G Angular measurements II.

Angular standards II. D.

Antenna gain V.C

Antenna pattern V.C

Antenna polarization V.C

Antennas V.C

Attenuation V.B

Attenuation, electromagnetic V.B

Attenuation, network (see network

attenuation) V.B

Attenuator, coaxial, fixed V.B.

Attenuator, variable, waveguide V.B.

Attenuator, waveguide below cutoff

(piston) V.B.4

Ball diameters II.C.5 Barometers VII. B.

Basic photometric calibrations VIII. A.

Basic radiometric calibrations VIII.A. 1 Beta-particle source calibration IX.C.2

Calorimeter, laser V.E.I

Capacitance bridges IV.G.2

Capacitance, three terminal (see

impedance) V.D.3

Capacitance, two terminal (see

impedance) V.D.2

Coaxial impedance V.D.7

Coaxial phase shift V.G.2

Collaborative reference program XI.

Computer science X

Cryogenic measurements VII. D,

VII.D.l, VII.D.3

Current transformers IV.F.2

DC voltage standards IV.D.2

Density II. A

Density (cryogenic fluids) VII.D.

Density determinations of liquids

and solids II. A.

Diameter, external II.C.3 Diameter, internal II.C.6 Diameter, measuring instruments II.C.IO Diameter, spherical II.C.5

Dimensional metrology II.

Dissemination services, energy V.E.2

Dissemination services, laser V.E.2

Distributed parameter measurement V.D.7

Dosimeters, thermal neutron IX.A.

Dosimetry of high-energy electron

beams IX.C.3

Dosimetry, x-rays, gamma-rays, and

electrons IX.C

Effective noise temperature V.F

Electrical instruments (ac-dc) IV.

Electromagnetic fields V.C

Electromagnetic interference V.K

Electromagnetic measurements V

Electromagnetic radiation V.K

Electron-beam dosimetry IX.C.3

EM field strength measurements V.C

EMI emission measurements V.C

EMI susceptibility measurements V.C

EM near-field measurements V.C

End standards of length II.C.l Energy measurements, laser V.E.2

Expansion of length standards II. B.

External diameter standards

and gages II.C.3 Extrapolation range measurements V.C.

Flatness II.D Flowrate meters III.E.l Fluid flow III.E Fluid quantity III.E.l Force III.

Frequencies, comparison VI

Frequencies, infrared VI

Frequencies, multiplication VI

Frequencies, optical VI

Frequency and time VI

Frequency and time dissemination

services VI

Frequency and time measurements VI

Frequency and time standards VI

Frequency stability VI

Gage block comparator stylus tip radius ... II. B.

Gage blocks II.B.l Gages, conical plug and ring II. C.

Gain, antenna (see antenna) V.C

Gain, network (see network gain) V.J.I,

V.J.3

Gamma ray source calibration IX.C.2

99
Haemacytometers II.B.8 Hazards (non-ionizing) V.K

High-dose dosimetry IX. D

High voltage and energy measurements IV.

G

High voltage capacitors IV. G.

2

High voltage field calibration IV.G.

4

High voltage resistors IV.G.l

Holmium oxide glass standards VIII. B.

2

Humidity measurements VII.

C

HydrauHcs III.F Hygrometers, coulometric VII. C.4

Hygrometers, dew-point VII. C.l

Hygrometers, electric VII.C.

2

Hygrometers, pneumatic bridge VII.C.

5

Image optics VIII.

C

Immittance (see impedance) V.D

Impedance IV.C,

V.D

Impedance, coaxial V.D.

7

Impedance MAP service IV.C. 1 Impulse spectrum amplitude V.J. 1 Inductance (see impedance) V.D.

4

Inductive voltage dividers IV.B.l

Industrial radiation source IX.

D

Infrared/optical frequencies VI

Instrument comparators IV.

F

Instrument components requiring

dimensional control II.C.ll Instrument transformers IV.

F

Interference (electromagnetic) V.K

Internal diameter standards II.C.6 Ionizing radiation IX

Irradiation of foils, neutron IX.A.

3

Kerr electro-optical measurements IV.G.3

Laboratory Accreditation Program XI.

C

Laser energy V.E

Laser power V.E

Laser standards V.E. 1 Length II.B Length and diameter measuring

instruments II.C.9 Length, end standards of II. C.l

Length, line standards of II.B.4 Length measuring elements II.C.IO Length, step gage standards of II.C.2 Linear thermal expansion of

length standards II.B.

2

Line standards of length II.B.4 Liquefied Natural Gas Program VII. D.

3

Low pass filter transition time V.J. 1 Magnetic disk calibration X.A

Manometers VII. B.4

Mass II

Mass, reference standards of II. A. 1 Mass, volume and density II.

A

Measurement seminars XI.

G

Mechanics and acoustics Ill Mercury thermometers VILA. 1 Microcopy resolution test charts VIII.C.

3

Microphones III.B.l Micropotentiometers (rf voltage) V.I.

2

Microwave antenna measurement V.C

Microwave power V.H

Network attenuation (wideband,

coaxial) V.B

Network gain (wideband, coaxial) V.B

Neutron irradiation of foils IX.A.3

Neutron physics IX.

A

Neutron sources IX.A. 1 Noise source, coaxial V.F

Noise source, waveguide V.F

Noise temperature, effective V.F

Optical/infrared frequencies VI

Optical measurements VIII

Optical reference planes: flats II. D. 1 Oscillator (precision) noise VI

Pattern, antenna (see antenna) V.C

Peak pulse power V.H

Phase shift V.G

Phase shift difference V.G

Phase shifters V.G

Photographic calibration VIII.C.2

Photography VIII.C

Photometry VIII.A

Piston gages VII.B.l

Piston gages, controlled clearance VII. B.2

Pitot-static tubes III.F. 1 Planar near-field scanning method V.C.I

Plug gages II.C.3,

n.C.7, II.C.8 Polarization, antenna (see antenna) V.C

Power, electromagnetic V.H

Power measurements V.H

Power measurements, laser V.E

Power meter calibrations, laser V.E.

2

Power meters, directional couplers V.H

Power meters, electromagnetic (rf

and microwave) V.H

Power meters, pulse (coaxial and

rectangular waveguide) V.H

Power, pulsed V.H

Power, rf and microwave V.H

Precision apparatus IV. B

Precision circles II.B.9 Precision oscillator frequency VI

Precision oscillator noise VI

100
Pressure gages VII. B. 5 Pressure measurements VII.

^Pressure transducers VII. B.

^Prototype examination II.F.l Psychrometers VII.C.3

Pulsed power V.H

Pulse generator transition time V.J.

Pulse power meters (coaxial and

rectangular waveguide) V.J. 1 Pulse (time, clock) VI

Pulse time delay V.J.I,

V.J.4

Pyrometer indicators VII.A.

Q-standards (see impedance) V.D.6

Radiation (electromagnetic) V.K

Radiation hazards (non-ionizing) V.K

Radiation physics IX

Radiation processing dosimetry IX.D

Radiation sterilization dosimetry IX.D

Radioactive samples, alpha-particle

emitting IX.B.2

Radioactive samples, beta-ray

emitting IX.B.3

Radioactive samples, gamma-ray

emitting IX.B.l

Radioactivity ••• IX.

Radiochromic dosimetry IX.D ^ Radiometric and photometric gage

f calibrations VIII.A.3

Radiometry VIII. A

Reference data (see standard reference

data) XI.E

Reflection coefficient (see

impedance) V.D

Refractive index VIII.C.l

Resistance (see impedance) V.D.5

Resistance MAP services IV.A. 1 Resistance measurements IV.

Resistance standards IV.A.2,

IV.A.3

Resistance standards (high current) IV.A.4

Resistance thermometers VII.A.3

Rf power V.H

Ring gages II.C.6,

II.C.7, n.c.8

Rise time (see transition time) V.J.

Roundness II.D.3,

II.D.4

Sieves II.B.7 Signal sources (see oscillator) VI

Special attenuation measurements V.B.I

Spectrophotometric measurements VIII.B.4

Spectrophotometric standards VIII.

Spectrum amplitude, pulse V.J.I Spherical diameter standards II. C.

Stabihty, frequency VI

Stability, oscillator VI

Standard capacitors IV.C.

Standard inductors IV.C.3

Standard reference data XI.E

Standard reference materials XI.

Standards information services XI.

Standards of spectral transmittance

for checking the photometric

scale of spectrometers VIII. B.l

Step gage standards of length II. C.

Step height measurements II.E.3 Straight edges II.D.2

Structural engineering III.D Surface plates II.D.2

Surface roughness measurements II.E.2 Surface texture II.

Surveying and oil gaging tapes II.B.5 Surveying leveling rods II. B.

Tapes, Invar II.B.5 Tapes, oil gaging and surveying II.B.5 Tapes, steel II.B.5 Technical publications XI.

Temperature VII

Thermal neutron dosimeters IX.A.2

Thermal voltage converters (rf) V.I.I Thermistors VII

Thermocouples VII. A.

Thermodynamics quantities VII

Thermometer systems VII. A.4

Thermometers, laboratory VII.A.l

Thermometers, standard platinum

resistance VII.A.3

Thermometry VILA

Time and frequency measurements VI

Time and frequency standards VI

Time delay, pulse V.J. 2, V.J.4

Time pulses VI

Transition time, low pass filter V.J.I Transition time, pulse, generator V.J.2

Ultrasonic reference blocks III.B.4 Ultrasonic transducer and

system calibration III.B.2 Vacuum gages VII. B.7

Vibration services—pickups III.C Voltage dividers I V.G.I

Voltage, electromagnetic V.I Voltage MAP services IV.D.l

Voltage measurements IV. D

Voltage, micropotentiometers V.I

101
Voltage, rf V.I

Voltage, thermal voltage converters (TVC) V.I Voltage transformers I V.F. 1 Volt ratio MAP services IV. B.

Volt ratio standards IV. B.

Volume II.

Volumetric apparatus, reference

standards of II. A.

Water current meters III.F.l Watthour meters IV.G.5

Waveguide devices V.G.3

Weights and measures II.

Wideband attenuation (pulsed) V.J.I,

V.J.3

Wires for threads and gears II.C.4 Working standards of spectral

reflectance factor VIII. B.

X-ray and gamma-ray measuring

instruments IX.C.l

X-ray, gamma-ray, and electron

dosimetry IX.C

102
BS-1MA (REV.

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. $P 250 r 1980 Edition

2. Performing Organ. Report No 3. Publication Date March 1981

TITLE AND SUBTITLE

Calibration and Related Measurement Services of the National Bureau of Standards

5. L. J. Kieffer, Editor

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

m

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Same as No. 6

10. SUPPLEMENTARY NOTES

Supersedes flBS Special Publication 250 - 1978 Edition

Library of Congress Catalog Card Number: 63-60099

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sumn\ary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This publication provides detailed descriptions of the currently available NBS

calibration services, measurement assurance programs, and other measurement

services. In addition, each section describing specific services contains references to additional publications giving even more detail about the measurement

techniques and procedures used. This revised edition reflects the services available as of the second quarter of 1980. NBS Special Publication 250 was last

issued in 1978. The Appendix to SP250 is reissued every six months (June and

December). It lists current prices for the services described in this publication

and the NBS points of contact (addresses and phone numbers) from whom additional

information can be obtained.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration; measurement assurance; measurement services; standards; traceability

13. AVAILABILITY

[~X] Unlimited

I I For Official Distribution. Do Not Release to NTIS

flC] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

105

15. Price

$4.50

<rU.S GOVERNMENT PRINTING OFFICE: 1981- 340-997:1547 USCOMM-OC 6043-PRO
4i

t#i
Changes in Calibration and Measurement Services

As services are initiated or discontinued, or fees are changed, an- nouncements will appear in the appendices of SP 250. If you wish to be

placed on the mailing list to receive these updates as they are issued, please

complete the post card below, detach it, and mail to the National Bureau

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If you have previously submitted a card to the Bureau, it is not

necessary to furnish a duplicate card, unless your address has changed.

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Date

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Please place the foregoing name on your special

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250, Calibration and Related Measurement Services of

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(SP 250-1980 Edition)
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Office of Measurement Services

Washington, DC 20234
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JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

$13; foreign $16.25. Single copy, $3 domestic; S3. 75 foreign. NOTE; The Journal was formerly published in two sections; Section A "Physics and Chemistry" and Section B "Mathematical

Sciences." DIMENSIONS/NBS—This monthly magazine is published to in- form scientists, engineers, business and industry leaders, teachers,

students, and consumers of the latest advances in science and

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highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution abatement,

health and safety, and consumer product performance. In addition, it reports the results of Bureau programs in measurement

standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and

automatic data processing. Annual subscription; domestic $11;

foreign $13.75. NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies. Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work,

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

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published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

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environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

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useful background knowledge for shopping in today's tech- nological marketplace. Order the above NBS publications from: Superintendent of Documents. Government Printing Office, Washington, DC 20402. Order the following NBS publications—FIPS and NBSIR's—from

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in paper copy or microfiche form.
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