U.S. Department of Commerce

National Bureau of Standards
Center for Basic Standards

The Center for Basic Standards develops and maintains the scientific competences and laboratory facilities nec- essary to preserve and continue to refine the base physical quantities upon which the Nation's measurement

system is constructed. The Center is responsible for the standards of voltage, current, resistance and

impedance, temperature, vacuum, leak rate, pressure, mass, length, time and frequency. The Center collaborates closely with the standardizing laboratories of other countries to ensure that measures used in international

trade are fully compatible. The Center also performs research at the frontiers of physics in order to improve

measurement capability and quantitative understanding of basic physical processes that underlie measurement

science.

Electricity Division

Maintains and improves the national standards of electrical measurements; conducts experiments; conducts ex- periments to realize the electrical units in terms of the mechanical units; develops stable electrical standards

and improved methods for the dissemination of the units of electrical measurements; determines fundamental

physical constants related to electrical units; develops instrumentation for precise electrical measurements di- rectly referenced to the national standards; implements techniques for their use and puts them into practical

applications; assists in applying scientific and technological development of measurement problems especially in other areas of NBS research.

Temperature and Pressure Division

Maintains and improves the national standards of temperature and pressure; conducts research towards the development of new concepts for these standards; calibrates practical standards for the U.S. scientific and technical community in terms of the primary standards' develops methods and devices to assist user groups in the

assessment and enhancement of the accuracy of their measurements of temperature and pressure; prepares

and promulgates evaluations and descriptions of temperature and pressure measurement processes; coordi- nates these standards and measurement methods nationally and internationally; and determines the accuracy of the national standards of temperature and pressure with respect to fundamental thermodynamic relations. Length and l\/lass Division

Develops and uses the competence necessary to implement a more accurate measurement system for length

and mass; applies research, utilizing the most advanced technology available, to the improvement of our present

standards and devises new standards and new methods for their transfer. Time and Frequency Division

Maintains, develops and improves the national standards for the unit of time, the unit of frequency, and time

scales based on these standards; carries out research in areas of importance to the further fundamental improvement of frequency standards and their applications focusing on microwave and laser devices, atomic and

molecular resonances, and the measurement of fundamental physical phenomena and constants; adapts time

and frequency standard devices and concepts to special scientific and technological demands; develops time

and frequency measurement methods in the radiofrequency, microwave, infrared, and visible radiation regions;

coordinates the NBS time scales with international time scales and the time scales maintained by the United

States Naval Observatory; collaborates with national and international organizations in the development of improved methods for the measurement of time related quantities; operates time and frequency dissemination services, such as radio stations and broadcasts, for the purpose of traceability to the national standards of time and

frequency.

Quantum Physics Division

Engages in research in atomic and molecular physics at the forefront of the field, and performs basic, highly

accurate measurements and theoretical analyses that help establish a reliable foundation for scientific and technological measurement and data.

Quantum IVIetroiogy Division

Engages in forefront research in quantum metrology; contributes to new determinations of the fundamental

physical constants, and the extension and refinement of the electromagnetic scale and devises, where possible,

tests of basic symmetries and invariances.
NBS MEASUREMENT SERVICES:

STANDARD CELL CALIBRATIONS

Bruce F. Field

Center for Basic Standards

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Clarence J. Brown, Acting Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1987
Library of Congress Catalog Card Number: 87-619872

National Bureau of Standards Special Publication 250-24

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-24, 52 pages (Oct. 1987)

CODEN: XNBSAV

Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment

identified are necessarily the best available for the purpose.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

Calibrations and related measurement services of the National Bureau of

Standards provide the means for makers and users of measuring tools to achieve

levels of measurement accuracy that are necessary to attain quality,

productivity and competitiveness. These requirements include the highest

levels of accuracy that are possible on the basis of the most modern advances

in science and technology as well as the levels of accuracy that are necessary

in the routine production of goods and services. More than 300 different

calibrations, measurement assurance services and special tests are available

from NBS to support the activities of public and private organizations. These

services enable users to link their measurements to the reference standards

maintained by NBS and, thereby, to the measurement systems of other countries

throughout the world. NBS Special Publication 250, NBS Calibration Services

Users Guide . describes the calibrations and related services that are offered,

provides essential information for placing orders for these services and

identifies expert persons to be contacted for technical assistance.

NBS Special Publication 250 has recently been expanded by the addition of

supplementary publications that provide detailed technical descriptions of

specific NBS calibration services and, together with the NBS Calibration

Services Users Guide, they constitute a topical series. Each technical

supplement on a particular calibration service includes:

o specifications for the service

o design philosophy and theory

o description of the NBS measurement system

o NBS operational procedures

o measurement uncertainty assessment

error budget

systematic errors random errors

o NBS internal quality control procedures

The new publications will present more technical detail than the information

that can be included in NBS Reports of Calibration. In general they will also

provide more detail than past publications in the scientific and technical

literature; such publications, when they exist, tend to focus upon a particular element of the topic and related elements may have been published in

different places at different times. The new series will integrate the

description of NBS calibration technologies in a form that is more readily

accessible and more useful to the technical user.

The present publication, SP 250-24, NBS Measurement Services: Standard Cell

Calibrations, by Bruce F. Field, is one of approximately 20 documents in the new series published or in preparation by the Center for Basic Standards. It

describes calibration technology and procedures utilized in connection with NBS

Service Identification Numbers from 53110 to 53150 listed in the NBS

111
Calibration Services Users Guide. Inquiries concerning the contents of these

documents may be directed to the author or to one of the technical contact

persons identified in the Users Guide (SP-250)

.

Suggestions for improving the effectiveness and usefulness of the new series

would be very much appreciated at NBS . Likewise, suggestions concerning the

need for new calibration services, special tests and measurement assurance programs are always welcome.

Joe Simmons, Acting Chief

Office of Physical Measurement Services

Peter L. M. Heydemann, Director

Center for Basic Standards

iv
Table of Contents

page

List of Figures vii

List of Tables viii

Abstract 1

1. Introduction 1

2. Description of Service 2

Regular Calibration 2

Volt Transfer Program 4

3. Calibration Apparatus and Measurement Approach 4

Voltage Difference Measurements 4

Measurement Apparatus 7

Redundant Measurement Designs 9

Temperature Measurements 9

4. Maintenance of the Volt 10

The ac Josephson Effect 10

The Josephson Array 11

Microwave Apparatus 11

dc Measurement Apparatus 11

Standard Cell Comparisons 13

Uncertainty 14

5. Operational Procedures 15

Calibration of Laboratory Primary Cells 15

Calibration of Working Cell Groups 17

Regular Calibration 20

Calibration of Client Saturated Standard Cells 20

Assignment of Final Value 20

Volt Transfer Program 22

Measurement of the Transport Standards at NBS ... 23

Comparison of the NBS Transport Standard to

the Client Standard 23

Assignment of Final Value 24

V
Table of Contents (cont)

page

6. Quality Control Procedures 26

Measurement System Verification 26

Measurement Assurance for Client Standards 31

7. Bibliography 31

Appendices

A Redundant Measurement Designs 35

B Regular Calibration Test Report 37

C Volt Transfer Program Test Report 40

D Software 42

vi
List of Figtires

Figure page

1. Simplified measuring circuit 5

2. Two measurement configurations to eliminate positional errors (P) 6

3. Block diagram of the two measurement systems used for

comparison of pairs of standard cells 7

4. Simplified circuit diagram of the measurement system

used for 2e/h measurements 12

5. Measurement sequence used to assign values to the

primary groups based on 2e/h measurements 13

6. Mean emf of Primary Group A as determined from 2e/h

measurements 16

7. Measurement sequence used to compare the Regular

Calibration and VTP workload to NBS working groups 17

8. The difference between the two daily values assigned to

Working Group A from the two primary groups 18

9. Mean emf of a tj^Dical transport standard as measured at

NBS and a client laboratory 24

10. Daily DVM readings of the 1000 (iV Zener source 27

11. Within-day standard deviations calculated from the

redundant measurement designs 28

12. Typical daily log printout from the VTP measuring

system 29

13. The difference between the daily value assigned to the

mean emf of Working Group A from the two primary

groups, and the predicted value from the Working Group 30

vii
List ofTables

page

Table I. Uncertainties in the Assignment of the Mean emfs

of the Primary Reference Groups 14

Table II. Uncertainties in the Assignment of the Mean emf

of the Working Group 19

Table III. Uncertainties in the Assignment of the Mean emf

ofa Client Cell 21

Table IV. Uncertainties in the Determination of Vl^B ' ^NBS 25

viii
NBS MEASUREMENT SERVICES: STANDARD CELL CALIBRATIONS

Bruce F. Field

Abstract - This document describes the procedures used at NBS to cahbrate

standard cells in terms of the U.S. Legal Volt. The two calibration services that

are offered by the Electricity Division are discussed; Regular Calibration of client

standards at NBS; and the Volt Transfer Program which is a process to

determine the difference between the U.S. Legal Volt and the volt as maintained

by a client laboratory. The operational procedures used to compare standard

cells and to maintain the U.S. Legal Volt via the ac Josephson effect are discussed in detail.

L Introduction

To maintain a unit of electromotive force (emf), standards laboratories typically

use one or more groups of electrochemical cells called standard cells. Standard

cells are physical representations of the unit of emf and are used as standards

against which the emf of other cells and systems are compared. At present,

saturated cadmium-sulfate-type (Weston) cells are used when high accuracy,

1 ppm or less, is required in maintaining a unit traceable to NBS.

Standard cells are electrochemical systems composed of two dissimilar

electrodes immersed in an electrolytic solution. They are not intended to supply

electric current and, therefore, are of different design from those

electrochemical systems which are intended for such purpose. The stability of

the emf of the cell depends on the chemical equilibrium within the cell.

Although the emf of a cell is highly reproducible and generally exhibits a fairly

constant emf, it must be periodically recalibrated to eliminate the effects of long

term drift. The frequency with which recalibrations are required is a function of

the accuracy requirement, the number of cells used to maintain the volt, and the

stability of the individual cells. This must be determined by the client laboratory

using the cells.

Calibration of cells in terms of the U.S. Legal (or NBS) Volt is accomplished via

two services offered by the Electricity Division of the NBS.

Regular Calibration

Clients may send cells to NBS for direct comparison to NBS standards which are

in turn calibrated in terms of the NBS Volt. At the conclusion of the test a report

is issued with a statement of the values of the emfs of the cells and estimated

uncertainties for the emf values, while the cells were at NBS. No additional

1
uncertainty is included for transportation effects, long term drifts of the emfs of

the cells, or measurement errors in the client's laboratory.

Volt Transfer Program

The Volt Transfer Program (VTP) is a measurement assurance program

designed to determine the difference in the unit of voltage maintained by a client

laboratory using standard cells and the NBS Volt as maintained by NBS. In this

program, a group of NBS-owned cells in a thermoregulated enclosure is

measured at NBS, shipped to the client laboratory, compared to the client cell

group, and returned to NBS for final measurements. The measurements to be

made at the client laboratory are specified by NBS and the data is reduced and

reviewed by NBS. A report is issued containing the difference of the client unit

of voltage from the NBS Volt and specifying an uncertainty of that difference

observed during the time of the test. In this experiment the uncertainty in the

transportation of the standard cells is estimated and included in the final

uncertainty.

2. Description of Service

' . Regular Calibration

NBS offers a voltage calibration service for saturated and unsaturated standard

cells of the Weston type. All cells are compared to NBS standards which are

calibrated in terms of the U.S. Legal Volt defined via the ac Josephson effect.

Cells accepted for test include shippable or imshippable saturated cells that are either designed for immersion in an oil bath or that are housed in a

thermoregulated enclosure. NBS maintains two oil baths, one at 28 °C and one

at 30 °C, for testing immersible-type cells.

Routine calibrations of standard cells involve the following considerations:

1) Unsaturated cells require approximately 3 weeks for a complete calibration.

The emfs of such cells are read daily for a period of 10 days. If the

measured emfs fluctuate unduly or are unusually low, or if the cells show

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.

2) 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 degrees in any direction. Unshippable

saturated cells contained in portable, temperature-regulated enclosures

2
should also be transported by messenger, and with the enclosure activated

or under power if possible.

3) 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. To prevent overheating, enclosures

should not be energized by the ac power line while they are packed in

shipping containers. When an enclosure is shipped under temperature

control, arrangements should be made by the client to have it promptly

delivered to and picked up from the airport (if shipping by air freight) to

avoid exceeding the capacity of the batteries powering the enclosure.

4) 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.

5) 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 one month after they are initially brought to

temperature, daily readings will be taken to 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 unless special

arrangements are made.

6) Prior to each daily reading of the cell emfs the temperature of the enclosure

(or the oil bath) will be read using the temperature measuring device

included in the enclosure, or special arrangements can be made to have the

temperature monitored by a calibrated NBS platinum resistance

thermometer. A calibrated platinum resistance thermometer is always

used to monitor the temperature of the oil bath with the temperature

reported relative to the International Practical Temperature Scale (IPTS68).

3
Volt Transfer Program

The Volt Transfer Program experiment consists of four phases.

1) Preliminary Laboratory Study . During this phase NBS personnel will

discuss with client laboratory personnel various aspects of the experiment,

suggest experiments to evaluate the measurement process, and establish a

tentative shipment and measurement schedule. If necessary NBS

personnel may visit the laboratory. NBS may restrict participation in the

Volt Transfer Program to those clients who use quality- control procedures

to monitor their representation of the volt. Guidelines for setting up control

chart procedures can be found in VTP Control Chart Requirements [40].

2) Shipment of an NBS Transport Standard. An NBS transport standard, a

group of 4 cells (usually) in a thermoregulated enclosure with a battery

pack, is shipped via air freight to the client laboratory. The success of the

experiment depends on keeping the transport standard under power

continuously and expediting its transport. Detailed instructions for

transport are given to the client.

3) Measurement of the Transport Standard in the Client Laboratorv. Measurements comparing the cell emfs of the transport standard to the

emfs of the client laboratory cells are done in accordance with NBS specified

procedures. Usually measurements are made daily over a two to three

week period. Data is sent to NBS immediately after each measurement for

reduction and review. The conclusion of this phase is determined by NBS

after review of the client data; in some cases additional measurements may

V be required.

4) Analvsis of the Data and Issuance of a Final Report. Upon return to NBS

the transport standard will be remeasured to determine if it changed

during the experiment. After sufficient measurements have been made the

data is analyzed to determine the difference between the units of voltage at

NBS and the client laboratory (VlaB " ^NBS^ uncertainty. If the

difference exceeds the estimated uncertainty, new emf values for the client

cells will be recommended to reduce the difference to zero.

3. Calibration Apparatus and Measurement Approach

Voltage Difference Measurements

All cells at NBS are calibrated by comparing them directly to NBS owned

standard cells which are in turn calibrated in terms of the U.S. Legal Volt. All

comparisons use redundant measurement designs and a series opposition

method, as described in NBS Technical Note 430 [16]. Figure 1 shows a

simplified schematic of the measuring circuit.

4
Measuring

Instrument

+

-

+ +

Fig. 1. Simplified measuring circuit.

Two cells are connected in series opposition (one unknown, one reference) and

the small voltage difference is read using a digital voltmeter (DVM). In an ideal

situation the difference in emf as measured by the voltmeter is: E — Ej - E2

where E2 and E2 are the emfs of the two cells being compared. However, in the

real situation there may be spurious emfs in the circuit. In general these can be

classified into two categories:

1. Those emfs that remain constant, or relatively so, in relation to the interval

over which a complete set of measurements is made.

2. Those emfs that vary rapidly (referenced to the interval over which a complete

set of measurements is made).

If the emfs are of the second type they will have the effect of decreasing the

precision of the process. On the other hand, if they are of the first type they will

have the effect of introducing a systematic error into the measurement result

such that

E = Ej-E2 + P

where P is the constant, and as yet unknown, emf. It is possible to estimate P by

taking a second measurement E' as shown in Fig. 2, where

E = E2 " Ej + P .

5
Taking the difference between the two expressions gives

E - E = 2,{Ei - E2)

,

thus yielding an estimate of E^ - E2 free of P. The pair of measurements are said

to be "left-right" balanced. That is, if there is a positional effect it is balanced out

of the final result. This technique is analogous to that used to eliminate the

inequality of balance arms in precision weighing on a two-pan equal-arm

balance. In order to designate the cell positions from the operational point of

view, they are frequently designated as unknown and reference (or left and

right) relative to the input terminals of the measuring instrument.

Measuring

Instrument

+

Measuring

Instrument

+

-I- -I- -II 1

+

(a) (b)

Fig. 2. Two measurement configurations to eliminate positional errors (P).

With classical potentiometers the left-right effect P does not vary significantly over

the time of the measurements and it is sufficient to require that the total design be

left-right balanced, i.e., each cell appears on the left side and right side of the

measuring system an equal number of times irrespective of their order of

appearance in the design. Digital voltmeters, however, have been found to

introduce a small time-varying dc offset that must be eliminated by immediately

reading every cell pair twice, once in the normal mode and once with the polarity

of the input reversed. The algebraic difference of these two measurements divided

by two is considered to be a single measure of the cell difference with the DVM

offset eliminated.

6
Measurement Apparatus

Two voltage measuring systems are currently in use at NBS for cell

comparisons. Except for the switching systems, they are identical. One system

(System VTP) is used for comparison of NBS primary standards with NBS

working standards and for comparison of the working standards to VTP

transport standards. The second system (System REG) is used for comparison

of NBS working standards to client cells sent in for regular calibration. Figure 3

is a block diagram of the measurement apparatus of the systems.

Cell emf inputs

1000 |iV source

CO

File Server &

16MB Hard Disk

Fig. 3. Block diagram of the two measurement systems used for

comparison of pairs of standard cells.

Using a measurement design, the computer commands the crossbar scanner to

connect two cells in series opposition to the DVM; the DVM takes a number of

readings of the voltage difference (usually three); the crossbar scanner

reconnects the cells with reversed polarity; and the DVM takes three more

readings. The computer records the readings, and when the experimental

design is complete, it calculates the estimates of the cell differences using a

least-squares method and records the cell emfs on the hard disk and on a floppy

disk for backup. The hard disk is shared among several desktop computers to

permit the data from both measurement systems to be stored in one location,

7
and to permit access to the data from other machines even while measurements

are being made. A summary of the basic statistics is printed after each

measurement set.

The digital voltmeters used to make the difference measurements are 6-1/2 digit

high-resolution voltmeters. Two performance parameters of the voltmeters are

critical to this application: low input bias current and small random

measurement error. The design of the saturated standard cell is such that the

emf stability depends on electrochemical equilibrium within the cell. Small

charging or discharging currents that may be present at the input of the DVM

will produce changes in the cell emf that may be significant. The voltmeters

used at NBS have a measured input bias current of 12 pA which contributes a

negligible error to the measurement. On the lowest voltage range of the

voltmeter the standard deviation of a single measurement is 0.034 ]iV with an

integration time of 18 seconds. The integration time was chosen specifically to

obtain a measurement standard deviation of less than 0.04 |iV. Since only small

voltage differences are being measured (the worst case is about 600 |J,V when

comparing cells at 37 °C to cells at 30 °C), only a modest DVM reading accuracy

of 17 ppm (parts-per-million) is required for 0.01 |i.V accuracy in the voltage

differences. (Unless otherwise specified, all uncertainties in this document are meant to correspond to a one standard deviation estimate.) To ensure that the

voltmeter is within the required accuracy, it is checked every day by reading a

calibrated 1000 |iV source. A commercial Zener standard that produces 1000 |iV is connected to one input of

the scanner switch, and a piece of copper wire (to provide a low-thermal short) is

connected to a second input. Connecting the first input minus the second input

presents the 1000 |J.V source to the DVM. Actually, two Zener standards and two

copper wires are used, one set for each measuring system. Each 1000 |J.V source

is read daily with the DVM to perform a one point calibration on the gain

accuracy of the DVM, The sources are periodically calibrated using a resistive

divider and a standard cell.

Pairs of cells are connected to the digital voltmeters by one of three low-thermal

switching systems. The first system consists of a 10x10 matrix crossbar switch

with latching solenoids. This switch is used to connect up to 80 VTP transport

cells to measuring system VTP. A second modular switching system is also

used to connect NBS reference cells to measuring system VTP. This system

consists of up to five switch modules each capable of connecting one or two of ten

cells to a common 4-wire signal bus [27]. The output of this bus is connected to

the DVM of measuring system VTP. The third system, consisting of two 10x10

matrix crossbar switches with latching solenoids and a 2x5 selector switch, is

used to connect up to 300 client cells to measuring system REG.

All the switching systems are based on a low-thermal crossbar switch; however,

each system is constructed somewhat differently with capabilities for switching

different numbers of cells. Common to all three systems is the capability to

randomly select any two cells under computer control and connect them in

8
series opposition to the DVM in "normal" or "reversed" polarity. The matrix

crossbar switches are actuated by momentarily energizing the switch solenoids,

and then permanent magnets within the switches latch the contacts closed until

a second release solenoid is energized. This momentary action is designed to

minimize the heat input to the switch which in turn would produce thermal

gradients and generate large thermal emfs in the switch. The uncompensated

thermal emfs in these switching systems have been measured and found to be

less than 20 nV. The solenoids in the modular switch system (used for

intercomparison of NBS reference cells) are continuously energized during

contact closure, but the switch has been modified to locate the solenoids outside

the chamber containing the switch contacts to minimize the heat input to the

contacts. The uncompensated thermal emfs have been determined to be less

than 6 nV.

Redundant Measurement Designs

Redundant measurement designs are used to compare cell emfs because they

provide diagnostic information about potential errors or problems with the

measuring system. Two important parameters are estimated by the designs:

the within-day standard deviation and the left-right (positional) effect in the

measuring system. The within-day standard deviation permits laboratory

personnel to estimate the quality of the measurements and, if necessary, to

eliminate anomalous observations from the measurement design and re- compute the cell emfs with little loss in accuracy.

Two specific designs are generally used, one for comparison of two groups of

four cells, and one for comparison of one group of six cells to one group of four

cells. These designs are described in detail in Appendix A. On occasion

additional designs are required for groups with varying numbers of cells; these

are constructed similarly to the ones in the Appendix. The cell emfs are estimated from the cell difference measurements by solving the overdetermined

set of equations using the least-squares method and including the constraint

that the mean emf of a given group of cells (i.e., the reference group) is known.

This results in assigning individual emf values to all the cells in terms of the

mean emf of the reference group. This calculation is performed by the computer

program READBOX which is discussed in Appendix D.

Temperature Measurements

All saturated standard cells exhibit an emf change with temperature and must

be maintained at a constant temperature. Cells sent to NBS for calibration are

either housed in portable thermoregulated enclosures, or are immersed in NBSprovided constant-temperature oil baths. Thermoregulated enclosures

generally contain a temperature sensing element which can be used to monitor

the temperature variations within the cell enclosure. Typical devices are mercury-in-glass thermometers, thermistor bridges, and platinum resistance

thermometers. NBS follows the manufacturer's recommended procedure for

9
monitoring the temperature using these devices. The temperature scale

embodied in the temperature device is taken as correct, primarily because

accurate knowledge of the temperature is unnecessary. Cell emfs are corrected

only for small changes in temperature, referenced to a nominal temperature as established by the temperature device.

For cells housed in the NBS oil baths, calibrated platinum resistance

thermometers and an ac resistance thermometer bridge are used to measure

the temperature. The temperature of the oil bath is stable and uniform to at

least 0.001 °C. The estimated uncertainty of the temperature measurement with

respect to the International Practical Temperature Scale (IPTS-68) is 0.005 °C (3

standard deviation estimate), and includes uncertainty for the power dissipation

in the thermometer, errors in the determination of the triple point of water, and

errors in the drift of the calibration constants between calibrations.

Cell emfs are corrected for small day-to-day temperature changes by monitoring

the temperature as described above and applying an emf correction based on the

International (or WolfO Temperature Formula [13]:

E>r = E20 -0.00004060(T-20) -0.000000950(T-20)2+0.00000001 0(T-20)3

in volts where E-p is the emf at temperature T and E20 is the emf at 20 °C. This

equation is used to correct the cell emfs to any arbitrary nominal temperature by

computing the correction for the actual temperature with respect to 20 °C and

subtracting the correction for the nominal temperature with respect to 20 °C.

The formula is an approximation and is not exact for all cells although it

provides reasonable accuracy (better than 0.1 |iV) if the total correction is less

than 1 laV.

4. Maintenance of the Volt

The ac Josephson Effect

The U.S. Legal (or NBS) Volt is presently defined in terms of the atomic

constants h (the Planck constant) and e (the elementary charge) via the ac Josephson effect [26]. Critical to this definition is the role played by a Josephson

junction which may be regarded as a frequency-to-voltage converter, where the

frequency-to-voltage ratio is precisely equal to the combination of physical

constants 2e/h. The current value of 2e/h used for maintaining the NBS Volt is

483593.420 GHzA^nbs- The U.S. Legal Unit of voltage is known to be smaller

than the SI unit by about (9 ±1) ppm (3a) [41]. (The dominant system of units

used throughout the world to express the results of physical measurements is

Le System International d' Unites or International System of Units, abbreviated

SI.)

10
When two weakly coupled superconductors are irradiated with microwave

energy, the assembly (a Josephson junction) can be used to produce precise

voltages described by the following equation:

E = nf/(2e/h)

where n is an integer and f is the frequency of the irradiating microwave

energy. A variety of experimental tests (for material dependence, temperature

dependence, etc.) and theoretical investigations of the Josephson relation have

been made which indicate that for ordinary Josephson devices (particularly

tunnel junctions) with conventional current-voltage lead configurations the

ratio is exact to at least a few parts in 10^ [23]. In practice, frequency

measurements of the microwave energizing signal are based on the NBS unit of

time interval, the atomic second.

The Josephson Array

Arrays consisting of from 1500 to 2076 Josephson junctions have been used to

produce a total voltage of up to 1.2 V [42,43]. These arrays do not require

individual control of the bias currents as is usual, but avoid the multiple bias

problem by using constant-voltage steps which cross the zero current axis of the

junction I-V curve. This arrangement allows a large array of junctions to share

a common current bias at or near zero. The arrays are fabricated using

niobium and lead alloys and are stable at room temperature.

JVIicrowave Apparatus

The microwave radiation applied to the Josephson array is supplied by a 60 mW

Gunn diode oscillator at 94 GHz which is frequency-stabilized by a frequency

locking counter containing a quartz-crystal oscillator. The short term frequency

stability of the microwave radiation (15 min) is about 1 part in 10^. The

frequency is measured by the frequency counter with a resolution of 1 part in

10^0 The accuracy of the counter time base is regularly checked against the

U.S. frequency standard by comparing the counter time base to a 100 kHz highstability oscillator which is simultaneously compared to the signal from WWVB

using a VLF comparator.

DC Measurement Apparatus

Figure 4 shows some of the dc measurement apparatus used to compare the

array voltage at 1.018 V to a 1.018 V Zener reference standard. (Not shown is a

low-thermal-emf crossbar selector switch used to connect one of three Zener

references to the measurement system.) To measure the voltage of the Zener

reference, the array is adjusted to produce a voltage nearly equal to the Zener

reference by (a) adjustment of the bias current and microwave power to select a

voltage step, and (b) adjustment of the microwave frequency to fine-tune the step

voltage. The step is observed on the oscilloscope to check for any abnormalities.

11
The difference between the array and Zener reference is then measured by

averaging several readings (Ej) of the digital voltmeter. Low-thermal-emf

reversing switch A is reversed and several more readings (E2) are taken to

eliminate offsets in the digital voltmeter. The bias current to the array is

reversed and adjusted to produce -1.018 V and reversing switch B is switched.

Two more sets of readings (Es and E4 respectively) are taken with switch A in its

normal and reversed positions. This action is required to eliminate thermal

emfs in the leads from the array to the reversing switches. The Zener voltage is

calculated as (Ej - E2 - E3 + E4 )/4 + n£^(2e/h) where n is the integer step

number, f is the microwave frequency, and 2e/h is as defined above. This

measurement sequence is repeated five times for each Zener reference, with a

typical standard deviation of 0.009 |J.V for the five measurements, and takes

about 12 minutes.

A final check is made on the thermal emfs in the leads from the Zener reference

to reversing switch B by replacing the Zener reference with a short and

adjusting the array to operate on the zero-voltage-step with the microwave power

set to zero. The same measurement sequence is run as for the 1.018 volt

measurement. The residual thermal emfs thus determined are subtracted from

the 1.018 volt measurements. (The Zener references are connected to one of the

modular crossbar switches described in section 3. The output of this switch is

considered to be the output of the Zener references as all measurements of these

references are made through the switch. Any stable thermal emfs within the

switch, or between the switch and the Zener reference, are cancelled as they add

equally to both the 2e/h measurements and the cell comparisons.)

reversing

switch A

bias supply

and oscilloscope

reversing

switch B

liquid Helium

Dewar

1 V Josephson

Array

Fig. 4. Simplified diagram of the measurement system used for

calibration of three Zener references.

12
Standard Cell Comparisons

Two standard cell groups (consisting of 10 cells total and considered to be the

NBS primary groups) are calibrated in terms of 2e/h at weekly intervals. The

calibrations consist of making cell comparison measurements (using

redundant measurement designs) between the primary groups of cells and a

group of three Zener reference standards each producing 1.018 V. The pattern

of group comparisons currently in use is shown in Fig. 5. A first set of cellZener comparisons are made and then the three Zener standards are each

calibrated in turn by the Josephson Array. After the Array calibrations the cell

comparisons are repeated to determine if the Zener standards changed during

the measurements.

Only Zener standards are compared to the Array because the Array voltage

occasionally and unpredictably jumps abruptly to a slightly different voltage

step. If the Array were being compared to a cell while this shift occurred, a small charging or discharging current would be introduced into the cell,

slightly changing it's emf. Zener standards, however, are not significantly

affected by this type of current pulse.

Zener standards

(3 references)

Primary Group

A (6 cells)

Primary Group

B (4 cells)

'iillli, ,(i,illii:

Working Group

A (6 cells)

Fig. 5. Measurement sequence used to assign values to the primary

groups based on 2e/h measurements.

13
TABLE I UNCERTAINTIES IN THE ASSIGNMENT OF THE MEAN EMFS

OF THE PRIMARY GROUPS

Source of uncertainty 1 std. dev. estimate

(ppm)

Microwave frequency 0.005

Assignment of Zener reference group using Array 0.007

Random uncertainty of comparison of Zener reference

group to the mean of Primary Group A 0.009

Change in the Zener reference group during the 2e/h

measurement 0.019

Uncompensated thermal emfs in the cell switches 0.005

RSS total 0.023

Uncertaintv

Table I summarizes the sources of uncertainty in assigning a value to the mean

emf of Primary Group A at the time of a 2e/h measurement. The microwave

frequency is measured by a frequency counter which is calibrated in terms of

WWVB. An uncertainty is included for measurement uncertainty and drift of

the oscillator frequency.

For each daily 2e/h measurement each Zener reference is compared to the

Array five times. The pooled standard deviation of the mean calculated from

individual comparisons is 0.0042 |j,V (60 degrees of freedom). This value, divided

by the square root of three (0.0024 |iV, 0.002 ppm), is used as the limit of the

random component of uncertainty in comparing the mean of the three Zener

standards to the Josephson Array. The thermal emfs in the leads to the Zener

reference are measured and subtracted from the Zener values. The uncertainty

of determining the thermal emfs is 0.0042 |iV (0.004 ppm). All other known

sources of systematic error are negligible.

The random component of uncertainty in comparing the Zener standards to the

primary group is determined from the 0.034 |j.V pooled standard deviation of a

14
single cell-Zener comparison. Because of the redundant measurements of the

measurement design, the uncertainty of the difference of the mean of the Zener

reference group to the mean of the primary group is 0.012 p.V. Two

measurement designs are made for Primary Group A (before and after the 2e/h

measurements); thus, the uncertainty of it's mean emf is reduced by the square

root of two for a total uncertainty of 0.009 |J.V (0.009 ppm).

The difference of the mean Zener group emf minus the mean primary group

emf typically shows a change of (0.00810.011) |iV in the before and after

comparisons described above. These appear to be caused by small random shifts

in the Zener references emfs. Taking a conservative approach, we assign an

imcertainty of 0.019 ppm (0.008+0.011) for changes in the mean Zener group emf

during the measurements.

Modular low-thermal switches (described in Section 3) are used to connect the

various cells to the measuring system. The thermal emfs in these switches that

do not cancel on cell reversal have been measured to be 0.005 jiV (0.005 ppm).

The Root-Sum-Square (RSS) total is an estimate of the uncertainty in assigning a value to the mean emf of Primary Group A based on one 2e/h measurement.

This is an estimate of how well the present system would agree with another

totally independent 2e/h system. Based on data from actual 2e/h measurements

(see Fig. 6, for example) the reproducibility with which the mean emf of a single

group of standard cells can be determined using the present Josephson

apparatus is about 0.020 ppm.

5. Operational Procedures

Calibration of the Laboratorv Primarv Cells

Standard cell emfs drift with time so the use of a simple time invariant model

for the cell emf can lead to unacceptably large step changes in the disseminated

volt each time the volt is reassigned from Josephson Array comparisons (2e/h

measurements). In addition, it is desirable to "average" several 2e/h

measurements to reduce the random error of the measurements. Thus, we use

a model for the primary cell group emfs that predicts a linear drift with time

and new model coefficients calculated after every fourth 2e/h measurement.

2e/h measurements are made at approximately weekly intervals. For every

fourth measurement (a cardinal measurement) additional cell comparisons

between the primary and working cell groups are made. These data are used to

reassign the emfs of all the primary cell groups. In general, least-squares lines are fit to the last five or so cardinal measurements and these lines are used to

predict weekly values for the means of the cell emf groups for the coming

month. The exact number of measurements chosen for the fitted lines depends

on (a) how well the cell emfs fit a linear model, and (b) the random scatter in the

2e/h measurements of the cell emfs. The judgement and experience of the

15
laboratory staff are used to determine the models. Figure 6 shows the typical

behavior of the mean emf of six cells in Primary Group A plotted against time.

The line plotted in Fig. 6 is the least-squares fitted line to the data; the residual

standard deviation of the fitted points about the line is 0.020 |J,V.

The three 2e/h measurements made on "off-weeks" each month are used to

check the prediction of the assignment of the cells. The collection of cell groups

that constitute the primary groups changes as cells need replacement or enclosures need repair. In general, the primary groups consist of two or three

groups of four to six cells each, with about 10 to 12 cells total. Generally, these

standard cell emfs drift with time; drift rates of the different cells range from

-1-0.1 to -0.7 ppm/year.

8192.0 T

> 8191.9 4

00

T

o

T 8191.8

>

:C

'

LU 8191.7 -.

8191.6

5/15/86 8/14/86 11/13/86 2/12/87 5/15/87

Fig. 6. Mean emf of Primary Group A as determined from 2e/h

measurements.

The primary cell groups also serve as a check standard for the 2e/h

measurements. If an individual 2e/h measurement assigns values to the

primary cells that are inconsistent with the predicted values, all the

measurement systems are investigated to determine the source of the

inconsistency. If the problem cannot be resolved, the 2e/h measurement is

usually repeated. In the exceptional circumstance where later 2e/h

measurements confirm the deviation from the predicted model (usually more

than 0.07 ppm) appropriate past workload data are corrected to reflect the

change.

Note: Prior to February 1987 two series-connected Josephson junctions

producing 10 mV were used as the basis for the U.S. Legal Volt. The small

voltage produced by the junctions was "stepped-up" to 1.019 V by special

16
potentiometers and compared to unsaturated standard cells. These unsaturated

cells were compared to the primary cells using a manual potentiometer.

Because of the difficulty of these measurements they were made only monthly

and they are described in detail in reference 26. Some data (particularly figures

8 and 13) are based on these older measurements. The use of the Josephson

Array has reduced the total uncertainty of Table I from 0.036 ppm for the old

system to 0.023 ppm for the Array.

Calibration of Working Cell Groups

To minimize the possibility of disturbing the primary cells, the client cells are not compared directly to the primary cells. Instead, as shown in Fig. 7, the

primary cells are used to calibrate working groups of cells once each day,

immediately before calibration of the client cells or the NBS transport standards

for the Volt Transfer Program. The working cell emfs are determined from the

predicted values of the primary cells, and are used for that day. The working

cells are then compared to client cells and the NBS transport standards. This is

feasible because of the small within-day uncertainty of 0.004 to 0.007 ppm which

is introduced by the intercomparison of standard cells. Measurements are made using redundant designs and the VTP automated measuring system.

Primary Group A Primary Group B

Working Group B

Volt Transfer

Program

workload

regular

calibration

workload

Fig. 7. Measurement sequence used to compare the Regular

Calibration and VTP workload to NBS working groups.

17
This procedure is modified as necessary when cells show erratic behavior or temperature-regulated enclosures fail. At present the primary cells consist of

two groups, in different thermoregulated enclosures, that are independently

compared to each of the working groups. This results in two assigned values to

each working group. In an ideal situation the two values should agree; the

difference is a measure of the error of the predicted values for the primary cells.

Figure 8 shows the typical behavior of this difference. If the assigned values

disagree by more than 0.08 |iV (as they occasionally do in Fig. 8), direct

comparisons are made between the two primary groups to try and resolve the

disagreement. In many cases the disagreement can be traced to abnormal

behavior of the temperature controller of one of the enclosures, or a rapidly

changing cell emf.

0.1

0.05

0

-0.05

-0.1

-0.15

A Li i \ i

2e/h cardinal

measurements

• • •

......

:^ t^t^^

A

1/1/86 4/2/86 7/2/86 10/1/86

Fig. 8. The difference between the two daily values assigned to Working group A from the two Primary groups. The arrows

indicate dates on which the cell emfs were reassigned based

on 2e/h measurements.

If the lines fit to the emfs of the primary groups are poor predictors of the emfs,

it would be expected that the difference would be nearly zero immediately after a

2e/h measurement and gradually diverge thereafter. The data of Fig. 8 do not

indicate this. The data do show correlation with changes in room temperature,

even after correcting for the indicated cell temperatures [29]. This appears to be

related to less than optimum temperature control of Primary Group B, which is

a commercial enclosure. Plans are underway to replace this enclosure with an

NBS-designed enclosure similar to the one for Primary Group A [29].

18
TABLE II UNCERTAINTIES IN THE ASSIGNMENT OF THE

MEAN EMF OF THE WORKING GROUP

Source of uncertainty 1 std. dev. estimate

(ppm)

Assignment of primary group (from Table I) 0.023

Day-to-day fluctuation of primary groups 0.028

DVM scale error 0.005

Random uncertainty in comparison of primary

group to working group 0.004

Uncompensated thermal emfs in cell switches 0.015

Uncertainty of assignment to working group

RSS total 0.040

Table II summarizes the uncertainties in assigning a daily value to the mean

emf of the working groups. Although several 2e/h measurements are combined

to predict values for the primary groups, the uncertainty of the primary group

assignment is not reduced from the value quoted in Table I because part of the

uncertainty may be systematic to all the 2e/h measurements. An uncertainty is

included for the day-to-day fluctuation of the primary groups and the additional

uncertainty due to predicting the values of the primary groups up to one month

ahead. This is estimated by taking the standard deviation of the difference of the

two assigned values to a working group, calculated from the daily comparisons

with the primary groups (see Fig. 8).

The DVM scale error is estimated based on readings of the calibrated 1000 |J.V

source which show a scale error of 70 ppm and a maximum cell difference of

64 |iV. The random uncertainty of the cell comparisons is estimated from the

redundant measurement designs as before with a pooled standard deviation of a

single measurement of 0.034 )iV.

Low-thermal switches connected to measuring system VTP (described in

Section 3) are used to connect the various cells to the measuring system. The

19
thermal emfs in these switches that do not cancel on cell reversal have been

measured to be 0.015 |iV (0.015 ppm).

The RSS total uncertainty in assigning a value to the mean emf of either

working group is estimated to be 0.040 ppm.

Regular Calibration

Calibration of Client Saturated Standard Cells

Cells received for calibration may be subjected to a stabilization period before

measurements are begun. The length of the stabilization period depends on

whether the cells were shipped to NBS under constant temperature control and

may be as long as 4 weeks. If space is available on the measuring system, and

cells have been shipped under temperature control, they are generally connected

to the measuring system immediately. However, later review of the data may

result in these early measurements being discarded.

Three types of saturated standard cells are calibrated: cells in oil at 28 °C or 30 °C and groups of cells in temperature-controlled enclosures. The first two

types are placed in oil baths whose temperature is stable and uniform to at least

0.001 °C and whose temperatures are determined using NBS-owned platinum

resistance thermometers. Temperature-controlled standard cell enclosures are

tested under the following ambient conditions:

Temperature (23 ± 1 ) °C

Relative Humidity 50% or less

The operating temperature of the cells in temperature-regulated enclosures is

determined using the temperature measuring device supplied with the

enclosure. If the enclosure has a temperature indicating bridge, an NBS owned

null detector is used to make the readings to within the resolution of the bridge,

usually 0.001 °C. An NBS or customer owned platinum resistance thermometer

may be used if requested. Temperature measurements are made each day

before the cell emf measurements are started.

Assignment of the Final Value

Client cells are compared to NBS working cells for a minimum of one

measurement design per day for ten working days. The cell emfs are plotted

and the data is reviewed by two staff members who decide if the cell emfs exhibit

sufficient stability for a report to be issued. If so, the means of the ten

measurements are used as the final assigned values of emf of the cells. If the

cells show excessive drift or other unusual behavior, additional measurements

may be taken.

20
TABLE III UNCERTAINTIES IN THE ASSIGNMENT OF THE

MEANEMF OF A CLIENT CELL

Source of uncertainty 1 std. dev. estimate

(ppm)

Uncertainty of assignment to working group 0.040

Change in working group during the day 0.040

DVM scale error 0.010

Random uncertainty of comparison of working

group to Client cell 0.007

Uncompensated thermal emfs in the cell switches 0.030

Uncertainty of assignment to Client cell

(not including temperature

measurement errors; see text)

RSS total 0.065

Table III summarizes the sources of uncertainty in calibrating a client cell in

terms of the U.S. Legal Volt.

The DVM scale error is calculated as described for Table I, except cell

differences as large as 120 |iV may be measured. The random measurement

uncertainty of the working group - client cell comparison is estimated from the

redundant measurement design as before.

No allowance for temperature has been included in the above uncertainties.

Fluctuations in cell emfs due to inexact measurement and correction of

temperature changes of the primary and working groups are included in the

day-to-day fluctuation of the primary groups, and the within-day fluctuation of

the working groups. The uncertainty of the cell emfs due to the imprecision of

the temperature monitoring device is estimated by calculating the change in cell

emf (according to the International Temperature formula) for a change of one

least count of the temperature monitoring device. In the case of cells in

enclosures containing internal thermistor bridges, where a least count of

0.001 °C is possible, an additional uncertainty of approximately 0.05 ppm is

21
included. Enclosures monitored by mercury-in-glass thermometers are

assigned an uncertainty equivalent to half the smallest graduation marked on

the thermometer, approximately 0.5 ppm for a resolution of 0.01 °C. For cells

calibrated in NBS oil baths an emf uncertainty equivalent to a temperature

uncertainty of 0.005 °C is used to account for possible calibration errors in the

platinum resistance thermometers used at NBS.

The day-to-day random component of the uncertainty of the client cell emf is

determined by calculating the standard deviation of the measured emfs of the

cell from the ten daily measurement designs and comparing it, at the 99%

confidence interval (CI) using an F-test, with the pooled standard deviation of a

large population of measurements of similar standards (0.135 - 0.188 \xV,

depending on the type of enclosure). If the statistic is determined to belong to

that population, then the population standard deviation of the mean is used as the estimate of the limit of the random component. If not, the computed

standard deviation of the mean is used as the estimate.

The final reported uncertainty is the direct sum of this random component, the

total assignment uncertainty from Table III, and the temperature uncertainty,

multiplied by a factor of three.

The calibration service has recently been extensively modified, with completely

new automated measuring systems and new quality control procedures. The

values quoted in the tables above are estimates of the uncertainties of the new

system. Until a complete evaluation is completed, the following (previously

determined) uncertainties are being used. The client cell assignment

uncertainty is taken as 0.075 ppm, the pooled standard deviation for random

error is 0.135 |iV, and the temperature uncertainty is the same as described.

The reported uncertainty contains no allowance for long term drift of the cells

under test. Long term behavior must be determined by the client by analysis of

the history of each individual standard. In addition, no allowances are made for

the possible effects of transporting the standard between laboratories or the

possible existence of a gross temperature dependence on ambient (room)

temperature of a standard cell enclosure.

Volt Transfer Program

The Volt Transfer Program involves sending an NBS owned transport voltage

standard to the client laboratory. The transport cells are compared to NBS

working standards at NBS before and after shipment to the client laboratory,

and to the client reference group of standard cells while at the client laboratory.

The process is described in more detail below.

22
Measurement of the Transport Standard at NBS

The transport standard consists of a commercial thermoregulated enclosure

usually containing 4 saturated shippable standard cells, operating at an

internal temperature of 30 °C or 32 °C. The temperature is measured using the

internal temperature bridge and an external null detector to provide 0.001 °C

resolution. Corrections to the cell emfs are applied according to the discussion

given in Section 3. Transport standard cells are compared daily to Working

Group A using redundant measurement designs and the VTP measurement

system (Fig. 7). Before shipment to a client laboratory, the cell emfs are plotted

and examined for stability. In any case, at least 15 daily measurements are required after the enclosure's return to NBS and before it is shipped to another

laboratory.

Upon receipt of a purchase order NBS schedules the shipment of a transport

standard to the client laboratory. The transport standard is shipped with an

external power supply containing a battery to power the standard during

shipping, and in a special container designed to provide physical shock

protection and temperature lagging. The battery supply can maintain

temperature control of the standard for about 24 hours during normal ambient

shipping conditions. Since the temperature control system cannot supply

cooling to the enclosure, shipment to warmer locations is avoided during

extremely hot weather to prevent the enclosure from overheating. Shipment is

normally via air freight with special delivery service from NBS to the airport just

in time for the scheduled flight. The delivery service may also be able to

arrange, with an affiliated service, for pickup and delivery of the enclosure to

the client laboratory. If not, client laboratory personnel are notified of the time of

arrival of the standard and are expected to provide transportation from the

airport to their laboratory within the 24 hour lifetime of the batteries.

Comparison of the NBS Transport Standard to the Client Standard

Each laboratory participating in the Volt Transfer Program must identify a

group of saturated standard cells that are considered to be the "laboratory

reference group" and constitute the "laboratory volt". The transport standard

cells are compared to the cells of the laboratory reference using redundant

measurement designs similar to the ones used at NBS. Each laboratory uses its

own measuring equipment in the manner it would normally use to calibrate

saturated cells, except for the requirement that an NBS-specified measurement

design be used. NBS provides data sheets to record the measurements which

are returned to NBS where the cell comparison observations are reduced to

determine the cell emfs in terms of the client laboratory volt. Laboratories are required to make daily design measurements and the data is reviewed by NBS,

as it is taken, to determine when sufficient data has been obtained to permit

shipment of the standard back to NBS. A minimum of 10 measurement designs

over 10 days is required for the transfer; however, the number is more typically

in the range from twelve to twenty. In addition, the within-day standard

deviation and left-right components from the measurement designs are checked

23
to see if they are statistically in control with respect to the expected values

determined from measurements at NBS and other laboratories.

Upon its return to NBS, the standard is again compared to Working Group A as before and, when the standard has stabilized and sufficient data has been taken,

a final report is issued. Typical measurement data is shown in Fig. 9 where the

mean emf of a transport standard over the period of one transfer is plotted. The

emf in the figure is expressed in microvolts with 1.01 volts subtracted from the

total cell emf.

8158.4 T

5 8158.2 •

8158.0

>

00

>

3 8157.8

LJJ 8157.6

8157.4

NBS

before

Client

Lab

o

o O o

o

o

> o

30 60 90

Time in days

NBS

after

120 150

Fig. 9. Mean emf of a typical transport standard as measured at NBS

and a client laboratory.

Assignment of Final Value

Emf assignments at NBS and the client laboratory for each cell of the transport

standard are plotted as in Fig. 9 above. Data showing cell emf changes due to

shipment or other effects are removed from the analysis; often the first

measurement after shipment is anomalous. The data are also reviewed for

abnormal temperature readings and to determine if the data reasonably

conform to a linear model. As the before and after NBS data do not usually

exactly agree, a least-squares line is fitted to the NBS data for each transport

cell, and values are predicted for each of the times the cell was compared to the

client laboratory cells. VlaB'^nbS is determined using each cell in the

transport enclosure by subtracting the NBS predicted values of the transport cell

from the respective client laboratory emf assignments, and taking the mean of

these differences.

2A
Although the difference in the units as determined from each of the four cells in

the transport standard should agree, on occasion some cells show abnormal

behavior, usually excessive drift, or excessively long recovery from physical

shock or electrical disturbance. The judgement and experience of the laboratory

staff are used to determine when to exclude a cell from the analysis or, on rare

occasions, use only the NBS data taken before or after the transfer.

TABLE IV

UNCERTAINTIES IN THE DETERMINATION OF Vlab - ^nbs

Source of uncertainty 1 std. dev. estimate

(ppm)

Uncertainty of NBS assignment to transport group 0.061

Correlated temperature effects of the transport 0.050

Temperature resolution 0.050

Random component due to individual cell

assignments and changes during transport 0.093

Uncertainty of Vl^B ' ^NBS determination

using a four cell transport enclosure

RSS total 0.132

Table IV summarizes the sources of uncertainty in the determination of Vl^B"

VnbsThe uncertainty in assigning a value to the transport standard while at NBS is

obtained through an analysis identical to Table III, except the switches used for

the VTP transport standards have somewhat lower uncompensated thermal

emfs (0.020 |iV as opposed to the 0.030 ^V shown in Table III).

The transport standards used for the Volt Transfer Program all contain

thermistor bridges with 0.001 °C resolution; an uncertainty for the cell emf

equivalent to this temperature change is included. In addition, the cell

temperature within the enclosures changes slightly with changes in room

25
temperature. The thermistor bridge does not properly reflect the change of the

temperature of the cell in this case, probably due to a temperature sensitive

component of the bridge circuitry that is at room temperature. An uncertainty

estimate of 0.05 ppm is included to account for a difference between the client

laboratory ambient temperature and the NBS laboratory ambient temperature.

Each of the four cells in the transport standard is used to determine a value for

the difference in laboratory units, Vlab " ^nbs- 'The standard deviation of these

four values includes an uncertainty in predicting the cell emfs while at the

client laboratory, an imcertainty in comparing the client laboratory reference to

the transport standard, and an uncertainty for random changes in the cell emfs

due to shipment. A pooled standard deviation of 0.19 |iV has been computed

from 50 transfers and is used as the population standard deviation. The

calculated standard deviation for each new transfer is compared to the

population standard deviation at the 99% confidence interval using an F-test. If

the statistic is determined to belong to that population, then the population

standard deviation of the mean is used as the uncertainty estimate; if not, the

actual standard deviation of the mean is used. Thus the uncertainty estimate

for a four cell transport standard, based on the population standard deviation, is

(0.19 ^V)/2 or 0.095 |iV (0.093 ppm).

If the difference Vl^B ' ^NBS exceeds 0.20 ppm (1.5 times the uncertainty based

on the population standard deviation), the report will recommend adjusting the

assigned values of the client reference cell emfs to reduce the difference to zero. Adjusted values for the cell emfs are calculated based the measurements made

in the client laboratory comparing the client reference group to the NBS

transport standard. Each comparison results in a determination of the

difference of each client reference cell emf from the mean emf of the client

reference group. The average difference for each cell, over all the comparisons,

is added to the newly determined mean emf of the reference group to calculate

the new values for each of the client reference cells.

6. Quality Control Procedures

Measurement Svstem Verification

Potential sources of error in the cell measurement systems include scaling or gain errors in the digital voltmeters, uncompensated thermal emfs in the

crossbar switches and cell leads, leakage currents to ground from the

measurement apparatus, and circulating ground currents. Several quality

control procedures are periodically performed to estimate or eliminate these

uncertainties.

26
998.300

998.200 -

>

998.000

998.100 -

.

E

LU 997.900

997.800

12/25/85 2/25/86 4/25/86 6/25/86 8/25/86 10/25/86

Fig. 10. Daily DVM readings of the 1000 |iV Zener source.

Before each daily set of cell comparisons the measurement system reads the

output of a 1000 |J.V Zener source to monitor the gain error of the 0.1 volt range of

the digital voltmeter. All voltmeter measurements are taken with the applied

voltage in the normal polarity and again with the polarity reversed by the

crossbar switch to eliminate any uncertainty due to zero offset of the voltmeter

and thermal emfs in the leads to the voltmeter. The daily measured values of

the 1000 |iV source are plotted on a control chart and compared to

predetermined limits. Figure 10 is a plot of the voltage of the Zener source that

is connected to measuring system REG. The control limits correspond to a

worst case cell comparison uncertainty of approximately 0.01 ppm. If the limits are exceeded, the reason for the out-of-tolerance condition is investigated by

laboratory personnel. Usually the problem is found to be that the voltage of the

1000 |iV Zener source has drifted (as it has in Fig. 10), and the source is

recalibrated.

After each measurement design, the computer controlling the measurement

system reduces the data using a least-squares technique and computes the

standard deviation for a single measurement for that design. This standard

deviation is compared to a control limit based on the population standard

deviation to determine if it is in statistical control. If it is out of control the

measurement design may be repeated. Measuring system VTP will repeat the

measurement design once if the standard deviation is out of control. If the

standard deviation of the second design is also out of control the system does not

repeat it again. Measuring system REG does not repeat the design under any

27
circumstances. The standard deviation is a function of the individual cell group

being measured, and control limits for newly arrived client cell groups are not

well known. Each day, a pooled standard deviation from all the measurement

designs that are in control is calculated and plotted on a control chart for each of

the measurement systems (REG and VTP). Figure 11 is the control chart for

measurement system REG.

0.050 T

0.040

^ 0.030

0.020

0.010

0.000

• •<•••••* •

•

Pooled Standard

Deviation = 0.029 (iV

-I 1 1 1 1 \- A 1 1 12/25/85 2/25/86 4/25/86 6/25/86 8/25/86 10/25/86

Fig. 11. Within-day standard deviations calculated from the

redundant measurement designs.

Thermal emfs in the leads from the cell enclosures to the crossbar switch are evaluated regularly. The positive and negative leads are shorted together at the

end where they would normally connect to the cells and a redundant

measurement design is made between two sets of shorted leads. These thermal

emf measurements are performed frequently, usually every two months or so as

cell enclosures leave the laboratory and leads become free. Experience has

shown that the thermal emfs are usually small and fairly constant

(approximately 15 nV and 30 nV for measuring systems VTP and REG

respectively).

Both measurement systems print out a daily log of the measurements

performed by each system. Figure 12 is a typical example produced by the VTP

measurement system. This log contains the measured value of the 1000 |J.V

source, and a list of the groups of cells that were measured. The time of the

measurement, the reference group used, the measured temperature of the

group, the change in temperature correction, the change in the group mean

emf, the standard deviation of the redundant design, and the measured DVM

28
offset voltage are printed for each cell group measured. The change in group

temperature correction and mean emf are calculated by subtracting the values

obtained for the previous day's readings from the current day's readings. If any

of the values exceed predefined limits the measurements are flagged on the log

and lab personnel investigate the problem. The limits are set at 0.1 |J.V for both

the change in temperature correction and mean emf, and are based on

laboratory personnel experience. The exact values are not overly important

since they serve only to alert the operator to a potential problem.

VTP MEASUREMENT LOG FOR: 29 May 1986 [1.4] WEEK: 8622

DVM check Zener measurement = 1000.014 (+/- 0.005) uV

UNK Change Change DVM

Time STD-ID UNK-ID Temp T-corr Mean SD Offset

(C) (uV) (uV) (uV) (uV)

10:20 18 2800 30.0070 -.113 .084 .020 -.251

10:31 18 19 29.9790 -.034 .028 .009 -.274

10:42 18 2000 31.9965 -.030 -.000 .021 -.299

10:53 18 1400 30.0080 -.038 .002 .029 -.271

11:05 18 2100 32.0055 -.059 .007 .027 -.300

MEAN of daily runs: -.053 .024 -.279

Standard deviation: .036 .035 .022 .021

Some runs were marked for exceeding predefined limits. The letters in the right hand column mean:

T - the temperature correction changed by more than .1 uV

M- the mean of the unknown group changed by more than .1 uV

8 - the std. dev. exceeded 1 .4 time the accepted std. dev.

D - the magnitude of the dvm offset exceeded 2 uV

ALL CALCULATIONS COMPLETED AND SRM UPDATED

Fig. 12. Typical daily log printout from the VTP measuring system.

As discussed earlier, two working groups of cells are calibrated daily in terms of

the primary groups. Cell emf values are predicted for these working groups at

the same time predictions are made for the primary cell groups, i.e., once every

4 weeks when 2e/h measurements are made. Occasional problems with

temperature regulated enclosures, oil baths, and unstable cells have made long

term predictions of either the primary or working groups meaningless.

However values for up to one month ahead are predicted for the primary and

29
working groups based on four or more months of data. Each day the values of

the working groups are calculated based on the daily intercomparisons of the

primary groups against the working groups and the predicted values of the

primary groups. The measurements or measurement systems are scrutinized

whenever there is significant (0.08 ppm) disagreement between the directly

predicted values of the working groups and the values assigned through the

primary groups. Figure 13 is a plot of the difference between the daily values

assigned to Working Group A and the predicted values.

0.10 T

0.05

0.00

-0.05

0.10

5/10/86 6/9/86 7/9/86 8/8/86 9/7/86 10/7/86

Fig. 13. The difference between the daily value assigned to the mean

emf of Working Group A from the two Primary groups, and

from the predicted value of the Working Group.

An enclosure that can be used exclusively as a long term check standard is not

available because of a shortage of good quality enclosures and cells. Instead, the

average of the changes in emfs of the workload from the previous day's

measurements is calculated daily and noted by laboratory personnel.

Occasional additional tests are performed on the automated systems. Selected

cell enclosures are calibrated with both systems to ensure that their results

agree, and the automated systems are compared to a calibrated millivolt

potentiometer . Measurements of the insulation/leakage resistance of the

measuring systems are done occasionally and recorded in laboratory notebooks.

Closure experiments are done monthly (at least) to detect systematic errors due

to leakage resistance. Closure experiments consist of redundantly comparing

three cell enclosures with the pattern, A-B, B-C, and C-A. Summing the three

mean emf differences should yield a value of zero; the disagreement of the

actual value is an indication of measurement error. Using twenty-four such

experiments performed on different enclosures between July 17, 1986 and

30
October 1, 1986, the mean closure error was -0.0009 |iV with a standard deviation

of the mean of 0.0017 |iV.

Measurement Assurance for Client Standards

Standards shipped to NBS in temperature regulated enclosures under power are hand carried from the Building 301 Receiving Room as soon as Electricity

Division personnel are notified. The temperature is checked by qualified

laboratory technicians and the enclosure is brought to the Volt Facility

Laboratory. Laboratory personnel within the Volt Facility connect and

disconnect the standards to the measuring systems to minimize the possibility of

incorrect connections.

After stabilization and measurement of the standard, the data are plotted for

each standard cell emf in the enclosure. These data are reviewed by laboratory

personnel and anomalous data usually related to stabilization are removed. The

remaining data are used to generate a calibration report. The final calibration

report and intermediate emf plots are also reviewed by the leader of the

Electricity Division Dissemination Services Group.

7. Bibliography

1. ASTM Manual on Quality Control of Materials, Special Technical

PubHcation 15-C, Part 3 (Jan., 1951).

2. Harris, F. K., Electrical Measurements, J. Wiley & Sons, pp. 168-185 (1952).

3. Field, B. F., Eicke, W. G, and Kleimann, R. E., unpublished computer

programs for reducing standard cell data.

4. Linnik, J. W., Method of Least Squares and Principles of the Theory of

Observations, Translated from the Russian by Regina C. Elandt, Pergamon

Press, New York (1961).

5. Brodd, R. J. and Eicke, W. G., "Effect of vibration and shock on unsaturated

standard cells," J. Res. Natl. Bur. Stand. (U.S.), Vol. 66C (Eng. and Instr.),

No. 2, pp. 85-97 (Apr.-June 1962).

6. Finch, D. I., "Temperature: Its Measurement and Control in Science and

Industry," General Principles of Thermoelectric Thermometry, Vol. 3, Part

2, Reinhold Publishing Corporation, New York (1962).

7. Eicke, W. G., "Comments on Zener diodes as voltage standards," Comite

International des Poids et Mesures, Comite Consultatif d'Electricite, 10th

Session, p. 27 (1963).

a.
8. Eicke, W. G., "Making precision voltage measurement on Zener diodes,"

IEEE Trans., Paper CP 63-416 (1963).

9. Natrella, M. G., Experimental Statistics, NBS Handbook 91, pp. 18-1, 18-4

(1963).

10. Dunfee, B. L., "Method for calibrating a standard volt box," J. Res. Natl.

Bur. Stand. (U.S.), Vol. 67C (Engr. and Instr.), No. 1, pp. 1-13 (Jan.-Mar.

1963).

11. Ramaley, D., "Practical method for calibration of potentiometers," Natl.

Bur. Stand. (U.S.) Tech. Note 172, 44 pages (1963).

12. Eicke, W. G, "The Operating Characteristics of Zener Reference Diodes and

their Measurement," ISA Transactions, Vol. 3, No. 2, pp. 93-99 (1964).

13. Hamer, W. J., "Standard cells, their construction, maintenance, and

characteristics," Natl. Bur. Stand. (U.S.) Monogr. 84, 38 pages (1965).

14. Pontius, P. E., "Measurement Philosophy of the Pilot Program for Mass

Measurement," Natl. Bur. Stand. (U.S.) Tech. Note 288 (1966).

15. Hamer, W. J., "The Volt Standard Moves To Gaithersburg, Maryland," J.

Wash. Acad. ofSci.,Yo\. 56, pp. 101-108 (1966).

16. Eicke, W, G. and Cameron, J. M., "Designs for Intercomparing Small

Groups of Saturated Standard Cells," Natl. Bur. Stand. (U.S.) Tech. Note

430, 19 pages (1967).

17. Volt Transfer Program Instructions, Electricity Division, Natl. Bur. Stand.

(U.S.) (1968).

18. Belecki, N. B., "Transfer of the unit of voltage," Proc. ISA Ann. Conf., Vol.

5, p. 608 (1968).

19. Belecki, N. B., "A Guarded Shielded Standard Cell Measuring System,"

ISA Preprint 706-70 (1970).

20. Dziuba, R. F. and Dunfee, B. L., "A resistive voltage-ratio standard and

measuring circuit," IEEE Trans. Instrum. Meas., Vol. IM-19, No. 4, pp

266-277 (1970).

21. "NBS offers new calibration service for the volt," Natl. Bur. Stand., NBS

Special Publ. 250, Users Bull., No. 1 (1971).

22. Eicke, W. G., "The Volt, Yesterday, Today and Tomorrow," ISA Reprint 71-

705(1971).

32
23. Finnegan, T. F., Denenstein, A., and Langenberg, D. N., "ac Josephson

Effect Determination of e/h: A Standard of Electrochemical Potential Based

on Macroscopic Quantum Phase Coherence in Superconductors," Phys.

Rev. B, Vol. 4, No. 5, pp. 1487-1522 (1971).

24. Eicke, W. G., and Taylor, B. N., "Summary of international comparison of

as-maintained units of voltage and values of 2e/h," IEEE Trans. Instrum.

Meas.,Yo\. IM-21, No. 4., pp. 316-319 (1972).

25. Popenoe, C. H., and Campbell, M. S., "MIDAS: Modular interactive data

acquisition system - description and specification," Natl. Bur. Stand. (U.S.)

Tech. Note 790 (1973).

26. Field, B. F., Finnegan, T. F., and Toots, J., "Volt maintenance at NBS via

2e/h: A new definition of the NBS volt," Metrologia, Vol. 8, pp. 155-166

(1973).

27. Braudaway, D. W. and Kleimann, R. E., "A high-resolution prototype

system for automatic measurement of standard cell voltages," IEEE Trans.

Instrum. Meas., Vol. IM-23, No. 4, pp. 282-286 (1974).

28. Eicke, W. G., and Auxier, L. M., "Regional maintenance of the volt using

NBS volt transfer techniques," IEEE Trans. Instrum. Meas., Vol. IM-23,

No. 4, pp. 290-294 (1974).

29. Cutkosky, R. D. and Field, B. F., "Standard cell enclosure with 20-uK

stabihty," IEEE Trans. Instrum. Meas., Vol. IM-23, No. 4, pp. 295-298

(1974).

30. Eicke, W. G. and Kleimann, R. E., "An automated standard cell

comparator controlled by a desk calculator," NBS Preliminary Report

(1975).

31. Field, B. F. and Hesterman, V. W., "Josephson junction based voltage

standard," Proc. 5th Cal Poly Meas. Sci. Conf. (1975).

32. Field, B. F. and Hesterman, V. W., "Laboratory voltage standard based on

2e/h," IEEE Trans. Instrum. Meas., Vol. IM-25, No. 4, pp. 509-511 (1976).

33. Eicke, W. G., Leedy, T., et al., "Measurement assurance programs in a

field environment," Natl. Conf. of Stand. Lab. Newsletter, Vol. 23, No. 1, pp.

51-55 (1983).

34. "Supplementary Instructions for Group Volt Transfer Programs,"

Electricity Division, Natl. Bur. Stand. (U.S.) (1984).

35. Huntley, L., "The Fluke direct voltage maintenance program," Proc. 1984

Meas. Sci. Conf, pp. 147-151 (1984).

33
36. Koep, K. J., "The use of a solid state dc voltage transport standard to

transfer the dc volt in a regional standard cell MAP program," Proc. 1984

Meas. Sci. Conf., pp. 142-146 (1984).

37. Murray, W, and Dencher, P., "High-accuracy voltage transfer using a

modified commercial solid-state reference," J. Phys. E.: Sci. Instrum., Vol.

17, No. 5, pp. 354-356 (1984).

38. Spreadbury, P. J. and Everhart, T. E., "Ultra-stable portable voltage

sources," lEE Conf. Publ, Vol. 174, pp. 117-120.

39. Field, B. F., "A sub-ppm automated one-to-ten volt measuring system,"

IEEE Trans. Instrum. Meas., Vol. IM-34, No. 2, pp. 327-330 (1985).

40. VTP Control Chart Requirements, Electricity Division, Natl. Bur. Stand.

(U.S.) (1984).

41. Taylor, B. N., "Possible Changes in the U.S. Legal Units of Voltage and

Resistance," J. Res. Natl. Bur. Stand. (U.S.), Vol. 91, No. 5, pp. 299-305

(Sep.-Oct. 1986).

42. Hamilton, C. A., Kautz, R. L., and Lloyd, F. L., "A practical Josephson

voltage standard at one volt," IEEE Elec. Dev. Lett., EDL-6, pp. 623-625.

43. Hamilton, C. A., Kautz, R. L., Lloyd, F. L., Steiner, R. L., and Field, B. F.,

"The NBS Josephson Array Voltage Standard," IEEE Trans. Instrum.

Meas., Vol. IM-36, No. 2, pp. 258-261 (1987).

34
Appendix A - Redundant Measurement Designs

Redundant measurement designs are used for all standard cell comparisons as they are efficient in providing the maximum amount of information per

measurement and provide information about possible systematic effects in the

measuring system. The designs are used to provide estimates of:

1) the cell emfs with respect to a group mean emf,

2) the within-day standard deviation of a single observation,

3) the left-right (positional) effect,

4) the standard deviations of the cell emfs,

5) and the deviations of individual observations from the predicted

values.

In general, when comparing two or more cell groups, a "full" design is used

where all possible pair differences that involve two cells from different groups

are measured. The 4x6 design and 4x4 design below illustrate this process. For

intercomparison of cells within a single group the appropriate design from NBS

Technical Note 430 is used [16].

On occasion an anomalous reading may be discovered during the data reduction

of the cell comparison data. Since some redundancy is provided by the design it

is possible to remove one or two observations and recalculate with little loss in

accuracy in determining the cell emfs with respect to the group mean.

For the examples below, the cells in the groups are designated Rl - R4 (R for

reference) and XI - X4 or XI - X6 (X for unknown) depending on the design. The

voltage difference measured is "left cell" - "right cell".

4x4 Design - 16 observations

Observation Left Right Observation Left Right

nimaber cell cell number cell cell

1 Rl XI 9 X3 R4

2 Rl X3 10 XI R4

3 R2 X2 11 X4 R3

4 R2 X4 12 X2 R3

5 R3 XI 13 X3 R2

6 R3 X3 14 XI R2

7 R4 X2 15 X4 Rl

8 R4 X4 16 X2 Rl

Appendix A - Measurement Designs 35
4x6 Design - 24 observations

Dservd.i'ion nigni <^ D s ervd. tion ijeix nigni

nuniDer ceil ceil nuniDer ceil ceil

1 ivi VIAJ. io Yf^ AO rift 9 rvl A-O 1 A YQAO

q A.O iO Y1A_L rv'* A Y9 ID YRAD rvo co 1 7 Yd. A^ rvo D xvZ Y^^ AD 1 ft Y9A.Z xvO

7 R3 XI 19 X5 R2

8 R3 X3 20 X3 R2

9 R3 X5 21 XI R2

10 R4 X2 22 X6 Rl

11 R4 X4 23 X4 Rl

12 R4 X6 X2 Rl

Appendix A - Measurement Designs 36
U. S. Department of Commerce

NATIONAL BUREAU OFSTANDARDS

National Measurement Laboratory

Gaithersburg, MD 20899

REPORT OF CALIBRATION

DC Voltage Standard

Description of Standard:

Standard Cell Enclosure

Model XXXXXXXXXXXXXXXXX

Containing 4 Saturated Standard Cells

Submitted By:

XXXXXXXXXXXXXXXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXX

Tflis standard cell enclosure was received 21 Mar 1986, not under power witfi an internal temperature

far below its normal operating temperature.

The data from which the values in the table below are computed are the result of ten daily

measurements of the differences between the EMF's of the cells under test and those of NBS

working standards during the period between 5 May 1986 and 16 May 1986.

POSITION EMF UNCERTAINTY EMF UNCERTAINP/

NUMBER (volts) (microvolts) (volts) (microvolts)

The electromotive forces on the left above were corrected to nominal temperature (30.0 degrees

Celsius) using the International Temperature formula proposed by F.A. Wolff. The electromotive

forces on the right are at the mean operating temperature during the test (30.0019 degrees C) as determined by use of a temperature deviation measuring device mounted in the enclosure.

The above uncertainties include components for random fluctuations in the cell under test and in NBS equipment and standards, for a systematic error of 0.076 ppm in the measurements of NBS

working standards in terms of the Josephson effect, and for the systematic error in transfer due to the

finite resolution of the apparatus used to determine the temperature of the cells under test. In the case of standard cells tested in NBS oil baths, the latter uncertainty is replaced by the EMF equivalent

of 0.005 degrees Celsius which is the uncertainty of the temperature measurements in this laboratory. The random error component is computed from the standard deviation of the mean EMF

and is at the three sigma level.

12

3

4

1.0181270

1.0181271

1.0181272

1.0181268

0.27

0.27

0.27

0.27

1.0181269

1.0181270

1.0181271

1.0181267

0.27

0.27

0.27

0.27

Appendix B - Regular Calibration Report 37
Standard Cell Enclosure

Model XXXXXXXXXXXXXXXXX

Page 2

These uncertainty figures contain no allowance for the effects of transportation upon this standard.

The minimum uncertainty due to such effects under very carefully controlled transport conditions has

been seen to be 0.42 ppm (3 sigma). Any valid uncertainty statement applying to the above values

when the standard has been moved from the NBS Volt Facility must contain such a component of

error. If data from which to estimate the transport error are not available, one part-per-million is not an

unreasonable value. Also not included in the above uncertainties is an allowance for long term drift of the values of the outputs of this standard. This must be determined from historical data on a case by case basis.

A summary and analysis of the data upon which the above values are based is appended. A

complete explanation of the uncertainty statements given above as well as additional information

regarding NBS calibration of such voltage standards is also included.

For the Director

National Measurement Laboratory

Norman B. Belecki, Physicist

Center for Basic Standards

Electricity Division

Test No. XXXXXXXX

Date: 21 May 1986

Appendix B - Regular Calibration Report 38
U. S. Department of Commerce

NATIONAL BUREAU OFSTANDARDS

National Measurement Laboratory

Gaithersburg, MD 20899

REPORT OF CALIBRATION

DC Voltage Standard

Description of Standard:

Unsaturated Standard Cell Model No. XXXXXXXXXXXX

Serial No. XXXXXXXXXXXX

Submitted By:

XXXXXXXXXXXXXXXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXX

XXXXXXXXXXXXXXXXXXXXXXX

The electromotive force of this cell at 22.4 degrees Celsius was, at the time of test, 1.01923 volts. This value, correct to 0.005 percent, is the mean of a series of measurements concluded 24 Apr

1986. The stated uncertainty (0.005%) includes allowance of plus or minus 50 microvolts for

variability in the EMF of the cell during test.

This is an unsaturated cell of the cadmium sulfate type, suitable for work requiring no greater accuracy

than 0.005 percent. Such cells have a temperature coefficient that is negligible within the ordinary

range of room temperature. Rapid changes in temperature may, however, produce temporary

alterations of several hundredths of one percent in the electromotive force.

Precautions in using standard cells: (1) the cell should not be exposed to temperatures below 4

degrees Celsius, (2) abrupt changes in temperature should be avoided, (3) all parts of the cell should

be at the same temperature, (4) current in excess of 0.0001 ampere should never pass through the

cell, (5) unsaturated cells should be recalibrated at intervals of a year or two because the

electromotive force of an unsaturated cell usually decreases with time.

For the Director

National Measurement Laboratory

Norman B. Belecki, Physicist

Center for Basic Standards

Electricity Division

Test No. XXXXXXXX

Date: 1 May 1986

Appendix B - Regular Calibration Report 39
U. S. Department of Commerce NATIONAL BUREAU OF STANDARDS

National Measurement Laboratory

Gaithersburg, MD 20899

REPORT OF TEST

Determination of V'p5qq - V]<\-qs

Laboratory: (LAB)

xxxxxxxxxxxxxxxxxxxxxxxx

xxxxxxxxxxxxxxxxxxxxxxxx

xxxxxxxxxxxxxxxxxxxxxxxx

Reference No. XXXXXXXXXX

Reference Cells:

5403 5404 5402

5406

The difference between the unit of electromotive force

maintained by the above laboratory and the U.S. Legal Volt, as

maintained by the National Bureau of Standards (NBS), was

experimentally determined during the period between November

11, 1985 and February 28, 1986. The results, given in the

equation below, are based on the previously-assigned mean emf

of the reference cells listed above.

Vlab - VnbS = 0.15 X 10-6 V^bs

The above number is based upon the experiment described in

Appendix A and the data summarized in Appendix B of this

report

.

The uncertainty of this difference, Vl^b " ^NBS' 0.15

microvolts at the 99% confidence level. It was determined by

combining in quadrature random error components resulting from

variations among the cells in the transport standard due to the

transfer, day-to-day fluctuations in the results of NBS

measurements of the transport standard, and day-to-day

variations in the difference between the results of the

client's measurements and the corresponding NBS-predicted

values of the emf's of the cells in the transport standard. As

far as is presently know, systematic effects are negligible.

The pertinent data and their reduction are given in Appendix B,

along with plots of all data taken both at NBS and in the

client laboratory.

Test No- XXXXXXXX

Appendix C - Volt Transfer Program Report 40
Page 2

This uncertainty, the difference cited above, and the results

of any adjustments made to the values of the reference cells

whose numbers are given above are valid only under conditions

approximating those under which this experiment was performed.

In particular, the operating temperature of the reference cells

must be within 0.05 degree C of that measured during the course

of the experiment. This condition must be met in order that

errors in the algorithm used to compute temperature corrections

not affect the experimental uncertainty.

On July 1, 1972, NBS changed from using saturated standard

cells to maintain the U.S. Legal Volt to the use of the ac

Josephson effect. Based upon data taken to date, it is

estimated that the U.S. Legal Volt is constant, with respect to

time to at least 0.1 ppm. This figure is not included in the

above uncertainty. As further data are obtained, the

uncertainty for this type of experiment will be refined to take

into account the improved method for maintaining the volt.

If the measured difference between the unit of voltage as

disseminated by this laboratory and the U.S. Legal Unit of

voltage is greater in magnitude than the characteristic

standard deviation of a measurement of this type (0.20 ppm), it

is recommended that the assigned mean value of the laboratory's

reference group be adjusted so that ^i^ab ~ ^NBS ^ 0. Appendix

C has be included in this report to facilitate recommended

adjustments

.

For the Director

National Measurement Laboratory

Norman B. Belecki, Physicist

Center for Basic Standards

Electricity Division

Test No. XXXXXXXX

Date: May 14, 1986

Appendix C - Volt Transfer Program Report 41
Appendix D - SOFTWARE

All software for controlling the measurement systems, reducing the cell data,

and producing test reports, was written in-house specifically for the intended

application. At present the software is in transition; a minicomputer used to do

data reduction is in the process of being replaced by a number of dedicated

desktop computers. Software has been completed for these computers to make

standard cell comparisons and calculate the cell emfs from the cell difference

observations. These data are stored on a shared hard disk. Existing software

for the minicomputer to maintain a data base of cell emfs of NBS and client

standards, produce control charts, and produce test reports is currently being

converted to run on a new, more modern, minicomputer. This is only an

interim step since these programs need to be substantially rewritten to provide

better quality control charts and easier access to the data base.

The following programs are complete and operate on the desktop computers.

READBOX - This program intercompares standard cell voltages using a

redundant measurement design, reduces the data to determine the cell emfs,

and saves the data to disk. The actual voltage differences are measured with

measuring system VTP, using the digital voltmeter, with the cells connected to

the voltmeter by a crossbar switch. The data collected by the program is stored

on a floppy disk in drive 0 and optionally on the Shared Resource Management

(SRM) hard disk. The files on the floppy disk are designed to hold approximately

one week's worth of data. A printed log is generated for each day's

measurements listing the boxes compared, the standard deviation of the

measurements, the change from the previous day's measurements, and the

DVM offset. At the end of all the measurements, the above quantities are

summarized.

READCAL - This program is a slight modification of READBOX to run on

measuring system REG. The crossbar switch used with system REG is a 300

position switch rather than the 80 position switch used with system VTP, and is

addressed slightly differently by the computer.

STDCEL - This program is contained within READBOX (and READCAL) and is

used to reduce standard cell difference observations to standard cell emfs. This

program is used when individual cell difference observations, within a

measurement design, are out of control. It permits the operator to remove

selectively those suspect observations from the design and store the corrected

cell emfs on floppy and hard disks.

EDIT - This program is a general purpose text (ASCII) file editing program and

is used to create or modify an existing "runfile". The "runfile" contains a list of

standards and their temperatures that are to be read by READBOX. This file is

updated daily using EDIT to contain the current enclosure temperatures, cell

identification numbers, and crossbar circuit numbers.

Appendix D - Software 42
This program can also be used to view or modify any of the data files containing

cell difference observations or cell emfs.

The following programs operate on the old minicomputer and are in the process

of being converted to run on the new minicomputer.

VFORMAT - This program takes data in the format produced by READBOX and

converts it to "Analysis" format for the program described below,

ANALYSIS - This program (and a number of variants created for special

purposes) produces control charts of cell emfs and cell temperatures on a line

printer. The program also performs a linear fit to the cell emf and can be used

to predict the emf of cells at a given time. This is used for calculating VlaB " ^NBS from the Volt Transfer Program transport cells.

REPORT - This program in conjunction with ANALYSIS above is used to

produce a final VTP report. This program performs text formatting, combining

the test report text with the values obtained from ANALYSIS.

BFORMAT - This program converts the READBOX data into a format

compatible with EMF below. This is necessary as EMF was originally written

for operation with an entirely different measurement system. When completely

converted to the new minicomputer, BFORMAT will be bound with EMF to

produce one program.

EMF - This program compiles standard cell emf data for the regular calibration

service. The data are printed out, statistical tests are applied to determine if the

data are sufficiently stable and if the temperature correction is reasonable, and

a calibration uncertainty is calculated.

CELL REPORT - This program in conjunction with EMF produces the final

calibration report for the regular calibration service.

Appendix D - Software 43
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/SP-250/24

2. Performing Organ. Report No, 3. Publication Date October 1987

4. TITLE AND SUBTITLE

NBS Measurement Services:

Standard Cell Calibrations

5. AUTHOR(S)

Bruce F. Field

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619872

I I Document describes a computer program; SF-185, PIPS Software Summary, is attached.

11. ABSTRACT fA 200-word or less factual summary of most si gnifi cant information. If document includes a si gnificant

bi bliography or literature survey, mention it here) This note describes the procedures used at NBS to calibrate standard cells in terms

of the U.S. Legal Volt. The two calibration services that are offered by the

Electricity Division are discussed. Regular Calibration of client standards at NBS,

and the Volt Transfer Program which is a process to determine the difference between

the U.S. Legal Volt and the volt as maintained by a client laboratory. The

operational procedures used to compare standard cells and to maintain the

U.S. Legal Volt via the ac Josephson effect are discussed in detail.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calibration services; dc volt; standard cells; voltage standards

13. AVAILABILITY

lyyl Unlimited

I I For Official Distribution. Do Not Release to NTIS

Kx] 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

52

15. Price

USCOMM-DC 6043-P80
PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 NBS Measurement Services: SP 250-12

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L,R. Canfield and N. Swanson

NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet SP 250-14

J.Z. Klose and J.M. Bridges

NBS Measurement Services:

Irradiation of Ferrous-Ferric Dosimeters

(Fricke- Dosimetry) SP 250-15

M. Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations SP 250-16

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz, SP 250-17

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg SP 250-18

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia SP 250-19

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "4Tt" Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

SP 250-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

NBS Measurement Services:

Neutron Personnel Dosimetry

R.B. Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with Californiixn Fission Sources

G.P. Lamaze and J, A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor

Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and O.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamna-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services:

Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation

M. Ehrlich, J.S. Pruitt, and C.G. Soares

PUBLICATIONS FROM THE CENTER FOR BASIC STANDARDS

SP 250-22 NBS Measurement Services: Platinum Resistance Thermometer

Calibrations

B. W. Mangum

SP 250-23 NBS Measurement Services: LiquicHn-Glass Thermometer

Calibration Sen/ice

J. A. Wise

SP 250-24 NBS Measurement Services: Standard Cell Calibrations

B. F. Field

SP 250-25 NBS Measurement Services:

Calibration Service for Inductive Voltage Dividers

A. 0. McCoubrey
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty tor Private Use $300