NIST HANDBOOK 150-28 

National 

Voluntary 

Laboratory 

Accredit ation 

Program 

Calibration 

Laboratories 

Technical Guide 

for 

Electromagnetic DC 

Low Frequency 

Measurements 

C. Douglas Faison and 

Carroll S. Brickenkamp, Editors 

March 2004 

U.S. Department of Commerce 

Donald L. Evans, Secretary 

Technology Administration 

Phillip J. Bond, Under Secretary for Technology 

National Institute of Standards and Technology 

Hratch Semerjian, Acting Director 
National Institute of Standards and Technology 

NIST Handbook 150-2A 

62 pages (March 2004) 

CODEN: NIHAE2 

U.S. GOVERNMENT PRINTING OFFICE 

WASHINGTON: 2004 

For sale by the Superintendent of Documents 

U.S. Government Printing Office 

Internet: bookstore.gpo.gov 

Phone: (202) 512-1800 

Fax: (202) 5 12-2250 

Mail: Stop SSOP, Washington, DC 20402-0001 

NVLAP AND THE NVLAP LOGO 

The term NVLAP (represented by the NVLAP logo) 

is a federally registered certification mark of the 

National Institute of Standards and Technology and 

the federal government, who retain exclusive rights 

to control the use thereof. Permission to use the term 

andor logo is granted to NVLAP-accredited 

laboratories for the limited purposes of announcing 

their accredited status, and for use on reports that 

describe only testing and calibration within the scope 

of accreditation. NIST reserves the right to control 

the quality of the use of the term NVLAP and of the 

logo itself. 

THE USE OF BRAND NAMES 

Brand names cited here are used for identification 

purposes and do not constitute an endorsement by 

NIST. 
Contents 

Preface ...................................................................... v 

Acknowledgments ........................................................... vii 

... Summary .................................................................. viii 

1 Generalinformation ........................................................ 1 

1.1 Purpose ............................................................. 1 

1.2 Organization of handbook ............................................... 1 

1.3 Description of Calibration .Laboratories Accreditation Program ................. 1 

1.4 References ........................................................... 2 

1.5 Definitions ........................................................... 3 

1.6 NVLAP documentation ................................................. 5 

1.7 Assessing and evaluating a laboratory ..................................... 6 

2 Criteria for accreditation .................................................... 11 

2.1 Introduction ......................................................... 11 

2.2 Areas of commonality among all dc/low frequency parameters ................. 11 

2.3 Directvoltage ....................................................... 19 

2.4 Capacitance - low frequency ............................................ 28 

2.5 

2.6 Inductance - low frequency ............................................. 34 

2.7 Fast electrical pulse waveforms ......................................... 36 

2.8 Dc resistance ....................................................... 38 

2.9 Ac-dc difference - thermal voltage converters .............................. 40 

2.10 High voltage capacitors ................................................ 46 

2.11 High-voltage transformers .............................................. 47 

2.12 Digital multimeters and multifunction calibrators ........................... 48 

2.13 Watt/watthour meters .................................................. 49 

2.14 Phasemeters ......................................................... 50 

2.15 Dc high voltage dividers and dc high voltage resistors ......................... 51 

Inductive voltage dividers (decade transformer dividers) - low frequency ......... 31 

iii 
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Preface 

The Calibration Laboratories Accreditation Program was developed by the National Voluntary Laboratory 

Accreditation Program (NVLAP) at the National Institute of Standards and Technology (NIST) as a result 

of interest from private industry and at the request of the National Conference of Standards Laboratories 

(now the NCSL International). The goal of the program is to provide a means by which calibration 

laboratories can be assessed for competency. This voluntary program is not designed to serve as a means of 

imposing specific calibration procedures or minimum uncertainties on applicant laboratories; instead, the 

program allows for all scientifically valid calibration schemes and requires that laboratories derive and 

document their measurement uncertainties. 

To accomplish this goal, NVLAP employs technical experts on a contract basis, to serve as assessors in each 

of the following eight fields of physical metrology calibration: 

electromagnetic dc/low frequency, 

electromagnetic rf/microwave frequency, 

time and frequency, 

ionizing radiation, 

optical radiation, 

dimensional, 

mechanical, and 

thermodynamics. 

NIST Handbooks 150-2A through 150-2H are technical guides for the accreditation of calibration 

laboratories, with each handbook corresponding to one of the eight fields of physical metrology calibration. 

They are intended for information and use by: 

NVLAP technical experts in assessing laboratories, 

staff of accredited laboratories, 

those laboratories seeking accreditation, 

other laboratory accreditation systems, 

users of laboratory services, and 

9 others needing information on the requirements and guidelines for accreditation under the NVLAP 

Calibration Laboratories Accreditation Program. 

NOTE The Calibration Laboratories Accreditation Program has been expanded to cover chemical calibration for 

the providers of proficiency testing and certifiers of spectrophotometric NTRMs. (See NIST Handbooks 150-19 and 

150-21.) Other NVLAP handbooks in the chemical calibration area are expected in the future. 

The assessor uses NIST Handbook 150, NVLAP Procedures and General Requirements, and the appropriate 

guides (NIST Handbooks 150-2A through 150-2H) to validate that a laboratory is capable of performing 

calibrations within the laboratory’s stated uncertainties. These technical guides and other relevant technical 

information support assessors in their assessments of laboratories. Along with inspecting the fxilities, 

documentation, equipment, and personnel, the assessor can witness a calibration, have an item recalibrated, 

and/or examine the results of measurement assurance programs and round-robins to collect objective 

evidence. 

NIST Handbooks 150-2A through 150-2H supplement NIST Handbook 150, which contains Title 115 of the 

US. Code of Federal Regulations (CFR) Part 285 plus all general NVLAP procedures, criteria, andpolicies. 

The criteria in NIST Handbook 150 originally encompassed the requirements of ISO/IEC Guide 25: 1990 and 

V 
the relevant requirements of IS0 9002 (ANSYASQC 492-1 987). These handbook criteria have been updated 

to incorporate the requirements of ISO/IEC 17025: 1999. The entire series of Handbooks 150-2A through 

150-2H comprises information specific to the Calibration Laboratories Program iind neither adds to nor 

detracts from requirements contained in NIST Handbook 150. 

Any questions or comments on this handbook should be submitted to the National Voluntary Laboratory 

Accreditation Program, National Institute of Standards and Technology, 100 Bureau Drive, Stop 2140, 

Gaithersburg, MD 20899-2140; phone (301) 975-4016; fax (301) 926-2884; e-mail NVLAP@nist.gov. 

vi 
Acknowledgments 

NIST Handbook 150-2 was first available as a draft covering all eight fields ofphysical metrology calibration 

in one volume. It has been separated into eight handbooks to allow easier updating and electronic 

downloading from the NVLAP web site. The preparation of these documents has been a joint effort, with 

input from representatives of other government agencies, laboratories, and the private: sector. 

Acknowledgment of their efforts is in order; however, the listing of individual names is impractical. The 

submissions by individuals and companies offering suggestions for improvement to this document were also 

very welcome, as were the contributions of those who attended the public workshops. 

We thank all the NIST measurement divisions for their work in writing or contributing to the individual 

handbooks. Listed below are those from the NIST measurement divisions who deserve special thanks for 

input to Handbook 150-2A: 

Mr. Norman B. Belecki (Capacitance, Inductance, AC Resistance, DC Voltage, and Inductive 

Mr. Ronald F. Dziuba (DC Resistance), 

Mr. William L. Gans, Mr. Donald Larson, and Mr. Barry A. Bell (Fast Electrical Pulse Waveform), 

Mr. Thomas E. Lipe (AC-DC Difference), 

Dr. Martin Misakian (DC High Voltage Divider and Resistance), 

Mr. Thomas L. Nelson (Watt-Watthour Measurements), 

Mr. Nile M. Oldham (Digital Multimeters and Phase Meter Measurements), and 

Dr. James K. Olthoff (High Voltage Capacitors and High Voltage Transformers). 

Voltage Dividers), 

Additional thanks go to those who actively participated in the Technical Guide Workshop held November 

1993 and to those who served as points of contact within fields of calibration. They include: Mr. Georgia 

L. Harris, Mr. Norman B. Belecki, Dr. Theodore D. Doiron, Mr. Robert M. Judish, Mr. Thomas C. ILarason, 

Ms. Sally S. Bruce, and Dr. Donald B. Sullivan. A special thanks is owed to Mr. James L. Cigler for work 

in developing the content and format of this guide, and to Ms. Vanda White for her editorial expertise in 

making this a readable document. 

Above all, we wish to thank Mr. Jon M. Crickenberger, the editor of the first three drafts of this document, 

for literally hundreds of hours of his work in creating this guide. It was he who tasked the contributors to 

produce the technical content, assembled the results of their efforts into a consistent format, and provided 

the general commentary. Without Jon's dedicated effort to this monumental task, this guide would never 

have been published. 

NVLAF' has edited the individual handbooks and made changes resulting from comments by individuals to 

earlier draft versions. This editing has been to a different extent for each parameter. Every effort was made 

to include all pertinent information relevant to an ISO/IEC 17025-derived technical guide. 

C. Douglas Faison and Carroll S. I3rickenkamp, Editors 

National Voluntary Laboratory Accreditation Program 

National Institute of Standards and Technology 

100 Bureau Drive, Stop 2140 

Gaithersburg, MD 20899-2140 

vii 
Summary 

This guide presents the general technical requirements (i.e., on-site assessment and proficiency testing) of 

the laboratory accreditation program for calibration laboratories along with specific technical criteria and 

guidance applicable to electromagnetic dc/low frequency measurements. These technical guidelines are 

presented to indicate how the NVLAP criteria may be applied. 

Any calibration laboratory (including commercial, manufacturer, university, or federal, state, or local 

government laboratory) engaged in calibration in electromagnetic dc/low frequency measurements listed in 

this handbook may apply for NVLAP accreditation. Accreditation will be granted to a laboratory that 

complies with the criteria for accreditation as defined in NIST Handbook 150. Accreditation does not 

guarantee laboratory performance - it is a finding of laboratory competence. 

Fields of calibration covered: Specific calibration parameters and related stimulus and measurement devices 

in areas of electromagnetic dc/low frequency measurements. 

Scope of accreditation: 

1) 

2) 

3) 

Calibration parameter(s), range, and uncertainty level 

Types of measuring and test equipment 

Quality assurance system for measuring and test equipment. 

Period of accreditation: One year, renewable annually. 

On-site assessment: Visit by an assessor(s) to determine compliance with the NVLAP criteria before initial 

accreditation, in the first renewal year, and every two years thereafter. Preassessment and monitoring visits 

are conducted as required. All calibration parameters or general areas of calibration within the specific scope 

of accreditation requested will be assessed. 

Assessors: Selected from technical experts with experience in the appropriate areas of calibration and 

quality systems assessment. 

Proficiency testing (measurement assurance): Each laboratory is required to demonstrate its capability to 

successfully perform calibrations as part of on-site assessment or by documented successful completion of 

an approved Measurement Assurance Program (MAP) or round-robin intercomparison. Proficiency testing 

may be required for initial accreditation, or where other evidence of measurement assurance is not evident, 

and may be conducted annually thereafter. Advance notice and instructions are given before proficiency 

testing is scheduled. 

Fees: Payments are required as listed on the NVLAP fee schedule, including the initial application fee, 

administrative/technical support fee, on-site assessment fee, and proficiency testing fee. 

viii 
1 General information 

1.1 Purpose 

The purpose of this handbook is to amplify the general requirements for accreditation by NVLAP of 

calibration laboratories in the area of electromagnetic dc/low frequency measurements covered by the 

Calibration Laboratories Program. It complements and supplements the NVLAP programmatic procedures 

and general requirements found inNIST Handbook 150, NVLAP Procedures and General Requirements. The 

interpretive comments and additional guidelines contained in this handbook make the general NVLAP 

criteria specifically applicable to the Calibration Laboratories Program. 

This handbook does not contain the general requirements for accreditation, which are listed in NIST 

Handbook 150, but rather provides guidelines for good calibration laboratory practices, which may be useful 

in achieving accreditation. 

1.2 Organization of handbook 

The handbook is organized in two sections. The first section provides additional explanations to the general 

procedures and requirements contained in NIST Handbook 150. The second section provides de:tails and 

guidance very specific for electromagnetic dc/low frequency measurement calibration laboratories. 

1.3 Description of Calibration Laboratories Accreditation Program 

On May 18, 1992, .as a result of the petition and public notice process, the Director of the National Institute 

of Standards and Technology published in the Federal Register a notice of intent to develop the Calibration 

Laboratories Accreditation Program under the procedures of the National Voluntary La.boratory 

Accreditation Program . On June 2, 1994, the procedures and general requirements under which NVLAP 

operates, Title 15, Part 285 of the U.S. Code of Federal Regulations (CFR), were revised to: 

expand the procedures beyond testing laboratories to include accreditation of calibration laboratories, 

update the procedures to ensure compatibility with generally accepted conformity assurance and 

conformity assessment concepts, 

incorporate international changes, especially with relevant International Organization for 

Standardization/International Electrotechnical Commission (ISOLEC) documents (e.g., ISO/IEC Guides 

25 (now ISO/IEC 17025:1999), 38,43, and 58, and the IS0 9000 series), and 

facilitate and promote acceptance of the calibration and test results between countries to avoid barriers 

to trade. 

Calibration laboratory accreditation is offered in eight fields of physical metrology calibration covering a 

wide variety of parameters and includes accreditation in multifunction measuring and test equipment 

calibrations. Specific requirements and criteria have been established for determining laboratory 

qualifications for accreditation following prescribed NVLAP procedures. The criteria address quality 

systems, staff, facilities and equipment, test and calibration methods and procedures, manuals, records, and 

calibration reports. 

NIST Handbook 150-2A 1 March 2004 
On September 18, 1992, a public workshop was held at NIST Gaithersburg and attended by a mix of private 

sector and government personnel. The workshop reviewed a draft handbook, which included general 

requirements, as well as very specific technical requirements for dc voltage calibrations at all lev& As a 

result of the workshop, the draft handbook was revised to take the form of a Calibration Laboratories 

Program Handbook, which included the general requirements for laboratories (using ISO/IEC Guide 25 as 

a basis), and eight companion Technical Guides covering the specific requirements for each field of 

calibration offered for accreditation. 

On May 18, 1993, a public workshop on the revised draft program handbook was held at NIST Boulder and 

attended by more than 60 industry and government personnel. Comments from this workshop, as well as 

responses to a survey/checklist mailing, were used to prepare the final draft of the handbook, now entitled 

NVLAP Procedures and General Requirements (NIST Handbook 1 SO), published in March 1994. NIST 

Handbook 150 has since been revised to incorporate ISO/IEC 17025:1999. 

A public workshop for the Calibration Laboratories Technical Guides was held at NIST Gaithersburg, on 

November 22 through 24, 1993. More than 60 industry and government personnel attended and provided 

comments on the draft version of the Technical Guide for each of eight fields of calibration. As a result, the 

eight Technical Guides were incorporated into a draft Handbook 150-2, Calibration Laboratories Technical 

Guide, covering the fields being offered for accreditation. [In 2000, Handbook 150-2 (draft) was divided into 

eight handbooks, one for each calibration area.] 

The need for technical experts to serve as assessors was advertised, and the first group of assessors was 

selected and trained during a four-day session held from November 16 through 19, 1993, in Gaithersburg, 

using materials developed by NVLAP. 

The Calibration Laboratories Accreditation Program officially began accepting applications when 

notification was given in the Federal Register dated May 1 1,1994. Applications are accepted and processed 

following the procedures found in NIST Handbook 150. 

1.4 References 

1.4.1 The following documents are referenced in this handbook. 

a) NIST Handbook 150, NVLAP Procedures and General Requirements; available from: 

National Voluntary Laboratory Accreditation Program 

National Institute of Standards and Technology, 

100 Bureau Drive, Stop 2140 

Gaithersburg, MD 20899-2140 

Phone: (301) 975-4016 

Fax: (301) 926-2884 

E-mail: nvlap@nist.gov ' 

NVLAP Web site: http://www.nist.gov/nvlap 

b) ISO/IEC/BIPM (BIPM is the Bureau International des Poids et Mesures, the International Bureau of 

Weights and Measures) Guide to the Expression of Uncertainty in Measurement (GUM), 1993. 

c) ISO/IEC 17025 : 1999: General requirements for the competence of testingand calibration laboratories. 

NZST Handbook 1.50-2A 2 March 2004 
ISO/IEC Guide 43: 1997, ProJiciency testing by interlaboratory comparisons, Part I and Part 2. 

ISO/IEC/BIPM International Vocabulary of Basic and General Terms in Metrology (VIM), 1993. 

IS0 documents b) through e) are available from: 

Global Engineering Documents (paper copies) 

Order phone: (800) 854-7179 

American National Standards Institute (ANSI) (electronic copies) 

Electronic Standards Store 

ANSI web site: http://www.ansi.org 

ANSYIEEE Std. 100-1 996, IEEE Standard Dictionary of Electrical & Electronics Terms. 

NIST Technical Note 1297, Guidelines for Evaluating and Expressing the Uncertainty of NIST 

Measurement Results. Available on-line at http://physics.nist.gov/Document/tn1297.pdf. 

NCSL Recommended Practice RP-15: Guide for Interlaboratory Comparisons, 1999. 

ANSIINCSL 2540- 1-1 994, Calibration Laboratories and Measuring and Test Equipment--General 

Requirements. 

NCSL Recommended Practice RP-7: Laboratory Design, 1993. 

NCSL documents h) through j) are available from: 

NCSL International 

2995 Wilderness Place, Suite 107 

Boulder, CO 80301-5404 

Phone: (303) 440-3339 

Fax: (303) 440-3384 

E-mail : orders@ncsli .org 

Web site: http://www.ncsli.org 

Ehrlich, C. D. and Raspberry, S. D., “Metrological Timelines in Traceability,” ,I Res. Natl. Inst. Stand. 

Technol. 103, 93 (1998). 

Croarkin, M. C., Measurement Assurance Programs, Part II: Development and Implementation, NBS 

Special Publication 676-11 (U.S. Government Printing Office, Washington, DC, 1985). 

1.4.2 

Section 2. 

Additional references specific to electromagnetic dc/low frequency measurements are listed in 

1.5 Definitions 

Definitions found in NIST Handbook 150 apply, but may be interpreted differently or stated differently, 

when necessary to amplify or clarify the meaning of specific words or phrases as they apply to1 specific 

technical criteria. 

NIST Handbook 150-2A 3 Mtzrch 2004 
Electrical/electronic terminology is consistent with ANSUIEEE Std. 100, IEEE Standard Dictionary of 

Electrical & Electronics Terms. Other terms with meanings specific to this document are defined below. 

1.5.1 

via a transport standard for the purpose of calibrating, or assigning values to one in terms of the other. 

Calibration transfer: A comparison of two measurement systems or standards either directly or 

1.5.2 Control environment: A controlled environment provided locally to mitigate the effects of 

ambient conditions on the performance of a standard, by for example, a thermostatted oven as a standard cell 

enclosure. 

1.5.3 

directly or via a transport standard for the purpose of evaluating their equivalence. 

Evaluation transfer: A comparison of the performance of two measurement systems either 

1.5.4 Josephson array: Two or more Josephson junctions in series. 

1.5.5 

system (Josephson array system). 

Josephson array standard: A Josephson array and its accompanying support and measurement 

1.5.6 Josephson array system: The auxiliary apparatus including switches, cables, waveguide, 

instrumentation, operator, software (where applicable), and any other hardware required to operate a 

Josephson array properly and use it to perform measurements outside its cryostat. 

1.5.7 Manufacturer’s stated accuracy: Generalized statement or collection of statements describing 

the qualitative performance of an instrument under very broad conditions. (These statements sometimes 

contain such terms as “typical” or “nominal” accuracy, where accuracy is a qualitative term.) Such statements 

are made by the manufacturer for sales or warranty purposes, or by the user to facilitate categorizing the 

instrument within an inventory or making broad statements regarding its suitability for use in making a 

particular measurement. 

Use of this terminology does not excuse the laboratory from demonstrating, through uncertainty analysis, 

that it has adequate measurement capability to perform the measurement needed either to assign values or 

corrections to standards or to verify the performance of instruments and standards it is responsible for 

calibrating. 

1.5.8 Nominally equal: Within 5 x of equality. 

1.5.9 Proficiency testing: Determination of laboratory performance by means of comparing and 

evaluating calibrations or tests on the same or similar items or materials by two or more laboratories in 

accordance with predetermined conditions. For the NVLAP Calibration Laboratories Accreditation Program, 

this entails using a transport standard as a measurement artifact, sending it to applicant laboratories to be 

measured, and then comparing the applicant’s results to those of a reference laboratory on the same artifact. 

1.5.10 Situation 1 : Calibration of high-accuracy standards using an intrinsic standard or system. 

1.5.1 1 Situation 2: Calibration of secondary standards in circumstances where attainment of a four-toone ratio between the manufacturer’s stated accuracy of the secondary standard and that of the reference 

standard (other than an intrinsic standard or system) is made impossible by the demanding specific:ations of 

the former. 

NIST Handbook 150-2A 4 March 2004 
1.5.12 Situation 3: Calibration of working standards or instruments in circumstances where attainment 

of a four-to-one ratio between their manufacturers’ stated accuracies and that of the scaling standard 

combined with that of the secondary or reference standard (other than an intrinsic standard or s,ystem) is 

made impossible by the demanding specifications of the former. 

1.5.13 

the calibration workload to that of the working standard equals or exceeds four to one. 

Situation 4: Any calibration situation where the ratio of the manufacturers’ stated accuracies of 

1.5.14 

secondary standards to calibrate working standards or instruments over a large range of voltages. 

Standard, scaling: A ratio standard, which may be self-calibrating, used with reference or 

1.5.15 Standard, solid-state: A complete instrument in one box that is based on a solid-state reference, 

is powered by the ac mains or internal batteries, and continuously produces one or more stable voltages. The 

solid-state reference is a semiconductor device or ensemble of devices such as a temperature-compensated 

Zener diode, a band-gap device, or a reference amplifier. 

1.5.16 Standard, transport: A standard especially designed to be minimally affected by the rigors of 

transportation or by laboratory influence factors, and to have very predictable behavior over a period of time 

adequate for the evaluation of one measurement system in terms of another (its long-term stability may be 

sacrificed for ruggedness). 

1.5.17 Standard, working: A multiple-valued standard, such as a precision digital voltmeter or 

calibrator, used for the calibration of a substantial workload; it may be calibrated in terms of scaling 

standards and either reference or secondary standards. 

1.5.18 Traceability: Property of the result of a measurement or the value of a standard whereby it can 

be related to stated references, usually national or international standards, through an unbroken chain of 

comparisons all having stated uncertainties. [See reference 1.4.1 e), section 6.10.1 

A single measurement intercomparison is sufficient to establish uncertainty relationships only over a limited 

time interval (see reference 1.4.1 k)); internal measurement assurance (see reference: 1.4.1 l)), using control 

(check) standards, is required to fully demonstrate that uncertainties remain within stated levels over time. 

For the purposes of demonstrating traceability for NVLAP accreditation, a laboratory must demonstrate not 

only that there is an unbroken chain of comparisons to national standards, but also that this chain is supported 

by appropriate uncertainties, measurement assurance processes, continuous standard maintenance, proper 

calibration procedures, and proper handling of standards. In this way, traceability is related to these other 

areas of calibration. 

1.6 NVLAP documentation 

1.6.1 Accreditation documents 

Laboratories granted NVLAP accreditation are provided with two documents: Scope of Accreditation and 

Certificate of Accreditation. 

The Scope of Accreditation lists the “Best Uncertainty” that an accredited laboratory can provide for a given 

range or nominal value within a given parameter of measurement. This “Best Uncertainty” is a statement 

of the smallest uncertainty that a laboratory has been assessed as capable of providing for that particular 

range or nominal value. The actual reported value of uncertainty for any particular measurement service that 

NIST Handbook 150-2A 5 March 2004 
the accredited laboratory provides under its scope may vary depending on such contributors as the statistics 

of the test and uncertainties associated with the device under test. 

1.6.2 Fields of calibration/parameters selection list 

The Calibration Laboratories program encompasses eight fields of physical metrology calibration, with 

multiple parameters under each field. Each field is covered by a separate handbook (NIST Handbooks 150- 

2A through 150-2H). (Fields of accreditation under Chemical Calibration are covered by separate 

handbooks.) Depending on the extent of its calibration capabilities, a laboratory may seek accredi~tation to 

all or only selected fields and parameters within the scope of the program. The fields of calibration and their 

related parameters are given on the Fields of Calibration and Parameters Selection List, which is ]provided 

to a laboratory seeking accreditation as part of the NVLAP application package for the program. Additional 

fields of calibration and/or parameters may be added to the Calibration Laboratories program upon request 

of customer laboratories and/or if decided by NVLAP to be in the best interest of the Calibration 

Laboratories program. 

The laboratory is requested to indicate on the Fields of Calibration/Parameters Selection List the parameter(s) 

for which accreditation is desired, along with appropriate ranges and uncertainties. There is also provision 

for an applicant laboratory to request accreditation for parameters not currently listed on the Selection List, 

or for accreditation of the quality system employed for assuring Measurement and Test Equipment (M & TE) 

used in support of product certification. Request for accreditation of quality assurance systems for M & TE 

will be treated as a separate field of calibration for the purpose of setting appropriate fees. Once a laboratory 

meets all the requirements for accreditation for the Fields of CalibratiodParameters Selection List, this 

information will become the basis for the Scope of Accreditation document. 

1.6.3 Checklists 

Checklists enable assessors to document the assessment of the laboratory against the NVLAP requirements 

found in NIST Handbook 150. The NVLAP Calibration Laboratories Accreditation Program incorporates 

the NVLAP General Operations Checklist. The questions are applicable to evaluating a laboratory's ability 

to operate a calibration program, and address factors such as the laboratory's organization, management, and 

quality system in addition to its calibration competency. 

TheNVLAP General Operations Checklist is numbered to correspond to the requirements in NIST Handbook 

150. Comment sheets are used by the assessor to explain deficiencies noted on the checklist. Additionally, 

the assessor may use the sheets to make comments on aspects of the laboratory's performance other than 

deficiencies. 

1.7 Assessing and evaluating a laboratory 

1.7.1 On-site assessment 

1.7.1.1 The NVLAP lead assessor will schedule with the laboratory the date for onsite evaluation, and will 

request the quality manual and documented quality and calibration procedures in advance of the visit to 

reduce time spent at the laboratory; such materials will be returned by the assessor. NVLAP and the: assessor 

will protect the confidentiality of the materials and information provided. The laboratory should be prepared 

to conduct routine calibrations, have equipment in good working order, and be ready for examination 

according to the guidance contained in this handbook, the requirements identified in NIST Handbook 150, 

and the laboratory's quality manual. The assessor will need time and work space to complete assessment 

NIST Handbook 150-2A 6 March 2004 
documentation while at the laboratory, and will discuss these needs at the opening meeting of the on-site 

assessment. 

1.7.1.2 

ensure the completeness, objectivity, and uniformity of the on-site assessment. 

NVLAP technical assessors are provided with the NVLAP General Operations Checklist to help 

1.7.1.3 When accreditation has been requested for a considerable number of fields of calibration and 

parameters, the assessment may range from observing calibrations in progress, requiring repeat 

measurements on completed calibrations, to listening to laboratory staff describe the calibration process. The 

depth into which the assessor performs the assessment depends on the number of fields of calibration and 

associated parameters for which accreditation is requested and the time required to perform a given 

calibration. 

1.7.1.4 The assessor, or the assessment team, does the following during a typical on-site assessment: 

Conducts an entry briefing with the laboratory manager to explain the purpose of the on-site visit and 

to discuss the schedule for the day(s). At the discretion of the laboratory manager, other staff may 

attend the briefing. 

Reviews quality system manual, equipment and maintenance records, record-keeping procedures, 

laboratory calibration reports, and personnel competency records. At least one laboratory staff member 

must be available to answer questions; however, the assessor may wish to review the documents alone. 

The assessor(s) does not usually ask to take any laboratory documents with hidher, and previously 

supplied documents will be returned. 

Physically examines equipment and facilities, observes the demonstration of selected procedures by 

appropriate personnel assigned to perform calibrations, and interviews the personnel. The 

demonstrations must include preparation for calibration of devices, and the setup and use of measuring 

and test equipment, standards and systems. 

Holds an exit briefing with the laboratory manager and staff to discuss the assessment findings. 

Deficiencies are discussed and resolutions may be mutually agreed upon. Item:; that must be addressed 

before accreditation can be granted are emphasized, and outstanding deficiencies require response to 

NVLAP within 30 days. Items that have been corrected during the on-site and any recommendations 

are specially noted. 

Completes an On-site Assessment Report, as part of the exit briefing, summarizing the findings. The 

assessor(s) attaches copies of the completed checklists to this report during the exit briefing. The report 

is signed by the lead assessor and the laboratory’s Authorized Representative to acknowledge the 

discussion. This signature does not necessarily indicate agreement; challenge(s) may be made: through 

NVLAP. A copy is given to the representative for retention. All observations made by the NVLAP 

assessor are held in the strictest confidence. 

1.7.2 Proficiency testing 

1.7.2.1 Background 

Once the quality system review and on-site assessment steps have been satisfactorily completed, it is 

necessary to gather another set of data points to aid in deciding whether or not the applicant laboratory is 

competent to perform calibrations within the fields of interest to the uncertainties claimed. In the eight fields 

NIST Handbook 150-2A 7 March 2004 
of calibration covered by Handbooks 150-2A through 150-2H, there are approximately 85 parameters of 

interest. Under most parameters there are several subsets, referred to as ranges. For example, in the 

electromagnetic dc/low frequency field, the dc resistance parameter can range from 0.001 SZ up to the 

terraohm (1 x lo'* SZ) level in decade values. When considering all possible ranges, there are thousands of 

areas in which proficiency testing can be conducted. NVLAP reserves the right to test by sampling in any 

area; hence, applicant laboratories must be prepared, with reasonable notice, to demonstrate proficiency in 

any of a number of parameters. 

1.7.2.2 Proficiency testing vs. measurement assurance 

There is an important difference between proficiency testing and measurement assurance. The objective of 

proficiency testing is to determine through a measurement process that the laboratory's measurement results 

compare favorably with the measurement results of the audit laboratory (NIST or one designated by WLAP), 

taking into account the relative uncertainties assigned by both the applicant and audit laboratories. The 

objective of proficiency testing is not to determine and certify the total uncertainty of the applicant laboratory, 

as is done in a Measurement Assurance Program (MAP) with NIST, but to verify (through the assessment 

process) that the uncertainty claimed by the applicant laboratory is reasonable, and then use the claimed 

uncertainty to test that the measurement result obtained through the proficiency test is acceptable. 

It is neither the intention nor the mission of NVLAP to conduct MAPs or to otherwise provide traceability 

for laboratories. Laboratories obtain these services from the NIST measurement divisions. NVLAP assesses 

the implementation, application, and documentation of MAPs by laboratories. NVLAP accreditation 

encourages the use of MAPs by the calibration laboratory community, and MAP results produce objective 

evidence that NVLAP assessors look for as part of the assessment process. 

1.7.2.3 Requirements 

NVLM's proficiency testing program uses a sampling approach. All applicant laboratories are required to 

complete an annual proficiency test in one parameter under each field of calibration for which it has applied 

to be accredited. For the purposes of the NVLAP Calibration Laboratories Accreditation Program, the results 

of the proficiency test are considered as objective evidence, along with the on-site visit, of a laboratory's 

ability to perform competent calibrations. Proficiency testing is conducted annually using different 

parameters in each field; however, those laboratories accredited in only one parameter within a field are 

retested in the same parameter. 

1.7.2.4 Uncertainty determination 

The applicant laboratory is required to perform a measurement or series of measurements on an artifact using 

the same calibration method, apparatus, and personnel that it uses to calibrate its cusl.omers' equipment. The 

laboratory must be able to identify and quantify all sources of uncertainty that affect the measurement. The 

laboratory should attach an overall uncertainty to the measurement by combining all Uncertainty contributions, 

in their type A and type B components, in the root-sum-squared method as described in the Guide to the 

Expression of Uncertainty in Measurement (see reference 1.4.1 b)). The confidence limit used should be k 

= 2, which is equivalent to a 95 % confidence probability. 

1.7.2.5 Pass/fail criteria 

The performance of the proficiency test is judged by calculating the error of the measurement, normalized 

with respect to the uncertainty of the measurement, using the following equation: 

NIST Handbook 1 SO-2A 8 March 2004 
E,,-, = I (Value,, - valueref) / ( Uncertainty,,: + Uncertaintyla~>~l’2 I 

where 

Enorma, = normalized error of the applicant laboratory 

Value,, = the value as measured by the applicant laboratory 

Value,,, = the value as measured by the reference laboratory 

Uncertainty,,, = the uncertainty of the reference laboratory 

Uncertainty,, = the uncertainty of the applicant laboratory 

To pass the proficiency test, the applicant laboratory must have a value for Enormal less than 1 (i.e., Enomal < 1). 

The results may be plotted graphically, with lines representing the limits of Uncertainty of the measurements. 

The anonymity of each applicant laboratory will always be preserved. 

1.7.2.6 Scheduling and handling 

Proficiency testing is scheduled by NVLAP-designated reference laboratories. These sites are NIST 

laboratories or NVLAP-accredited laboratories that have been found to have the: ability to perform the 

required proficiency tests to an uncertainty level appropriate for the laboratories they evaluate. The 

proficiency test is scheduled independently andnot to correspond with the on-site visit.. Applicant laboratories 

are notified in advance as to the approximate arrival time of the measurement artifact. Instructions for 

performing the test, reporting the results, communicating with the reference laboratory, and shipping are 

included along with the artifact as part of the proficiency test package. Applicant lalioratories are iinstructed 

to perform all required measurements within a reasonable time and are told where to ship the artif dcts once 

the testing has been completed. 

1.7.2.7 Notification of results 

NVLAP notifies each laboratory of its own results in a proficiency test. If a laboratory has been evaluated 

prior to the completion of the proficiency test, the status of that laboratory’s accreditation is contingent upon 

successful completion of proficiency testing. The laboratory’s accreditation status may be changed to reflect 

a partial accreditation, or may be completely suspended pending demonstration of the laboratory’s ability to 

successfully complete the proficiency test at a later date. 

1.7.3 Traceability 

1.7.3.1 Establishing traceability 

Laboratories must establish an unbroken chain of comparisons leading to the appropriate international or 

national standard, such that the uncertainties of the comparisons support the level of uncertainty that the 

laboratory gives to its customers. Generally speaking, the uncertainties of the comparisons increase as they 

move from a higher (international or national level) to a lower level standard. This uncertainty chain is the 

evidence of traceability and must be documented accordingly. Traceability does not simply mean having 

standards calibrated at the national laboratory, but must consider how a measurement, with its corresponding 

uncertainty, is transferred from the national level to the calibration laboratory’s customers. 

1.7.3.2 Considerations in determining traceability 

Without some type of measurement assurance process, one cannot be reasonably certain that the comparisons 

have been transferred properly to the laboratory’s customers. The measurement process itselfmust be verified 

NIST Handbook 150-2A 9 Ma,rch 2004 
to be in control over time. Therefore, traceability is not a static concept that, once established, may be 

ignored; it is dynamic. Process control exercised in each calibration provides the assurance thiat a valid 

transfer of the international or national standard has taken place. This assurance may be accomplished 

through the use of tools such as check standards and control charts. Also, the laboratory’s primary standards 

must be maintained in such a way as to verify their integrity. Examples of this may be 

1) having more than one primary standard to use for intercomparisons, 

2) monitoring the primary standard with a check or working standard (looking for changes:), and 

3) verifying a primary standard on a well-characterized measurementhalibration system. 

Using scientifically sound measurement procedures to transfer the primary standard value to the working level 

and the customer’s item is essential to establishing traceability. If the procedure itself‘ yields the wrong result, 

there is no way the laboratory can perform a calibration traceable to the international or national standard. 

Handling the laboratory’s standards affects the measurement process, and therefore the ability to transfer the 

standard’s value to the customer. Examples of handling problems are 

1) dirty or improperly cleaned standards, 

2) maintaining standards in an improper environment, 

3) not maintaining custody and security, and 

4) improper handling of standards during the measurement process. 

1.7.3.3 Relationship to existing standards 

The above discussion illustrates how traceability is dependent on many aspects of the measurement process 

and therefore must be considered in all phases of calibration. It is not just coincidental that the factors 

addressed above are main topics of concern in ISO/IEC 17025: 1999. 

1.7.4 Uncertainty 

NVLAP recognizes the methodology for determining uncertainty as described in the Guide to the Expression 

of Uncertainty in Measurement, published by ISO. To be NVLAP-accredited, a laboratory must document 

the derivation of the uncertainties that it reports to its customers. These uncertainties will appear on the scope 

issued to each accredited laboratory to an accuracy appropriate to the standards, procedures, and measuring 

devices used. 

NIST Handbook 150-2A 10 ’ March 2004 
2 Criteria for accreditation 

2.1 Introduction 

2.1.1 Applicant laboratories are assessed using the requirements in NIST Handbook 150, NVLAP 

Procedures and General Requirements. This guide, NIST Handbook 150-2A, was developed from a NIST 

measurement laboratory perspective and provides examples and guidelines, not requirements, to assessors and 

interested calibration laboratories on good laboratory practices and recommended standards. Therefore, the 

guide language reflects this philosophy through the use of “shoulds” instead of “shalls” (along with other less 

prescriptive language) when describing criteria. The requirements presented here are not absolute since 

specific requirements depend on the measurement uncertainty for which an applicant laboratory wishes to be 

accredited. This is a business decision for each laboratory and beyond the scope of WLAP. Simply stated, 

to be accredited, an applicant laboratory must have a quality system and be able to prove (and document) that 

it is capable of doing what it says it does (i.e., correctly calibrate to a stated uncertainty) within the framework 

ofNIST Handbook 150. Accreditation will be granted, and therefore may be referenced in calibrationreports, 

etc., only for those specific parameters, ranges and uncertainties using calibration methods and procedures 

for which a laboratory has been evaluated. Calibrations performed by a laboratory using methods and 

procedures not considered appropriate for the level of measurements being made, and which have not been 

evaluated by the accreditation process, are outside the scope of accreditation and may not be referenced as 

“accredited” calibrations on calibration reports, etc. 

2.1.2 Sections 2.2 through 2.15 provide specific interpretations of the NIST Handbook 150 criteria for 

electromagnetic dc/low frequency measurement calibration laboratories. This guide: is dynamic in that new 

parameters may be added and existing criteria updated and improved. 

2.2 Areas of commonality among all dc/low frequency parameters 

2.2.1 Background 

A given laboratory in the electromagnetic dc/low frequency area of calibration can rely on one or inore of a 

number of different standards types, depending upon the nature of the calibrations to be performed, the 

calibration volume, and the levels of measurement uncertainty required. The laboratory’s standards imay have 

values assigned as the result of calibrations, MAP services, or measurements involving intrinsic standards. 

At all except the highest accuracy levels, standards or instrumentation are available whose performance 

exceeds the demands for accuracy of the calibration process by such a large margin that only minimal quality 

assurance techniques are required to ensure the adequacy of the standard. At the highest accuracy levels, 

instrumentation is specified to be so close in performance to the standards themselves; that considerable effort 

is required to ensure both that the standards are behaving properly and that the instrumentation used to 

compare the standards with the units being calibrated has adequate uncertainty, resolution, and precision for 

the task. 

2.2.2 Scope 

2.2.2.1 This section contains specific technical criteria in accordance with which a laboratory should 

demonstrate that it operates, if it is to be recognized as competent to carry out electromagnetic dc/low 

frequency calibrations. 

2.2.2.2 

implementation of their quality systems. 

NIST Handbook 150-2A 11 March 2004 

This section may also be used as a guide by calibration laboratories in the development and 
2.2.2.3 

accuracies, this section will treat four distinct types of calibration situations: 

To cover the complexity of the calibration process in going from the highest to the lowest 

a) Situation 1 : Calibration of high-accuracy standards using an intrinsic reference standard or system. 

b) Situation 2: Calibration of secondary standards in circumstances where attainment of a four-to-one ratio 

between the manufacturer’s stated accuracy of the secondary standard and that of the reference standard 

(other than an intrinsic standard or system) is made impossible by the demanding specificatioas of the 

secondary standard. 

c) Situation 3: The similar situation where working standards or instrumentation are being calibrated. 

d) Situation 4: The situation where a four-to-one ratio or higher is readily attainable. 

2.2.2.4 In laboratories where calibrations of the highest accuracies are performed, all four types may exist; 

in laboratories where instruments of only modest accuracy are calibrated, only the last is likely to be 

encountered. It should be noted that each of these situations requires skills and knowledge different from the 

others, and, therefore, demonstrated competence in one may not be sufficient to guarantee adequate 

performance in the others. 

2.2.3 Quality system 

2.2.3.1 

noted, and evidence pertaining to each should be available for review. 

Items specified in this section are minimum recommendations for accreditation for the applications 

2.2.3.2 Situation 1 (Intrinsic standard or system as a standard) 

a) General recommendations for monitoring measurement quality include: 

Eithedor: 

a. Direct intrinsic standard or system-to-intrinsic standard or system comparison with NIST 

or another accredited laboratory under conditions set forth in Situation 1 under 

Measurement Traceability and Calibration 

b. Indirect system-to-system comparisons using appropriate transport standards (reduced 

uncertainty). 

Calibration history of device used to measure differences between intrinsic standard or system and 

unknown values. 

Calibration history of the intrinsic standard or system components (for example, the time base of 

the reference frequency counter in a Josephson voltage array system). 

Evidence of system to check periodically on system precision and stability (for example., leakage 

currents, ground loops, thermal emf s, step integrity, trapped magnetic flux., noise, and microwave 

power impinging on a Josephson voltage array). 

Laboratory environmental monitoring. 

NIST Handbook 150-2A 12 March 2004 
b) When an intrinsic standard or system is used for calibration of working standards (for example, Zener 

voltage standards calibrated with Josephson voltage array), additional recommendations include: 

1) Control charts on appropriate check standard(s). 

2) Data analysis on workload. 

c) When an intrinsic standard or system is used for the calibration of working standards requiring a 

secondary transfer of measurements (for example, calibration of standard cells and solid-state standards 

at 1.02 V using a Josephson voltage array), additional recommendations include: 

1) 

2) 

When an intrinsic standard or system is used for the calibration of test equipment (for example, digital 

voltmeters and calibrators up to 10 V), additional recommendations include: 

Control charts on secondary check standards (saturated standard cell bank used as check standard), 

Data analysis on workload and transfer standard (Zener voltage standard in the example). 

d) 

1) Control charts on test equipment and transfer standard used as check standards. 

2) Data analysis on workload. 

2.2.3.3 Situation 2 (Calibrations of secondary standards at accuracies where four-to-one 

manufacturer’s stated accuracy ratios cannot be met or exceeded) 

General recommendations for monitoring measurement quality include: 

a) An active process for maintaining check standards on a regular basis. 

b) Redundant measurements for statistical control of precision. 

c) A system for checking independence from operator effects. 

2.2.3.4 

manufacturer’s stated accuracy ratios cannot be met or exceeded) 

Situation 3 (Calibrations of working standards or instruments at accuracies where four-to-one 

General recommendations for monitoring measurement quality include: 

a) Single value check standards. 

b) Evaluation of scaling processes. 

c) Like-item check standards. 

d) 

2.2.3.5 

ratios are met or exceeded) 

A system for checking independence from operator effects. 

Situation 4 (Calibrations at accuracies where four-to-one manufacturer’s stated accuracy 

General recommendations for monitoring measurement quality include: 

NIST Handbook March 2004 150-2A 13 
a) A history of the standard. 

b) A system to ensure the integrity the of standard (mechanism for ensuring no significant loss of accuracy 

due to transport, if applicable). 

c) A system for checking independence from operator effects. 

2.2.4 Personnel 

2.2.4.1 

one or more of the following: 

Those with technical responsibility for electromagnetic dc/low frequency calibrations should have 

a) Formal statistics training including concepts of randomness; understanding of results of t-tests, F-tests, 

and linear regression; rudiments of experimental design; and error analysis. 

b) Ready access to a statistician specializing in experimental data analysis. 

c) A permanent, trained quality-assurance specialist on the staff. 

2.2.4.2 The laboratory should provide training in elementary statistics to all personnel engaged in data 

reduction for Situations 1,2, and 3. Technicians making measurements need a good understanding of basic 

electrical principles and those factors which influence measurements and measurement results. 

2.2.4.3 The intrinsic standard or system operator should be trained on a NIST intrinsic standard lor system 

or one in an accredited laboratory, with 3 or 4 days of hands-on experience, and should have expertise in 

electronics, cryogenics (if appropriate), and computer usage. Professional support should include someone 

with strong physics background and experimental skills. 

2.2.4.4 Personnel responsible for calibrations of secondary and working standards and instruments of the 

highest accuracy should have good experimental skills and understand low-level measurements, including 

factors affecting measurement sensitivity and accuracy. 

2.2.4.5 In addition to the provisions listed above under secondary and working standards, personnel 

responsible for calibration of lower level standards and test equipment should understand the metrological, 

chemical, and thermal properties ofthe items under test and their interaction with circuits and instrumentation. 

2.2.5 Accommodation and environmental conditions 

2.2.5.1 Environment 

The environment (acoustic noise, dust, lighting, relative humidity, temperature, and vibration) of the 

calibration laboratory should be controlled according to applicable provisions of NCSL Recornmended 

Practice RP-7, with the exception that relative humidity should have a maximum value of 55 % (see 1.4.1 j)). 

Temperature and relative humidity should be recorded on a continuous or sampled basis; records of abnormal 

conditions (as a minimum) should be retained for a period to be determined by the laboratory, but no less than 

one year. 

2.2.5.2 Electrical power 

The quality of electric mains supplying power for instrumentation should be in compliance with applicable 

NIST Handbook 1.50-2A 14 March 2004 
provisions of NCSL Recommended Practice RP-7; or alternatively, these provisions may be met through the 

use of uninterruptible power supplies. 

2.2.5.3 Grounds 

For circumstances outlined above, a dc instrumentation ground is required. This ground should meet or 

exceed the specifications given in NCSL Recommended Practice RP-7. 

2.2.5.4 Electromagnetic interference 

Electrical and magnetic fields should be controlled to be in compliance with the provisions of NCSL 

Recommended Practice RP-7. 

2.2.6 Measurement traceability 

2.2.6.1 Situation 1 

a) For circumstances requiring measurement uncertainties approaching that of NIST, an evaluation transfer 

employing a portable intrinsic standard or system to compare the system's measurements with those of 

the system used at NIST for the U.S. representation or to another, similar intrinsic standard whose use 

has been accredited should be made: 

1) Prior to initial accreditation; 

2) Upon the change of personnel responsible for the operation of the system; 

3) Following any significant change in the system structure; and 

4) Immediately following any measurement problem revealed by check standard data and riot proven 

to be a problem inherent to the check standard(s). 

b) For circumstances requiring measurement uncertainties in a range sufficiently above the NIST 

uncertainty, evaluation transfers using well-characterized transportable transfer standards to ccimpare the 

system's measurements with those of the NIST system used for the U.S. representation, or another, 

similar intrinsic standard whose use has been accredited, may be used. 

2.2.6.2 Situation 2 

a) Traceability should be established for the reference standard through the use of a carefully evaluated 

transport standard to be calibrated at NIST or at an accredited laboratory having an intrinsic standard or 

system as a reference standard. Except under extraordinary conditions, the reference standard should 

not be removed from the laboratory and should be maintained continuously at operational environmental 

conditions to sustain traceability. 

b) Calibration transfers should be made at least annually for the first 5 years of use of the reference standard 

or until a sufficient history of data can be accumulated to support the use of a longer interval between 

transfers. 

c) After an appropriate data history is accumulated, a statistical model of the behavior of the reference 

standard should be developed and used to assign its values and their uncertainties. This model is to be 

NIST Handbook 150-2A 15 March 2004 
verified on a regular basis via calibration transfers and by rigorous analysis of the behavior of the 

reference standard, the transport standard, and the check standards from both statistical and metrological 

points of view. This process, with a regular re-evaluation of all components of measurement uncertainty 

involved, should constitute traceability for this situation. 

2.2.6.3 Situation 3 

a) Provisions of Situation 2 are applicable. 

b) Additionally, provisions for scaling to the limits of parameters covered from those provided by the 

reference or secondary standard should be subjected to the same types of calibration and statistical 

control with the exception that scaling techniques or standards may be verified by self-comparison or 

“boot-strap” techniques, where appropriate, in lieu of extra-laboratory calibrations. The use of such 

techniques should be rigorously and formally analyzed and the analyses documented. 

2.2.6.4 Situation 4 

The reference standards should be calibrated on a regular basis at NIST or by any accredited calibration 

laboratory with capability adequate to sustain the uncertainties required of them. 

2.2.7 Test and calibration methods and method validation 

2.2.7.1 The measurement methods that are used should eliminate or take into account the followiing effects 

at a level of precision sufficient to comply with the requirements of the calibration: thermal emf 5;; leakage 

currents; electrostatic interference; electromagnetic interference; instrument offsets; and short-term 

temperature, relative humidity, and air pressure fluctuations. 

2.2.7.2 There are essentially three approaches to calibration under Situations 2 through 4, depending on the 

level of uncertainty to which the metrologist is working. These approaches are independent of the physical 

or parametric characteristics of the test item(s). The three approaches are: 

a) Direct intercomparison of standard(s) and unknown(s). 

b) Measurement of unknown(s) on a system characterized against standard(s). 

c) Measurement of unknown(s) using standard(s) with smaller uncertainties (four-to-one rule olf thumb). 

2.2.7.3 This section describes each approach and provides guidance concerning the expression of the 

resulting total uncertainty. The aim is to provide insight into the process by which an assessor can determine 

whether or not the approach is valid for any given application, and then to assess whether or not the stated 

uncertainty and its associated traceability have been satisfactorily achieved. 

a) Direct intercomparison 

1) Some typical measurement parameters using this approach include: capacitance (using ac: bridges), 

voltage (using direct opposition), and resistance (using current comparator bridges). In thi s process, 

the unknown is measured either directly or indirectly (using correction factors) with respect to the 

known value of the standard(s). The degree to which the standard is actually known, along with 

transfer uncertainties arising from the act of measurement, determines the uncertainty in 

measurement of the unknown. This technique is prevalent in higher echelon laboratoriles such as 

NIST Handbook 1.50-2A 16 March 2004 
those obtaining values for their standards from NIST, and usually results in the achievement of a 

total measurement uncertainty relatively close to that which NIST reports on its certificate as total 

NIST uncertainty. This is due to the fact that the performing laboratory goes to great lengths to 

quantify Type A (evaluated by statistical methods) and Type B (evaluated by other means) 

uncertainties through a rigorous program of statistical process control (SPC) involving much data 

gathering and reduction. The comparator is maintained in control through the use of control charts 

which are analyzed continuously as part of the measurement process (experiment). 

Critical to determining the suitability of this process is the understanding of the measurement 

scheme (multiple standards versus multiple unknowns, the ability of the measurement design to 

identify or eliminate measurement effects such as reversal error, left-right effect, etc.), and the 

statistical techniques employed (Student t and F testing, standard deviation computation, coverage 

factor used, etc.). 

It is important to state again that the process remains unchangedregardless of the parameter(s) being 

measured, and the assessor should have a clear understanding of the quantification of Type A and 

B uncertainties as they are combined into total uncertainty for the purpose of reporting. In all cases, 

the policy and procedures of the IS0 Guide to the Expression of Uncertainty in Measurement will 

be complied with. Finally, the total uncertainty will be specified using a coverage factor (of two on 

the report of calibration. 

An example of the quantification and enumeration of an uncertainty budget for this type of 

measurement is shown as Example H. 1, End-gage calibration, at Annex H of the IS0 Guide to the 

Expression of Uncertainty in Measurement. 

b) Unknowns measured on a characterized system 

NIST Handbook 150-2A 17 March 2004 

In this technique, a measurement system is characterized by a systematic analysis of the results 

which it produces when used to measure a standard or group of standards. In some cases, where it 

is determined that there are short term systematic errors present, these are often “calibrated” or 

adjusted out so that the system reads correctly when a standard is measured. One should be careful 

to ensure that the systematic error is quantifiable both in magnitude and direction. In many cases, 

such as with vector quantities (magnitude and phase) found in many ac measurements, the 

magnitude (real plus imaginary components) of the offset may not be subtracted arithmetically. In 

those cases, the system is verified to read within an uncertainty band before the system is certified 

to be capable of reading the unknown to the desired uncertainty. 

Having standardized the measurement system, the unknown(s) are measured taking enough 

redundant measurements to develop an estimate of system randomness which is added as a Type B 

component to the uncertainty band appropriate to the measurement system. 

These systems are usually comprised of several stimulus and measurement components which, 

though all candidates for individual calibration, usually are not maintained on a periodic recall 

schedule for calibration. If the system is documented properly, it should be readily apparent to the 

assessor that the individual items in fact do not require individual calibration as long as the system 

is maintained in measurement process control through the use of reference and check standards and 

appropriate control chart analysis and corrective action. If this is the case, the appropriate 

components should be identified as not requiring periodic calibration, and that they are verified 

upon performance of the system standardization process. 
4) As in the first case, all levels of management should be committed to the chosen rnethod of 

standardization and uncertainty determination as clearly specified in the quality manual. It should 

be explainable to the assessor, and all control charts and other supporting measurement diita should 

be readily available for on-site assessment. 

c) Test accuracy ratio or test uncertainty ratio methods 

In this process, an unknown used to make measurements of relatively large specified uncertainty, 

referred to as the “tolerance,” is verified to perform within that tolerance by comparison with a 

standard or standards capable of much smaller measurement uncertainty compared to the tolerance 

(at least 1/4 the tolerance of the unknown). The probability of incorrectly measuring the unknown 

without correcting for the uncertainty component of the standard, as being either within tolerance 

when it is in fact not, or out of tolerance when it is actually within, is very low (unlikely) when the 

standard(s) is at least four times less uncertain than the tolerance of the unknown. This is referred 

to as a test accuracy ratio (TAR) of four-to-one; even better is a TAR of ten-to-one in term:; of safety 

in reporting the measured value of the unknown without applying any corrections for the uncertainty 

of the standard(s). 

Using a test uncertainty ratio (TUR), the uncertainty of the measurement must be 4 times better (or 

more) than the measurement uncertainty of the instrument being calibrated. 

Some examples of this type of testing are calibrations of high uncertainty working standards (1 % 

to 10 % as opposed to 0.1 % or 4 x etc.) such as power setting attenuators, multifunction 

meters, calibrators (some are more accurate than others), etc. The assessor should be made fully 

aware of the measurement requirement by statements in the quality manual. In addition, valid’test 

procedures should contain, or be accompanied by, easily understood matrices showing the relative 

uncertainties of the measurement of the device under test and the specified accuracy of stimulus or 

measurement devices (test and measuring equipment) used. 

In assessing competence in this field of calibration, particular attention should be given to the 

calibration status of measuring and test equipment (M & TE) being used, documentation and 

approval of calibration procedures, technical qualifications and training of the metrologist(s) and 

documentation of test results. It is impossible to verify to the assessor’s complete satisfaction every 

possible combination of measurement parameters (e.g., voltage vs. frequency vs. voltage range vs. 

frequency range, etc.) that exist for all the different types of devices that may be encountered in a 

typical calibration laboratory. It is important that the assessor become convinced that the 

measurement philosophy as dictated by the quality manual is well established throughout the 

laboratory under evaluation, so that by sampling a representative number of calibration stations, 

confidence in the competency of the total operation is achieved. 

Unique areas of concern 

The following sections (2.3 through 2.15) cover each of the metrology disciplines that comprise the 

electromagnetic dc/low frequency area, and provide specific guidance for assessors and laboratoriies. 

NIST Handbook 150-2A 18 March 2004 
2.3 Direct voltage 

2.3.1 References 

EN 45001: General criteria for the operation of testing laboratories (1989). 

NCSL Recommended IntrinsicDerived Standards Practice RISP- 1 : Josephson Voltage Standard (1 993). 

Morrison, R., Grounding and Shielding Techniques in Instrumentation, John Wiley & Sons, New York, 

1967. 

Bertone, G. A., et al, “Elimination of the Anomalous Humidity Effect in Precision Capacitance Based 

Transducers,” IEEE Trans. Instrum. Meas., 40, (6), pp. 897-901, Dec. 1991. 

Braudaway, D. W., “Standards Laboratory Environments,” Sandia Report, Sand90-1962 UCC90. 

Field, B. F., “Solid-state Voltage Standard Performance and Design Guidelines,” NBS Tech. Note 1239, 

September 1987. 

Field, B. F., “Solid-state DC Voltage Standard Calibrations,” NBS Special Publication 250-28, 1988. 

Field, B. F., “Standard Cell Calibrations,” NBS Special Publication 250-24, 1987. 

2.3.2 Equipment 

2.3.2.1 Table 1 gives an indication of the standards available to support dc voltage calibration processes 

and their best accuracy, range of use, and general application. A number of such standards types may be used 

in a single calibration system to support a variety of dc voltage accuracy levels. 

2.3.2.2 Because of the large range of voltages covered by typical voltmeters, measurement systems based 

on Josephson arrays, standard cells, or solid-state standards must make complementary use of accurate means 

of scaling in order to provide full calibration support. Scaling techniques in general need not be traceable to 

national standards; on the other hand, to use them properly demands a high degree of knowledge and skill on 

the part of the laboratory staff, especially for measurements at the highest accuracies. These techniques are 

not generally useful to sustain a production calibration process directly because of the time consumed in their 

execution. They are universally used for the calibration of a working standard-a precision dc calibrator or 

digital voltmeter-which is then used to calibrate a large workload. The frequency of reverification of this 

working standard depends on the accuracy demands of the calibration workload and the performance with 

time of the working standard itself. 

NIST Handbook 150-2A 19 March 2004 
Table 1. 

Examples of standards, uncertainties, ranges, and applications for dc voltage calibration processes. 

Best Specified 

Uncertainty, Value 

<0.5 x at 1.02 V 

<0.5 x at 10 V 

2 x at 10 V 

4 x at 10 V 

Range 

200pV to 2.5 V for 1 V 

array; 200 pV to 11 V 

for 10 V array 

1.02 v 

lVtolOV 

(one to four fixed 

voltage values) 

0.1 V to 1 kV 

10mVto 1 kV 

1.02 v 

Application 

Primary (intrinsic) standard for 

calibration of highest-accuracy 

artifacts (solid-state standards, 

standard cells), and linearity of 

precision digital voltmeter:; and 

calibrators. 

Reference standard for 

calibration of other cells, sldidstate standards, and instrunnents 

(at one point). 

Calibration of calibrators and 

meters; transport standards for 

intercomparison of banks of 

standards (cells or solid-state 

standards); intermediary and 

check standards for Josephson 

arrav svstems. 

- 

- 

Calibration of large populations 

of digital meters, either 

automaticallv or manuallv. 

Calibration of less accurate 

DVM's or accurate source:;; 

calibration of thermometers, 

other sensors. 

Calibration of low-accuracy 

meters; reference for classical 

potentiometers. 

- - 

- - 

2.3.2.3 Saturated standard cells are extremely sensitive to temperature level and temperature changes. For 

measurements of the lowest uncertainties, temperature level effects should be compensated by converting the 

measured emf of the standards to that at a nominal temperature using the Wolff International Temperature 

Formula, 

E (e) = E (20) - 40.6 x (e - 20) - 0.95 x (e - 20)~ + 0.01 x (e - 20)~ 

where E is the emf of a saturated standard cell and 8 is the temperature in degrees Celsius, providedl that the 

level change being compensated does not exceed 0.05 "C. This serves to eliminate or greatly reduce the 

effects of short-term temperature changes and allows the laboratory to better determine and control the 

behavior of the reference and check standards. 

NIST Handbook 150-2A 20 March 2004 
2.3.2.4 Because of the small reaction rates and large time constants of the chemical processes inside a 

standard cell, the cell’s emf and its short-term stability are functions of the temperature history of thLe cell for 

the past 60 days or longer. Accordingly, means should be provided to maintain the temperature constant 

within 0.005 “C on a continuous basis to ensure against deterioration of the cell’s performance for any 

measurements with required uncertainties of 51 x or better. 

2.3.2.5 Auxiliary equipment 

The auxiliary equipment used with a Josephson array voltage standard should at a minimum be 

comprised of equipment with performance equivalent to the items described in NCSL RISP-I, Chapters 

D and E (see 2.3.1 b)). 

Table 2 indicates reference standards commonly used for the Calibration of 10 V standards. Auxiliary 

equipment for use with a 10 V standard based on solid-state voltage standards for Calibration of likevalued standards includes the following items. A calibrated meter or potentiometer and null detector 

with a minimum uncertainty and resolution at the *1 x level, capable of discriminating 0. :I pV and 

having a precision of ‘r0.08 pV or better should be used to measure the differences between the emf s 

of standards and references. A means of monitoring the temperature of the reference devicei[s) at the 

0.1 “C level is required. 

Table 3 indicates reference standards commonly used for the calibration of 1.02 V standards. Auxiliary 

equipment for use with a 1.02 V standard based on saturated standard cells for the calibration of likevalued standards includes the following items. A calibrated meter or potentiometer and null detector 

with a minimum uncertainty and resolution at the k5 x 10” level, capable of discriminating 0.05 pV and 

having a precision of k0.05 pV or better should be used to measure the differences between the emf s 

of standards and references for calibrations at accuracy levels of *’2 x or better. FiDr lower 

accuracies these requirements are relaxed, but instruments capable of discriminating at least 0.1 pV 

should be used. A means of monitoring the temperature of the standard cells at the mK level of 

resolution and stability is required. 

Examples of typical auxiliary equipment for use with either 10 V or 1.02 V standards to perform 

calibrations throughout the range of voltages covered by the unit being calibrated, e.g., scaling or ratio 

standards, are given in Table 4. 

Auxiliary equipment for use with instrumentation standards for calibrating a workload comprised of 

instruments for measuring or supplying the range of voltages covered by this document includes: 

1) For the case where both the reference and the item under calibration are voltmeters, a source 

capable of supplying dc voltages throughout the calibration range with a one-hour short-term 

stability at a level such that its effect on the calibration is at a minimum 10 % of the uncertainty 

sought. 

2) For the case where the reference is a calibrator and the item being calibrated is a voltag,e source 

such as another calibrator, the laboratory should either use a voltmeter or potentiometer with 

sufficient accuracy that the uncertainty of the measurement of the difference between the voltage 

of the reference and the nominally equal voltage of the unit being calibrated is less than 10 % of 

the accuracy requirement of the calibration, or a voltmeter or potentiometer whose linearity and 

stability are such that the uncertainty of the comparison of the voltage from the reference and a 

nominally equal voltage from the unit being calibrated does not exceed 10 % of the uncertainty 

requirement of the calibration. 

NIST Handbook 150-2A 21 March 2004 
Table 2. Reference standards commonly used for the calibration of 10 V standards. 

Best 

Uncertainty 

20.1 x 

h0.5 x lov6 

Minimum 

Standards 

Required 

10 V Josephson array 

standard 

Four solid-state standards 

having individual control 

environments; or 

Two standards each 

comprised of three or more 

solid-state reference units, 

each set contained in an 

individual control 

environment; or 

Two groups of four each 

saturated standard cells, 

each group being 

contained in a separate 

oven, and an adequate, 

verifiable self-calibrating 

means of scaling to 10 V 

from 1.02 V 

IRemar ks 

A Josephson array staindard should be a system that 

includes one or more check standards at the 10 V 

level. 

Only the 10 V outputs of the solid-state standards 

should be used. 

The solid-state device,, amplifiers, ratio resistors, 

and other temperature-sensitive components upon 

which the stability of ii solid-state standard is based 

should be contained in a oven having provision for 

monitoring the stability of its temperature with 

adequate precision to ensure proper performance of 

the standard. The “control environment” also 

includes the power supplies for the above. 

Provision should also lbe made for battery back-up 

power in the event of a failure of the power mains. 

The saturated standard cell groups should each be 

contained in an oven capable of controlling the 

temperature of the group to be constant within 

0.002 K over a working day and within 0.01 K over 

a one-month period. The temperature should be 

capable of being monitored at the 0.001 K level of 

precision. These groups should not be used as ii 

reference standard for a period of two months 

following a loss of temperature control in the 

absence of evidence that emf recovery is complete. 

This evidence should include measurements 

comparing the cell emf’s with those of stable sources 

not subjected to the same or similar environmental 

perturbations. 

Neither group should be removed from the 

laboratory for purposes of calibration or traceability 

to support calibrations at uncertainty levels of 

*l x loF6 or better; traceability should be achieved 

with the use of a transport standard of adequate 

performance to support the required measurements. 

- 

NIST Handbook 150-2A 22 March 2004 
Table 2. (cont.) Reference standards commonly used for the calibration of 10 V standards. 

Best 

Uncertainty 

-12 x 

*40 x 

Minimum Standards 

Required 

Three solid-state standards 

each in individual control 

environments; or 

One set of three or more solidstate standards; or 

A dc voltage calibrator; or 

One group of at least four 

saturated standard cells in an 

oven and an adequate, 

verifiable self-calibrating 

means of scaling to 10 V from 

1.02 V; or 

A digital voltmeter 

Three solid-state standards 

packaged individually, used at 

10 V; or 

One set of three or more solidstate standards, used at 10 V; 

or 

One group of at least four 

saturated standard cells in an 

oven and an adequate, 

calibrated means of scaling to 

10 V from 1.02 V 

Remarks 

The above requirements apply. 

The calibrator should be adequately specified and 

should be supported by the use of check standards 

for uncertainties better than *12 x 

The voltmeter should be adequately specified and 

should be supported by the use of check standards 

for uncertainties better than *12 x 10". 

Neither ovens for controlling the temperature of 

critical components nor back-up battery power is 

required for the use of solid-state standards. 

Oven stability requirements for saturated standard 

cells are relaxed to h0.05 K, for both short- and 

long-term requirements. 

NIST Handbook 150-2A 23 March 2004 
Table 3. Reference standards commonly used for the calibration of 1.02 V standards. 

Best 

Uncertainty 

I rto.l x 

i0.3 x 

h1 x 

Minimum Standards 

Required 

A Josephson array 

Two groups of four each 

saturated standard cells, 

each group being 

contained in a separate 

oven 

Two groups of four each 

saturated standard cells, 

each group being 

contained in a separate 

oven 

~ 

Remarks 

A Josephson array standard should be a system that 

includes one or more check standards at the 1.02 V 

level. 

The saturated standard cell groups should each be 

contained in an oven capable of controlling the 

temperature of the group to be constant within 

0.002 K over a working day and within 0.01 K over a 

one-month period. The temperature should be 

capable of being monitored at the 0.001 K level of 

precision. These groups should not be used as a 

reference standard for a period of two months 

following a loss of temperature control in the absence 

of evidence that emf recovery is complete. This 

evidence should include measurements comparing the 

cell emf s with those of stable sources not subjected 

to the same or similar environmental perturbations. 

Neither group should be removed f?om the laboratory 

for purposes of calibration or traceability to support 

calibrations at uncertainty levels of hl x lo-' or 

better; traceability should be achieved with the use of 

a transport standard of adequate performance to 

support the required measurements. 

One of the groups may be used as a transportable 

standard for traceability purposes. 

Standard cell oven perforrnance requirements are 

relaxed to hO.0 1 K for one-day performance and to 

rt0.02 K for one month. 

NIST Handbook 150-2A 24 March 2004 
Table 3. (cont.) Reference standards commonly used for the calibration of 1.02 V standards. 

Best 

Uncertainty 

*1 x 

*4 x 

-17 x 

Minimum Standards 

Required 

Four solid-state standards 

having individual control 

environments and an adequate, 

verifiable self-calibrating 

means of scaling from 10 V to 

1.02 V; or 

Two standards, each comprised 

of three or more solid-state 

reference units, each set 

contained in an individual 

control environment and an 

adequate, verifiable selfcalibrating means of scaling 

from 10 V to 1.02 V 

One group of at least four 

saturated standard cells in an 

oven; or 

Three solid-state standards, 

each in individual control 

environments; or 

One set of three or more solidstate standards in a control 

environment 

A dc voltage calibrator; or 

a digital multimeter 

~ ~~ 

Remarks 

Only the 10 V outputs of the solid-state standards 

should be used. 

The solid-state device, amplifiers, ratio resistors, 

and other temperature-sensitive components 

upon which the stability of the standard is based 

should be contained in a oven having provision 

for monitoring the stability of its temperature 

with adequate precision to ensure proper 

performance of the standard. 

The “control environment” also includes the 

power supplies for the above. 

Provision should aliso be made for battery backup power in the event of a failure of the power 

mains. 

The 1.02 V outputs of the solid-state standards 

may be used in lieu of an external scaling 

arrangement. 

These instruments should be used in conjunction 

with appropriate chleck standards for calibrations 

of instruments of 3: 15 x 1 0-6 uncertainty or 

NIST Handbook 150-2A 25 March 2004 
Table 3. (cont.) Reference standards commonly used for the calibration of 1.02 V standards. 

Best 

Uncertainty 

*160 x 

3~250 x 

Minimum Standards 

Required 

Three solid-state standards 

packaged individually; or 

One set of three or more 

solid-state standards; or 

One group of at least four 

saturated standard cells in 

an oven 

One group of at least three 

unsaturated standard cells 

]Remarks 

The 1.02 V outputs of'the solid-state standards may 

be used in lieu of an external scaling arrangement. 

Neither ovens for controlling the temperature of 

critical components nor back-up battery power is 

required for the use of solid-state standards. 

Oven requirements for saturated standard cells are 

relaxed to k0.05 K, fo'r both short- and long-term 

requirements. 

Unsaturated cells are inot in ovens. They should not 

be used as references during the three days 

immediately following a loss of laboratory 

environmental control. 

Current draw permanently reduces the emf s of 

unsaturated cells; intercomparisons among the 

group should be made prior to any calibration at 

uncertainty levels of k100 x or better. 

NIST Handbook 150-2A 26 March 2004 
Table 4. Scaling standards. 

I 

Type Ratio(s) 

Binary 

resistive 

divider 

Direct current 

comparator 

potentiometer 

Hamon devices 

Oto 1 

continuous 

in lo-' steps 

to 1 

continuous 

in lo-' steps 

10: 1 

100: 1 

Hamon-based 

reference 

divider 

I 

10: 1 

100: 1 

Kelvin-Varley 

divider 

0 to 1 

continuous 

in steps 

Resistance 

transfer 

standard 

Voltbox i : 1.5 

(Volt Ratio where i is 

Standard) multiples of 

i: 1 

where 

11i512 

Range 

0 to 20 v 

Oto 1ov 

Oto3V 

Oto 1ov 

(10: 1) 

(100 : 1) 

0 to 100 v 

0 to 1000 v 

0 to 750 V 

0 to 2300 V 

depending on 

resistance 

0 to 1200 v 

Min. Specified 

Ratio 

Uncertainty 

f 0.03 x 

(of input) 

f 0.03 x 

(of input) 

f 0.5 x 

f 0.01 x 

(10: 1) 

(100 : 1) 

f 0.2 x 

f0.5 x 

fO.1 x of 

input voltage 

f10 x 

Remarks 

Used for within-range 

linearity calibrations; must 

be used with a device with 

larger ratios to calibrate an 

instrument completely. 

See above. 

Precision resistors 

networks configurable in 

series, series/parallel, or 

parallel with high 

accuracy; for calibration 

of other ratio devices, selfcalibrating. 

Instrument; used directly 

for range calibrations of 

calibrators and voltmeters, 

self-calibrating. 

Used both for in-range 

linearity calibrations and 

range-to-range 

measurements; may be 

self-calibrating. 

Based on Hamon scaling; 

used for calibration of 

fixed-ratio dividers; may 

be used directly or with a 

Kelvin-Varley divider for 

workload measurements. 

Series resistive divider 

with fixed ratios chosen to 

be additive. 

NIST Handbook 150-2A 27 March 2004 
2.4 Capacitance - low frequency 

2.4.1 Scope 

This section gives specific technical criteria for measurements of capacitance in the frequency range from 

50 Hz to 20 kHz. 

2.4.2 References 

Hague, Bernard, Alternating Current Bridge Methods, sixth ed., London, Pitman & Sons, 197 1. 

Harris, Forest K., Electrical Measurements, New York, John Wiley & Sons, Inc., 1952. 

Kibble, B. P., and Raper, G. H., Coaxial AC Bridges, Bristol, Adam Hilger, Ltd., 1984. 

Cutkosky, R. D., “Techniques for Comparing Four Terminal-Pair Admittance Standards,” J. Res. Nat. 

Bur. Stand. C., 74C, (3 & 4), pp. 63-78, July-Dec. 1970. 

Shields, J. Q., “Absolute Measurement of Loss Angle Using a Toroidal Cross Capacitor,” IEEE 

Trans. Instr. Meas., IM-27, (4), pp. 464-466, Dec. 1978. 

Jones, R. N., “A Technique for Extrapolating the 1 KC Values of Secondary Standard Capacitors to 

Higher Frequencies,” NBS Tech. Note 21 0, Nov. 1963. 

Various manufacturers’ catalogs and specification sheets, particularlly from ESI, GENRAD, HewlettPackard, and QuadTech. 

Chang, Y. M., “NIST Measurement Assurance Program for Capacitance Standards at 1 KHz,” Nat. 

Inst. Of Stand. and Tech. (US.), Tech. Note 141 7, 19 pg., Mar. 1996. 

Waltrip, B. C. and Oldham, N. M., “Digital Impedance Bridge,” IEEE Trans. Instru. Meas., 44, (2), 

pp. 436-439, Apr. 1995. 

Chang, Y. M. and Tillett, S. B., “NIST Calibration Service for Capacitance Standards at Low 

Frequencies,” Nut. Inst. Of Stand. and Tech. (U.S.), Spec. Pub. 250-47, 77 pg., Apr. 1998. 

Koffman, A. D., Waltrip, B. C., Oldham, N. M., and Avramov-Zamurovic, “Capacitance and 

Dissipation Factor Measurements from 1 KHz to 10 MHz,” 1998 Nat’l. Con$ Stds. Labs. Workshop 

& Symposium (NCSL), Jul. 19-23, 1998, Albuquerque, NM, pp. 63-618, Jul. 1998. 

NIST Handbook 150-2A 28 March 2004 
2.4.3 Assuring the quality of test and calibration results 

2.4.3.1 

level normally measured or calibrated, regardless of whether the calibration is a comparison of an 

unknown capacitor with a standard capacitor or is the result of deriving capacitance from other impedance 

standards, unless the instrumentation used for the measurement or calibration has an uncertainty onefourth or smaller than the required measurement uncertainty. The check standards should be at least at the 

level of quality of the best items being calibrated, and preferably at a higher level of quality. The 

statistical results of these controls should indicate a process precision comfortably smaller than the 

Type A uncertainty so that the variability of the measuring system can be distinguished from that of the 

item being calibrated. 

The laboratory should have controls based on artifact capacitors (check standards) at each 

2.4.3.2 Environmental aspects of the laboratory capable of affecting the measurement uncertainty 

should be monitored on a continuous basis, and measurements relevant to a calibration should not be 

made within three days following a significant environmental disturbance. 

2.4.4 Accommodation and environmental conditions 

2.4.4.1 

such a level that the changes of capacitance resulting from temperature changes during the measurement 

process do not contribute in excess of 10 % of the stated (or required) measurement uncertainty. 

Approximate temperature coefficients of various types of capacitance standards are given below: 

The temperature of the standard capacitors being used and/or calibrated should be controlled at 

Approximate Temperature 

Coefficient 

Dielectric (micro farad/farad/K) Remarks 

Fused silica 12*2 

Dry nitrogen 

(Sealed) 

Air 

2*2 Suffers from temperature 

hysteresis 

10 to 20 Loss and capacitance affected 

by humidity 

Mica 35 f 10 

2.4.4.2 

large changes in temperature create non-restorable changes in capacitance. These effects are generally not 

serious in air and mica capacitors because of their limited accuracies but can be serious in nitrogendielectric capacitors. Therefore, the latter should be protected from large: changes in ambient temperature 

by being kept in temperature-lagged enclosures such as the foam boxes in which they are supplied. 

Extreme care should be taken in transporting them. 

The bottom three types of capacitors are subject to temperature hysteresis at some level; i.e., 

NIST Handbook 150-2A 29 March 2004 
2.4.4.3 

cantilever when the capacitor is placed on its side. Gravity may then cause significant dimensional 

changes in the capacitor’s structure which can affect its capacitance. Because of the nature of the design, 

there is no equivalent force to restore its prior dimensions when the capacitor .is again upright. Such 

standards should always be stored, carried, and used in an upright position. 

Typical nitrogen-dielectric standard capacitors have a support structure that becomes a 

2.4.4.4 

shock or vibration. The existing empirical data shows that changes as lairge as 20 x can result from 

relatively small shocks under controlled conditions; changes on the order of 200 x 1 0-6 or more have been 

seen between calibrations and are presumed to be the result of shock and vibration that occurred during 

shipment. 

A significant minority of this type of capacitor is sensitive to physical disturbances, either 

2.4.5 Equipment 

2.4.5.1 

to other impedance standards, or combinations of impedance standards. In any case, the standards that are 

used should have values with sufficiently low uncertainty to support the uncertainties of the measurements 

or calibrations being carried out. The standards should have been calibrated at each frequency at which 

they are used. If this cannot be done, the laboratory should provide the analysis from which the 

capacitance values are obtained and the documentation of the uncertainty of such values. 

Capacitance measurements and calibrations may be made referenced to standard capacitors or 

2.4.5.2 

measured, or have a calibrated system for scaling to cover the entire range of values calibrated from a few 

standards (since one standard is insufficient to check the scaling capability). 

The laboratory should maintain standards covering each level of capacitance calibrated or 

2.4.5.3 

instruments (LCR meters, for example) are designed to make four-terminal or four-pair-terminal 

measurements. The laboratory should have jigs to be used for the calibration of such instruments or for 

their use in making two- and three-terminal measurements. These jigs should be analyzed and the results 

of the analyses used to correct for lead and shielding effects when making these types of measurements. 

These analyses should be available for review. 

Standard capacitors are most commonly of two- or three-terminal design whereas newer 

2.4.5.4 

standards. The de facto standard for this is the General Radio - GENRAII - QuadTech - ESI nitrogen 

dielectric parallel-plate capacitance standard whose dissipation factor is taken to be 0 f 

factor measurements should be carefully checked. 

At the present time, dissipation factor calibrations are not available at NIST for high-accuracy 

Dissipation b 

2.4.6 Test and calibration methods and method validation 

Calibration procedures for standard capacitors should show the background method for deriving 

capacitance from other impedance standards unless the calibration is a cclmparison of two or more 

capacitors. In the latter case, the type of measurement technique that is used should be clearly described, 

including shielding, grounding and other considerations. 

- 

NIST Handbook 150-2A 30 March 2004 
2.4.7 Handling of test and calibration items 

2.4.7.1 

large temperature excursions and physical shock and vibration. See 2.4.4 above. 

Standard capacitors of the nitrogen-dielectric, parallel-plate type should be protected from 

2.4.7.2 

environment controlled to within 0.01 K of a nominal fixed temperature to reach their potential accuracy 

level. 

Standard capacitors having fused-silica dielectric are extremely stable, but should be kept in an 

2.4.8 Reporting the results 

2.4.8.1 

verification should include: 

In addition to the requirements of ISO/IEC 17025, certificates or reports of calibration or 

a) The value(s) of measured capacitance, 

b) The frequency at which the measurements are made, 

c) The ambient temperature at the time of the measurement, and 

d) (Optionally) the dissipation factor or loss. 

2.4.8.2 

terminal, four-terminal, or four-pair terminal measurement. 

The certificate or report should also clearly state whether the result is of a two-terminal, three2.4.8.3 

of the jig or fixture used to modify the terminals should be given, unless the calibrations are at such a 

frequency and uncertainty level that the results are not affected by the use of the jig, in which case this 

should be clearly stated. 

If the measurement type does not match the terminal type of tjhe calibrated item, a description 

2.4.8.4 The description should include both the physical and electrical characteristics of the jig. 

2.5 Inductive voltage dividers (decade transformer dividers) - low frequency 

2.5.1 Scope 

This section describes specific technical criteria for calibrations and use of inductive voltage dividers 

(IVDs) in the audio-frequency range. 

NIST Handbook 150-2A 31 March 2004 
2.5.2 References 

Fink, Donald G. and Beaty, H. Wayne (eds.), Standard HandbookJw Electrical Engineers, 

Thirteenth Ed., New York, McGraw Hill, 1993. 

Lisle, R. V., and Zapf, T. L., “Comparison Calibration of Inductive Voltage Dividers,” ISA 

Transactions, 3, (3), July 1964. 

Sze, W. C., Dunn, A. F., and Zapf, T. L., “An International Comparison of Inductive Voltage Divider 

Calibrations at 400 and 1000 Hz,” IEEE Trans. Instr. Meas., IM-14, (3), pp. 124-131, Sept. 1965. 

Sze, W. C., “Comparator for Calibration of Inductive Voltage Dividers from 1 to 10 Hz,” ISA 

Transactions, 6, (4), pp. 263-267, 1967. 

Deacon, T. A., and Hill, J. J., “Two-Stage Inductive Voltage Dividers,” IEEE Proc., 115:272, 1968. 

Sze, W. C., “An Injection Method for the Calibration of Inductive Voltage Dividers,” J. Res. Nat. 

Bur. Stand. C., 72C, pp. 49-50, Jan.-Mar. 1968. 

Homan, D. N., and Zapf, T. L., “Two Stage, Guarded Inductive Voltage Divider for Use at 100 kHz,” 

ISA Transactions, 9, (3), pp. 20 1-209, 1970. 

Various manufacturers’ catalogs and specification sheets, particularly from Dytronics, ESI, Gertsch, 

and Tegam. 

2.5.3 Assuring the quality of test and calibration results 

For the calibration of NDs, a second divider should be employed as a check standard and backup. While 

NDs are extremely stable, a second ND with a known calibration history permits identifying and 

correcting problems with the injection circuit used to balance the bridge formed by the standard and 

unknown dividers. 

2.5.4 Accommodation and environmental conditions 

Inductive voltage dividers should be protected against the influence of ac magnetic fields and should never 

be excited by direct current. 

2.5.5 Equipment 

Inductive voltage divider calibrations may be made relative to other inductive voltage dividers, specially 

designed transformers, resistive voltage dividers (at low frequencies), or capacitive voltage dividers. 

ND’s may be used as ratio arm pairs in impedance calibration bridges; therefore, these criteria may be 

required for assessment of calibrations of capacitance or inductance stanclards. 

NIST Handbook 150-2A 32 March 2004 
2.5.6 Test and calibration methods and method validation 

2.5.6.1 IVD corrections and uncertainties are typically expressed in tenns ofproportional parts ofthe input 

rather than the ratio. This has to be taken into account when the divider is used to provide a ratio which has 

to meet uncertainty specifications expressed in ratios or voltages. 

2.5.6.2 

effects of loading on the ratios of the divider. 

Calibration and use procedures for inductive voltage dividers should take into account the possible 

2.5.6.3 

the inductive voltage divider. 

Procedures should provide for means of identifying and correcting magnetization of the cores of 

2.5.6.4 Procedures should take into account the fact that the ratios associated with the dial settings of 

single-stage dividers (especially for the three most significant dials) are not additive. The ratio associated 

with any given setting of a dial depends on the settings of the less significmt dials. Accordingly, one should 

increase the uncertainty of the divider ratio when more than one dial has a rion-zero setting. Most IVDs 

manufactured in the U.S. are of single stage construction. Two-stage dividers are characterized by having two 

sets of input terminals. 

2.5.6.5 

are normally given as a fraction of the frequency of the applied signal. 

Procedures should call for strict observance of the input voltage restrictions of the dividers; these 

2.5.7 Handling of test and calibration items 

Inductive voltage dividers and transformers have no special handling requirements beyond those in the 

General Requirements for DC and Low Frequency Measurements and Calibrations. 

2.5.8 Reporting the results 

2.5.8.1 

of inductive voltage dividers should include: 

In addition to the reporting requirements of ISO/IEC 17025, certificates or reports of calibration 

a) Corrections or errors for both in-phase and quadrature components of ratio, and 

b) The input voltage and frequency. 

2.5.8.2 Multiple measurements should be made of at least a sampling of the settings having quadrature 

values of unusually high value, with some index of the range of their values reported as an indicator of the 

condition of the divider’s switches. 

- 

NIST Handbook 150-2A 33 March 2004 
2.6 Inductance - low frequency 

2.6.1 Scope 

This section describes specific technical criteria for calibrations of self- and mutual-inductance air-core 

standards and iron-core inductors in the frequency range from 50 Hz to 2:O kHz. 

2.6.2 References 

Hague, Bernard, Alternating Current Bridge Methods, sixth ed., London, Pitman & Sons, 1971. 

Fink, Donald G. and Beaty, H. Wayne (eds.), Standard Handbook for Electrical Engineers, Thirteenth 

Ed., New York, McGraw Hill, 1993. 

Zapf, Thomas L., “Calibration of Inductance Standards in the Maxwell-Wien Bridge Circuit,” J. Res. 

Nat. Bur. Stand. C., 65C, pp.183-188, Sept. 1961. 

Homan, D. N., “Some Techniques for Measuring Small Mutual Inductances,” J. Res. Nat. Bur. Stand. 

C., 70C, pp.221-226, 1966. 

Dunn, Andrew F. and Tsao, Hoi, “Ratio Comparisons of Impedance Standards,” IEEE Trans. Instr. 

Meas., IM-18, pp. 276-283, Dec. 1969. 

Hanke, R. and Droge, K., “Calculated Frequency Characteristic of GR1482 Inductance Standards 

Between 100 Hz and 100 kHz,” IEEE Trans. Instr. Meas., 40, pp. 893-896, Dec. 1991. 

Recommended Practice RP-52.1, Recommended Environments for St<andards Laboratories, Instrument 

Society of America, Research Triangle Park, NC, 1975. 

Various manufacturers’ catalogs and specification sheets, particularly from ESI, GENRAD, HewlettPackard, and QuadTech. 

2.6.3 Assuring the quality of test and calibration results 

2.6.3.1 The laboratory should have controls based on artifact inductors (check standards) at nominal 

values for each range of inductance normally measured or calibrated, regardless of whether the calibration 

is a comparison of an unknown inductor with a standard inductor or is the result of deriving inductance from 

other impedance standards, unless the instrumentation used for the measurement or calibration has an 

uncertainty one-fourth or less than the required measurement uncertainty. The check standards should be at 

least at the level of quality of the best items being calibrated, and preferably at a higher level of quality. The 

statistical results of these controls should indicate a process precision comfortably smaller than the Type A 

uncertainty so that the variability of the measuring system can be distinguished from that of the item being 

calibrated. 

NIST Handbook 150-2A 34 March 2004 
2.6.3.2 Environmental aspects of the laboratory capable of affecting the measurement uncertainty should 

be monitored on a continuous basis, and inductance measurements relevant to a calibration should not be 

made within 24 hours following a significant environmental disturbance. 

2.6.4 Accommodation and environmental conditions 

2.6.4.1 The temperature ofthe standard inductors being used andor calibrated should be controlled at such 

a level that the changes of inductance resulting from temperature changeis during the measurement process 

do not contribute in excess of 10 % of the stated (or required) measurement uncertainty. (Note: the 

temperature coefficient of inductance is of the order of 30 x K.) 

2.6.4.2 For the calibration or measurement of standard inductors at thle highest accuracy, a stand should 

be provided such that the standard is more than 50 cm from the nearest rnagnetic material while it is being 

measured. 

2.6.5 Equipment 

2.6.5.1 Inductance measurements and calibrations may be made referenced to standard inductors or to 

other impedance standards, or to combinations of impedance standards. In ;my case, the standards used should 

have inductance values of sufficiently low uncertainty to support the uncertainties of the measurements or 

calibrations being carried out. The standards should have been calibrated at each frequency at which they are 

used. If this cannot be done, the laboratory should provide the analysis from which the inductance values are 

obtained and the documentation of the uncertainty of such values. 

2.6.5.2 The laboratory should maintain standards covering each level of inductance calibrated or 

measured, or have a calibrated system for scaling to cover the entire range of values calibrated from a few 

standards. (One standard is insufficient to check the scaling capability.) 

2.6.5.3 Standard inductors are most commonly oftwo- or three-terminal design whereas newer instruments 

(LCR - inductance (L), capacitance (C), resistance (R) - meters, for example) are designed to make fourterminal or four-terminal-pair measurements. The laboratory should use jigs for calibrating such instruments 

or for making two-terminal measurements. These jigs should be analyzed and the results of the analyses used 

to correct for lead and shielding effects when making these types of measurements. These analyses should 

be available for review. 

2.6.5.4 

instrumentation to control, measure, and report the current level used in such measurements. 

The inductance of iron-core inductors is current dependent. The laboratory should have adequate 

2.6.6 Test and calibration methods and method validation 

2.6.6.1 Calibration procedures for inductors should show the background method for deriving inductance 

from other impedance standards unless the calibration is a comparison of' two or more inductances. In the 

latter case, the type of measurement technique used should be clearly described, including shielding, 

grounding and other considerations. 

NIST Handbook 150-2A 35 March 2004 
2.6.6.2 Calibration procedures and methods for mutual inductance should show the principle of 

measurement and the derivation of equations used to obtain the mutual inductance from the measured values 

of inductance . 

2.6.6.3 

in the inductor being measured. 

Calibration procedures for iron-core inductors should include techniques for measuring the current 

2.6.7 Handling of test and calibration items 

Typical inductance standards are relatively robust and therefore do not require special handling. 

2.6.8 Reporting the results 

2.6.8.1 

or verification should include: 

In addition to the reporting requirements of ISO/IEC 17025, certificates or reports of calibration 

a) The value(s) of inductance measured, 

b) The frequency at which the measurements are made, 

c) The ambient temperature at the time of the measurement, and 

d) (Optionally) the equivalent series resistance. 

2.6.8.2 

terminal, four-terminal, or four-terminal-pair measurement. 

The certificate or report should also clearly state whether the result is of a two-terminal, three2.6.8.3 If the measurement type does not match the terminal type of the calibrated item, a description of 

the jig or fixture used to modify the terminals should be given, unless the calibrations are at such a frequency 

and uncertainty level that the results are not affected by the use of the jig, in which case this should be clearly 

stated. 

2.6.8.4 The description should include both the physical and electrical characteristics of the jig. 

2.6.8.5 

stated. 

If the inductor is of a iron-core type, the current level used during the calibration should also be 

2.6.8.6 If the inductor is a mutual inductor, the mutual inductance should be given. 

2.6.8.7 

Q, or the resonance frequency of the inductor. 

The report may optionally contain the equivalent series resistance, a value for the storage factor 

2.7 Fast electrical pulse waveforms 

2.7.1 Scope 

This section contains specific technical criteria that a laboratory should follow if it is to be recognized as 

competent to carry out fast electrical pulse waveform calibrations. These c:alibrations include those for pulse 

parameters, pulse time delay, and impulse spectrum amplitude. 

NIST Handbook 150-2A 36 March 2004 
2.7.2 References 

IEC Publications 469-1 and 469-2, Pulse techniques and apparatus, Part 1: Pulse terms and definitions; 

Part 2: Pulse measurement and analysis, general considerations. IEC, Geneva, Switzerland, 1974. 

IEEE Std. 18 1-2000, IEEE standard on transitions, pulses, and related waveforms, IEEE, New York, 

July 2003. 

IEEE Std. 376, IEEE standardfor measurement of impulsestrength aizd impulse bandwidth, 1975, IEEE, 

New York, 1975. 

“Time domain automatic network analyzer for measurement ofrf and microwave components,” Nat. Bur. 

Stand. (US.) Tech. Note 672, Sept. 1975. 

“Spectrum amplitude - Definition, generation andmeasurement,” Nat. Bur. Stand. (U.S.) Tech. Note 699, 

Oct. 1977. 

“Deconvolution of time domain waveforms in the presence of noise,” Nat. Bur. Stand. (US.) Tech. Note 

1047, Oct. 1981. 

“Impulse generator spectrum amplitude measurement techniques,” IEEE Trans. Instrum. and Meas., 

IM-25, (4), Dec. 1976. 

“The measurement and deconvolution of timej itter in equivalent-time waveform samplers,” IEEE Trans. 

Instrum. and Meas., IM-32, (l), Mar. 1983. 

Gans, W. L., “Dynamic calibration of waveform recorders and oscilloscopes using pulse standards,” 

IEEE Trans. Instrum. andMeas., IM-39, (6), Dec. 1990. 

Deyst, J. P., Paulter, N. G., Jr., Daboczi, T., Stenbakken, G. N., Souders, T. M., “A fast-pulse 

oscilloscope calibration system,Instrumentation and Measurement,” IEEE Trans. Instrum. and Meas. , 

IM-47, (5), pp. 1037-1041, Oct. 1998. 

Souders, T. M., Andrews, J., Caravone, A., Deyst, J. P., Duff, C., and Naboicheck, S., “A pulse 

measurement intercomparison,” IEEE Trans. Instrum. and Meas., IM-47, (5), pp. 103 1-1036, Oct. 1998. 

Paulter, N. G., and Larson, D. R., “Pulse Parameter Uncertainty Analysis,”Metrologia, 59, pp. 143-1 55, 

2002. 

2.7.3 Statistical process control 

The laboratory should possess and periodically calibrate at least one control standard for pulse parameters, 

at least one control standard for pulse time delay, and at least one control standard for impulse spectrum 

amplitude. 

2.7.4 Accommodation and environmental conditions 

All fast electrical pulse waveform calibrations should be performed under the following environmental 

conditions: 

NIST Handbook 150-2A 37 March 2004 
a) 

b) 

Laboratory ambient temperature nominally 23 “C, 

Laboratory ambient relative humidity between 30 % and 50 %, and 

c) Test system line voltage within f 3 % of nominal with less than 1 ’% total harmonic distortion (THD). 

2.7.5 Equipment 

2.7.5.1 All fast electrical pulse waveform calibrations should be performed on a system which consists, 

at the least, of a computer-controlled equivalent-time sampler. In addition, adequate standards should be used 

periodically to calibrate the system’s time base delay, gain, and linearity; voltage axis offset, gain, and 

linearity; and sampler dynamic response. 

2.7.5.2 

concerning calibration equipment may be directed to the appropriate NIST personnel. 

Because these calibration systems are still in a state of rapid change, unresolved questions 

2.7.6 Test and calibration methods and method validation 

2.7.6.1 

as possible to those described in the references. 

The methods utilized for all fast electrical pulse waveform calibrations should adhere as closely 

2.7.6.2 

concerning calibration methodology may be directed to the appropriate NIST personnel. 

Because these calibration methods are still in a state of rapid change, unresolved questions 

2.7.7 Reporting the results 

2.7.7.1 In addition to the reporting requirements of ISOAEC 17025, the calibration report should include: 

a) The laboratory ambient temperature, with tolerances if applicable, 

b) Relative humidity, with tolerances if applicable, and 

c) Line voltage, with tolerances if applicable. 

2.7.7.2 

and scale factors for both time and voltage axes clearly labeled. 

The calibration report should include a plot of all significant pulse waveforms with the dimensions 

2.8 Dc resistance 

2.8.1 Scope 

This section describes specific technical criteria for the calibration of dc iresistors. 

2.8.2 References 

a) Dziuba, R. F., Boynton, P. B., Elmquist, R. E., Jarrett, D. G., Moore, T. P., and Neal, J. D.,”NIST 

Measurement Service for DC Standard Resistors,” Natl. Inst. Stand. dt Tech., Tech. Note 1298,65 pages 

(U.S. Government Printing Office, Washington, DC, November 199:2). 

NIST Handbook 150-2A 38 March 2004 
Elmquist, R. E., Wood, B. N., Jaeger, K., “Quantized Hall Resistance Recommended Intrinsic-Derived 

Standards Practice,” National Conference of Standards Laboratories, Boulder, CO, Aug. 1997. 

Elmquist, R. E., and Dziuba, R. F.,“Loading effects in resistance scalling,” IEEE Trans. Instr. Meas. 46, 

(2), pp. 322-324, Apr. 1997. 

Jarrett, D. G., “Automated guarded bridge for calibration of multimegohm standards resistors from 

10 MQ to 1 TQ,” IEEE Trans. Instr. Meas. 46, (2) pp. 325-328, Aps. 1997. 

Boynton, P. B., Sims, J. E., and Dziuba, R. F., “NIST Measurement Assurance Program for Standard 

Resistors,” Natl. Inst. Stand. & Tech., Tech. Note 2434, 53 pages, Nov. 1997. 

Jarrett, D. G., “Evaluation of guarded high-resistance Hamon transfer standards,” IEEE Trans. Instr. 

Meas. 48, (2), pp. 324-328, Apr. 1999. 

Dziuba, R. F., and Kile, L. L., “An automated guarded bridge system for the comparison of 10 k!2 

standard resistors,” IEEE Trans. Instr. Meas., 48, (3), pp. 673-677, June 1999. 

Jarrett, D. G., Marshall, J. A., Marshall, T. A., and Dziuba, R. F., “Design and evaluation of a low 

thermal electromotive force guarded scanner for resistance measurements,” Rev. Sci. Instr. (Am. Inst. 

Phys.) 70, (6), pp. 2866-2871, June 1999. 

Delahaye, F., Witt, T. J., Elmquist, R. E., and Dzuiba, R. F., “Comparison of Quantum Hall Effect 

Resistance Standards of the NIST and the BIPM,” Metrologia, 37, pp. 173-176,2000. 

2.8.3 Accommodation and environmental conditions 

2.8.3.1 The temperature of a standard resistor should be controlled at a level such that changes of 

resistance of the standard resistor resulting from temperature changes during the measurement process 

contribute 10 % or less to the measurement uncertainty. 

2.8.3.2 The relative humidity in the laboratory should not exceed 45 ‘XI. 

2.8.3.3 

standard resistors (e.g., Thomas-type 1 Q) with measurement uncertainties of 0.5~ 

The barometric pressure in the laboratory should be monitored when measuring pressure-sensitive 

or less. 

2.8.4 Equipment 

2.8.4.1 The laboratory should maintain a local unit of resistance (traceable to the quantum Hall effect) 

with three or more standard resistors, one of which may be used as a check: standard during the measurement 

process. 

2.8.4.2 The laboratory should have the capability to scale from its local unit of resistance to other 

resistance levels that will encompass its range of calibration services. At these other resistance levels, the 

laboratory should maintain one or more standard resistors, one of which is used as a check standard during 

the measurement process. 

NIST Handbook 150-2A 39 March 2004 
2.8.5 Test and calibration methods and method validation 

2.8.5.1 

measurement run. 

An indication of the temperature of the resistor under test should be recorded during the 

2.8.5.2 

measurement uncertainties of 0.5 x 

should also be measured and recorded in order to determine the additional pressure effect on the resistor. 

The barometric pressure should be recorded when measuring pressure sensitive resistors with 

or less. If the resistor is immersed in a liquid, the depth of immersion 

2.8.5.3 

should be recorded. 

For voltage-sensitive resistors (e.g., film type), the test voltage applied across the test resistor 

2.8.5.4 For high-current resistors (shunts), the test current applied to the resistor should be recorded. 

2.8.5.5 A check standard should be used during each measurement run. 

2.8.6 Assuring the quality of test and calibration results 

For each measurement run, the laboratory should examine the difference between the check standard and its 

predicted value based on previous history to determine whether the measurement system is operating under 

statistical control. In addition, the standard deviation of a measurement run and the previous history of the 

test resistor (if available) should be examined to determine the quality of the measured value. 

2.8.7 Reporting the results 

The calibration certificateheport issued by a laboratory should contain the following: 

a) Resistor temperature, 

b) Effective pressure, if applicable, 

c) Relative humidity, if applicable, 

d) Test voltage, if applicable, and 

e) Test current, if applicable. 

2.9 Ac-dc difference - thermal voltage converters 

2.9.1 Scope 

2.9.1.1 This section contains specific technical criteria that a laboratory should meet in order to 

demonstrate competence in performing calibrations ofthermal voltage converters (TVCs). These instruments 

include: 

a) Coaxial thermal voltage converters, 

b) Battery-powered multirange thermal voltage converters, and 

NIST Handbook 150-2A 40 March 2004 
c) Mains-powered multirange thermal voltage converters. 

2.9.1.2 This section covers a voltage range of 0.5 V to 1000 V, and a frequency range of 5 Hz to 1 MHz. 

It may be of general use outside this parameter range, but the reader should seek specific guidance for 

measurements conducted beyond these ranges. 

2.9.2 References 

a) NCSL Recommended Practice RP-11: Reports and Certificates of Calibration, 1991 

b) ISA RP52.1, Recommended Environments for Standards Laboratories, Instrument Society of America, 

1975. 

c) SAND90-1962, Standards Laboratories Environments. 

d) Kinard, J. R. and Lipe, T. E., “Recharacterization of thermal voltage converters after thermoelement 

replacement,” IEEE Trans. Instrum. Meas., 38, pp. 35 1-356. 

e) Kinard, J. R, Hastings, J. R., Lipe, T. E., and Childers, C. B., “AC-DC Difference Calibrations,” Natl. 

Inst. Stand. & Tech., Spec. Publ. 250-27,308 pages, May 1989. 

f) Lipe, T. E., “A re-evaluation of the NIST low-frequency standards for ac-dc difference in the voltage 

range of 0.6 V - 100 V,” IEEE Trans. Instru. Meas., 45, (6), pp. 913-917, Dec. 1996. 

2.9.3 Assuring the quality of test and calibration results 

2.9.3.1 Sources of error or variability for the calibration should be monitored by the calibration of the 

subsystems involved in the calibration. These subsystems may include the voltage source(s) used in the 

calibration system, the detectors (voltmeters, for example) used to measure the output emf of the TVCs, and 

any auxiliary instruments (voltmeters, frequency counters, temperature sensors) used to monitor the functions 

of the system. In addition, check standards should be used to ensure that the calibration system is in 

statistical control. The laboratory should maintain some form of statistical process control in accordance 

with the uncertainty levels of the calibration; the lower the uncertainty of the calibration, the more stringent 

the controls required to ensure that the process is under statistical control. In general, if a laboratory is 

attempting to maintain uncertainties of less than four times the uncertainty quoted by NIST for a similar 

measurement, some sort of statistical process control is required. The frequency and thoroughness of check 

standards measurements should be commensurate with calibration uncerUainty levels. 

2.9.3.2 The laboratory should have control standards which adequately span the voltage and frequency 

ranges normally calibrated by the facility. Historical records of intercomparisons between control artifacts 

and between control artifacts and calibration standards should be kept, and all new data acquired from these 

intercomparisons compared to the historical record to ensure that the process is in statistical control. For 

thermal voltage converters, checks between control artifacts and calibration standards should be performed 

at least once per year, assuming that the devices are not damaged in any ’way. 

2.9.3.3 Should a thermoelement be damaged, or fail and be replaced, the laboratory should assign the acdc difference values of the new TVC in accordance with the uncertainties quoted by the laboratory. For 

quoted uncertainties comparable to the standard NIST uncertainties, the TVC should be intercompared to 

existing standards at appropriate voltages and frequencies to determine the ac-dc differences of the repaired 

TVC. For quoted uncertainties up to about four times the NIST uncertainties, a thermoelement replacement 

NIST Handbook 150-2A 41 March 2004 
algorithm (2.8.2 d)) may be employed to calculate the new corrections, with spot checks at one or two points 

to confirm the results of the calculations. For larger uncertainties, a tlhermoelement may in general be 

replaced without affecting the ac-dc difference. It is also worth noting that the ac-dc difference of a thermal 

voltage converter depends upon the characteristics of the thermoelement only at low voltages; at high 

voltage, the resistor makes the overwhelming contribution to the ac-dc diifference and its uncertainty. 

2.9.4 Accommodation and environmental conditions 

2.9.4.1 Thermal voltage converters are generally tolerant of gradual teimperature changes; nevertheless, 

the temperature of the calibration area should comply with ISA Rp52.1 or Sandia Report SAND90-1962. 

This requires a nominal temperature of 23 "C. In addition, the time derivative of the temperature should be 

as small as possible, less than 0.5 "C per hour. In general, fluctuations i:n temperature may cause thermal 

expansion of the resistors in a TVC. This affects high-voltage TVC ranges, where the change in capacitance 

between the resistor and the overall structure may lead to changes in the ac-dc difference, particularly at 

frequencies of 50 kHz and above. 

2.9.4.2 The relative humidity in the calibration area should comply with the requirements listed in ISA 

RP52.1 or Sandia Report SAND90-1962. These requirements include a nominal relative humidity of 

20 %-55 %. The time derivative of the relative humidity in the Calibration area should be kept small - less 

than 2 % per hour. Changes in relative humidity may cause changes in the characteristics of high-voltage 

wire-wound resistors in some TVCs, which may lead to variations in the ac-dc differences of the TVC. 

2.9.4.3 Reasonable care should be taken to avoid mechanical shock to TVCs. Excessive mechanical 

shockmay alter the geometry of the internal structure of a TVC, which in turn may lead to variations in ac-dc 

difference, particularly at high frequencies. In addition, TVCs that have been damaged by shock may show 

intermittent contact problems and pose a safety hazard if the shell of the TVC comes in contact with any 

high-voltage structures. 

2.9.4.4 Electrical power supplied to the calibration system should conform to the guidelines in ISA 

RP52.1 for voltage regulation. An uninterruptible power system or line conditioner may be required to 

achieve these standards. 

2.9.4.5 Thermal voltage converters are generally immune to electromagnetic interference; however, 

instruments that are operated under ac line power may be susceptible to noise along ground leads and are 

more likely to be affected by EMI. These considerations are addressed in ISA RP52.1. 

2.9.5 Equipment 

2.9.5.1 

standards. 

The laboratory should have the equipment necessary to intercornpare both working and reference 

2.9.5.2 The ac and dc voltage source(s) should be in calibration w'hen measurements of TVCs are 

performed. Instruments for detecting the thermoelement output (voltmeters, for example) should also be in 

calibration. 

2.9.5.3 Instruments (voltmeters, temperature probes, counters, oscilloscopes, etc.) for monitoring the 

ambient temperature of the calibration area and for monitoring the ac and d.c voltage and frequency supplied 

to the TVCs during a calibration should be functional and within specifications. 

NIST Handbook 150-2A 42 March 2004 
2.9.5.4 If possible, multiple TVCs should be available for use as reference or check standards. In 

addition to making intercomparisons of similar voltage ranges possible, rriultiple TVCs offer redundancy in 

the event of a failure of a TVC, or when a TVC is being calibrated by an external laboratory. 

2.9.5.5 If the calibration is controlled by and calibration data reduction performed by automated means 

(that is, by computer), the hardware and software should be thoroughly checked to ensure that the data 

derived by this system is accurate, and that the system itself does not introduce any bias into the 

measurements. In addition, software should have consistency checks to ensure that over-ranging the TVCs 

by accidentally entering an incorrect voltage does not occur, or that program loops do not increment the 

applied voltage beyond the range of the TVCs. 

2.9.6 Handling of test and calibration items 

2.9.6.1 

potential for structural or electrical damage. 

Thermal voltage converters should be stored and handled in such a way as to minimize the 

2.9.6.2 

to the measurement are sound enough to enable the instrument to perform as required. 

Instruments operating from batteries should be checked to ensure that the batteries appropriate 

2.9.6.3 The output of ac and dc calibrators may be lethal. The laboratory should be aware of this danger 

and take steps to avoid situations where contact with exposed high-voltage points may occur. This is 

particularly relevant for ac power amplifiers, which can deliver up to 160 mA and which may have only 

rudimentary protection. 

2.9.6.4 When voltages greater than 120 V are present in the calibraltion area, the laboratory should 

display an obvious warning to that effect. In addition, the TVCs may be isolated fkom the world by a box 

or covering, which makes contact with the voltage difficult. Such isolation will also be beneficial for 

temperature control. 

2.9.7 Test and calibration methods and method validation 

2.9.7.1 The laboratory should maintain documentation describing the calibration system, software, and 

procedures, as well as documentation concerning the determination of the uncertainty assigned to calibrations 

and procedures for performing check standard measurements. 

2.9.7.2 If a variety of instruments are calibrated at the laboratory, the laboratory should maintain 

documentation concerning testing methodology (connections, grounding, etc.) to supplement the user's 

manuals of the instruments. 

2.9.7.3 All ac-dc difference calibration results should by computed using the formula 

where 6 is the (uncorrected) ac-dc difference of the UUT, Q, is the difference between the outputs of the 

standard TVC and UUT with ac applied, and Qdc the difference between the: outputs of the standard TVC and 

UUT for the average of both polarities of dc voltage that produces the same output from the TVC as that 

produced by Q,. This convention states that the ac-dc difference is positive when more than the nominal 

ac voltage is required to match the average of the polarities of the dc reference voltage. 

NIST Handbook 150-2A 43 March 2004 
2.9.7.4 The polarity of the dc voltage should be reversed either by an isolated switch or by internal 

switching in the dc source. The dc voltage should never be reversed by connecting the shell of the input 

connector to high voltage. Not only is this a dangerous safety hazard, but this practice may also result in 

thermoelement bead failure. 

2.9.7.5 All measurements of ac-dc difference should be performed using the voltage sequence: ac, dc+, 

dc-, ac, dc-, dc+, ac or dc+, ac, dc-, ac, dc+. In either case, the averatge of both positive and negative 

polarities of applied dc voltage should be used to account for dc reversal error in the thermoelement. If no 

dc reversal is done, the uncertainty of the measurement should be expanded appropriately. 

2.9.7.6 All measurements should be made at equal time intervals after switching from ac to dc or dc to 

ac, or when reversing the dc voltage. This effectively cancels out any 1ine;ir drift in the TVCs. Waiting until 

the TVC output stabilizes (at possibly very different time intervals) introduces error from drift in the TVCs. 

2.9.7.7 The time required to switch voltages or polarities should be minimized so as to minimize the 

cooling of the TVCs while no signal is applied. A waiting period may be required for the TVCs to stabilize 

after switching, but this waiting period should be equal for every occurrence of switching. This time period 

may vary according to applied voltage and switch dead time. In addition, any safety delays built into voltage 

sources should either be disabled or accounted for in the waiting period. 

2.9.7.8 All leads connecting the voltage source(s) to the TVCs shouldbe as short as possible to minimize 

capacitance in the leads. Any structures (adapters, for example) required to connect the TVCs to the voltage 

leads should also be as short as possible. 

2.9.7.9 

application of voltage. For higher voltage ranges, quite a long warmup time may be necessary. 

A suitable warmup period should be observed in order for the TVCs to stabilize after the 

2.9.7.10 

one (and only one) of the voltage sources. 

For coaxial single-range TVCs, the outer shell of the input connector should be grounded through 

2.9.7.11 For multirange TVCs that are powered by a battery pack, the outer shell of the input connector 

should be grounded through one of the voltage sources. For some instnuments, this requires that the shell 

of the uhf-type input connector be grounded, but the dc ground stud left floating. For other instruments, the 

outer shell of the Type N input connector should be grounded. 

2.9.7.12 Grounding schemes for instruments calibrated under ac line power vary with the calibration 

system. In general, the ground should be made at the ground terminal of the instrument, and the supplies in 

the calibration system should be floated. The laboratory should be aware of the potential for common-mode 

signals entering the instrument and should take steps to minimize such signals. 

2.9.7.13 During a calibration, the ac supply and dc supply should be matched to within 50 pVN to ensure 

that any ac-dc difference arising from source mismatch will be negligible. Records ofthe voltage adjustment 

are useful in monitoring the performance of the sources. 

2.9.7.14 The ambient temperature should be recorded at some point in the calibration procedure. 

2.9.8 Records 

2.9.8.1 All measurements should be recorded, either manually or by automatedmeans. Corrections which 

are applied manually should be checked against a master correction table to ensure accuracy. Information 

NIST Handbook 150-2A 44 March 2004 
concerning the unit under test (UUT), standard TVC, voltage ranges of the test and standard TVCs, applied 

voltage, and the raw data, correction and final number at a given frequency should be recorded. In addition, 

the ambient temperature of the calibration area should be recorded. 

2.9.8.2 

and maintained for the life of the check standard or calibration artifact. 

Information concerning check standards or range-to-range intercomparisons should be recorded 

2.9.8.3 

at least 3 years. 

Calibration information on individual TVCs should be maintained as long as practicable, but for 

2.9.9 Reporting the results 

2.9.9.1 

Practice RP-11 as well as ISOAEC 17025. 

All content of reports or certificates of calibration should c'onform to NCSL Recommended 

2.9.9.2 

may include some or all of the following: 

The uncertainty should be derived from a detailed analysis of the measurement system, which 

a) Type B uncertainties in thermal voltage converters: 

1) Uncertainty in calibration of primary standards or reference artifacts, including uncertainty 

quoted from calibration of these standards by a higher-echelo-n laboratory, 

2) Uncertainty in range-to-range intercomparisons between reference and working standards or in 

voltage of frequency build-up measurements between adjacent voltage or frequency ranges, 

3) Effect of short-term drift in the TVCs, 

4) Effect of long-term stability of the TVCs, and 

5) Effect of dc reversal error in TVCs, if not measured directly. 

b) Type B uncertainties in measurement system: 

1) Uncertainty in voltage output from ac and dc voltage source(s), 

2) Uncertainty in applied frequency, 

3) Uncertainty in detector performance (stability, linearity, etc.), and 

4) Effects of grounding on various types of TVCs. 

c) Type A (statistical) uncertainties. 

d) Environmental considerations. 

2.9.9.3 

calibration report or certificate. 

The method of calibration, particularly the connection scheme used, should be included in the 

NIST Handbook 150-2A 45 March 2004 
2.10 High voltage capacitors 

2.10.1 Scope 

This section contains specific technical criteria that a laboratory should meet if it is to be recognized as 

competent to carry out high-voltage capacitor calibrations. 

2.10.2 References 

a) Anderson, W. E., “A Calibration Service for Voltage Transformers andHigh-Voltage Capacitors,,’ NBS 

Special Publication 250-33, 1988. 

b) Harris, F. K., Electrical Measurements, John Wiley & Sons, New York, 1966. 

2.10.3 Assuring the quality of test and calibration results 

The laboratory should have standard capacitors for use as check standards. 

2.10.4 Accommodation and environmental conditions 

2.10.4.1 Temperature variations should be less than 1 “C during the actual testing period. 

2.10.4.2 

be used. 

Appropriate methods for determining changes in check standard values with temperature should 

2.10.5 Equipment 

2.10.5.1 

range of rated voltages and fiequencies specified for the capacitors to be calibrated. 

The laboratory should have the equipment needed to generate and measure voltages over the 

2.10.5.2 

factors angle of a high-voltage capacitor. 

The laboratory should have the equipment needed to measure the capacitance and dissipation 

2.10.5.3 The uncertainties of all check standards and calibration techniiques should be documented. 

2.10.6 Test and calibration methods and method validation 

High-voltage capacitors may be calibrated by either direct or comparative methods. 

2.10.7 Handling of test and calibration items 

Check standards should be handled with care in order to avoid changes in the calibration. 

2.10.8 Reporting the results 

Calibration reports should specify: 

a) The calibration technique used, 

b) The temperature at the time of the test, 

NIST Handbook 150-2A 46 March 2004 
c) The voltage frequency, 

d) Uncertainties of the measurements, and 

e) The test results, including the capacitance and dissipation factor. 

2.11 High-voltage transformers 

2.11.1 Scope 

This section contains specific technical criteria that a laboratory should meet if it is to be recognized as 

competent to carry out high-voltage transformer calibrations. 

2.11.2 References 

a) Anderson, W. E., “A Calibration Service for Voltage Transformers and High-Voltage Capacitors,” NBS 

Special Publication 250-33, 1988. 

b) ANSIIIEEE (257.13-1 978, IEEE Standard Requirements for Instrument Transformers, American 

National Standards Institute. 

c) Harris, F. K., Electrical Measurements, John Wiley & Sons, New York, 1966. 

2.11.3 Assuring the quality of test and calibration results 

The laboratory should have check standards, such as standard capacitors, voltage transformers, and/or voltage 

sources, appropriate for the calibration method used. 

2.1 1.4 Accommodation and environmental conditions 

2.11.4.1 Temperature variations should be less than 1 “C during the actual testing period. 

2.1 1.4.2 

be used. 

Appropriate methods for determining changes in check standard values with temperature should 

2.11.5 Equipment 

2.11.5.1 

range of primary voltages and frequencies required by the transformers to be calibrated. 

The laboratory should have the equipment needed to generate and measure voltages over the 

2.1 1.5.2 The laboratory should have the equipment needed to measure tlne ratio correction factor and phase 

angle of a voltage transformer, including a range of secondary burdens appropriate for the transformers being 

calibrated. 

2.11.5.3 The uncertainties of all check standards and calibration techniques should be documented. 

2.11.6 Test and calibration methods and method validation 

Voltage transformers may be calibrated by either direct or comparative ]methods. 

NIST Handbook 150-2A 47 March 2004 
2.11.7 Handling of test and calibration items 

Check standards should be handled with care in order to avoid changes iin the calibration. 

2.11.8 Reporting the results 

Calibration reports should specify: 

a) The calibration technique used, 

b) The temperature at the time of the test, 

c) The voltage frequency, 

d) Uncertainties of the measurements, and 

e) The test results, including the ratio correction factor and the phase single of the transformer. 

2.12 Digital multimeters and multifunction calibrators 

2.12.1 Scope 

This section contains specific technical criteria that a laboratory should meet to be competent to calibrate 

digital multimeters (DMMs) and multifunction calibrators on all functioiis. 

2.12.2 References 

a) Oldham, N. M., and Parker, M. E., “NIST Multifunction Calibration System,” NISTSpecial Publication 

250-46, February 1998. 

b) Oldham, N. M., Parker, M. E., Young, A. M., and Smith, A. G., “A High Accuracy 10 Hz - 1 MHz 

Automatic AC Voltage Calibration System,” IMTC/87 Conference Proceedings, IEEE 

InstrumentatiodMeasurement Technical Conference, April 27-29,1987, Boston, MA, digest published 

in IEEE Trans. Instrum. Meas., IM-36, (4), pp. 279-281, Dec. 198‘7. 

2.12.3 Equipment 

The laboratory should have a calibration system capable of calibrating all of the ranges of the test DMMs 

and calibrators normally used by the laboratory. The system may consist o lcalibrated artifact standards such 

as dc voltage references, resistors, voltage dividers, thermal voltage and current converters, suitable for 

calibrating the reference and DMMs and calibrators. Alternatively, the reference DMMs and/or calibrators 

may be directly calibrated by an accredited laboratory. 

The test instruments should be connected and programmed according to documented procedures. 

2.12.4 Assuring the quality of test and calibration methods 

The laboratory should periodically calibrate at least one control DMM over the range normally used by the 

laboratory to calibrate test DMMs. 

NIST Handbook 150-2A 48 March 2004 
2.12.5 Accommodation and environmental conditions 

2.12.5.1 

a) 

Calibrations should be performed under the following conditi.ons: 

Ambient temperature of (23 *l) “C. 

b) Ambient relative humidity between 30 % and 50 %. 

c) Mains voltage within &lo % of nominal, with less than 3 % THD. 

2.12.5.2 

% of the error budget. 

These requirements may be relaxed to the point that their combined influence will not exceed 25 

2.12.6 Reporting the results 

The calibration report should denote the following: 

a) Test parameters (function, amplitude [range and applied], and frequency), 

b) DMM correction, 

c) Test uncertainty and description of how the uncertainty was calculated, 

d) Ambient (or instrument) temperature and humidity with tolerances, and 

e) Brief description of the standard used to perform the calibration. 

2.13 Watt/watthour meters 

2.13.1 Scope 

This section contains specific technical criteria that a laboratory should meet to be competent to calibrate 

power frequency wattmeters (WMs) and watthour meters (WHMs). 

2.13.2 References 

a) “Code for Electricity Metering,” ANSI C-12. 

b) Ramboz, J. D., and McAuliff, R. C., “A Calibration Service for Wattmeters and Watthour Meters,” NBS 

Tech Note I 179, July 1983. 

c) Oldham, N. M., Laug, 0. B., Waltrip, B. C., and Palm, R. H.,“The BUST Digitally Synthesized Power 

Calibration Source,” NIST Tech Note 1281, Aug. 1990. 

2.13.3 Equipment 

The laboratory should have the necessary power/energy calibrator to calibrate all of the ranges of the test 

WMs and WHMs. This calibrator should be calibrated to an uncertainty smaller (by a factor of 2 or more) 

NIST Handbook 150-2A 49 March 2004 
than the desired test WM/WHM uncertainty. The test WM/WHM should be connected to the calibrator in 

accordance with the calibrator or test WM/WHM specifications. 

2.13.4 Assuring the quality of test and calibration results 

The laboratory should periodically calibrate at least one control WM/WHM over the range normally used 

by the laboratory to calibrate test WM/WHMs. 

2.13.5 Accommodation and environmental conditions 

2.13.5.1 

a) 

b) 

c) 

2.13.5.2 

% of the error budget. 

2.13.6 Reporting the results 

The calibration report should denote the following: 

Calibrations should be performed under the following conditions: 

Ambient temperature of (23 f 1) “C. 

Ambient relative humidity between 30 % and 50 %. 

Mains voltage within f10 YO of nominal, with less than 3 % THD. 

These requirements may be relaxed to the point that their com‘bined influence will not exceed 25 

a) 

b) WMIWHM correction, 

Test parameters (function, amplitude [range and applied], and frequency), 

c) 

d) 

e) 

Test uncertainty and description of how the uncertainty was calculated, 

Ambient (or instrument) temperature and humidity with tolerances, and 

Brief description of the standard used to perform the calibration. 

2.14 Phase meters 

2.14.1 Scope 

This section contains specific technical criteria that a laboratory should meet to be deemed competent to 

calibrate digital phase meters (DPM) with voltage and/or current inputs. 

2.14.2 References 

Turgel, R. S., “NBS 50 Hz Phase Angle Calibration Standard,” NBS Tech Note 1220, 1986. 

NIST Handbook 150-2A 50 March 2004 
2.14.3 Equipment 

The laboratory should have a phase angle calibrator capable of calibrating all of the ranges of the test DPMs. 

This calibrator should be calibrated to an uncertainty smaller (by a factor of 2 or more) than the desired DPM 

uncertainty. The test DPM should be connected to the calibrator in accordance with the calibrator or test 

DPM specifications. 

2.14.4 Assuring the quality of test and calibration results 

The laboratory should periodically calibrate at least one control DPM over the range normally used by the 

laboratory to calibrate test DPMs. 

2.14.5 Accommodation and environmental conditions 

2.14.5.1 Calibrations should be performed under the following conditions: 

a) Ambient temperature of (23 f 1) "C. 

b) Ambient relative humidity between 30 % and 50 %. 

c) Mains voltage within &lo % of nominal, with less than 3 % THD. 

2.14.5.2 

% of the error budget. 

These requirements may be relaxed to the point that their combined influence will not exceed 25 

2.14.6 Reporting the results 

The calibration report should denote the following: 

a) Test parameters (signal amplitudes [range and applied], and frequency), 

b) DPM correction, 

c) Test uncertainty and description of how the uncertainty was calculated, 

d) Ambient (or instrument) temperature and humidity with tolerances, and 

e) Brief description of the standard used to perform the calibration. 

2.15 Dc high voltage dividers and dc high voltage resistors 

2.15.1 Scope 

This section contains specific technical requirements in accordance with which a laboratory should 

demonstrate that it operates, if it is to be recognized as competent to carry out calibrations of dc high voltage 

dividers and dc high voltage resistors. 

NIST Handbook 150-2A 51 March 2004 
2.15.2 References 

a) ISA RP52.1, Recommended Environments for Standards Laboratories, Instrument Society of America, 

1975. 

b) Misakian, M., “High Voltage Divider and Resistor Calibrations,” N13S Technical Note 1215, National 

Institute of Standards and Technology (formerly National Bureau of Standards), July 1985. 

c) Park, J. H., “Special Shielded Resistor for High-Voltage DC Measurements,” J. Res. NBS, 66C, 

pp. 19-24, 1962. 

d) Wenner, F., “Methods, Apparatus, and Procedures for Comparison of Precision Standard Resistors,” 

J. Res. NBS, 25, pp. 229-294, 1940. 

2.15.3 Assuring the quality of test and calibration results 

2.15.3.1 Components of the calibration apparatus, e.g., high voltage and low voltage resistors, should be 

calibrated periodically at intervals depending in part on the stability of the components. The uncertainties 

associated with the component values should be commensurate with the uncertainty levels claimed when high 

voltage dividers and resistors are calibrated with the apparatus. Records shLould be maintained of component 

values to identify trends and instabilities. 

2.15.3.2 The laboratory should have one or more check standards that can be used to verify the satisfactory 

performance of the calibration apparatus over its operational range of volt,ages, ratios and resistance values. 

Provision should be made for evaluation of the check standard. This evaluiation should be made periodically 

at intervals dependent in part on the stability of the check standard. Evaluations should be performed at least 

yearly. Records of calibration apparatus performance and check standard performance should be maintained 

to identify trends and instabilities. 

2.15.4 Accommodation and environmental conditions 

2.15.4.1 Excessive vibration in the calibration room should be avoided. If an obvious source of vibration 

exists, precautions should be taken to prevent adverse effects on the calibration apparatus and on the 

laboratory’s measurements. 

2.15.4.2 All high voltage should be confined within grounded or properly insulated enclosures. Instrument 

cabinets containing high voltage should have safety interlocks on access doors. If confinement is impossible, 

exposed high voltage should be enclosed with safety fences with working interlocks. Entrance into the high 

voltage area should be impossible (except by deliberate breach of the enclosure) without tripping one or more 

interlocks. 

2.15.4.3 “DANGER HIGH VOLTAGE” signs should be on display on all entrances to high voltage areas. 

A warning light, preferably flashing, should indicate when the voltage is on. In all cases where there is direct 

access from the outside hallway to the high voltage area, there should be a warning light, a “DANGER HIGH 

VOLTAGE” sign, a safety interlock, and a locked door. 

2.15.4.4 Before touching a high voltage circuit or leaving it unattended, it should be de-energized and 

grounded. When possible, the grounding stick should be left on the high voltage terminal until the circuit 

is re-energized. Grounding sticks should be available near entrances to high voltage areas. Automatic 

NIST Handbook 150-2A 52 March 2004 
grounding systems, or systems that employ audible warning signals to remind personnel to ground the high 

voltage equipment, are encouraged. 

2.15.4.5 Circuit breakers, disconnects, or contacts used to energize a high voltage source should be left 

open whenever the supply is not used. Laboratories should always be left in such a configuration that at least 

two switches should be used to energize the high voltage circuits. Whenever possible, a “return-to-zerobefore-energizing” interlock should be incorporated into the high voltage supply. 

2.15.4.6 

when: 

Two people should be present when high voltage is on. Exceptions to this rule are permitted 

a) There are no exposed high voltages. This means that all high voltage conductors are inside grounded 

or insulated enclosures and are inaccessible to the operator when the high voltage circuit is energized. 

b) There are exposed high voltages, but doors or openings that allow the operator to reach the high voltage 

conductors have interlocks that not only disconnect the high voltage power source, but also discharge 

the stored energy in the high voltage circuit to ground. 

2.15.4.7 The temperature should be maintained at (23 f. 1.5) “C arid sufficiently stable so that the 

calibration apparatus and unit under test are in thermal equilibrium with the ambient temperature. The 

relative humidity should be constant for the duration of the calibration and remain below 50 % so that 

leakage currents over critical insulating surfaces are negligible. 

2.15.5 Equipment 

2.15.5.1 The laboratory should have sufficient standard resistors, both high voltage and low voltage, for 

its calibration apparatus, with values necessary to determine the ratio and resistance values calibrated by the 

laboratory. Periodic checks, e.g., quarterly, of the calibration apparatus should be made using appropriate 

check standards. 

2.15.5.2 

interest in a safe manner. 

The laboratory should have means for generating high dc voltages over the voltage range of 

2.15.5.3 

calibration procedure. 

The laboratory should have temperature and humidity measuring capabilities suitable to the 

2.15.5.4 The laboratory should have equipment needed to make auxiliary measurements on components 

of the calibration system and for comparative measurements (e.g., precision voltmeter for comparative 

voltage measurements). 

2.15.5.5 

a (two-sided) measurement uncertainty that does not exceed 33 % of the specified uncertainty. 

A laboratory that certifies high voltage dividers and/or high voltage resistors should demonstrate 

2.15.5.6 

high voltage conditions. 

The laboratory should have equipment needed to make ratio and resistance measurements under 

2.15.5.7 Voltages applied to the unit under test should be known within ~t2 %. 

2.15.5.8 

requirements of the calibration. 

The high voltage power supply should be stable to the extent necessary to meet the uncertainty 

NZST Handbook 150-2A 53 March 2004 
2.15.5.9 The total uncertainty associated with the resistors used in the calibration process, taking into 

account heating effects, voltage coefficients, lead and contact resistances, leakage currents, and corona 

should be no larger than 30 % of the uncertainty quoted in reports issued by the laboratory. 

2.15.6 Handling of test and calibration items 

2.15.6.1 

standard to mechanical shock or vibration. 

Precautions should be taken to avoid exposing components of the calibration apparatus or check 

2.15.6.2 High voltage dividers and resistors should be protected from moisture, mechanical shock and 

vibration during handling in the laboratory. Provision should be made for similar protection during shipment 

of the device to and from the laboratory (responsibility for this provision rests primarily with the sender of 

the device). 

2.15.7 Test and calibration methods and method validation 

2.15.7.1 

calibration. 

The laboratory should have a manual outlining procedures to be followed for each type of 

2.15.7.2 Ratios and resistance values may be determined using bridge techniques or by direct comparison 

methods. The schedule of applied high voltage as a function of time should be agreed to by the laboratory 

and the owner of the device under test. 

2.15.8 Records 

2.15.8.1 All measurements should be recorded. If the calibration results are recorded by hand, the work 

sheets should show the measurement data, corrections, equation used folr calculating the divider ratio or 

resistance, and final answers. The components used in the calibration apparatus, temperature, relative 

humidity, unit under test, operator, date, and other pertinent information for the calibration should be 

included. 

2.15.8.2 

calibration should be maintained for a minimum of five years. 

Information relating to the procedures, equipment, results, and personnel involved for aparticular 

2.15.8.3 

during the lifetime of the components or check standard. 

Records associated with components ofthe calibration apparatus or check standard should be kept 

2.15.9 Reporting the results 

2.15.9.1 

Recommended Practice RP-11, as well as the requirements of ISO/IEC 1 :7025. 

All content of certificates or reports of calibration should conform to the guidelines of NCSL 

2.15.9.2 

determined from measurement data and which agrees with the laboratory'k stated definition. 

All certificates or reports should contain an uncertainty statement which is scientifically 

2.15.9.3 

applicable) the uncertainties associated with: 

The uncertainty should be derived from a model of the measurement system that includes (as 

a) Resistors in calibration apparatus, 

NIST Handbook 150-2A 54 March 2004 
Lead and contact resistances, 

Sensitivity of detector(s) (e.g., voltmeter or null indicator), 

Heating effects of components, 

Corona, 

Leakage currents over critical insulating surfaces, and 

Voltage coefficients. 

2.15.9.4 

in Measurement or another well-documented method. 

2.15.9.5 

a) 

b) 

2.15.9.6 

The uncertainty should be computed according to the IS0 Guide to the Expression of Uncertainty 

The calibration report should indicate: 

The ratio and/or resistance values as a function of time and dc high voltage, and 

Ambient temperature and relative humidity at the time of the measurements. 

Voltage dividers and resistors that are out of tolerance need not be reported. 

1 

NIST Handbook 150-2A 55 March 2004