| Table of Contents |
| Foreword *
Program Description *
Introduction *
Recognition Requirements for Certificates of Measurement Traceability *
Recognized Measurement Areas *
Mass *
Volume *
Length *
Grain Moisture *
Temperature *
Time *
Other *
Unevaluated Areas *
OWM Technical Assistance *
Training and Standard Procedures *
Laboratory Quality System Support *
Integrated Measurement Assurance Support *
Regional Measurement Assurance Programs (RMAPs) *
Figure 1. Map of Regional Measurement Assurance Programs *
Summary *
References *
Directory
Laboratory Contacts *
Calibration Scope Summary
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| Forward |
| In support of its mission to promote uniform standards of measurement throughout the country, the National Bureau of Standards (NBS), now the National Institute of Standards and Technology (NIST), received funding from Congress in 1965 to provide new standards of mass, length, and volume and the equipment needed to perform calibrations in these measurement areas to State weights and measures laboratories. This program, called the [New] State Standards Program, was established at NBS (now NIST), and is managed by the NIST Weights and Measures Division. To maintain the validity of these standards and to ensure that the standards are used properly, OWM: |
| • | provides technical assistance to State metrologists and provides training in the test procedures used in the NIST laboratories; and |
| • | conducts a voluntary Recognition program for State weights and measures laboratories where States that comply with program criteria (summarized in this document) receive a Certificate of Measurement Traceability, along with a letter identifying critical components of the program that is often requested by laboratory clients. |
This electronic document describes the components of the Recognition program and the technical assistance that NIST/OWM provides to State weights and measures laboratories. It also provides laboratory contact information and lists the recognized services. A link is provided to the laboratory Certificates of Measurement Traceability; these certificates detail the scope and period of recognition.
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Note regarding SI Units:
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One objective of the Omnibus Trade and Competitive Act of 1988 was to make the International System (SI) of Units the primary measurement system in the United States. SI Units are the preferred measurement system in the State laboratories. State laboratories have primary standards in both "customary" and SI units, they have laboratory equipment that primarily uses SI units, and training for State metrologists primarily uses the SI System of Units. However, the foremost system for commercial equipment for sales to the U. S. public is that of "customary" units. Descriptions of measurement services in this directory refer to whichever system is typically predominant for the type of service provided, although capabilities exist for measurement services in both systems of units.
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Excerpt regarding laboratory quality system standards:
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State laboratories have participated in a voluntary program of certification/recognition that has followed international quality system standards for many years. The 1986 version of NBS Handbook 143, Program Handbook used the 1982 version of ISO/IEC Guide 25 (4) as a model. During 1992, the Weights and Measures Division began the process of upgrading the program to include adoption of ISO/IEC Guide 25 and incorporation of components of ISO 9001 with cooperation from the NIST National Voluntary Laboratory Accreditation Program (NVLAP). Program requirements were published in 1996 and revised in 1997. General and specific technical criteria incorporated ANSI/NCSL Z 540-1-1994 (3) and NVLAP technical criteria to ensure consistency in the program with international recommendations for calibration laboratories. Laboratories have been implementing these quality system requirements since prior to the 1996 edition of NIST Handbook 143. A template quality manual (NIST IR 5802) and draft administrative procedures were developed and provided to the State laboratories and are available for downloading for any interested laboratory ( see laboratory quality systems ). During 1999, work began to gain laboratory compliance to ISO/IEC 17025 (2) (the replacement for ISO/IEC Guide 25) and is ongoing. Technical resources to assist laboratories in this latest upgrade are also available via the Internet. OWM believes that laboratories that are recognized under these criteria meet customer requirements for measurement traceability under the ISO 9000 and QS 9000 standards and also interim requirements of accrediting bodies.
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| Program Description |
| Introduction |
| NIST manages the State Laboratory Program that began with the New State Standards Program, established by Congress in 1965, as part of its continuing support to the State weights and measures programs. This program of the NIST Weights and Measures Division is designed to provide guidance, technical support, training and assistance to State metrology laboratories to ensure accurate and traceable measurements from NIST to local regulatory jurisdictions and for other local government and industry measurement needs. The program operates through continued partnership with the State laboratories to manage numerous measurement-related activities as described here. |
| State weights and measures laboratories are custodians at the State level of measurement standards that serve as the basis for ensuring equity in the marketplace and as reference standards for calibration services for domestic industry. Some 350,000 calibrations are provided each year for nearly 20,000 customers throughout the country by State weights and measures laboratories. About 50 percent of these measurements are in support of regulatory weights and measures activities. The other 50 percent of the measurements provide accessibility to reference measurement standards for local government, educational institutions and a wide variety of industries. This nationwide availability of measurement services forms a valuable link in the United States’ national measurement infrastructure. |
| Recognition Requirements for Certificates of Measurement Traceability |
| Although OWM does not provide formal accreditation according to ISO/IEC Guide 58, and the program is operated independently from the National Voluntary Laboratory Accreditation Program (NVLAP), the general and technical criteria used in both programs are identical. OWM offers laboratory recognition, through the issuance of a Certificate of Measurement Traceability. The requirements for recognition are documented in NIST Handbook 143, Program Handbook (1) . |
| The Handbook 143 criteria include the ANSI/NCSL Z 540-1-1994, U. S. Standard for Calibration Laboratories and the National Voluntary Laboratory Accreditation Program (NVLAP) technical criteria from the Draft NIST Handbook 150-2, Calibration Laboratories Technical Guide. In summary, to be recognized, the laboratory must have: |
| • | a documented quality system and procedures; |
| • | adequate facilities, appropriate equipment, and traceable standards with evaluated uncertainties that are sufficiently small to perform the claimed measurements; |
| • | established periodic calibration intervals, with check standards and measurement assurance systems acceptable to NIST/OWM in place for all recognized measurement areas; |
| • | metrologists who have completed the necessary NIST/OWM training and have submitted data demonstrating the results of applying test procedures; and |
| • | participation on an annual basis in the Regional Measurement Assurance Programs; each meeting includes updated training and inclusion in interlaboratory comparisons to provide evidence of the capability to provide reliable, NIST-traceable measurements. |
| OWM recognition also indicates that the metrologist has submitted additional technical data requested by NIST each year to allow for a thorough evaluation of measurement traceability and metrological proficiency. Following review and successful evaluation of the laboratory’s ability to make reliable measurements (principally mass, volume, length, and temperature), which includes annual proficiency testing and may include an on-site assessment, NIST/OWM issues a Certificate of Measurement Traceability. While a certificate indicates that the laboratory is capable of providing traceable measurements, each laboratory is responsible for verifying measurement traceability for all of its measurements. |
| Laboratories are recognized according to specific measurement areas. If the laboratory is recognized, it is capable of providing the measurement services indicated. The ranges of standards that can be tested by different laboratories vary and are dependent on the levels of training and competence that have been demonstrated, the achievable uncertainties, and the equipment and standards that are available. |
| If a laboratory has NOT been recognized, this does not necessarily indicate that the laboratory is not adequate or that its measurements are not valid. In some cases, a laboratory has not submitted the information and technical data to permit NIST to evaluate its capability; therefore, NIST cannot take a position regarding its capability. NIST/OWM discourages the use of laboratory services that are not recognized or accredited. |
| Recognized Measurement Areas |
| Laboratories providing any recognized measurements have evaluated and declared measurement capabilities in terms of uncertainties for each nominal value and each type of procedure. Submitted uncertainties must conform to the ISO Guide to the Expression of Uncertainty in Measurement: 1993. Detailed technical criteria, and associated links, are found in NIST Handbook 143, Program Handbook. In addition to reporting calibrated values and uncertainties, the calibration of many standards often requires reference to NIST, ASTM, OIML or other national or international standards. A brief description of recognition parameters and associated references are provided in the following sections. |
| Mass HB 143, Section 7.2 Technical Criteria |
| A laboratory may be recognized at three levels for mass: Echelon I, II, and/or III. The type and quality of standards and facilities, choices of measurement procedures and measurement assurance systems, a metrologist's demonstrated training and competence, and claimed measurement uncertainties all factor into which designation a laboratory is able to achieve. Echelon I requires complex measurements with advanced weighing designs, air buoyancy and temperature corrections. Echelon II is routinely used for precision calibrations and is essential for evaluating the ongoing stability of mass standards. Echelon III is the level that is routinely needed for legal metrology or weights and measures enforcement activities. The echelons loosely correspond to the specifications and tolerances of the following standards as noted: |
| • | NIST Handbook 105-1 (5), Class F (Echelon III); |
| • | American Society for Testing and Materials (ASTM), E 617-97 (6), Classes 0 and 1 (Echelon I), Classes 2 through 4 (Echelon II), and 5 through 7 (Echelon III); and |
| • | International Organization of Legal Metrology (OIML) R 111 (7), Classes E1, E2 (Echelon I), F1, F2 (Echelon II), M1, M2, and M3 (Echelon III). |
| Laboratories may also calibrate to NBS Circular 547 (8) Classes S, S-1, P, Q, and T (out of print) or to NBS Circular 3 (9) Classes A, B, C (out of print); however, these classes have been superseded by ASTM classifications since 1978 and the practice is discouraged. |
| Volume HB 143, Section 7.3 Technical Criteria |
| Recognition in the area of volume (or capacity) is provided according to the type of calibration procedure. Volume in State laboratories is usually determined by gravimetric (weighing procedure) or volume transfer (comparative) methods. Volume may also be determined dimensionally, but use of that method would be recognized under Length. The ability to be recognized for differing types of volume calibrations is dependent upon the metrologists’ level of training and demonstrated competence. Volume transfer is a fairly simple measurement and is required for weights and measures enforcement standards. Traceability in volume may be achieved through mass standards using the gravimetric methods, but it is a more complex measurement. |
| The volume standards listed below are commonly referenced as weights and measures field standards: |
| • | NIST Handbook 105-2, (glass flasks) (10); |
| • | NIST Handbook 105-3, (metal test measures and provers) (11); and |
| • | NIST Handbook 105-4, (liquefied petroleum gas provers) (12). |
| Volume standards may also be referenced to ASTM, OIML, or the American Petroleum Institute (API) standards, though these standards are not as commonly requested. |
| Length HB 143, Section 7.1 Technical Criteria |
| Steel tapes up to several hundred feet long may be calibrated by State laboratories. The uncertainties are typically a few thousandths of a foot. Rigid rules up to 12 inches long can be calibrated with uncertainties of several thousandths of an inch. Longer rules may be tested with slightly larger uncertainties. Most requests for length calibrations in State laboratories have not been in metric units, although States are capable of making measurements in metric units. Length measurements are all made by optical comparison methods at this time. Due to the reduction in requests for length measurements, all laboratories do not maintain services in this area. |
| Grain Moisture HB 143, Section 7.6 Technical Criteria |
| Laboratories may be recognized for moisture measurements in four categories based on procedures and reference methods. Having a recognition program for grain moisture testing promotes uniformity in grain moisture testing and provides a recognition program for State (government) grain moisture laboratories that are sometimes outside the scope of the weights and measures metrology laboratories. |
| The specific technical requirements were developed by consensus among participants from the Grain Inspection, Packers and Stockyards Administration (GIPSA) of the U. S. Department of Agriculture, the National Conference on Weights and Measures (NCWM) and the NIST Weights and Measures Division. The program follows the same process as that of the State weights and measures metrology laboratories. |
| Temperature HB 143, Section 7.5 Technical Criteria |
| Recognition in temperature measurements may be granted in any of 5 echelons. The 5 echelons in thermometry are associated with levels of performance based on the uncertainty associated with the standards and procedures used. Services in State laboratories for thermometry have been developed outside the scope of the New State Standards Program where equipment and standards were provided by NBS or NIST. Those States with programs have obtained the standards, equipment, and the training necessary to provide for the needs of their clients on a local level. Services range from simple comparison of liquid-in-glass clinical thermometers to comparisons of standard platinum resistance thermometers and the use of defined standards based on the ITS 90 Temperature Scale. |
| For specifications and tolerances related to the use of thermometers in weights and measures field enforcement activities or industry standards see: |
| • | NIST Handbook 105-6 (thermometers) (13); and |
| • | ASTM E1 Standard Specifications for ASTM Thermometers (14). |
| Time |
| NIST disseminates traceable free time measurements through a variety of media and services. Information regarding services can be obtained at http://www.time.gov . These services can be easily used by laboratories testing such simple devices as watches, clocks and stopwatches. Stopwatches used for law enforcement activities (e.g., for testing parking and taxi meters) have expanded uncertainties that are significantly less than the tolerances required for enforcement (e.g., 2 seconds in 24 hours). |
| Specifications and tolerances related to the use of stopwatches in weights and measures field enforcement activities may be found in: |
| • | NIST Handbook 105-5 (stopwatches) (15). |
| Other |
| A number of other items are tested or calibrated by State weights and measures laboratories. These areas include the testing of tuning forks used in testing radar speed devices, testing of wheelload weighers used in testing large trucks for road weight restrictions, and the testing of hydrometers for testing sugar content of syrup. Specific technical criteria have not been developed in these areas. However, procedures are available and laboratories are recognized for these areas in a limited number of cases. |
| Unevaluated Areas |
| In addition to the recognized measurement areas that have been described, many laboratories perform measurements for which the NIST Weights and Measures Division has not established guidelines for recognition. These measurement areas can include the testing of dial gauges used to test polyethylene sheeting (an extension of dimensional measurements), the testing of lottery balls for State lottery programs, or entire programs such as watthour meter testing and petroleum quality testing. |
| OWM Technical Assistance |
| One of the primary reasons the OWM program does not meet ISO/IEC Guide 58, which is used to evaluate accrediting bodies, is that the extensive level of technical assistance provided to State laboratories is considered consultancy. OWM expends a significant effort in ensuring uniformity in measurements performed in the State laboratories through various types of technical assistance which include the following: |
| • | Training and standard procedures; |
| • | Laboratory quality system support and publications; |
| • | Integrated measurement assurance program support; and |
| • | Regional Measurement Assurance Programs (RMAPs). |
| Training and Standard Procedures |
| NIST recommends to the States minimum qualifications for metrologists. A suitable technical degree, such as Engineering, Physics, Mathematics, or Chemistry, is highly recommended. Working knowledge of algebra and basic statistics is essential. NIST training is offered at the Basic, Intermediate, and Advanced levels, but it is expected that metrologists will have spent time in the metrology laboratory familiarizing themselves with the equipment prior to attending the Basic metrology seminar. |
| The Basic metrology seminar for States is a two-week course that covers measurement areas in mass, length, and volume as needed for providing basic services in weights and measures enforcement activities. NIST test procedures are covered with hands-on measurement opportunities for each metrologist in the OWM training and demonstration laboratory in Gaithersburg, MD. Test procedures are published in NIST Handbook 145, Handbook for the Quality Assurance of Metrological Measurements (16). After the training seminar is completed, a series of Laboratory Auditing Program (LAP) problems are assigned to the metrologists for completion in their home laboratories to document their understanding of the procedures and calculations. LAP problems are submitted to NIST for review. Successful completion of the Basic seminar for States and the LAP problems are prerequisites for the minimum level of recognition that will meet weights and measures enforcement requirements. |
| Each metrologist is expected to attend the Intermediate seminar within 2 years of successfully completing the Basic seminar to receive additional training in laboratory procedures needed for proper surveillance of laboratory standards and for precision calibrations. Following the Intermediate seminar, additional LAP problems are assigned. However, the Intermediate problems evaluate the metrologist's level of understanding and ability to effectively apply concepts to laboratory measurements. LAP problems are again submitted to NIST for review. Successful completion of the Intermediate seminar and LAP problems is essential for recognition for precision calibrations such as is required for Mass, Echelon II and Gravimetric Volume calibrations. |
| The Advanced Mass seminar, NIST Thermometry seminar and NIST Pressure seminar provide additional levels of training for metrologists wishing to work at the highest levels of precision. Once training is completed, laboratories must establish appropriate check standards and measurement assurance methods to ensure proper application of procedures and maintenance of traceability. Round robins are conducted in advanced or specialized areas whenever possible. |
| Laboratory Quality System Support |
| State laboratories have participated in a voluntary program of certification/recognition that has followed international quality system standards for many years. The 1986 version of NBS Handbook 143, Program Handbook used the 1982 version of ISO/IEC Guide 25 as a model. Training has been conducted at the regional measurement assurance program meetings since 1993 on how to write quality manuals and how to perform internal auditing. Results of audits are often shared among the State laboratories to help improve the entire State Laboratory Program. |
| During 1992, the Weights and Measures Division began the process of upgrading the program to include adoption of ISO/IEC Guide 25:1990 and incorporation of components of ISO 9001:1987 with cooperation from the NIST National Voluntary Laboratory Accreditation Program (NVLAP). Program requirements were published in 1996 and revised in 1997. General and specific technical criteria incorporated ANSI/NCSL Z 540-1-1994 and NVLAP technical criteria consistent with international recommendations. Laboratories have been implementing these quality system requirements since prior to the 1996 publication of NIST Handbook 143. A template quality manual (NIST IR 5802) and draft administrative procedures were developed and provided to the State laboratories and are available for downloading for any interested laboratory ( see laboratory quality systems ). |
| During 1999, work began to gain laboratory compliance to ISO/IEC 17025 (the replacement for ISO/IEC Guide 25) and is ongoing. Technical resources such as a database comparing the current program to the ISO/IEC 17025 standard, a modified template quality manual with ISO/IEC 17025 notes included, and training slides to assist laboratories in this latest upgrade are also available via the Internet. |
| Integrated Measurement Assurance Support |
| NIST criteria require State weights and measures laboratories to use formal measurement assurance concepts and procedures to ensure that measurements are sufficiently accurate to meet their needs on a continuing basis. This includes several facets of measurement assurance, all integrated in such a way as to assist in ensuring measurement traceability, quantifying uncertainties, supporting decisions regarding calibration intervals, and substantiating results of interlaboratory comparisons. |
| Components of an integrated measurement assurance system include: |
| • | Periodic calibration at NIST for direct traceability. Tracking historical data for calibrations provides an assessment of drift rates, stability of standards, system uncertainties, and data that may be used to justify calibration intervals. |
| • | Annual conduct of round robins (interlaboratory comparisons) using artifacts with accepted reference values. Evaluation of bias in the results of round robins provides a data point for correlation with NIST calibration results and surveillance testing of laboratory standards. |
| • | Surveillance testing of laboratory reference standards with external check standards. This is a special test in a single laboratory using check standards that belong to NIST or another laboratory with NIST reference values and specific procedures to evaluate the ongoing stability and traceability of laboratory standards. |
| • | Surveillance testing of laboratory reference standards with internal check standards. This is a special test in a single laboratory using check standards from within the laboratory that have NIST reference values and using specific surveillance procedures to evaluate the ongoing stability and traceability of laboratory standards. |
| • | Periodic calibration of working standards within the laboratory using standard procedures and check standards. The history of calibration values for working standards and the data for the check standards both provide valid points of reference in ensuring accurate measurements or in identifying problem areas. |
| • | Measurement assurance control charts and check standards used for the routine work of the laboratory. The data obtained from these charts provides ongoing assurance of the stability of working standards, justification for calibration intervals of working standards, and a way to measure the actual uncertainty of the measurements. |
| Software tools such as spreadsheets have historically been used to gather and analyze all of the data from the sources identified above. Standardized PMAP software has also been provided to all of the State laboratories by NIST/OWM to provide methods for standardizing the collection and analysis of this data and means for sending the data to NIST electronically (through e-mail attachments) for evaluation. |
| Regional Measurement Assurance Programs (RMAPs) |
One of the foundational elements for collaboration, partnership, training, conduct of interlaboratory comparisons, and continued improvement among NIST and the State laboratories is the regional measurement assurance programs (RMAPs). The RMAPs are groups of States, NIST, and to a limited extent private laboratories, in given geographical locations that have joined together to: 1) conduct meetings for the purpose of training, demonstration of procedures, and addressing mutual measurement concerns; and 2) conduct round robin experiments to promote uniform measurements in mass, length, volume, and other measurement areas where possible. Participation of at least one person from each laboratory is required for ongoing NIST/OWM recognition. The RMAP groups provide an opportunity for all States, the District of Columbia, Puerto Rico, and the Virgin Islands to participate. County (Los Angeles) or Federal (USDA, GIPSA) weights and measures laboratories participate as well. The RMAPs were organized primarily between 1980 and 1982, with CaMAP being added in 1992 to further assist metrology efforts in the Caribbean. The six RMAP groups are:
- Northeastern Measurement Assurance Program (NEMAP);
- Southeastern Measurement Assurance Program (SEMAP);
- Western Regional Assurance Program (WRAP);
- Mid-America Measurement Assurance Program (MidMAP);
- Southwestern Assurance Program (SWAP); and
- Caribbean Measurement Assurance Program (CaMAP).
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| In the regional measurement assurance programs, standards external to the laboratory area are submitted to the laboratory for test. These external standards are normally measured by NIST (or USDA, GIPSA in the case of grain moisture) to provide an accepted reference value whenever possible. Other statistical methods are used to determine accepted reference values when NIST values are not available. These standards are circulated as part of round robin experiments conducted and coordinated by the RMAP participants. Although NIST provides artifacts and sometimes calibrated values for the circulating standards, the experiments are developed, coordinated, analyzed, and reported by the RMAP participants. NIST participates in the final data analysis and at regional meetings to identify any problems and to provide training to eliminate discrepancies. Information gathered through the RMAP training and round robins is vital to ongoing improvement of the program and for laboratory recognition. The RMAPs reduce the level of effort required for NIST to monitor the output of State weights and measures laboratories. |
| Training is coordinated for each RMAP meeting between NIST and the host laboratory. Since 1993, a full training program has been coordinated by NIST, with member metrologists rounding out a full, nearly week-long training effort. Training has consisted of laboratory quality requirements, technical management, measurement assurance practices, statistics training, and specific measurement-related training covering laboratory procedures in detail. |
| Representatives from each RMAP group interact further at the metrology meetings at the National Conference on Weights and Measures annual meeting. Members report on activities from their regions and discuss ways of improving the quality of the laboratories on a national level. |
| Figure 1. Map of Regional Measurement Assurance Programs * |
| Summary |
| In order for a State weights and measures laboratory to be recognized in a particular measurement area, each State must have a trained and competent metrologist and an adequate facility, and must demonstrate on a continuing basis that it is capable of providing valid measurements. Recognition by NIST indicates that the laboratory is capable of providing a traceable measurement service, but each State must document and provide evidence of measurement traceability. |
References
- Harris, Georgia L. editor; State Weights and Measures Laboratories Program Handbook, NIST HB 143 .
- ISO/IEC 17025:1999 – General Requirement for the Competence of Testing and Calibration Laboratories.
- ANSI/NCSL Z 540-1-1994, American National Standards Institute, National Conference of Standards Laboratories, American National Standard, General Requirements for Calibration Laboratories and Measuring and Test Equipment, 1994.
- ISO/IEC Guide 25, International Organization for Standardization, Guide 25, General Requirements for the Competence of Calibration and Testing Laboratories, 1990.
- NIST Handbook 105-1 (Revised 1990), Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures. 1. Specifications and Tolerances for Field Standard Weights (NIST Class F) (Echelon III).
- ASTM (American Society for Testing and Materials) E 617-97 Classes 0 and 1 (Echelon I), Classes 2 through 4 (Echelon II), and 5 through 7 (Echelon III).
- OIML (International Organization of Legal Metrology), R111, Classes E1, E2 (Echelon I), F1, F2 (Echelon II), M1, M2, and M3 (Echelon III).
- NBS Circular 547, Section 1, Precision Laboratory Standards of Mass and Laboratory Weights, 1954, out of print.
- NBS Circular 3, Classes A, B, C, Design and Test of Standards of Mass, out of print.
- NIST Handbook 105-2 (Revised 1996), Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures. 2. Specifications and Tolerances for Field Standard Measuring Flasks.
- NIST Handbook 105-3 (Revised 1997), Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures. 3. Specifications and Tolerances for Graduated Neck Type Volumetric Field Standards.
- NIST Handbook 105-4 (1997), Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures. 4. Specifications and Tolerances for Liquefied Petroleum Gas and Anhydrous Ammonia Liquid Volumetric Provers.
- NIST Handbook 105-6 (1997), Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures. 6. Specifications and Tolerances for Thermometers.
- ASTM E1 Standard Specifications for ASTM Thermometers.
- NIST Handbook 105-5 (1997), Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures. 5. Specifications and Tolerances for Field Standard Stopwatches.
- Taylor, J.K., Oppermann, H.V., Handbook for the Quality Assurance of Metrological Measurements, NBS/HB 145 (1986).
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