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GLP 4

Good Laboratory Practice

for

Periodic Recalibration of State Standards 

1 Purpose 

Many States have provisions in their weights and measures laws that require the periodic 

submission of their State standards to NIST for calibration. Those provisions are based on 

an early version of the Model Law (1911), which was considered appropriate for the 

circumstances that prevailed prior to the establishment of the new State Standards Program 

by NIST. Periodic calibration is necessary on a defined regular, periodic basis, and when 

measurement control results from internal control charts or external interlaboratory 

comparisons indicate questionable data. Good Measurement Practices 11 and 13 provide 

guidance regarding appropriate calibration intervals and documentation of metrological 

traceability.

Standards of mass, volume, and length, fabricated from modern materials, kept in the 

controlled environment of a State metrology laboratory under the custody of trained 

metrologists, are generally stable and not subject to excessive change. The cooperative 

NIST-State audit programs often identify changes in ample time for corrective action in 

the unlikely event that such a change should occur. These same programs provide the 

necessary evidence of the ability to provide metrological traceability of measurement 

results at a level of confidence sufficient for the need. 

The process of packing, shipping, and unpacking exposes the standards to unnecessary 

hazards that could result in damage, compromising their integrity. The return and recalibration could take several months, causing an unavailability of State services that would 

be disruptive to the performance of the mission of the State laboratories. 

2 Policy Recommendations

To develop a policy for the guidance of and implementation by all 50 States regarding this subject, 

the following actions are recommended:

States should recognize the fact that periodic return of their State standards to NIST 

(or another suitable and accredited calibration laboratory) for re-calibration is 

essential to comply with State laws regarding metrological traceability and with 

ISO/IEC 17025, GMP 11, and when: 1) data regarding traceability is unavailable;

2) charted measurement results indicate that the standards are out of control; 3) 

measurement results on interlaboratory comparisons or performance tests are 

suspect; or 4) NIST advises the State of the need.

GMP 11 and GMP 13 provide the basis for documenting metrological traceability 

and provide baseline calibration intervals. These publications are templates that 

must be modified to specific measurement parameters and applications in each 

laboratory. Documented calibration intervals may state that calibrations are 
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obtained whenever evidence from outside evaluations, proficiency tests, and/or 

internal controls indicate that significant changes may have taken place, or when 

measurement results are suspect, but shall not exceed a specified number of years 

(with the number specified) and may not exceed limits defined in GMP 11 and 13 

without significant statistical and technical analysis by the laboratory and by the 

NIST Office of Weights and Measures. Statements such as “calibration as needed” 

are unacceptable for both laboratory measurement equipment and measurement 

standards. 

References to the periodic re-calibration of State standards in the law such as, 

"He (the director) shall maintain the State standards in good order and shall submit 

them, at least once in ten years, to the National Institute of Standards and 

Technology for calibration,"

should be followed, if present, and may be amended to reflect a more frequent 

interval, plus calibration based on identified needs. Alternatively, the wording of 

Sections 3 and 12 of the present Uniform Law from NIST Handbook 1301 may be 

substituted:

“SECTION 3. PHYSICAL STANDARDS. -- Weights and measures that are 

traceable to the U.S. prototype standards supplied by the Federal Government, or 

approved as being satisfactory by NIST, shall be the state reference and working 

standards of weights and measures, and shall be maintained in such calibration as 

prescribed by the NIST as demonstrated through laboratory accreditation or 

recognition. All field standards may be prescribed by the Director and shall be 

verified upon their initial receipt, and as often thereafter as deemed necessary by 

the Director.”

“SECTION 12. POWERS AND DUTIES OF THE DIRECTOR. -- The director 

shall: 

12 (a) maintain traceability of the state standards as demonstrated through 

laboratory accreditation or recognition.”

The approach described above is recommended by NIST because many States that participate in 

the Office of Weights and Measures Laboratory Metrology Program have the capability of 

evaluating their own State standards, with adequate statistically valid data, and with the necessary 

documentation referencing metrological traceability to the International System of Units (SI) (or 

“national standards”, when required by State laws). The Laboratory Metrology Program provides 

interaction between the State standards laboratories and NIST, ensuring satisfactory laboratory 

conditions suitable for reference standards and/or working standards in addition to the proper use 

of NIST procedures in standards calibration. Thus, each State program is evaluated and, if found 

in compliance, is Recognized to perform the measurements listed on the Certificate of 

 1 NIST Handbook 130, Uniform Laws and Regulations in the Areas of Legal Metrology and Engine Fuel Quality 

(Latest Edition). This text was originally published in the 2005 version of Handbook 130 and has not been updated 

as of the publication of this Good Laboratory Practice.
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Metrological Traceability according to the requirements in ISO/IEC 17025 as referenced in NIST 

Handbook 143, Program Handbook (latest edition). 
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