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April 1, 2013

Field Standards for Weights and Measures

Byline: Georgia L. Harris

A number of questions and issues have recently been raised that caused me to pause – and think –

about field standards related to the entire weights and measures system. Normally, I would focus on 

just the laboratory applications and considerations, but I think it’s time for some integrated thinking 

about field standards, their acceptance, and use. A number of key questions are presented here related 

to manufacturing, calibration, and use of field standards, such as: Are the field standards “fit for 

purpose”? Do the field standards comply with a documentary standard or other specifications and

tolerances? What legal authority and responsibility do jurisdictions have regarding choice and purchase 

of field standards, compliance, metrological traceability, and acceptance of assessments or calibration 

reports? What should laboratories provide “the customer”?

Fit for Purpose

One of the big questions that should be asked in the measurement community is whether [field] 

standards are “fit for purpose.” The concept of fit for purpose is common in method validation practices 

to ensure that adequate quality and suitability is designed into procedures. The concept can also be 

applied to field standards. Two quick examples might be: Class F field standard weights (NIST Handbook 

105-1, Specifications and Tolerances for Field Standard Weights) are not suitable for evaluating a Class II 

weighing device (balance). They are not fit for purpose. Small Volume Provers (NIST Handbook 105-7, 

Specifications and Tolerances for Dynamic Small Volume Provers) are not suitable for evaluating Liquid 

Petroleum Gas (LPG) meters. They are not fit for purpose. While these statements could be debated, if 

one reviews the calibration uncertainty and tolerances for the field standards, it will quickly become 

obvious that Class F weights will not meet the Fundamental Considerations for evaluating a Class II 

device. The Fundamental Considerations require that the uncertainties in the standards be less than 

one third of the applicable tolerances. The tolerances on the mass standards are just too big. One could 

also argue that Small Volume Provers could be used for LPG meters; however, during the initial 

evaluation and development of the NIST Handbook 105-7, there was NO data collected or evaluated 

that demonstrated that Small Volume Provers could successfully be used to evaluate LPG meters. Thus, 

data are required to demonstrate suitability. 

An additional question that might be asked in this section is whether the field standard allows for 

replication of the use of the weighing or measuring instrument. An example here is whether or not it is 

suitable to remove the nozzle from a retail motor fuel dispenser to evaluate a meter – and whether 

there are sufficient steps in the evaluation process to ensure that the verification of the device complies

with NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and

Measuring Devices, and is fit for purpose (i.e., for consumers to use a meter tested this way to fill their

vehicles with adequate accuracy). This entire concept goes back to an article I wrote in June 2005 on the

Calibration of Rocks (http://www.nist.gov/pml/wmd/labmetrology/upload/h-006.pdf). A laboratory can

provide a perfectly valid mass value and uncertainty for a rock, but that does not mean the rock is suitable 

for use as a mass standard in testing a scale.

Compliance Assessments

A common practice among calibration laboratories is to evaluate compliance of the measurement

result and the uncertainties against published tolerances only. In fact, many calibration certificates say
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something along the lines of “the measurement results comply with the tolerances of XYZ Handbook.” A

key aspect of this practice to consider is that the calibration uncertainties must always be smaller than

the applicable field standard tolerances to claim any level of compliance. If a situation occurs such as

is shown in Figure 1, in Case A, the measurement uncertainty is bigger than the tolerance and a 

statement of compliance cannot be made with the usual confidence, in spite of the value being at

nominal! In Case B or Case C, there is doubt about the measurement result – which is why most

metrologists will also adjust a field standard and you may see “as found” and “as left” calibration results

reported on a calibration certificate.

What is also critical for weights and measures officials to know is that many laboratories do NOT evaluate

field standards for compliance to all of the specifications in a documentary standard. It is important to

make sure you know this when covering any of the subsequent sections in this article! For example,

cast iron 1 lb weights are manufactured, used by service companies, and calibrated by laboratories – yet

they do not comply with NIST Handbook 105-1 for field standard weights because of the material

requirements for weights between 0.01 lb – 10 lb (5 g – 5 kg). A volumetric prover or LPG prover may or

may not be evaluated against NIST Handbook 105-3, Specifications and Tolerances for Graduated Neck

Type Volumetric Field Standards, or 105-4, Specifications and Tolerances for Liquefied Petroleum Gas and

Anhydrous Ammonia Liquid Volumentric Provers, even if new and just being put into service – unless

the evaluation was requested.

Weights and measures jurisdictions may require compliance to NIST Handbook 105-series standards, or 

an ASTM standard (e.g., proving rings used for wheel load weigher testing), or OIML R 120 for volumetric 

standards. It is important to 

specifically request an evaluation 

of compliance to a standard for a 

weights and measures program

when evaluating the standards 

field staff use, when designating 

suitable field standards for service 

companies, or when accepting 

calibration reports from other 

calibration laboratories. NIST 

recommends full compliance 

evaluations whenever possible –

yet, we have no regulatory 

authority to ensure that 

laboratories perform this function. 

It is up to the jurisdiction to 

mandate compliance evaluations. 

ISO/IEC 17025, Section 5.10.4.2 

indicates that when compliance statements are included on a calibration report, they must specifically 

include what was or was not assessed: “If a statement of compliance with a specification is made, this 

shall identify which clauses of the specification are met or not met.” So, if a field standard was only 

evaluated for measurement results to tolerances, the report should specifically state that “no evaluation 

for compliance against the specifications was made” in addition to “all standards were found/left in 

tolerance according to this handbook.” An example here might be a volumetric prover made from

aluminum. Aluminum is not allowed according to NIST Handbook 105-3 due to the excessively large 

cubical coefficient of expansion. Yet, a laboratory could perform a valid calibration and find the prover in 

tolerance and produce an acceptable calibration certificate, as long as the report indicated no evaluation 

Figure 1. Example Values, Uncertainties and Tolerances.
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was done. In this case, not only would the prover NOT comply with the standard, it would also not be fit 

for purpose.

There is an ongoing challenge when no documentary standard exists that includes both specifications and 

tolerances. In the scheme of the NIST Handbook 105-series standards, Handbook 105-8, Specifications 

and Tolerances for Field Standard Weight Carts, for weight carts used to evaluate scales, is relatively new. 

Prior to this handbook being published, there were many weight carts already in use throughout the 

country by both weights and measures jurisdictions as well as by service companies. No one was able to 

assess weight carts for compliance, because no standard existed.

Legal Responsibilities

Most jurisdictions in the United States have adopted into law language similar to the model laws 

provided in NIST Handbook 130, Uniform Laws and Regulations in the Areas of Metrology and Engine Fuel 

Quality. In the current version (in fact going back to 2005 when some key changes were adopted), the

model law and the registered service persons program designate some key responsibilities and 

authorities to the weights and measures director. The director has the authority to accept calibration 

reports from other calibration laboratories and retains the right to inspect and evaluate the standards 

that will be used. This right is especially critical if the laboratory performing the calibration did not 

evaluate the field standards for compliance to a documentary standard that is required in your 

jurisdiction. When a calibration report is submitted which states that the standards were not evaluated 

for compliance, a weights and measures jurisdiction should conduct an inspection or evaluation of the 

field standard when specific documentary standards are required. 

The model laws also allow for specific calibration intervals or variable calibration intervals based on 

statistical analysis of stability of the standards. According to ISO/IEC 17025 for the laboratories, no 

calibration intervals are to be included on a calibration certificate unless requested by the customer or

unless there is a legal requirement. Service companies may not be aware of legally required calibration 

intervals when requesting calibration services, so it is a good idea for laboratories to ask customers where 

or how they intend to use the standards and if there are required calibration intervals. We have

previously published some baseline calibration intervals for a variety of field standards, where local 

authority allows for variable intervals based on data. 

See: http://www.nist.gov/pml/wmd/labmetrology/upload/h-009.pdf.

Supplier Evaluations – Calibration Laboratories

When accepting calibration certificates from a laboratory, there are some concepts from the ISO/IEC 

17025 standard (as applied to calibration laboratories through recognition and/or accreditation), that are 

useful. Compliance with ISO/IEC 17025 requires assessment of the evidence for metrological traceability 

of the measurement results. This is also defined in NIST Handbook 143, Weights and Measures 

Laboratories Program Handbook, Program Handbook for Laboratory Recognition, and in NIST Handbook 

130, which indicates that evidence of metrological traceability may be assessed through laboratory 

recognition or accreditation. 

However, even if a laboratory is or was recognized or accredited, it is important to ensure that they still 

are recognized or accredited at the time of calibration when accepting the calibration certificate AND that 

the measurements in question are actually on the laboratory’s measurement Scope. The Scope defines 

the measurement areas for which the laboratory has been recognized or accredited. For example, a 

laboratory only recognized or accredited for mass has not demonstrated metrological traceability and

proficiency for volume. It is also important to ensure that the laboratory has the capability of providing 

measurement uncertainties that are sufficiently small for your applications. Otherwise, a calibration 
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might have uncertainty bars similar to Case A (Figure 1). Laboratories have a responsibility to consider the 

needs and requirements of the user, so the example in Case A should never occur, but again, there is not 

a regulatory body that enforces this.

Contract Review and Customer Service

Another concept applied in the laboratories for ISO/IEC 17025 is that of contract review. We often hear 

the argument that, “we are a government laboratory and don’t have authority to sign contracts; 

therefore, this section is not applicable for us.” Not true! 

When a laboratory accepts a standard and provides a calibration, they have “agreed” to perform a 

calibration and provide a calibration report. The key questions that must be considered at every level 

are, “What does the customer need?” and “Are we recognized or accredited to provide that calibration

service – is it on our Scope?” NIST OWM has responded to a number of recent inquiries where a 

standard was calibrated by an accredited calibration laboratory in a jurisdiction, without a full compliance 

assessment. When the service company receiving that calibration submitted the calibration report to 

another jurisdiction, assumptions were made about compliance assessments of the field standards during 

the calibration. As noted before, unless the calibration report specifically claims that the standards were 

fully evaluated for compliance to a documentary standard, one should NOT assume this to be the case. 

Also, as noted before, service companies may not be aware of legally required assessments and/or 

calibration intervals. 

Suitability of Field Standards – in Use

I would like to conclude with some challenging thoughts and questions. Calibrations of field standards 

may or may not involve a compliance assessment. However, calibrations are also done at one point in 

time, under controlled laboratory conditions, using laboratory procedures. Yet, field standards are used 

in a wide variety of conditions for various applications, sometimes deviating from the specific 

examination procedure outlines. How do we know that the field standards are actually suitable for a 

wide variety of conditions in use? How confident are we in our measurement results when field 

standards knowingly do not comply with standards? How can we be sure a calibrated value can be 

replicated in field use? How can we be sure deviations from the field procedures produce reliable 

results? And finally, how can we responsibly take enforcement action with all of these questions and 

doubt? 

Some steps taken to provide assurance are to define specifications and tolerances that are intended to 

allow for or account for these various conditions and deviations. Every effort is made to consider suitable 

specifications and tolerances to ensure that when field standards are used, good quality measurements

are possible in a variety of conditions. However, additional factors often need to be considered. Yet, 

even a good specification and full compliance along with a good calibration is not the full answer and can 

leave a sense of doubt. Here are some specific examples and questions about standards used in the 

weights and measures system that should be considered, the impact should be evaluated, and 

procedures should be followed. 

Selection of weights that comply with NIST Handbook 105-1 and a current calibration might not address 

all of the user requirements.

• Laboratories have observed Class F weights being out of tolerance on the day they are brought 

into the laboratory for calibration, only to find that they are in tolerance once they have properly 

equilibrated with laboratory conditions (according to the standard operating practice [SOP]). 
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How often do laboratories calibrate field weights without allowing suitable equilibration? How 

often are weights used outside of normal laboratory operating conditions in the field?

• Laboratories have observed errors on precision weight values that were over 300 times the 

reported uncertainty of the calibration, due only to a 2 °C gradient in the calibration laboratory. 

This would be observable when evaluating a Class II balance. How often are the environmental

conditions measured when evaluating a Class II balance? Are the weights allowed to equilibrate?

Weights and measures officials or service agents have reported that they use Class F weights for 

evaluating Class II balances. How often is this done? Should a laboratory provide calibrations for 

weights to meet the required weight classification (e.g., OIML Class F2) if they are recognized for 

Mass Echelon III only (the answer is “no”)?

• Most state calibration laboratories do not have the ability to evaluate density, surface finish, or 

magnetism requirements in some mass specifications to determine compliance. Therefore, that 

laboratory is not able to perform a full compliance assessment. However, most manufacturers

are able to perform these additional assessments. 

Selection of glassware that is marked Class A might not meet all of your needs.

• Laboratories have reported that up to 50 % of the volumetric glassware purchased for use in 

package checking is out of tolerance when purchased. How often is the glassware used without 

calibration because it is purchased or marked as Class A by the manufacturer? 

Selection of a test measure or prover that complies with NIST Handbook 105-3 with a calibration might 

not address all of the user requirements. 

• Laboratories hear reports from service agents and field officials that they do not correct the 

volume of large volume provers (certainly not test measures) for the actual volume at time of 

test based on temperature. Most provers and test measures are calibrated to a reference

temperature 60 °F in the United States. Are field officials correcting the volume of the prover 

real-time? Are they using calibrated thermometers with evidence of metrological traceability?

• In some recent instances, the standards were being exported and needed to comply with OIML R 

120. Are laboratory staff familiar enough with the additional documentary standards that might 

be requested to assess compliance?

• How many large volume provers are grandfathered in because they don’t meet specifications 

and/or are calibrated in the field without suitable environmental controls?

Selection of a prover that complies with NIST Handbook 105-4 with a calibration might not address all of 

the user requirements.

• Laboratories observe that pressure gauges on LPG provers have been changed between 

calibrations. Provers are often calibrated without evaluation of the pressure gauge calibration. 

Are the replacement pressure gauges calibrated, and do they agree with the previous gauge? 

Again, are provers being used with calibrated thermometers with evidence of metrological 

traceability? 

Selection of a timing device or thermometer that complies with NIST Handbook 105-5, Specifications and 

Tolerances for Field Standard Stopwatches, or 105-6, Specifications and Tolerances for Thermometers, 

with a calibration might not address all of the user requirements.
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• Many laboratories are not recognized or accredited to evaluate timing devices and to calibrate 

thermometers or to assess these standards for compliance. How many are in use without 

evaluation or calibration? 

Use of a Small Volume Prover that complies with NIST Handbook 105-7 with a calibration might not 

address all of the user requirements. 

• Small Volume Provers were only assessed during development of Handbook 105-7 for refined 

fuel and fuel oil (and not LPG). The integral pressure sensors and thermometers are often not 

calibrated by the same laboratory when the volume is calibrated.

• Small Volume Provers are calibrated under laboratory conditions with controlled flow, pressure, 

and temperatures, but we don’t have data for their accuracy under a wide variety of flow rates 

and field conditions. When initial assessments were done, performing the slow flow test 

identified a leak in the system that was not obvious during the fast flow test. How often do short 

cuts take place when evaluating a meter to save time? 

• An industry laboratory was accredited to perform calibrations of a Small Volume Prover 

gravimetrically and inappropriately demonstrated their proficiency by calibrating a Class F mass 

standard. 

Use of a weight cart that complies with NIST Handbook 105-8 with a calibration might not address all of 

the user requirements.

• Weight carts are often used without the associated checklist in Handbook 105-8. Weight carts 

are often calibrated without evaluating them for compliance with the standard and have been 

grandfathered. What is the impact of deviations? Laboratories have data to show the impact of 

water on the surface that is wiped off and appears dry, with the cart being out of tolerance. Are 

weight carts ever used on days with precipitation? Handbook 105-8 instituted a smaller gas tank 

with the use of correction weights to account for the mass of fuel lost during use. Field staff 

often does not account for these mass changes and many weight carts found in service have not 

been modified to comply with Handbook 105-8. 

The examples provided here are just the tip of the iceberg and only cover those situations where a NIST 

Handbook currently exists. As you read this article, you may have had additional questions – and even 

examples! Feel free to contact me with additional examples and questions at gharris@nist.gov.