Without us realizing it, metrology and its associated standards and services play a big role in our daily lives. We buy groceries that are weighed or measured. Prepackaged products are weighed or measured during production. Medicine at the pharmacy is measured to make sure it contains the right dosage. Most people trust the scale at the supermarket or the pump at the gas station without giving it much thought.

They can because of an extensive legal metrology system throughout the U.S. that’s based on weights and measures program activities across all states, territories, and numerous local jurisdictions. Legal metrology has been at the heart of every modern society for centuries. Yet it’s mainly invisible to the larger public.

What is legal metrology?

What is the legal metrology system, exactly? According to the International Vocabulary of Terms in Legal Metrology (VIML), metrology is the science of measurement and its application. Metrology can be found in a wide range of applications, from fundamental science research to routine healthcare activities, as well as in the daily operations of nearly all industry and manufacturing sectors.

In practice, the focus of scientific metrology is concerned with the accuracy (including uncertainty) of the measurement result. It doesn’t include an explicit statement of the trustworthiness of the presented measurement result, such as the assurance that the measurement result hasn’t been tampered with or manipulated. 

Legal metrology, on the other hand, is defined as the practice and process of applying statutory and regulatory structure and enforcement to measurement and its applications. Legal control is thus the practice and process of this regulatory structure, which includes safeguards and security measures provided by weights and measures technical requirements (e.g., the code in NIST Handbook 44).  In other words, metrology + legal control = legal metrology, which is put in place to ensure measurement accuracy and reliability.

An illustration of the definition of legal metrology

Without this trust in the underlying measurements, one can argue that the functions in our daily lives and transactions in our economy could be in doubt. The practice of legal control in metrology (VIML) includes:
• Establishing legal requirements and, if necessary, documentary standards
• Control and conformity assessment of regulated products and activities by certification, inspection, or testing bodies
• Supervision by regulatory authorities of regulated products and regulated activities
• Providing the necessary infrastructure for the traceability of regulated measurements and measuring instruments to the International System of Units (SI) or other national standards.

Legislators decide which measurements are subject to legal control. These measurements generally concern safety, health, environment, and commerce. Examples include speed meters used by law enforcement, blood pressure monitoring devices used in healthcare, and pharmaceutical scales in the production of critical medicines. However, the largest group of regulated instruments in the U.S. includes those used in commerce (or via commercial transactions), such as gas pumps, grocery scales, and transportation network measuring system devices.

Legal control in trade and commerce

The legal control of commercial instruments in the U.S. has multiple aspects. The state (or local) jurisdiction must have a law or regulation that places commercial instruments under their control and selects specific documentary standards, such as NIST Handbook 44, to be used by their officials to regulate these instruments. Without this, the standards are nonbinding and thus powerless.

The documentary standards for weights and measures define the design and performance specifications, test procedures, tolerances, and, in the case of NIST Handbook 44, user requirements for the applicable instruments. These standards provide model guidelines and requirements for all stakeholders—manufacturers, calibration and test laboratories, and inspection bodies. NIST Handbook 44 is developed collaboratively with all stakeholders through the National Council on Weights and Measures (NCWM). However, only state regulators decide on which standards to adopt into their legislation. To learn more about this process, see the NIST OWM and NCWM Roles FAQs.

There are several places throughout the life cycle of a measuring instrument where legal control takes place. The manufacturers use the technical specifications within NIST Handbook 44 to design measuring instruments. When the instrument is completely developed, it then undergoes a type evaluation and certification performed by the National Type Evaluation Program (NTEP) through the NCWM. The type evaluation demonstrates compliance with the design specifications within NIST Handbook 44; however, it doesn’t guarantee compliance of every single unit placed on the market. That’s why every single instrument is evaluated during an initial verification by the state authorities before it’s placed into service. After a successful evaluation, the instrument is sealed and receives a mark of legal metrological compliance. Once in service, its performance is monitored through regular inspections by state regulators.

Although the initial verification guarantees 100% testing of every instrument entering the marketplace, performance tests and evaluations in the field can be limited. Not every aspect that may influence the instrument, such as temperature or radio interference, can be adequately tested in the field, because such tests require specialized equipment generally only available in a laboratory environment.

To test such influence factors, the NCWM introduced the Verified Conformity Assessment Program (VCAP). Under the VCAP program, the manufacturer tests random samples from its production while the test procedures and equipment are audited by the NCWM. The VCAP program currently covers only devices covered by NIST Handbook 44, Section 2.20 Scales; 2.21 Belt-Conveyor Scales; 2.22 Automatic Bulk Weighing Systems; and 2.24 Automatic Weighing Systems.

Points of legal control over a measuring instrument during its life cycle

Chain of trust

The legal metrology system in the U.S. has a constant challenge of incorporating the latest technological advances and new instrument designs, which are becoming increasingly sophisticated and multifunctional in performance and communication. Measuring instruments aren’t necessarily single stand-alone devices anymore. In fact, some of their functionality may not even be performed by a “physical device” at all. For instance, some functionality may be entirely digital. Modern grocery scale transactions may send digital receipts via email, generated by business software that weights and measures inspectors can’t fully access. Our legal metrological system must adapt to meet these challenges.

Every electronic (digital) measuring instrument can be modeled through a simple illustration that initiates a sensor that converts the physical measurement property into a measurable signal. In the case of a scale, the load cell senses force and produces a voltage signal compared to a calibrated mass scale.

Once an analog signal is obtained, it’s converted to a digital quantity by the analog-digital converter (ADC). The next step is to scale the obtained number to the right quantity in the correct measurement unit. This is where the calibration parameters come into play. For example, a signal of 1.75 mV coming from the load cell of a scale corresponds with 2.50 kg. After the measuring result has been converted to the correct measurement unit, further processing may take place. Continuing with the scale example, this could be a tare operation or price calculation. Finally, when all the measurement data are available, they’re presented to the user in the form of a display or a printed receipt.

The presentation of the measurement result is the final step in the measurement cycle in which all legal metrological tasks have concluded. This “first final indication” represents the end point of legal control. In disputes, the parties rely on this presentation of the measurement result, which is trustworthy due to legal oversight throughout the measurement process.

In the weights and measures community, this model has been referred to as the “chain of trust.” If we extrapolate from the common phrase “a chain is no stronger than its weakest link,” then an official presentation of a measurement result can only be trusted if every step in this model is secured.

“Chain of trust” in a measuring instrument or legal metrology device

The steps in the chain of trust don’t necessarily reside all in a single device and, in some cases, can be purely digital in nature. Current measuring instruments may leverage the customer’s mobile device to present the measuring result. A cellphone can act as a display, receipts can be sent to an email address, and in some cases, the customer receives a QR code to view their receipt online. There’s an ongoing debate about whether a QR code itself is an adequate mode of receipt presentation. It’s been argued that the receipt with the full details needs to be downloaded and/or printed and in the customer’s possession before the presentation is complete, and thus not subject to tampering.

Regardless of the physical appearance of a measuring instrument, the steps to reach the presentation of the measuring result must all be under legal control. All aspects necessary to create and present the measuring result are what’s termed “metrologically significant,” or internationally referred to as “legally relevant.” A measuring instrument can, therefore, be regarded as a set of metrologically significant functions.

And every function must be secured in some way. The chain of trust of an instrument is very similar to the traceability of a standard. A standard is only traceable if all calibrations in between are traceable. Coincidentally, the security of the measuring instrument is also one of the steps in the traceability chain of a measurement result.

Device type evaluation

Legal control over measuring devices and instruments used for trade and commerce in the U.S. is traditionally focused on device inspections in the field. Legal control over trade and commerce measuring devices traditionally involves field inspections. However, some U.S. states don’t mandate type evaluations for measuring instruments, creating challenges in regulating complex, technology-driven devices. As technology evolves and current measuring devices often resemble a more complex measuring system, the inspection of instruments in use becomes more difficult. 

Without type evaluation, the points of legal control for NTEP evaluation (point B) and VCAP (point C) in the prior illustration don’t exist.

Points of legal control without type evaluation

Consequently, the first moment of legal control on a measuring instrument is then during the initial verification (point D) when it’s placed into service. At this point, the inspection body must assure itself that the instrument complies with all regulations and technical specifications in NIST Handbook 44. Lack of the necessary tools (both software and hardware), in-depth knowledge, and time make it difficult—or even impossible—for regulatory inspectors to perform some necessary examinations to ensure that all metrologically significant functions are secured.

To maintain a high level of trust in measuring instruments used for trade, more emphasis must be placed on the type evaluation of the instrument. In a laboratory environment, the instrument and its design can be examined in depth by experts in state-of-the-art technology.

During a type evaluation, it’s established whether the instrument is accurate and all its metrologically significant functions are secured when the seals are correctly applied. The functions and sealing method are described in the type evaluation certificate (NTEP Certificate of Conformity).

Conducting adequate type approval of the instrument before it’s available on the market makes the verification and inspection of a measuring instrument simpler and reduces the burden on the regulatory inspector in the field. One can assume that a type-approved instrument is fully secured if three conditions are satisfied:
1. The instrument (including software version) and the sealing method conform to the type evaluation certificate
2. The seals are intact
3. There are no physical signs of manipulation

When these three conditions are fulfilled, the regulatory inspector must verify that the instrument is still accurate and correctly used by its operator. If necessary, the regulatory inspector may perform additional tests and spot checks.

Legal control over a measuring instrument or device places trust in the measurement result. One can only trust the measurement result if every step can be trusted (i.e., every step is secured). With today’s technological advances, it becomes more difficult (or even impossible) to verify the security of every link in this chain of trust during field inspections. Therefore, the type evaluation of measuring instruments and devices becomes more relevant than ever to maintain our trust in measurements that support a strong and thriving U.S. economy.

Published Jan. 8, 2025, by NIST.