Traceable Calibrations and Reference Standards

Here in the USA, most literature for flow instrumentation and/or instrumentation calibration services includes a statement explaining that the device’s calibration is NIST-traceable. Other countries may see similar statements such as PTB-traceable, or INMETRO-traceable calibrations. Such statements are an important part of meeting performance goals, but are not necessarily clear on how such traceability impacts the uncertainty of the instrument’s measurement after calibration.

National Reference Standards

The Bureau International des Poids et Mesures (BIPM) is a global, inter-governmental organization focused on all matters related to measurement science and standards. BIPM defines metrological traceability as: “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.” The reference in this definition refers to a particular national reference standard that is maintained by each country’s National Metrology Institute (NMI).

Most individual countries around the world maintain their own national reference standard(s) for flow and other types of calibrations at their respective NMIs. The following are examples of NMIs in several countries:

USA – National Institute of Standards and Technology (NIST)
Germany – Physikalisch-Technische Bundesanstalt (PTB)
Japan – National Metrology Institute of Japan (NMIJ AIST)
Brazil – Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMETRO)

As each individual country maintains their own reference standard(s), discrepancies between these can potentially have a negative impact on global commerce. For example, the measurements provided by 2 flow meters on a natural gas pipeline that crosses multiple countries may not match because their calibrations are traceable to the reference standards of 2 different countries. To minimize the impact of such problems, many NMIs around the world are a member state of the BIPM organization.

Among other goals, the BIPM works to ensure consistency and improve measurements across the national reference standards maintained by NMIs of all member states. This includes comparison of an NMI’s reference standard(s) against those of others using transfer standards to correct or quantify any differences in measurement results between countries. For example, Mesa’s ML-800 series primary flow calibration standard is currently being used as a transfer standard in a comparison study of the reference standards maintained by 12 different NMIs in Europe (EURAMET Project 1325).

Primary and Secondary Calibration Standards

Each NMI’s national flow reference standard(s) is classified as a primary calibration standard. A primary flow standard is not calibration for flow measurement specifically, but rather, is calibrated based on a unit of measure that is classified as “fundamental.” For primary flow standards, the most common fundamental calibration units include:

Length/Distance
Time
Mass
Temperature

A calibration standard classified as a secondary standard is a component that is calibrated directly against a primary standard, or in a traceable chain ending at a primary standard. NIST provides a great example of such a traceability chain in their natural gas flow meter calibration program. Critical Flow Venturi (CFV) nozzles are individually calibrated as secondary standards against NIST’s primary calibration standard, and the nozzles are gradually coupled together and calibrated as further secondary standards to scale up to higher flows. Large banks of these secondary standard nozzles are then used to calibrated large turbine meters as another set of secondary standards, which are then used to calibrated the actual natural gas flow meters.

The fundamental unit calibration is a major reason why the primary flow standards typically have tighter uncertainty statements compared to secondary flow calibration standards. In flow calibrations, several second-order effects impact the uncertainty of the calibrated instrument’s measurement. For example, temperature variations in gas flows change the density and viscosity of the gas, which increase the measurement uncertainty provided by many flow instruments used as secondary calibration standards.

Primary flow calibration standards that calibrate the distance a piston is displaced or compare the mass of the flow media before and after calibration are largely immune to such second-order effects. This is why calibrations directly against primary standards typically result in the tightest uncertainty statements. The ability of a primary flow standard to provide a measure of specified flows is typically verified, but the actual calibration is based solely on a fundamental unit of measure. This is a key distinction to recognize.

Maximizing the Benefits of NMI Traceable Calibrations

Users of new calibrated flow instruments or flow calibration services should consider the closing portion of the traceability definition to truly understand the impact NMI traceability has on the uncertainty of the calibrated instrument, “…each (calibration) contributing to the measurement uncertainty.” In other words, the higher the number of secondary standard calibrations in the traceability chain between the primary standard and the actual instrument, then the higher the calibration uncertainty should be expected to be. For example, if 10 secondary standard calibrations took place between the primary standard and the actual instrument, that means that second-order effects have been compounded into and ultimately added to the instrument’s final calibration uncertainty. Reducing the number of calibrations in the traceability chain to 5 (or fewer) will result in tighter calibration uncertainty for the instrument.

A user interested in maximizing the benefits of NMI traceable calibrations on the uncertainty of their flow instrument(s) can address their concerns in a couple different ways:

The use of a primary calibration standard to directly calibrate the instruments reduces the impact of second-order effects on calibration results and shortens the traceability chain. This is one reason many metrology labs and flow instrument manufacturers use the Mesa Metrology series of primary gas flow standards to perform their calibrations.
A secondary calibration standard is also well-suited for many flow calibrations, and a few practices are helpful for users of calibrations using these standards.

NMI traceability alone does not provide a measure of capability of the calibration provider. Calibration providers accredited to standards like ISO 17025 (like Mesa Labs) or ANSI Z540 have demonstrated their capability to perform calibrations to an accrediting agency. All providers of such calibrations can provide a document that outlines the calibration types and limitations included in the accreditation as well as a calibration uncertainty statement. Such accreditations are available for calibrations using both primary and secondary standard, so verification is a good practice for any consumer of calibration services.

For more information on calibrations and standards, please contact a Mesa representative.

(Written in collaboration with Brandon Hansen, US Sales Manager, DryCal)