We recently received this request:
LABORATORY CALIBRATION SERVICES SUPPLIER AGREEMENT
Laboratory Suppliers for Calibration Services must adhere to ISO/IEC 17025:2017 Standards for Metrological Traceability. This standard includes the following statements.
- Equipment Calibration supplier services must be traceable to NIST standards. The calibration service must note on their calibration certificate how they are traceable to NIST standards.
We informed this customer that we could not meet their request. Why?
ISO 17025:2017 section 6.5.2 The laboratory shall ensure that measurement results are traceable to the International System of Units (SI) through:
- a) calibration provided by a competent laboratory; or
NOTE 1 Laboratories fulfilling the requirements of this document are considered to be competent.
- b) certified values of certified reference materials provided by a competent producer with stated metrological traceability to the SI; or
NOTE 2 Reference material producers fulfilling the requirements of ISO 17034 are considered to be competent.
- c) direct realization of the SI units ensured by comparison, directly or indirectly, with national or international standards.
A.3 Demonstrating Metrological Traceability
- Calibration and measurement capabilities provided by national metrology institutes and designated institutes that have been subject to suitable peer-review processes. Such peer-review is conducted under the CIPM MRA (International Committee for Weights and Measures Mutual Recognition Arrangement). Services covered by the CIPM MRA can be viewed in Appendix C of the BIPM KCDB (International Bureau of Weights and Measures Key Comparison Database) which details the range and measurement uncertainty for each listed service.
- Calibration and measurement capabilities that have been accredited by an accreditation body subject to the ILAC (International Laboratory Accreditation Cooperation) Arrangement or to Regional Arrangements recognized by ILAC have demonstrated metrological traceability. Scopes of accredited laboratories are publicly available from their respective accreditation bodies.
You will notice that NIST is not mentioned. Are NIST standards not good enough? This is a common misunderstanding as to the role of NIST in calibration. Traceability is to the SI Unit not to a physical standard owned by an organization.
What is the difference between a NIST standard and the SI Unit?
Physical standards deteriorate over time. The SI units, more commonly referred to as the metric system are the basis for modern calibrations.
There are 7 base SI units:
- m - Meter - Length
- s - Second - Time
- mol - Mole - Amount of Substance
- A - Ampere - Electric Current
- K - Kelvin - Temperature
- cd - Candela - Luminous Intensity
- kg - Kilogram - Mass
SI units were chosen as they are considered independent of dimension.
Why does this matter and what impact does it have on measurement and calibration?
A key consideration in any measurement is measurement uncertainty. This is the +/- in any measurement. Think of using a ruler that has 1/32 inch divisions. While you can guess where a measurement might fall in between the divisions, you only really know 1/32 of an inch. If you could measure to 1 millionth of an inch you would have a smaller measurement uncertainty than 1/32 of an inch.
The desire for smaller measurement uncertainty has lead scientists to research involving SI units for measurement. As advances in science occur it is likely that measurement uncertainty will decrease.
That being said, when you purchase calibration services you do want traceability to the SI unit. In some cases, traceability can be through NIST. This is why you will see language that calibrations are traceable to the SI unit through NIST or another NMI(National Metrology Institute). NIST is the National Institute of Standards and Technology in the United States. The UK has the National Metrology Institute through NPL. There are many National Metrology Institutes in addition to NIST. The BIPM actually hosts meetings of the Directors of NMI's to discuss world wide issues in metrology.
The move away from physical standards to the SI unit was an effort to provide uniformity of calibration worldwide.
Measurement Traceability in Calibration
The VIM- International Vocabulary Of Metrology - 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". It also refers to a measurement hierarchy which we illustrated above.
Another way to visualize traceability is the practice of re gifting. You receive a gift, but instead of opening it and re wrapping it you simply wrap over the original gift and give it to someone else and write your name on the packaging. They do the same and so on. The original gift is the same, but all the packaging changes the dimensions of the package and you know who wrapped it each time. You have an unbroken chain because each person that gifted signed their name and left the previous intact. If someone opened the gift and removed the original or previous packaging the traceability would be gone.
In the United States, NIST- National Institute of Standards and Technology- has developed a policy on Traceability.
"Metrological traceability requires the establishment of an unbroken chain of calibrations to specified references. NIST assures the traceability of measurement results that NIST itself provides, either directly or through an official NIST program or collaboration. Other organizations are responsible for establishing the traceability of their own results to those of NIST or other specified references. NIST has adopted this policy statement to document the NIST role with respect to traceability."