What Is the Traceability of an Instrument?
In order to answer the question posed in the title: "What is the traceability of an instrument?", we will see:
- What is an international measurement standard?
- How is a laboratory accredited by NIST or NRC?
- What is a traceability certificate?
- When is such a document needed?
What is an international measurement standard?
According to the International vocabulary of metrology (VIM), it is "a measurement standard recognized by signatories to an international agreement and intended to serve worldwide" (1). Good. Now, what is a measurement standard? It is the "realization of the definition of a given quantity, with stated quantity value and associated measurement uncertainty, used as a reference" (1).
Let's take a closer look at this definition.
Realize a quantity by its definition. What is a quantity? The International System of Quantities (ISQ) has defined 7 basic ones: length, mass, time, electric current, thermodynamic temperature, quantity of matter and light intensity.
The International System of Units (SI) comes into play here. The SI provides a base unit for each of the quantities in the ISQ. As of 2019, all SI units are defined according to physical constants (2). The last unit defined in this way was the kilogram. The creation of a standard therefore consists of using these SI definitions and making them into an entity that can be used as a reference, with a value and an uncertainty.
In a schematic manner:
An international standard is therefore a reference entity recognized worldwide as being of "true" value.
How is a laboratory accredited by NIST or NRC?
The National Institute of Standards and Technology (NIST) in the United States or the National Research Council of Canada (NRC) are two federal agencies that have the second type of standard: a national measurement standard. This reference entity is one that is recognized for a given state or economy. It can generally be compared with the international measurement standard.
In Canada, laboratories are certified by the NRC Calibration Laboratory Assessment Service (CLAS) and accredited by the Standards Council of Canada (SCC) (3).
According to VIM, a calibration is the action of comparing X with a standard, in terms of value and uncertainty of measurement.
For the laboratory to be accredited, it must demonstrate that its working measurement standards are derived from an unbroken chain of calibration from an international measurement standard, with documentation of the uncertainty of each calibration level. It must also have the personnel and equipment necessary to perform calibrations with high accuracy. Finally, it must pass the CLAS assessment for ISO/IEC 17025 accreditation.(4)
A Type I laboratory is used for the calibration of standards.
A Type II laboratory is used for the calibration of measuring instruments from working measurement standards. It produces a document that establishes whether the equipment tested conforms to the manufacturer's standards.
What is a traceability certificate?
Metrological traceability is a property according to which the calibration of an instrument has been done according to an uninterrupted and documented chain of calibrations, each contributing to the uncertainty of measurement (1). This chain must be hierarchical with increasingly precise references. The documentation must include the unique identification number of the measurement standard used, as well as the date on which its calibration was performed. When we speak of "traceability to the SI", the top of the hierarchy must be an international measurement standard.
Here is, in a schematic way, the hierarchy necessary for the certification of a measuring instrument.
A certificate of metrological traceability to the SI, provided by a laboratory accredited by the Standards Council of Canada and the National Research Council of Canada (SCC-NRC), is therefore based on working measurement standards. The laboratory must also follow strict procedures defined to provide the necessary accuracy when calibrating an instrument. The NIST operates in a similar manner in the United States.
To obtain the certificate, the user of the instrument can choose calibration points. This allows an assurance of the accuracy of the instrument at targeted values. This choice should be made according to the final conditions of use of the instrument.
What is the difference between adjustment, calibration and traceability?
In common usage, a calibration is the act of correcting measurements to better match expected values. According to VIM, however, this definition corresponds to that of an adjustment.
Calibration, or the comparison of a measurement with a standard, is a prerequisite to adjustment. It is through calibration that a device can be adjusted to provide more accurate measurements at certain points.
A laboratory accredited to provide a certificate of traceability must therefore calibrate the device to verify its compliance. Sometimes it is also possible to have the laboratory adjust the device, if the device has an adjustment function (sometimes called a calibration function).
A traceability certificate is therefore not a guarantee that the device is adjusted to be more accurate. It is a certification that the measuring instrument conforms to the manufacturer's standards of accuracy, by comparing it to an unbroken chain of standards.
An example of a traceability certificate
This is what an NRC traceability certificate might look like. It was done on a USB RTD probe adapter (USB-RTD300-CAL). The temperature is then used as the quantity to calibrate this instrument.
Since this measuring device is adjustable (the manufacturer says "calibratable"), during certification, the instrument was also adjusted to give more accurate readings over a specific range (18°C to 36°C). Here is the document attesting to this.
You may notice a column with the title "Expanded Uncertainty". Briefly, this is the standard deviation of the measurement, multiplied by a constant K, which here is 2 (1).
When is such a document needed?
The International Organization for Standardization (ISO) defines International Standards approved by committees of experts. These standards are derived from the needs of a market and cover a very wide range. There are currently 24,495 standards. They are established by consensus of experts and other committee members, such as consumers. ISO Standards are a guarantee of confidence in the quality of a product or process.
We briefly mentioned ISO/IEC 17025 above. This international standard "enables laboratories to demonstrate their competence and ability to produce valid results [... and facilitates] cooperation between laboratories and other organizations by ensuring better acceptance of results in different countries." (5) In the field of metrology, this means that laboratories with this certification use reliable measuring instruments and procedures.
An NRC-accredited laboratory that passes the CLAS assessment for ISO/IEC 17025 is therefore certifying that the calibrated, SI-traceable measuring instrument will produce results that are internationally recognized as valid. The same is true for NIST accredited laboratories.
In some industries, it is necessary to demonstrate this validity. A traceability certificate assures the purchaser that the measuring instrument complies with this standard.
We hope to have clarified some basic questions about the traceability certificate, calibration, adjustment and the various international standards. If you have any questions or comments, we will be happy to answer them!
- BIPM, IEC, IFCC, ILAC, ISO, IUPAC, IUPAP, and OIML. International vocabulary of metrology | Basic and general concepts and associated terms (VIM). Joint Committee for Guides in Metrology, JCGM 200:2012. (3rd edition). URL: https://www.bipm.org/documents/20126/2071204/JCGM_200_2012.pdf/f0e1ad45-d337-bbeb-53a6-15fe649d0ff1.
- BIPM. Brochure sur le Système international d’unités (SI) v.1.08 (9e édition) ISBN: 978-92-822-2272-0
- Gouvernement du Canada (2022). Notes supplémentaires - Service d’évaluation de laboratoires d’étalonnage. [online] Conseil national de recherches du Canada. Available at: https://nrc.canada.ca/fr/certifications-evaluations-normes/service-devaluation-laboratoires-detalonnage/notes-supplementaires [Accessed 22 Sep. 2022].
- Gouvernement du Canada (2019). Obtenir la certification CLAS. [online] Conseil national de recherches du Canada. Available at: https://nrc.canada.ca/fr/certifications-evaluations-normes/service-devaluation-laboratoires-detalonnage/obtenir-certification-clas-1-commencer [Accessed 22 Sep. 2022].
- ISO. (2017). ISO/IEC 17025 - Exigences générales concernant la compétence des laboratoires d’étalonnages et d’essais. [online] Available at: https://www.iso.org/fr/publication/PUB100424.html [Accessed 26 Sep. 2022].