Traceability and the hierarchy of calibration

Advances in the science of measurement are simplifying the task of accurately quantifying uncertainty.

Michael Brown, Fluke Calibration
Accurate measurements are critical to every aspect of our lives; that’s why metrology is the oldest science in the world. Our ability to understand the world around us depends on how well we can measure it.

As time and technology have advanced, so too have metrology and the science of measurement. Each advance in metrology also requires a look back. Any calibrations or new measurement technologies must be compared against the International System of Units (SI). Until recently, these standards for comparisons were specially constructed physical artifacts that represented each measurement discipline. For example, the kilogram artifact was a cylinder of platinum-iridium alloy about the size of a plum. But in 2018, the SI was revised to base the measurements on constants of nature.

Why the change? Many of the artifacts were carefully kept under lock and key so they didn’t erode. Over the centuries, the materials used could change, erode, or become inaccurate which would throw off measurements down the line. That was just a level of uncertainty we all had to live with, making scientists unhappy for many years. But that uncertainty has vanished with the adoption of natural constants.

The change in standards practices involves the hierarchy of calibration and why it exists. Of course, ordinary technical personnel can’t directly check their

measurements against the measurement artifacts such as those stored at NIST. In the new SI many of the items needed to realize the new constants of nature in a laboratory are not readily available to all, or are cost prohibitive and large. So, not every measurement can be compared against these standards to ensure it’s accuracy. Instead, we rely on a sequence of calibrations all leading back to the SI. Every instrument used in the lineup has a spot on the hierarchy with the SI sitting at the top.

For example, a digital multimeter is calibrated by a multifunction calibrator. That multifunction calibrator was calibrated by a more accurate multi-product bench calibrator in a calibration lab. In turn, that bench calibrator was calibrated by even more accurate references in one of the National Metrology Institutes (NMI). And finally the references used in the NMI have been compared against the SI to determine their accuracy.

The necessity of traceability

Traceability documents the measurement hierarchy. The traceability pyramid creates an efficient and economic way for each lab or individual to access calibration standards. The series of tools and calibrators comprising the pyramid to the SI helps demonstrate the uncertainty level for each tool’s accuracy. Traceability steps form a chain of calibrations from the SI on down.

Below the SI sits National Metrology Institutes (NMI), the group that facilitates the promotion of the SI. However, every lab or individual can’t work directly with the NMIs. NMI-level calibration standards are used to calibrate primary calibration standards or instruments. Those primary calibration standards are then used to calibrate secondary standards. Those are then used to calibrate working standards. Each of these levels allows efficient and affordable dissemination of SI standards down the calibration chain.

The process of keeping traceability records is important for metrology as a whole, but also within industries. Many technical and quality industry standards require proof of the unbroken chain back to the pertinent SI unit.

The paper-based recording process has been cumbersome. As a tool is calibrated, the lab doing the work would print out a calibration certificate showing what was calibrated and how it was calibrated. This certificate would often include a statement of traceability or a list of calibration standards used for that particular calibration. The downside of this process was that each lab had their own way of recording calibration data. The calibration statement could vary from one lab to another–not all calibrations laboratories followed the same industry standards or fit in the same place on the calibration hierarchy.

As nearly everything is becoming digital, so too is traceability. By digitizing the calibration certificates, industry can have a database of traceability for each instrument. The traceability pyramid can become a digital calibration chain, containing information about measurements on the tools, as well as those above, below, or around them. The digitization of calibration certificates, in addition to the refreshed SI constants, brings the world one step closer to realizing the “for all times, for all people” idea behind the original SI.