“NIST-traceable” is often invoked as though it has some sort of magical metrological power, imparting accuracy to all it touches. In reality, it is only one of several elements of good calibration. What’s worrisome in the practice of equating NIST-traceability to accuracy is the perception that it supersedes the other, equally important elements of calibration.

The National Institute of Standards and Technology (NIST) is the USA’s federal agency responsible for ensuring that we adhere to a common measurement system. NIST, along with the world’s other National Measurement Institutes (NMIs), describes our shared metrological language, without which many critical industries—life sciences, meteorology, aerospace, and more—would live in a Tower of Babel situation, metrologically speaking. This is because, without global reference standards that are shared, agreed upon, and ratified, for instance, a component madeto- measure in Malaysia might cause a catastrophic failure when assembled as part of an airplane in the U.S. We need to agree on a standardized “vocabulary” of weights and measurements; in effect we agree to accept a reference.

BY ANY OTHER NAME
But, isn’t a meter always a meter? Unfortunately, no. There is no absolute truth in measurement. Measurement is really about probability. The percentages of times the measurement will be different are the uncertainties of that parameter. What we are left with is “close enough” or, the nearest we can currently get with the technologies and equipment available. Fortunately, for most applications, close enough is good enough. So long as measurements are standardized and communicated internationally—by our National Measurement Institutes (NMIs) like NIST,¹ PTB,² NPL,³ etc.—we have a global “mother tongue” of accuracy. But a shared language does no good unless it is applied. This is the goal of traceability. Traceability is what allows us to use international standards as the references for calibrating measurement instruments, so that mètre, even when called a “meter,” is still the same length.

TRACEABILITY V. ACCURACY
What is traceability? According to NIST, “[it] requires the establishment of an unbroken chain of comparisons to stated references.” NIST stands by its own measurements and can provide standard reference materials and calibrations that we can be assured conform to other NMIs around the world. Since it’s not possible to have NIST calibrate every instrument, traceability allows us to propagate measurement standards.

So we create a chain that can be traced back to the reference. If Cal Lab A has a thermometer calibrated by NIST, then Cal Lab B can have their thermometer calibrated by Cal Lab A, and be NIST-traceable. Cal Lab B can then calibrate thermometers for Lab C, and so forth.