Talk NISTy to Me: When Everyday Words Take On Different Meanings in Measurement Science

NIST is unique as the national measurement science institute. We are the U.S. agency responsible for maintaining measurement standards, from the second to the kilogram. 

We help ensure that these units of measure are consistent across our watches and our grocery scales. But this is not as straightforward as it may sound, and our activities sometimes lead us to use everyday words in unusual ways. In this post, I’ll introduce you to some of these different meanings, and who knows? At your next party, some people may think that you work at NIST (if you don’t already work here!).

For starters, you’ll often hear our measurement scientists talk about artifacts. In everyday terms, they’re historical objects that tell us about a particular culture at an earlier time. Think of the Rosetta stone, the Dead Sea Scrolls, and the mask of Tutankhamun. 

But in measurement science, “artifact” has a somewhat different meaning. It’s a physical object that represents a unit of measure, such as the metal kilogram or a platinum bar for the meter.

Unlike cultural artifacts, which we try to preserve, scientists have been working to completely get rid of measurement artifacts. That’s because measurement artifacts are inevitably imperfect. It’s impossible to make two kilogram artifacts exactly the same. A kilogram artifact in the U.S. will have a very slightly different mass than a kilogram artifact in Paris, so our physical definitions of the kilogram might disagree slightly.

Scientists have been aiming to define measurement units with constants of nature, which are the same everywhere in the universe. For example, we now use the speed of light to define a meter and an energy jump in the cesium atom to define the second. We also use the Planck constant to define the kilogram. (See the documentary The Last Artifact to learn a lot more about this.)

Realizing What I Didn’t Know About Realizing 

At first, I didn’t realize that measurement scientists use realize in a different way than we use it in everyday conversation. At NIST, the word often means “turning a concept into physical reality.” For example, we build an atomic clock to realize the definition of a second, and we build a Kibble balance to realize the kilogram based on the Planck constant.

And while we’re at it, if you’re ever on a game show, you should remember that measurement scientists are called metrologists, a term that is dangerously close to “meteorologist.” I’ve seen very smart and knowledgeable people outside of NIST stumble over the term, and even a repeat Jeopardy champion lost the final round after not quite getting the term “metrologist” right!

At NIST, we talk about lots of standards, but we’re not describing our expectations for fine restaurants or formal attire (though we do our best on special occasions!). 

In our world, a standard is generally one of two things. First, it could be an agreed-upon way to define a physical quantity. This is called a measurement standard. So, an atomic clock is a measurement standard for the second, and the Kibble balance is a measurement standard for the kilogram.

Second, a standard can refer to a way to carry out a technical process, such as a method for testing mattresses for being flame-retardant. This is known as a documentary standard. While we do make documentaries about NIST, we’re talking about something different here. “Documentary” simply means that these standards are documented, usually in written form. 

Measuring Sensitively and in Harmony 

People in the standards world often talk about harmonization, but they are not thinking of barbershop quartets (though talented amateur musicians are prevalent on our campuses). In the standards realm, harmonization means taking multiple standards on the same topic, perhaps from different nations or organizations, and making a single, consistent standard upon which all parties agree. Harmonization is very important in international trade. For example, there are international standards for USB-C ports that are used in phones, computers and other electronic devices. This makes it easier for a manufacturer in one country to sell products that work in other countries.

Metrologists use words that sound like emotional characteristics but really have rigorous scientific definitions. For instance, NIST researchers take great pride in their sensitivity. In general terms, this is a measurement of the smallest object or weakest signal that can be detected, whether it’s a single particle of light from an atom or the tiniest change in the position of a mechanical object. The word comes up a lot on the NIST website and in NIST-authored papers; sensitivity is often an important attribute in measurement. Our researchers are rightly proud when they reach new levels of sensitivity.

Measurement scientists also have specific meanings for accuracy and precision. Accuracy means how close a measurement is to its actual “true” value. Precision means how exact that measurement is — 4 meters is not as precise as 4.005 meters, for example. To understand the difference, think about a game of darts.  If you are trying to hit the center, an accurate dart throw might be one that lands in the inner ring of the dartboard. An accurate AND precise throw would be one that lands right on the bull’s-eye. 

Confidence in Uncertainty 

One of the most important things that measurement scientists grapple with is uncertainty. In everyday life, the term reflects a state of doubt. Its scientific definition is not too different. No measurement is perfect. So metrologists state not only a measurement (for example, the wall is 8.0 meters wide) but also its uncertainty (plus or minus 0.1 meters). As NIST’s Antonio Possolo says, uncertainty is like the fog surrounding the true value of the property you’re measuring. 

The uncertainty depends on multiple factors, such as the fineness of a measuring instrument (for example, if the ruler has markings every millimeter), parallax (in which a person’s reading of a ruler is different based on the angle in which they view it), and last but not least, the number of measurements they make of the wall. 

Researchers often talk about the uncertainty budget, which takes all the possible measurement errors into account. Uncertainty is such a major part of measurement science that NIST’s League of SI Superheroes includes a character called Major Uncertainty. (He’s a villain, but personally, I think he’s badly misunderstood.)

One of my favorite terms I’ve recently encountered is finesse, which I learned from a recent experiment involving NIST researchers who created high-performance mirrors, known as “supermirrors.”  I secretly wish that finesse described the flair with which the researchers performed the experiment. But in this context, finesse describes the quality of a mirror, namely how many times a particle of light can bounce between two mirrors before being absorbed. The more times it bounces, the higher the finesse. (And talk about impressive — the researchers built supermirrors that had a finesse of greater than 400,000!)

Setting the Standard for Reference Materials

Matrix is another term that often has a special meaning when you hear it in our halls. It comes up when talking about our standard reference materials (SRMs), including our ones for food. The matrix is the bulk of the material, such as carbohydrates, which contains other materials, such as vitamins and minerals, in smaller amounts. 

An example is our SRM for cat food. We measure the heck out of a carefully prepared sample of cat food so that we can provide accurate and precise values for how many grams of macronutrients (carbohydrates, fats, proteins) and how many milligrams of micronutrients (such as amino acids) it contains. Food manufacturers can use our SRM and measure these nutrients with their equipment. If their results match ours, they can be assured that their measurement equipment is accurate. 

I’ll let you in on a bit of a secret: The cat food SRM stands in for similar foods whose nutrient profiles are similar. So, if a manufacturer makes other kinds of pet foods, it may use the cat food to test its equipment. Because the cat food SRM works well for pet foods in general, we don’t have an SRM for dog food (very sorry to all the dog lovers out there!).

This is true for many foods. For example, in the case of spaghetti sauce, one of our SRMs for infant formula comes closest in terms of its nutrient profile. So a sauce manufacturer could use its equipment to measure the nutrients in our infant formula SRM to make sure it is providing accurate nutritional information for its sauce. Even though these are different foods, the results are generally very accurate.

So that’s a brief tour of what some ordinary words mean at NIST. Though our researchers may sometimes sound like they are living in an alternate reality, like the characters from The Matrix, they are very much talking about the world we live in — in their own NISTy way.