Now that the standard kilogram artifact has been replaced by a definition based on a reproducible arrangement, there’s a great DIY project for advanced students to consider.
Hackers, advanced hobbyists, makers, and hands-on physics experimenters are always looking for their next great project, and I think this is a good one: an apparatus to establish the “standard” kilogram. No, I am not talking about a physical artifact comparable to the one in Paris which was, until recently, the worldwide, formal SI kilogram. That’s so “yesterday.” Instead, I mean building a setup which, in theory, allows the re-creation of a “perfect” kilogram anywhere by anyone and at any time.
A little background: In November 2018, meeting in Versailles, France, representatives from the National Institute of Standards and Technologies (NIST) and 60 countries unanimously voted to officially transform the international system of units that underpins global science and commerce. The decades-long quest for a measurement system based entirely on unchanging fundamental properties of nature was officially confirmed for the defining metrology framework used throughout the world.
The seven base units in the SI are the meter, kilogram, second, ampere, kelvin, mole, and candela. These base units define the legal and scientific ultimate meaning of mass, distance, time, electric current, temperature, the amount of a substance, and luminous intensity. The goal of the new SI is to define all of these units completely in terms of seven fundamental constants with exact values, Figure 1.
Figure 1 All you need in theory to create non-artifact, reproducible primary standards are these seven definitions. (Image source: NIST)
For a long time, some of our well-known physical constants were defined by physical artifacts. For example, highest-level meter standard was a platinum-iridium bar in Paris with tiny scratch marks (it’s very hard to gauge those, that’s for sure!). That was replaced by a reproducible definition in terms of the number of wavelengths of a specific optical emission. Over time, most of the tangible “masters” were replaced by reproducible arrangements.
But the kilogram resisted replacement with a non-artifact standard. Until the 2018 vote, the primary kilogram – called the International Prototype Kilogram (IPK) – was a platinum-iridium cylinder kept in a vacuum jar (also in Paris) and only used every few decades to calibrate many secondary kilogram masters.
There were multiple problems with this IPK. First, calibrating your secondary standard against it was difficult, for obvious reasons. Second, the IPK itself had “issues” and was losing mass for a variety of reasons, some understood and some not. Whatever the cause, even a loss of a few micrograms or nanograms is significant at the desired level of precision. Finally, there’s a major philosophical question: if your primary kilogram is changing and you know that it is doing so, do all those secondary copies still have to go with the flow, so to speak?
The good news is that the new definition of physical constants defines the kilogram in terms of other physical constants rather than using a tangible artifact. A May 2020 article in the always interesting Physics Today — An atomic physics perspective on the kilogram’s new definition — discusses the principles of the arrangement needed to recreate your own mass standard using a Kibble balance (previously called the Watt balance, and that name is still in wide use), illustrated in Figure 2.
Figure 2 The cited article explains in some detail how the balance is set up to allow for establishment of a standard kilogram metric. (Image source: Physics Today)
The balance is set up so the electrical power required to levitate an object of mass (m) at velocity (v) is equal to the rate of change of mechanical energy, (mvg) and so allows for establishment of a standard kilogram metric. Although the article is not a build-it tutorial with a BOM or construction tips, it does outline the balance’s principles, operation, and functions. The NIST’s latest-generation balance is, as you would expect, an impressive instrument, Figure 3.Figure 3 This photo of the 4th-generation Kibble/Watt balance can only hint at the unit’s extreme complexity and sophistication. (Image source: Credit: Curt Suplee/NIST)
As a consequence of this new definition for the standard kilogram mass, it is possible to build a standard maker all by yourself. This is an intriguing technical and assessment challenge. It’s more than the issue of getting the balance to work in the gross sense; it also requires understanding the sources of error and how they can be eliminated, minimized, cancelled, or calibrated out. There’s also the question of determining how good your balance is when you are done: To what do you compare it? How many meaningful figures of accuracy can you achieve with confidence, and with what precision? What does it take to get to each additional digit of performance in a DIY version?
The DIY project has been tackled, of course, beginning with folks at NIST who have constructed a LEGO-based tabletop version capable of measuring a gram-level masses to 1% relative uncertainty, and there are other projects as well (see “Construction of a Kibble Balance—The Device that Redefined the Kilogram” and “The New Kilogram: Redefining the Kilogram with the DIY Watt Balance” examples). If done right, this type of DIY could be the core of a great learning experience in analog circuitry and error analysis/correction.
What’s your view on the new, non-artifact kilogram based on the Watt/Kibble balance? Extremely smart? Long needed? Too complicated? OK for its role as the ultimate primary kilogram? Irrelevant or don’t care?