Despite changes to the SI, don't throw away your fixed points

In November 2018 the global metrology community agreed on a revision to the International System of Units (SI) during the General Conference on Weights and Measures of the International Bureau for Weights and Measures (BIPM) in Versailles. With a unanimous vote, the 60 countries in attendance agreed to adjust the SI to be based entirely on unchanging properties of nature; a 150-year dream for the scientific community.

Starting on World Metrology Day 2019 (May 20), for the first time, all seven of the base units of the SI were defined in terms of constants of nature, or technical constants. This offers a fundamental constant whose numerical values are fixed and unvarying for each of the base units. For temperature, that means that instead of using the triple point of water to express the physical measurement, the Boltzmann constant takes its place.

Before 2018, the defining measurements for temperature and mass had to be something tangible. For temperature that was the triple point of water, and for the kilogram it was the mass of metal commonly called Le Grand K. In the original 1793 definition the kilogram was defined as one liter of water, and the meter was defined as one ten-millionth of the distance from the North Pole to the equator. Each of the base units had a physical item to compare it to such as a mass of metal for the kilogram. These objects would wear out or degrade over time, creating fewer and fewer precise measurements. In addition, these objects weren’t available for everyone to compare, leading to more opportunities for errors.

In a world where so much is dependent on precise measurements, the new system removes the dependance on physical objects for accuracy. The revised SI relies on constants of physical science which do not change over time and might truly be accessible to everyone.

What you should be doing to stay on top of SI changes?

The short answer is: nothing yet. Currently, there is no device that can tie the Boltzmann constant to temperature as accurately as a fixed point can, so don’t throw away your fixed points just yet.

For the average temperature calibration professional, the goal is to eventually move away from a Standard Platinum Resistance Thermometer (SPRT) where temperature is measured in fixed points. Eventually temperature calibration will transition to something like a photonic thermometer or Johnson noise thermometer that is based on quantum mechanics.

But, for now, the bottom line is to keep doing what you’re doing in temperature calibration; nothing changes–yet. Fixed points stay at the same temperatures, the ITS-90 does not change, and we’ll still use SPRTs for measuring temperature.

Better uncertainties are going to be developed. It may be 20 years until a device that can tie the Boltzmann constant to temperature more accurately than a fixed point can comes on the market, but keep your ears open as it will get there. Attending temperature conferences like TEMPMEKO is one way to stay up to date. TEMPMEKO is held every three years; the last one was held in 2019 with the next one originally scheduled for 2022, but it has been delayed.

SI redefinition and the rise of quantum photonic and Johnson noise thermometry

The future of temperature calibration, predictions from Frank Liebmann

In future, how the Kelvin will be realized in calibration labs is likely to change. The currently used fixed points will be replaced by a device that more accurately ties the Boltzmann constant to our work, perhaps a quantum device or a Johnson noise thermometer.

• A photonic thermometer generally uses lasers and locks a laser onto a certain wavelength of light based on what the thermometer is doing physically at a certain temperature.
• A Johnson noise thermometer uses electrical noise to take the center point (of the noise) and compares it to frequency to determine temperature.

The BIPM is working to define more fixed points for higher temperatures. Our future labs will see the new family of fixed points currently being designed, called eutectics. Eutectics, or a eutectic mixture is a set of substances that melt into one another as a liquid at a single temperature which is lower than the melting points of the separate substances or any other mix of them. Currently the highest fixed points are gold and copper, which heat to almost 1100 °C.

Eutectics will give metrologists and technicians better traceability into temperatures above the Copper Point, which is 1084.62 °C. Scientists are working to prove out eutectics by using radiation thermometry; taking a blackbody calibrator, operating on a similar principle to Fluke Calibration 4180/4181 Precision Infrared Calibrators, except with a cavity blackbody instead of a flat plate, along with the eutectic and comparing that to the silver triple point of copper, aluminum, silver, zinc, and/or gold. Once this new family is defined, it will make a big difference for thermocouple calibration.

In the thermocouple world, work is also being done to look at different wire combinations. This is to see if they can achieve higher temperature measurements. Before these new combinations can be worked into our cal labs, the work also has to be done to figure out how to calibrate these new thermocouples, so this advancement is probably a ways out still.

Recommended resources

• Tempmeko 2019 Temperature Metrology Conference – Fluke Calibration’s Overview of the Conference
• Temperature Simulation in Temperature Calibration
• Buying the right temperature calibration bath for your laboratory
• Temperature calibration application notes

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• Temperature calibrators