LAKE WALES, Fla. — Forget Kelvin, Fahrenheit, and Celsius, not to mention centigrade. The quantum-level accuracy of a temperature measurement unit called the SI (Système International d'unités, or International System of Units), the newest addition to the metric system, will obsolete the old Newtonian mechanics K, F, and C by 2019, according to the National Institute of Standards and Technology (NIST, Boulder, Colo.), which has developed a solid-state apparatus for measuring temperature in SI using the Boltzmann constant.

The shift to the SI standard should be especially helpful to electronics engineers seeking to measure hot spots on a die, increasing the accuracy of their measurements as well as speeding up the process. The Kelvin, for instance, can only be measured down to a parts per million with expensive traditional instruments, whereas the SI is hoped to spur inexpensive electronic instruments that can measure down to parts per billion.

Temperature is a tricky unit to measure, since it depends on the material used and becomes less accurate as you get further away from its “central” degrees unit: 273.15 K = 32˚F = .01˚C. Today that central unit is standardized as the point at which water freezes, but that metric is fraught with inaccuracies, stemming from impurities in the water and the inevitable presence of an undetermined amount of “heavy water” (H₂O with a neutron attached) in a given sample. Today, engineers must take into account the type of material as well as the temperature in order to compensate for the inaccuracies of using traditional instruments and units to measure heat.

The SI unit sidesteps these inaccuracies by using quantum measurements of the motion of electrons (Johnson noise) in the sample, thus standardizing on an easily obtainable quantum measurement of the Boltzmann constant (the average kinetic energy of particles). Every laboratory worldwide aims to begin using SI for quantum-accurate temperature measurements within the next two years.

NIST’s inexpensive apparatus depends on a quantum voltage noise source (QVNS) that basically measures the voltage noise from electrons rattling around in a resistor. The apparatus provides a rock-solid reference measurement of the Boltzmann constant which can then be used to determine the SI temperature of any substance.

The SI unit is the result of a worldwide effort. NIST is just supplying the prototype of a simple QVNS reference measurement of the Boltzmann constant, as it has done for other inexpensive methodologies, materials, and laboratory instruments that it has developed over the years for accurately calibrating virtually every unit of measurement used worldwide.

Representatives from around the world will vote on whether to redefine the temperature standard as the SI unit in November 2018 at the General Conference on Weights and Measures in Versailles, France. There is little doubt that the SI will replace the Kelvin as the international standard of temperature, but NIST is not waiting for the vote and is making its apparatus details available now.

“After the Boltzmann constant is defined, a successful Johnson noise thermometry [JNT] system will allow sub-50-parts-per-million measurements of temperature over a range of 100 K to 1,000 K using a rack-mountable system. Since this system is based on the definition of the Boltzmann constant and quantum phenomena, it would be self-calibrating. The other requirement for a successful JNT system would be disseminating it to other users outside of NIST,” institute researcher Nathan Flowers-Jacobs told EE Times in an exclusive interview.

“Shrinking the Johnson noise thermometry system, removing the need for liquid cryogens, and making it user-friendly are going to be a multiyear program. Therefore, we hope that it will begin to be used outside of NIST within five years,” Flowers-Jacobs said.

NIST scientist Horst Rogalla was leader of the Johnson noise thermometry project. The National Institute of Metrology in China also contributed to the work.

For all the details, see “A Boltzmann constant determination based on Johnson noise thermometry,” which is behind a paywall, or the free articles “Improved electronic measurement of the Boltzmann constant by Johnson noise thermometry” and “Spectral model selection in the electronic measurement of the Boltzmann constant by Johnson noise thermometry.”

— R. Colin Johnson, Advanced Technology Editor, EE Times

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