UF Researcher's Work To Result In New, Lower Temperature Scale

February 15, 2000

GAINESVILLE — The blue numbers on the thermometer are about to extend to a new low, thanks to the work of a University of Florida researcher.

A temperature scale pioneered by UF physics Professor Dwight Adams soon will become the world’s official standard for measuring the coldest temperatures known to man — temperatures just shy of absolute zero, or minus 459.67 degrees Fahrenheit.

Such temperatures are so extreme they do not occur naturally anywhere in the universe, not even in the empty space between galaxies. But physicists and chemists create the lows in laboratories because they shed light on the nature of matter and may turn materials into superconductors, superfluids or prompt them to undergo other unique changes. Now, an international committee of physicists is set to adopt Adams’ technique for measuring ultracold temperatures as the world standard to ensure scientists accurately measure and compare results no matter where they work.

“The importance of this is that researchers in Europe and the U.S. will all have the same temperature scale,” said Marten Durieux, a physicist at the University of Leiden in the Netherlands and one of the officials in charge of adopting the new scale.

The world’s current official temperature scale, the International Temperature Scale of 1990 (ITS-90), has a lower limit of 0.65 Kelvin, or minus 458.5 degrees Fahrenheit. When adopted later this year, the new scale will drop to 0.001 Kelvin, or minus 459.66 degrees.

Such temperatures had been out of reach to all but the most specialized laboratories in the world until the past two decades, when refrigerators that could lower temperatures to 0.01 Kelvin became commercially available. The result has been an explosion of research in this low temperature range, increasing the need for the new scale, Adams said.

“There’s lots of work going on in this range now, and there is no defined scale for the people doing it, so this will be quite useful to them,” he said.

Adams and two graduate students, Gerald Straty and Richard Scribner, laid the groundwork for the scale 35 years ago when they found a way to measure ultra cold temperatures using an isotope called helium-3.

When subjected to the extreme temperatures below 1 Kelvin, or 457.7 Fahrenheit, most materials freeze solid, Adams said. Helium, by contrast, remains a liquid all the way to absolute zero. When different pressures are applied to helium-3 at very low temperatures, however, it transforms into a changing mix of solid and liquid. The pressure at which it makes that change is known as the isotope’s “melting pressure.” If the melting pressure can be measured accurately, the scale can be calibrated.

To achieve that, Adams and Straty invented a measuring device known as the Straty-Adams gauge. Smaller than a Matchbox car, the gauge fits inside the inner chamber of a cryostat, the liquid nitrogen and helium refrigerator used to create ultralow temperatures. As the temperature drops and some of the helium-3 transforms from one state to another, the corresponding change in the melting pressure causes the diaphragm in the device to move slightly. Although the amount of movement is very small, totaling less than one-thousandth of an inch, it can be translated into temperature change via an equation.

Physicists have been using the gauge since Adams, Straty and Scribner pioneered it, but scientists disagreed about proper calibration of the scale. Two standards laboratories, the National Institutes of Standards and Technology, or NIST, in the United States and the Physikalisch-Techische Bundesanstalt in Germany, each proposed their own scale. Meanwhile, Adams and two UF colleagues, graduate student Win Ni and research scientist Jian-Sheng Xia, proposed a third scale. At a meeting at NIST’s Maryland headquarters in January, however, the three parties came together and agreed to one new scale.

Durieux, the University of Leiden physicist, is a member of a working group that is part of the International Committee of Weights and Measures, which governs temperature and weight measurements worldwide.

He said the group will recommend at a meeting in April that the full committee approve the new temperature scale, ITS-2000, at its next meeting in Paris in September. He said he is confident the full committee will approve the recommendation.