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UD physicists enable greater precision in pressure measurement

Krzysztof Szalewicz is conducting research to provide greater precision in the measure of pressure by using helium rather than mercury.

Most of us know helium as the lighter-than-air gas that puts lift into birthday balloons or makes a baritone's resonant voice sound more like Donald Duck's.

But helium -- a non-toxic element that is abundant in the universe but a scarce, non-renewable resource on earth -- has many significant uses in industry, medicine and science.

Researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, have proposed an improved method for measuring pressure by using helium rather than mercury. Mercury-based manometers/barometers are the standard tools of measurement, but helium-based approach offers significant advantages by avoiding the use of toxic mercury and making measurement devices more portable and more precise than those currently in use.

In an article published this week by Physical Review Letters, University of Delaware physics professor Krzysztof Szalewicz and his collaborators from the University of Warsaw and the University of Poznan show how they calculated a key property of the helium atom needed for a precise helium-based barometer -- its "polarizability" -- at a level of precision 100 times greater than previously possible either experimentally or computationally.

The helium-based barometer works the following way: A laser beam of light is sent through two cylinders, one empty, one filled with helium. Because light moves more slowly in helium than in a vacuum, it appears to move through a longer cylinder. The difference between the apparent and true length can be measured very precisely utilizing interference of light, and this can then be used to determine the speed of light in helium. Because this velocity depends on helium pressure, the result is an instrument measuring pressure.

However, the equation connecting the speed of light in helium and pressure contains a quantity called polarizability. This property of helium defines how easily it can be squeezed by an electric field, or how far its electron and nuclear charges separate to form what is called a "dipole moment."

Szalewicz compares it to trying to squeeze three common items -- a bowling ball, a football and a balloon. You can't squeeze the bowling ball. The football could be squeezed a bit, though, and the balloon a good bit more. So polarizability describes something like the hardness of an object.

Szalewicz' team developed new theory to calculate helium's polarizability to within 0.1 parts per million. To do it, they used quantum electrodynamics (QED) -- an advanced form of quantum mechanics -- new algorithms and high-performance computers.

In a few years, Szalewicz said, helium-based manometers will replace mercury-based ones as the standard of pressure, specifically replacing the 9-foot-high manometer that holds 551 pounds of mercury at NIST with a suitcase-sized helium-based manometer.

Ultimately, that could provide more accurate instruments for aviation, where atmospheric pressure is used to gauge altitude, and other processes requiring precise pressure measurements, such as manufacture of computer chips.

And the polarizability value his team developed will be part of that advance.

The ongoing work is supported by the National Institute of Standards and Technology, the National Science Foundation, the Polish Ministry of Science and the National Science Center.

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Krzysztof Szalewicz and his research collaborators have calculated a key property of the helium atom needed for a precise helium-based barometer.

UD physicist Krzysztof Szalewicz and his research collaborators have calculated a key property of the helium atom needed for a precise helium-based barometer, offering advantages over current mercury-based instruments.

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