Keith A Gillis (Fed)

Dr. Keith Gillis has major roles in three projects that exploit his expertise in physics and physical acoustics:

1. measuring the mass of gas in an unthermostated pressure vessel (with temperature gradients present) using acoustic and microwave resonances. Microwave resonances are used to determine the volume of the vessel as a function of pressure and temperature. The acoustic resonances provide a measure of the average gas temperature. When combined with the pressure, volume, and the equation of state, these measurements determine the mass of gas. Gillis has shown that the uncertainty in the average temperature and mass determination is dependent on the shape of the vessel, the acoustic mode measured, and the symmetry of temperature gradients that are present.
2. Gillis extended the method for mass measurement in a vessel to dynamically measure the mass as gas flows in or out, thereby determining mass flow. The method uses positive feedback to spontaneously excite a selected mode and tracks the resonance frequency as the speed of sound changes. This technique is particularly useful when the gas temperature is changing due to flow work as gas enters or leaves the vessel.
3. participating in an international collaboration using acoustic techniques to measure the thermodynamic temperature up to 1350 K and measuring the Boltzmann constant. This project led Gillis to extend the theory and modeling of acoustic resonators and the propagation of sound in waveguides.

In earlier research at NIST, Gillis developed a self-calibrating photoacoustic resonator to measure small concentrations of impurities in air or other gas. In contrast with other systems that measure molecular optical absorption, the response of these spectrometers is calculated from fundamental physical principles. Gillis developed acoustic techniques for accurately measuring the thermodynamic and transport properties of gases over wide ranges of temperature and pressure. He developed an all-metal cylindrical acoustic resonator with remote transducers to measure the speed of sound, the ideal-gas heat capacity, and the equation of state of benign refrigerants. This apparatus, for use between 240 K and 400 K and pressures up to 1 MPa, had acoustic transducers at room temperature and acoustic waveguides that transmitted sound to and from the resonator through metal diaphragms. Gillis perfected the Greenspan acoustic viscometer for accurate measurements of the shear viscosity of gases. These instruments were also used to characterize hazardous gases used by the semiconductor industry. With NASA sponsorship, Gillis developed a hybrid acoustic resonator optimized to measure the bulk viscosity and speed-of-sound dispersion in xenon near its liquid-vapor critical point. This resonator spanned a factor of 50 in frequency from 100 Hz to 5000 Hz. Before joining NIST, Gillis studied third sound in thin superfluid helium films and the heat capacity anomaly at the superfluid transition in helium confined within porous glass.

Previous Patents

“Photoacoustic spectrometer with calculable cell constant for quantitative absorption measurements of pure gases, gaseous mixtures, and aerosols,” U.S. Patent and Trademark Office, Pub. No. 2012/0118042 A1, Published May 17, 2012.

“Acousto-microwave System for Determining Mass or Leak of Gas in a Vessel and Process for Same,” U.S. Patent and Trademark Office, US Patent No. 10,036,683 issued July 31, 2018.

1979  B.S. Degree with high honors, CWRU
1979  Dayton C. Miller Prize, CWRU
1986 – 1987 IBM Postdoctural Fellowship
2003  US Department of Commerce Bronze Medal
2011  Acoustic Society of America Fellow
2015  US Department of Commerce Bronze Medal