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Acoustic Techniques in Fluid Metrology


The Fluid Metrology Group leads the world in designing and using gas-filled acoustic resonators for measuring the thermodynamic temperature T, the Boltzmann constant kB, and the thermophysical properties (e.g. heat capacity, shear viscosity, acoustic and density virial coefficients) of semiconductor process gases and environmentally benign heat transfer fluids (e.g. refrigerants). Now, we are using our expertise to solve problems related to greenhouse gases.


The NIST Acoustic Thermometer

Figure 1. Three-liter spherical cavity resonator used to measure the thermodynamic temperature from 273 K to 552 K.

  1. In collaboration with NIST's Chemical Sciences Division, we use photoacoustic resonators
    1. for continuous monitoring of the CO2 concentration in ambient air from a tall building rooftop; (see Fig. 2) [1]
    2. for measuring the optical properties of particles (e.g. soot) in air that affect the energy balance of the earth; [2]
    3. to develop a hybrid cavity-ringdown/photoacoustic spectrometer for measuring the abundance of C-14 in samples of CO2 to verify product origin.
  2. In collaboration with China's National Institute of Metrology, we use cylindrical acoustic resonators to re-determine the Boltzmann constant with fractional uncertainty of 3×10-6 from measurements of the speed of sound in argon. [3a, 3b]
  3. Photoacoustic resonator
    Figure 2. Photoacoustic resonator for optical absorption measurements in gases and aerosols.
    Design a Long-Wavelength Acoustic Flow Meter to measure the flux of carbon dioxide in the stack of a coal-burning power plant to facilitate a carbon tax or cap-and-trade regulations.

Major Accomplishments

  1. Measured the thermodynamic temperature using cavities acting as simultaneous acoustic and microwave resonators from
    1. 77K to 273 K using a quasi-spherical cavity resonator
    2. 271 K to 552 K using a spherical cavity resonator
    3. documented best-in-the-world techniques of acoustic thermometry
    4. "weigh" the quantity of gas in a large tank with large temperature gradients

tank with insulation
Figure 3. Acoustic resonances "weighed" the gas in this 300 liter tank while 13 K temperature differences were present. This is a model for a 30 m3 tank that will be used to calibrate flow meters.
  1. Resonator for measuring the bulk viscosity in xenon
    Figure 4. Hybrid cylindrical-Helmholtz resonator to measure speed of sound and 2nd viscosity over 27:1 frequency range.
    Designed cavity resonators (and associated ducts and transducers) for measuring the thermodynamic temperature from 4 K to 1350 K with fractional uncertainties as small as 3×10-6.[4,5,6,7,8]
  2. Developed a self-calibrating photoacoustic spectrometer with a calculable cell constant (Fig. 2) for absolute measurements of optical absorption in gases and aerosols.[9,10]
  3. Measured the 2nd viscosity (also called "bulk" or "dilation" viscosity) of xenon near its liquid-vapor critical point near 16 °C and 5.8 MPa using a novel hybrid acoustic resonator (Fig 4). [11]
  4. Measured the viscosities and viscosity virial coefficients of semiconductor process gases [NIST Standard Reference Database 134 - SEMIPROP] and environmentally-benign refrigerants [12, keith.gillis [at] (subject: reprint%20request, body: please%20send%20me%20a%20pdf%20of%20the%20article%3A%20af%20estrada-alexanders%20and%20jj%20hurly%2C%20kinematic%20viscosity%20and%20speed%20of%20sound%20in%20gaseous%20co%2C%20co2%2C%20sif4%2C%20sf6%2C%20c4f8%2C%20and%20nh3%20from%20220%20k%20to%20375%20k%20and%20pressures%20up%20to%203.4%20mpa%2C%20j%20chem%20thermodynamics%2C%20volume%2040%2C%20pages%20193%20to%20202%2C%202008) (13), REFPROP] using a novel acoustic viscometer.
  5. Cylindrical cavity resonator with acoustic waveguides
    Figure 6. Cylindrical cavity (14 cm long; 6.5 cm diameter) resonator used to measure the speed of sound in semiconductor process gases and refrigerants.
    Greenspan acoustic viscometer
    Figure 5. Greenspan acoustic viscometer for gases.
    Designed acoustic resonators equipped with acoustic waveguides and remote transducers, and used them to measure the thermodynamic properties of industrial gases under extreme conditions. [6,keith.gillis [at] (subject: reprint%20request, body: please%20send%20me%20a%20pdf%20of%20the%20article%3A%20accurate%20acoustic%20measurements%20in%20gases%20under%20difficult%20conditions%2C%20ka%20gillis%20et%20al%2C%20rev%20sci%20instruments%2C%20volume%2062%2C%20pages%202213%20to%202217%2C%201991) (14)]
Created March 6, 2014, Updated November 17, 2017