Acoustic Techniques in Fluid Metrology

Summary

The Fluid Metrology Group is a world leader in designing and using gas-filled acoustic resonators as tools to measure the speed of sound, the thermodynamic temperature T, thermodynamic and transport properties of gases, the mass of gas in a vessel, and gas leak from a vessel in the presence of significant temperature gradients. Now, we are using our expertise to develop gas flow standards using acoustic and microwave resonances in vessels.

Description

We use acoustic techniques in gases to measure

1. the thermodynamic temperature of a gas in a spherical or cylindrical cavity,
2. the optical absorption in gases and aerosols (in collaboration with NIST’s Chemical Sciences Division)
3. the quantity of gas in a large vessel or tank, and
4. gas flow into/from a large unthermostated vessel as a primary flow standard

Figure 1. A three-liter spherical cavity acoustic resonator used to measure the Universal Gas Constant and thermodynamic temperature from 273 K to 552 K

Credit: NIST

Figure 2. A photoacoustic resonator used for optical absorption measurements in gases and aerosols.

Credit: NIST

Major Accomplishments

1. Documented best-in-the-world techniques of acoustic thermometry
2. 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 (Fig. 1)
3. In collaboration with China's National Institute of Metrology, we used 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.
4. 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.
5. Developed a self-calibrating photoacoustic spectrometer with a calculable cell constant (Fig. 2) for absolute measurements of optical absorption in gases and aerosols.
6. 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. 3).
7. Measured the viscosities and acoustic virial coefficients of semiconductor process gases [Database of thermophysical properties] using a Greenspan acoustic viscometer. (Fig. 4)
8. Designed acoustic resonators equipped with acoustic waveguides and remote transducers, and we used them to measure the thermodynamic properties of industrial gases under extreme conditions.
9. Used microwave and acoustic resonances to measure the quantity of gas in an unthermostated
   1. 300-liter tank with large temperature gradients
   2. 1800-liter tank to dynamically measure gas flow (Fig. 5)

Figure 3. Hybrid cylindrical-Helmholtz resonator to measure speed of sound and 2nd viscosity over 27:1 frequency range.

Credit: NIST

Figure 4. Greenspan acoustic viscometer

Credit: NIST

Figure 5. Acoustic resonances "weighed" the gas in this 1800-liter spherical vessel to calibrate flow meters.

Credit: NIST