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James W. Schmidt

Dr. James Schmidt is a physicist with extensive experience in pressure metrology, measuring the thermophysical properties of fluids, and in the properties of liquid-vapor and liquid-liquid interfaces.

Current Research

  • Frequency-based standards for flow metrology: Schmidt is developing a system for calibrating gas flow meters that relies on simultaneous measurements of: (1) the gas pressure in a cavity, (2) acoustic and (3) microwave resonance frequencies of the cavity. (Typical volumes: 1 m3 to 10 m3.) These measurements are combined with reference data for the gas’s properties to accurately determine the quantity of gas remaining in the cavity while gas flows out of the cavity through the meter under test.
  • Atomic Standard of Pressure: The density of helium gas in a metal-walled cavity can be determined by measuring the gas’s temperature and the microwave resonance frequencies of the cavity. The pressure is determined by combining the density and temperature with an accurate equation of state calculated using quantum mechanics and the fundamental constants. This pressure standard based on atomic physics will be used to test piston gauges in the range 2 MPa to 7 MPa with a fractional uncertainty of 5×10–6. For more information, please read the description of the atomic standard of pressure.

Prior Research

  • Properties of Refrigerants and Gaseous Fuels: Schmidt measured the equation of state, refractive index, critical parameters, and surface tension of candidate alternative refrigerants. NIST’s characterization of candidate refrigerants allowed industry to maximize the efficiency of their products while reducing their depletion of the atmosphere’s ozone layer. The heating value of natural gas depends upon its composition which varies from well to well. The heating value of a gas can be determined by combining Schmidt’s measurements of the dielectric constants of the components of natural gas with other measurements of the properties of the gas.
  • Surface Effects in Fluids: The transition from incomplete wetting to complete wetting was recognized just before Schmidt joined NIST. Schmidt made the first measurements of the thicknesses of the wetting layers and also showed that the transition from incomplete wetting to complete wetting was first order. See First-order wetting transition at a liquid-vapor interface.
  • Primary pressure standards based on piston/cylinder assemblies: Schmidt characterized piston/cylinder assemblies that now anchor the NIST pressure scale. As part of the characterization, he measured differential pressure coefficients between two assemblies with variable coefficients and showed that they were consistent with pressure coefficients calculated from elasticity theory.


Sensor Science Division
Fluid Metrology Group

Phone: 301-975-2458
Email: james.schmidt@nist.gov