NIST has refined techniques to produce reference values for the viscosity and thermal conductivity of argon with a standard uncertainty of only 0.08%. The viscosity of hydrogen, methane, and xenon will be obtained with similarly small uncertainties. The low uncertainty of these results also advances the fundamental models of intermolecular potentials.
The manufacturers of semiconductor devices and producers and consumers of natural gas rely upon accurate measurements of gas flow rates for equitable transfer in markets. The range of requirements differ by factors as large as 109. Improved gas flow rate measurements increase confidence in market transactions and are enabled by gas property data of improved accuracy.
NIST’s quantum mechanical calculations of the viscosity of helium were combined with measurements made with two, newly developed viscometers. One viscometer used a coil of quartz capillary tubing of high uniform diameter.
These are normally used for gas chromatography. During its operation, gas flows through the capillary, transducers measure the pressures at the entrance and exit, and a hydrodynamic model converts the pressure measurements to a flow rate value.
The second viscometer used two capillaries in a series configuration, one maintained at 25 °C, and the other at temperatures ranging from 200 K to 400 K. Important features of the two-capillary viscometer include (1) electroformed nickel tubing with an extremely smooth internal surface, (2) voltage controlled piezoelectric leak valves, (3) pressure transducers maintained in thermostatically controlled enclosures, and (4) in situ calibration with helium at each temperature and time of use.
The two-capillary viscometer determined the ratio of the each gas’s viscosity to that of helium, which is known with an. uncertainty smaller than 0.1 %. For the noble gases argon and xenon, the measured viscosities were combined with calculations of the Prandtl number to yield the thermal conductivities with uncertainties of 0.08 %. Both viscometers used a hydrodynamic model for capillary flow, also developed recently at NIST. By incorporating the six most important corrections to the Hagen-Poiseuille equation for capillary gas flow, the model added negligible uncertainty to the final result.
The consistency of the measurements, and their agreement with the most accurate calculations of the viscosities of helium and argon, verified the accuracy of both viscometers. This improved measurement method has resulted in improvement in the accuracy of argon’s thermal conductivity that is 30 times better than prior knowledge of this quantity.
Analysis of the measurements of hydrogen, methane, and xenon is near completion and will form the basis for publication of the results. The new NIST reference values for viscosity and thermal conductivity will enable improvements of the thermal mass flow controllers that are used widely by the semiconductor industry.