Empirical enhancement factors for trace moisture in nitrogen and argon: Bridging measurement principles

Abstract

Accurate measurement of trace water vapor concentrations in gases is critical across various industries, specifically in semiconductor manufacturing where moisture impurities affect device reliability. The most sensitive and accurate methods for measuring ultra-low trace humidity concentrations, below 1 ppm, involve chilled mirror hygrometers and cavity ring-down spectroscopy sensors. The former measures dew/frost points directly, while the second detects particle number density or mole fraction. However, comparing measurements from different operating principles is challenging due to limited understanding of water vapor properties in real gases at very low humidity concentrations and elevated pressures.

The water vapor enhancement factor, which accounts for this non-ideal gas behavior, has so far been empirically determined within a restricted range, with values at low humidity extrapolated. This study extends water vapor enhancement factor measurements below 1 ppm to ultra-low frost points down to −90 °C (corresponding to 100 ppb water amount fraction) and high pressures of up to 1 MPa for carrier gases argon and nitrogen. Our findings reveal significant deviations from existing extrapolated water vapor enhancement factor values. With our measurements achieving lower uncertainties than previous models, this advancement supports the development of reliable, stable, accurate and low-cost humidity sensors essential for industrial applications requiring precise moisture control.