An expression for the uncertainty in the water vapour pressure enhancement factor for moist air

Abstract

Uncertainty in the empirically derived vapour pressure formulations must be incorporated in uncertainty budgets for a laboratory's humidity calibration and measurement capabilities and must be considered in any traceable humidity calibration. Uncertainty associated with the water vapour pressure enhancement factor is a strong function of pressure and temperature and may not be accurately reported since it is usually given in the form of a look-up table or as a single value that generally overestimates the true uncertainty. To facilitate automatic calculation and accurate reporting of uncertainty, an expression for the uncertainty in the vapour pressure enhancement factor for moist air is presented here. The expression is based on the analysis of Wexler and Hyland in their work for ASHRAE Project RP-216 and is valid from −100 °C to 200 °C and from 0.01 MPa to 10 MPa.Some implications of possible significant systematic error in the Wexler and Hyland values of the enhancement factor are discussed briefly.

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