National Physical Laboratory

An assessment of experimental data that underpin formulae for water vapour enhancement factor.

Bell, S A, Boyes, S J
TEMPMEKO 01, June 2001, Berlin, Germany, 603-608
NPL Doc. Ref:
PDB: 3629
Document Type:
Conference paper

Note: An asterisk after an author's name indicates a non-NPL author.


At any given temperature, the saturation vapour pressure of water within a gas is not generally the same as that for pure water vapour. In air, near room temperature and atmospheric pressure, it differs by almost 0.5%, and by more at higher pressures. The water vapour enhancement factor is used in humidity calculations to allow for this non-ideality of air-water mixtures.Published measured data for the water vapour enhancement factor in air are few in number, and have been reported at wide intervals over more than 70 years. They constitute a rather small data set to underpin the various derived formulae for calculating enhancement factor. Where doubt is cast on any of the measurements, this can lead to further uncertainties in the formulae.In this paper, the reports of measurements of enhancement factor over many years are compared and discussed. The apparent weaknesses and strengths of the data are considered by analysing derived values of the air-water cross-viral coefficient, leading to conclusions about the possible errors in the measured results.

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