The NIST Low Frost-point Humidity Generator (LFPG) produces water vapor - gas mixtures with mole fractions from approximately 5 nmol/mol to 4 mmol/mol for research, calibration of transfer standards, and testing and development of new humidity instrumentation. The LFPG generates these mixtures by saturating air or nitrogen with water vapor over a plane surface of ice maintained under conditions of constant temperature and pressure. Uncertainties in pressure and temperature determine the ultimate accuracy limit of the LFPG when humidity is expressed as frost-point temperature. Uncertainties in the correlations predicting vapor pressure and non-ideal gas effects (enhancement factor) dominate when knowledge of mole fraction is required. This paper describes the LFPG in sufficient detail to frame a discussion of inherent uncertainties. Uncertainties in the various subsystems are discussed and the expanded uncertainties in water vapor mole fraction and frost-point temperature are presented.
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