Christopher W. Meyer, Joseph T. Hodges, Gregory E. Scace, James R. Whetstone, R W. Hyland
A second-generation primary hygrometer has been completed at the National Institute of Standards and Technology (NIST). A gravimetric hygrometer measures humidity by separating the water from the carrier gas using desiccants and afterwards independently measuring the water mass and carrier gas mass. These combined measurements determine the mass ratio r (the ratio of the measured water mass to the measured dry-gas mass). The new gravimetric hygrometer design allows computer-automated continuous gas collection at up to 3 L/min. This enables the hygrometer to collect considerably larger amounts of gas and thereby measure lower humidity values than that measured by the previous NIST gravimetric hygrometer. When operated in an optimal thermal environment (minimal thermal loads in the laboratory), the total expanded relative uncertainty (k=2) of the gravimetric hygrometer is approximately 0.1 % for atmospheric-pressure frost points higher than −35 °C (r = 250 μg/g). Below this frost point the total expanded relative uncertainty gradually increases to approximately 1% at −55 °C (r = 13 μg/g). The hygrometer has measured the humidity of gas samples produced by the NIST Hybrid Generator and the NIST Low Frost-Point Generator with dew/frost points from −35 °C to 71 °C. For both generators the differences between the humidity generated and the humidity measured by the gravimetric hygrometer are less than the combined uncertainties of the respective generator and hygrometer.