Automated Continuous-Flow Gravimetric Hygrometer as a Primary Humidity Standard
Christopher W. Meyer, Joseph T. Hodges, R W. Hyland, Gregory E. Scace, J Valencia-Rodriguez, James R. Whetstone
A new primary gravimetric hygrometer has been completed at the National Institute of Standards and Technology (NIST). This apparatus determines the humidity in a steadily flowing gas stream by first separating the water from the carrier gas using desiccants and afterwards independently measuring the water mass and carrier gas mass. These combined measurements yield the mixing ratio r (the ratio of the measured water mass to the measured dry-gas mass) averaged over the collection time interval. The new gravimetric hygrometer design employs an automated, continuous-flow gas-collection and water-collection system for determination of the mixing ratio. The gas-collection system monitors the rate of gas flow by interferometrically measuring the displacement of pistons in two, precision-bore glass tubes. The mass of the water is determined by weighing the system s water-collection tubes before and after passage of the total amount of gas through the hygrometer. System automation enables gas collection at flow rates up to 3 L min 1 for time periods limited only by the patience of the operator, thus enabling the measurement of lower humidity levels than the previous NIST gravimetric hygrometer. The total expanded relative uncertainty (k=2) of the new gravimetric hygrometer is approximately 0.1 % for atmospheric-pressure dew or frost-point temperatures higher than 35 C (r = 250 g/g). Below this frost-point temperature the total expanded relative uncertainty gradually increases to approximately 1 % at 55 C (r = 13 g/g). The hygrometer has measured the humidity of air and N2 samples produced by the NIST Two-Pressure Generator and the NIST Low Frost-Point Generator, respectively.
International Symposium on Humidity and Moisture (ISHM) | 5th | | National Institute of Metrology, Standardization and Industrial Quality
May 2-5, 2006
Rio de Janeiro, BR
Proceedings for the International Symposium on Humidity and Moisture