The NIST Thermodynamic Metrology Group recently christened a second generation gravimetric hygrometer, and a description of the hygrometer was published in Metrologia. A gravimetric hygrometer measures humidity by separating the water from the gas and subsequently determining the masses of the water and the gas separately. These masses are then used to calculate the water mole fraction in the gas, one of the principal humidity quantities. Other humidity quantities, such as the dew point and relative humidity, can be easily determined from the mole fraction using additional measurements of the pressure and temperature of the gas made before it enters the hygrometer.
The new hygrometer uses an automated continuous-flow gas collection system. This makes it far easier to operate than its predecessor, which was rarely used because of its onerous nature. Also, an external cold trap allows the hygrometer to measure mole fractions a factor of 10 higher than before. The hygrometer uses a laser interferometry method in conjunction with a prover piston technique to increase the accuracy of the gas mass measurement. With the new design, uncertainty of the new gravimetric hygrometer is 35% lower than before. Currently, it is the only working gravimetric hygrometer in a national metrology institute. The hygrometer can measure mole fractions from 2.2 × 10−4 (a frost-point of −60 °C) to 0.31 (a dew point of 70 °C). Under optimal conditions, its relative uncertainty is within 0.09 % over most of its range.
The gravimetric hygrometer was constructed for the purpose of periodically verifying the performance of NIST standard humidity generators, which are used to calibrate customer hygrometers. These generators are based on thermodynamic principles, so knowledge of certain thermodynamic relations is essential for determining the humidity output. The Thermometry Group has performed comparisons between the gravimetric hygrometer and the NIST Hybrid Humidity Generator, and the results are also described in the Metrologia paper. These results show them to agree within the expanded combined uncertainties of the hygrometer and generator.
NIST's humidity standards are important to many types of end users. Hygrometers with calibrations traceable to NIST humidity standards are used by NOAA, the military, the pharmaceutical industry, semiconductor manufacturers, and the nuclear energy industry, to name a few examples.
While the gravimetric hygrometer was constructed for the support of NIST humidity standards, it can also be used as a research tool. For example, it may be used to help determine thermophysical properties of humid CO2 that are important for designing carbon capture and sequestration systems. Also, the gas collection system of the hygrometer can be used to help determine mercury vapor properties needed for accurate calibration of instruments that measure mercury emissions from coal-burning power plants.
For more detailed information, see The second-generation NIST standard hygrometer.