engleski

Improved method for pressure measurement in saturation chamber of primary dew/frost point generators

This paper describes two methods for measurement of air pressure inside the saturation chambers of two primary dew/frost point generators, used as national humidity standards at Laboratory for process measurement (HMI/FSB-LPM) in Croatia. Together, the generators cover the dew/frost point temperature range from −70 °C to + 65 °C. Low range generator operates at air temperatures between −70 °C and 5 °C, while high range covers the range between 1 °C and 65 °C. The saturation pressures considered are in the range between atmospheric and 1060 hPa, at the air flow rates between 1.0 L min−1 and 2.5 L min−1. The dew/frost point of the saturated air depends strongly on the saturation pressure, making improvement in this kind of measurements of great importance for humidity realizations. In the old method, the precise digital barometer is used for determination of saturated air pressure inside the saturation chamber. The barometer is traceable to the pressure balance, which is used as a national standard for pressure in Croatia. In the new method, the pressure was maintained directly by the primary pressure balance, achieving this way the significant improvement of related measurement uncertainties for both, pressure measurement and the dew/frost point realization. The paper will give a detailed description of both measurement setups as well as corresponding measurement uncertainty analyses. During the experiments it was also observed that in the second setup, balance stabilized the pressure oscillations as well as oscillations of the dew-point temperature measured by the chilled mirror hygrometer under calibration, making indirect improvement of related measurement uncertainties.

Humidity generators ; Saturation pressure ; Primary dew/frost-point realization

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