PRESSURE INFLUENCE IN CAPACITIVE HUMIDITY MEASUREMENT

We investigated the behaviour of a capacitive humidity transducer (Vai sala HMP 234) upon pressure exposure of nitrogen and helium up to 100 bar, using a humidity generator based on saturation of the gases at te mperatures below ambient. The increased saturated vapour pressure of w ater in the presence of nitrogen and helium (enhancement effect) was q uantitatively taken into account. The sensor output was found to decre ase with nitrogen pressure at fixed relative humidity (RH). Still, by taking the total pressure as an independent calibration parameter, the device can be used in nitrogen gas within error bounds of +/- 1% RH. For helium no deviation of the sensor output with pressure was observe d, showing that the deviation strongly depends on gas properties. For similar pressure calibrations in other gases, accurate data of enhance ment factors have to be available. (C) 1998 Elsevier Science S.A. All rights reserved.