Compensation of Thermal Transpiration Effect for Pressure Measurements by Capacitance Diaphragm Gauge

The thermal transpiration effect shown in intermediate flow and molecular flow regions influences the sensitivity of a capacitance diaphragm gauge (CDG) with a temperature controlled sensor head. This apparent change in the sensitivity of CDG owing to the thermal transpiration effect has to be compensated for precise pressure measurements in the range of lower than 150 Pa. In this study, the method and the uncertainty of the compensation are reported.   Seven types of equations for the thermal transpiration effect were compared with the calibration results of high accuracy CDGs by the static expansion system. Takaishi-Sensui equation, Miller equation, and Setina equation show good agreement with the calibration results.   The calibration results of CDGs also show that increasing the temperature of the vacuum chamber from 22.5°C to 26.0°C caused decreasing the sensitivity of CDGs by the thermal transpiration effect. The magnitude of the decrease in the sensitivity was from 0.09% for 10 Pa to 0.5% for 0.1 Pa.   Such apparent changes in the sensitivity owing to the thermal transpiration effect could be compensated within 0.2% by Takaishi-Sensui equation using parameters quoted from the catalog and the literature values. In addition, this deviation could be reduced to less than 0.1% by fitting Takaishi-Sensui equation using the parameter c or the sensor temperature T2.

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