Thermal Transpiration Error in Absolute Pressure Measurement with Capacitance Manometers

Readings of two Baratron capacitance manometers with sensors at unequal temperatures, when compared over a wide range of pressures in helium and nitrogen, demonstrate appreciable nonlinearity caused by thermal transpiration. The asymptotic indication ratio at low pressure, 1.026±0.002, is approximately double the value reported by Bromberg, but considerably less than the Knudsen ratio for apertures. The data can be fitted to the Liang Equation, but the coefficients are sensitive to the geometry of the apparatus in the region of temperature transition.