Thermal transpiration effects are commonly encountered in low pressuremeasurements with capacitance diaphragm gauges. They arise from the temperaturedifference between the measurement volume and the temperature stabilisedmanometer. Several approaches have been proposed to correct for the pressuredifference, but surface and geometric effects usually require that thecorrection is determined for each gas type and gauge individually. Common(semi) empirical corrections are based on studies of atoms or small molecules.We present a simple calibration method for diaphragm gauges and comparetranspiration corrections for argon and styrene at pressures above 1 Pa. Wefind that characteristic pressures at which the pressure difference reacheshalf its maximum value, are compatible with the universal scaling p_{1/2} = 2\{\eta} \cdot \{v_{th}} / d, thus essentially depending on gas viscosity \eta,thermal molecular speed v_{th} and gauge tubing diameter d. This contradictscurrent recommendations based on the Takaishi and Sensui formula, which show anunphysical scaling with molecular size. Our results support the Miller or\v{S}etina equations where the pressure dependency is basically determined bythe Knudsen number. The use of these two schemes is therefore recommended,especially when thermal transpiration has to be predicted for new molecules.Implications for investigations on large polyatomics are discussed.
展开▼
