Thermal transpiration of helium and nitrogen in 50-mu m bore silica capillaries

Thermal transpiration of helium and nitrogen in silica 50-mum capillaries b etween temperatures of 340 and 300K has been measured in the pressure range of 100Pa -to 10kPa by the differential method using apparatus in which the gases are back-filled into a clean UHV-compatible system. For helium at 10 0 Pa the magnitude of the effect is close to that predicted by kinetic theo ry. Calculations based on the semi-empirical formula of Takaishi and Sensui are in good agreement with the helium data over the whole pressure range b ut not for nitrogen below 1 kPa. Estimates of the thermomolecular pumping s peeds per capillary have been made. (C) 2000 Elsevier Science Ltd. All righ ts reserved.