| AVS 64th International Symposium & Exhibition | |
| Vacuum Technology Division | Tuesday Sessions |
| Session VT-TuP |
| Session: | Vacuum Technology Poster (and Student Poster Competition) |
| Presenter: | Tim Verbovšek, Institute of Metals and Technology, Slovenia |
| Authors: | T. Verbovšek, Institute of Metals and Technology, Slovenia B. Šetina Batič, Institute of Metals and Technology, Slovenia J. Šetina, Institute of Metals and Technology, Slovenia |
| Correspondent: | Click to Email |
A unique vacuum system for precise measurement of gas throughput through a tube connecting a vacuum chamber and a pump was constructed. A conductance of a duct between the pump and the vacuum chamber determines the rate of pressure decay p=p(t) when nonadsorbing gas is pumped. If gas back-streaming from the pump through the duct is negligible, and gas temperature in the duct is the same as in the chamber, the conductance C of the duct can be calculated from the time derivative of the logarithm of the pressure decay by C=V×d(ln(p))/dt, where V is volume of the chamber. The simplicity of this equation is also a basis for a very high precision of such measurement. Uncertainties which have to be considered are related to the volume of the chamber and deviations from isothermal conditions. Since logarithm of the measured p(t) curve is used to calculate conductance, any correction factor of the vacuum gauge cancels out. Moreover, different sensitivity of the vacuum gauge for different gases is totally unimportant, so only random noise of the measured pressure contributes to the uncertainty. Estimated relative uncertainty of measured conductance is less than 0.3%. Reproducibility (with volume V unchanged) is even less than 0.2%.
Very high precision of this method enables studies of the influence of gas-surface interaction on the tube conductance in molecular regime. Any changes of tangential momentum accommodation coefficient reflect in variation of the tube conductance. We will present results of measurements of conductance of a long stainless steel tube with inner diameter of 7.76 mm for initial state and after different treatments (all at 300 °C for 24 h): exposure to O2 at 0.1 Pa, vacuum bake, and exposure to H2 at 0.1 Pa. Conductance was measured for gasses He, Ne, Ar, Kr, CH4 and N2 in the range of Knudsen numbers from 0.01 to 1000. Variations of molecular scattering on the tube surface resulted in changes of tube conductance of more than 10 % for He, while for N2 and CH4 the observed changes were less than 2 %.