Abstract
The adsorption time of oil molecules on the vacuum chamber wall plays an important role on evacuation process of a vacuum system with oil diffusion pump. As the adsorption time of pump fluid molecules is considerably long, it takes a very long time until vapor of pump fluids establishes stationary flow through piping. This is the reason why ultimate pressure of a well-designed vacuum system evacuated by oil diffusion pump can be much less than the vapor pressure of pump fluids. Furthermore, such a pressure is kept for a long time, even when liquid nitrogen trap is not used, as was shown by D. Alpert.By measuring the time lag in setting up the stationary flow of oil vapor (di-2-ethyl-hexyl phthalate ; DOP), the adsorption time is obtained at 50 to 85°C on borosillicate glass. The activation energy of desorption calculated from the adsorption time is about 22 kcal/mole for DOP, which is almost equal to the heat of vaporization.Conditions for measurements are also discussed in detail.
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