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.
