If one assumes a monomolecular adsorbed layer on the inside wall of an evacuated sphere with 1 l volume, then the ratio of the number of adsorbed particles to the number of free molecules in the space will be as follows:įor this reason the monolayer formation time τ is used to characterize ultrahigh vacuum and to distinguish this regime from the high vacuum range. In the high and ultrahigh vacuum ranges the properties of the vacuum container wall will be of decisive importance since below 10 -3 mbar there will be more gas molecules on the surfaces than in the chamber itself. The limits for the individual pressure regimes (see Table IX) were selected in such a way that when working with normal-sized laboratory equipment the collisions of the gas particles among each other will predominate in the rough vacuum range whereas in the high and ultrahigh vacuum ranges impact of the gas particles on the container walls will predominate. It would make little sense to attempt to determine the vacuum pressure ranges as a function of the geometric operating situation in each case. As a result of reflection (but also of desorption following a certain residence period on the container walls) a gas particle can move in any arbitrary direction in a high vacuum it is no longer possible to speak of “flow” in the macroscopic sense. In the molecular flow range, on the other hand, impact of the particles with the walls predominates. The macroscopic speed of the gas is a “group velocity” and is not identical with the “thermal velocity” of the gas molecules. This alignment is compelled by the fact that the gas particles are densely packed and will collide with one another far more often than with the boundary walls of the apparatus. In the viscous flow range the preferred speed direction for all the gas molecules will be identical to the macroscopic direction of flow for the gas. This is always the case at a certain pressure differential and this value may be characterized as “critical”: In the case of viscous flow, however, this will be the case only until the flow velocity, which also rises, reaches the speed of sound. The intensity of the gas flow, i.e. the quantity of gas flowing over a period of time, rises with the pressure differential. Flow is turbulent where Re > 2200, laminar where Re 0. Viscous flow will generally be found where the molecules’ mean free path is considerably shorter than the diameter of the pipe: λ « d.Ī characteristic quantity describing the viscous flow state is the dimensionless Reynolds number Re. Re is the product of the pipe diameter, flow velocity, density and reciprocal value of the viscosity (internal friction) of the gas which is flowing. This special case is found frequently in vacuum technology. Laminar flow in circular tubes with parabolic velocity distribution is known as Poiseuille flow. If various layers of the flowing medium slide one over the other, then the term laminar flow or layer flux may be applied. If vortex motion appears in the streaming process, one speaks of turbulent flow. The character of this type of flow is determined by the interaction of the molecules. Consequently internal friction, the viscosity of the flowing substance, is a major factor. This will be found almost exclusively in the rough vacuum range. PC-Software for DRYVAC and RUVAC WH pumps Software for LEYSPEC Residual Gas Analyser Software for TURBOLAB high vacuum systems and TURBO CONTROL i display Leybold Fundamentals of Vacuum Technology
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