Speed, Speed Factor and Power Input of Different Designs of Diffusion Pumps, and Remarks on Measurements of Speed

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Abstract
The speed factor of a diffusion pump is defined as the ratio of its real speed to the ideal maximum speed. The fact that the speed factor is much less than unity is due to three causes, namely, the vapor dispersed through the slit, the nozzle chamber resistance and the diffuse reflection of gas molecules at the boundary of the vapor‐stream and at the condensing wall. Some effects on the speed are considered as being independent of the configuration. The following general conclusions result from the measurement of the speeds of different configurations. (a) In order to reduce the amount of dispersed vapor by the slit, the vapor nozzle should be divergent instead of being parallel; (b) in order to reduce the chamber resistance the space surrounding the slit should be large and the nozzle chamber should be cold; (c) there exists for each design a certain slit area at which the speed is maximum, but there has been no general rule to predetermine it; (d) in designing a large pump either a narrow throat annular nozzle or multiple nozzles can be used. In measuring speeds some precautions should not be overlooked, unless the results will be falsified: Either the determined speed will be too small or too large (pseudospeed).

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