Test header arrangements for determining the speed of pumps using an orifice type impact pressure transducer

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Abstract
Following an earlier paper on the measurement of the speed of pumps, in which the intrinsic pump speed was determined by choosing a certain position of a tubulated ionization gauge on a test dome, a similar calculation has been carried out for an orifice type pressure transducer. The results indicate that with a test dome of effective length equal to that of the pump diameter the correct gauge position is approximately the same as when using a tubulated gauge, i.e. at a position along the test dome wall from the pump inlet of 1.055 R for an orifice gauge compared with the 1.022 R previously found for a tubulated gauge, where R is the pump radius. For many purposes it is sufficiently accurate to place the gauge at R (i.e. half the diameter). As the test dome length is increased the gauge needs to be positioned further from the pump mouth but to a lesser extent for an orifice as compared with a tubulated gauge. An expression is derived for determining the error in speed measurement with gauge positioning indicating its dependence on the ratio of test header to pump diameter and the capture probability. It is shown that it is possible to arrange the gauge at a position such that measurements are accurate to within ±3% for all test header lengths likely to arise. Finally, the effect of variations in capture probability with the angle of molecular incidence at the pumping surface is briefly considered, although no experimental results are available to indicate the importance of these effects.