Determination of pump-down time from nomograms

3 Determination of pump-down time from nomograms 

The pump-down times of rotary vane and rotary piston pumps, insofar as the pumping speed of the pump concerned is constant down to the required pressure, can be determined by reference to example 1. 

In general. Roots pumps do not have constant pumping speeds in the working region involved. For the evaluation of the pump-down time, it usually suffices to assume the mean pumping speed. Examples 2 and 3 of the nomogram show, in this context, that for Roots pumps, the compression 

In the medium vacuum region, the gas evolution or the leak rate becomes significantly evident. From the nomogram 9.10 in Section 9, the corresponding calculations of the pump-down time in this vacuum region can be approximated. 

In many applications it is expedient to relate the attainable pressures at any given time to the pump-down time. This is easily possible with reference to the nomogram 9.7 in Section 9. 

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