Turbo pumps represent a very large conductance problem when they are not running because of the small clearances. You will have a long pumpdown because of that. Also, because you have such a low conductance, you will find it difficult to reach an acceptable base pressure with the pump off.
The turbo's components also have a large surface area so that only compounds the problem.
If you are going to run the device w/o the turbo spun up, better to either remove the turbo for the time being or put a bypass line around it.
Steve
Created on Tuesday, March 20, 2001 12:28 PM EDT by Steve Hansen