Pump down curve interpretation: Stopping of Turbo

[myID]

Hi-

when I keep my rotary pump running and switch on and off the turbo the curve looks like this (pressure range is from 5*10e-5 mbar (6.3) to 3*10e-4mbar (8) still have to do the calibration....time is from highest to highest pressure about 1000 sec).
The pump down looks like I would expect.
But why has the pressure rise curve with the turbo getting slower these "spikes"?
I have some ideas but any input would be welcome

Kind regards and thank you for you input
Roman

[steve_rb]

I think because pressure is too low you are getting random fluctuations of the pressure while air gets inside the chamber.

[myID]

[Chris Bradley]

Just to provide a response (so you know your question isn't being ignored!); I don't really see a particular reason for this. I can imagine several reasons for spikes during pump down (pockets of outgassing or controller phasing) but this data indicates the reverse.

The only thing that springs to my mind is to ask whether you are powering off the controller to turn it off, or switching it to 'off' on the controller. I'm just wondering if there is any active balancing in the design that needs to continue functioning for the pump to spool down smoothly - particularly in the horizontal configuration.

If no such requirements are needed, and it is powered down OK, then I would stab at a guess that the same reason may apply; free-wheeling resonances of the turbine? Can you hear periodic resonances during spool-down?

[Frank Sanns]

So from 0 seconds out to 1,100 seconds you were running your two stage mechanical pump and your turbo? At 1,100 seconds you switched your turbo off but let it spin down and kept your two stage pump pumping out to 2,400 seconds? Or did you valve off the chamber.

I think you could not have valved off the chamber or the spikes would not be there.

The pump down over the range of your graph to 1,100 seconds and the chamber returning to that in the same time period is not good. Either the turbo is not pulling as it should or you have a leak that approaches your pumping speed.

More info is needed to know what is really happening here.

Frank Sanns