In Reply to: Re: Helium Pump down time posted by Pat McMaho on 07/19/02 at 10:48 PM:
Yes, we connected a RGA to the system. The dominant residual gas is He (amu 4). I have saved some spectra as jpg files. These, however, are at work now, but I will try to send some at a later stage. What we also observed was that purging with Ar, suppressed the He peak while purging (feeding Ar to raise the chamber pressure to 1e-4 for about 3min), but when the Ar feed was closed the He peak reappeared at the same level that it would have without Ar purging.
I should also say that this system was contaminated by rotary vane pump oil. The system was cleaned twice with no improvement.
Also, Yesterday I disconnected the backing line from the turbo's and connected the He feed valve directly to the backing line. The pressure measured by the thermocouple showed the same behaviour as previously when the gas was feed via the vacuum chamber.
For me this points to unexpected behaviour of the backing pump (Alcatel ADP122). Pumping curves for the turbo's and ADP122 are available at http://www.alcatelvacuumus.com/.
The He pumping speed for the ATP900 is given as 540l/s and compression ratio as 2e4. The pumping curve for Nitrogen for this pump start to drop at 1e-4. The pumping speed of the ADP122 at 2 mBar for Nitrogen is 92m^3/h (25 l/s). My calculation says the turbo's (2 off) need 2*540/2e4 l/s backing at 2mBar ie. 0.054l/s. The pressure in the turbo backing line goes to 1.1e-1mBar. This indicates to me that the ADP122 has a He pumping speed at 1.1e-1 mBar of about 1l/s. Still more than sufficient to back the two turbos. Am I correct in stating that the pressure in the backing line could go as high as 2mBar for the turbo's to be sufficiently backed to run at 1e-4mBar inlet pressure? Ie. I am looking at the backing pumps only and assuming the turbos have sufficient pumping capacity. ( Which should be the case as long as we do not feed He to raise the chamber pressure to higher than 1e-4.)
However, what I have not confirmed is, that this is true for He. (ie. What is the actual He pumping curve for the ATP900 turbo's.)
Is the above calculation correct?
I would like to thank you for giving my problem a thought and for the well whishes in getting this system to work.