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Subject: Re: Backstreaming problem
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Date: 11/08/01 at 3:23 AM
Posted by: Vladimir Tchoutko
In Reply to: Backstreaming problem posted by Anneleen on 10/25/01 at 10:08 AM:
1.Strait or 90-degrees elbow won't change backstreaming.
2.Check up the direction of cooling water flow in DP cooling jacket. Usually it should be from top to bottom.
3.I suppose you use a good DP oil?
4. Do you have any special requirements to backstreaming level? If yes better change oil for the best one, Pentavac, for instance. It is more expensive but this liquid has much less vapour pressure.
5.Check up the heater power. It shall be nominal.
6.Check up the temperature of cooling water on the top of pump. It may be a little warm, but no more than 25 - 30C.
7. Check up the fore-line pressure. It should be usually no more than 0.1 torr, better 0.03 - 0.06 torr.
8.Check up the pumping time from high vac valve opening to 0,0002 torr. For a well balanced pumping system it shold be a couple minutes, no more.
9. Be careful trying to reach a lowest rough pressure. Practically all mechanical pumps have a strong backstreaming in the pressure range less than 1 torr. If you use Roots pump in mechanical group it doesn't matter. Are you sure that you have DP oil backstreaming?
10. And, of course, LN2 trap. Water trap can help too, but LN2 better. But it is not good to use traps when backstreaming is more than should be for normally working system.
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