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Subject: Helium Pump down time

Date: 07/18/02 at 9:31 PM
Posted by: Anton De Beer
E-mail: antondeb@mweb.co.za
Message Posted:

I have observed the following behaviour in a vacuum system. The system consist of a 125 liter chamber pumped down but two alcatel ATP900 turbo molecular pumps backed by a ADP122 alcatel drypump. Helium is fed into the chamber via a needle valve. The pressure in the chamber is measured by a penning gauge and at the outlets of the turbos by a thermocouple.

The chamber is pumped down for three days to lower than 1e-6 mBar. Helium is then introduced through the needle valve to bring the chamber pressure up to 1e-4 mBar. This chamber pressure is maintained for 2min. (An RGA connected to the chamber confirms that pure He is fed into the system). When the needle valve is closed, the chamber pressure drops to 1e-6 mBar. However maintaining the pressure (1e-4) for 30 min, the chamber only drops to 8e-6 mBar after the needle valve is closed and it takes about 1hr then to reach 1e-6.
Maintaining the 1e-4 pressure for 2hr and longer results in the chamber pressure only dropping to 5e-5 and a pump down time to 1e-6 of 2hr. The pressure at the Turbo outlets is 5e-3mBar when the chamber is at 1e-6. Initially when He is intorduced to take the chamber pressure to 1e-4, the turbo outlets go to 3e-2mBAr. However, this (indicated) pressure steadly rises to 15e-2mBar when the chamber is kept under Helium at 1e-4 for periods longer than 2hr.
Question: Is this normal behaviour for He? Why does the time that the chamber is kept under He influence the pump down time? Does this suggests that He is absorbed somewhere in the system when the chamber is at 1e-4. And release this He when the needle valve is closed? If so which materials absorb the He. We do have ceramics in the system. But this was taken out and the same behaviour was observed. Does He defuse into metal surfaces?

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