I am using an SS chamber, approximately 30 liters of volume, pumped using a TMP (Pfeiffer HiPace 300, 200l/s) with scroll pump (BOC Edwards XDS 10) used as backing pump. TMP is directly connected to chamber. A gate valve is connected to exit of TPM (between TMP and backing pump). I am using it to seal chamber when not in use. The chambers has CF joints all sealed using Teflon gaskets. The chamber is mostly empty except a translation stage and few Teflon screws.
During my experiments I pump chamber to 6*10^-7 mbar (4-hours of pumping) and inject Helium gas puffs. During shut down, I switch-off TMP, close the backing gate valve and turn off the backing pump. Then I wait for TMP to ramp down to 0 rpm before cutting the power to system.
One day after shutdown, I turned the backing pump on and open the gate valve. I pumped for 10minutes (5e-2) then closed it. Interestingly, I observed that the pressure was still reducing. It went all the way down to 8e-3mbar (in 5 minutes) before stabilizing.
At first, I suspected the faulty gate valve and completely disconnected the backing pump line. The TMP was also at stand still, but the gauge still showed reduction in pressure. I also tested with two different gauges, Pirani (MKS instruments, calibrated for Helium and air) and Pfeiffer full range gauge, but found the same thing. The chamber is never baked and is always at room temperature (24C) . Chamber is not vented for 30 days and the pumping is done daily for 8 hours.
Can someone explain what could be the reason? Is is possible that using Helium gas may be affecting the readings of the gauge. Is there a possibility of surface pumping through adsorption?
>Is it possible that using Helium gas may be affecting the readings of the gauge?
Yes, the type of gas measured does affect the reading.
https://www.wikiwand.com/en/Pirani_gauge
Check the graph "Curves to convert air readings to other gases".
Hope this helps.
Hello Sascha,
the gauge of Pirani instruments definitely depends on the gas. If the gauge is calibrated for air,and you let in helium, you get a lower value. (It is very important to know, that you play with your life, if you vent a chamber with He and read the pressure with an air calibrated Pirani). In the case of mixed gases the gauge is too low. If the Pirani is located in the upper parts of your chamber, a gas exchange can effect your observation.
Furthermore you must know, that the TMP pumps light gases worse than heavy gases. The rest gas in your chamber is dominated by He.
With Regard
R. Mitdank
Here still a gauge after venting a chamber with Argon. The difference arises from the lower thermal conductivity of Ar comparable to air.
Insofar I must correct my upper statement. It gives a higher gauge, than air due to the higher thermal conductivity of He comparable to air. So, a gas exchange can effect the change with time, but then, He must be substituted by air.
I ask to excuse me for my upper error.
R. Mitdank
Rüdiger Mitdank
Hello Rüdiger,
yes, that is what I meant to say. The first line of my comment was just repeating a part of the original question.
Thank you for the elaborate answer nonetheless.
Sascha.
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