Mass spectrometer failure and bake out

. zaterdag 9 juli 2011
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After cleaning every vacuum component (again) I tried bake out, for a short time, just heating the setup to about 60 deg. C for 10 to 15 minutes, using a paint burner.

Seemed to do the right thing to the H2O pressure, but then my mass spectrometer showed a very low pressure, about 8e-8, which was obviously wrong, the cold cathode gauge was still in the e-6 range.

The researchers at had the same problem with the same QMA 064 sensors. They tried cleaning with acetone, without any effect.

After posting it on the fusor board the idea is to start cleaning with H2SO4, NH4OH and H2O2.

We think that over the years various gasses got buried into the collector and were released during bake out, reacting and forming an insulating layer on the collector, so we have to do proper bake out before cleaning it, or the same affect would come up again.

But the bake out did have a positive effect, pressure is now in the e-7 range, to get to e-8 I need longer bake out.
I've ordered 50x Vishay PTC heating elements for 110 deg. C, they can be connected to 230 VAC directly and will heat up until reaching a specified temperature. Very simple and cheap. At 1200 ohms they will start at 44 watt and start giving of less power at about 100 deg. C, and switch off almost completely when the surface temperature is 140 deg. C.

Only a factor 12 to go (indication of the cold cathode is a bit on the high side, so t might be better than indicated):

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