[Sbs_gems] [Revised Logentry] BigBite GEM Gas Update

[ewertz at jlab.org]

Logentry Text:
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Around 8:30 this morning I received an email from Bob Michaels about an audible alarm in the counting house, noting that the GEM gas mix was above 80% Argon. In principle if this were shift conditions someone would have noticed this, but since the counting house is currently empty this went unnoticed. Before going into anymore details I would first note that the BigBite GEM HV was off when this occurred and will remain off until the mix ratio is reestablished and the source of this problem is understood. 

Currently what occurred overnight, is that progressively the gas ratio increased until it reached 100% Ar. This sounds really scary, but we were lucky that the GEM HV was off. This has not occurred before, but was a discussed hypothetical multiple times. I am currently monitoring and regulating the gas mix and gas flow to the GEMs. This afternoon we should be able to finish installing shutoff valves and the GEM modules, at which point we can close the GEMs temporarily and have Jack investigate the mass flow controllers and the mixing system at the gas shed to resolve this problem. 

We will need to monitor the GEM gas ratio for a couple of days once the system is resolved and to allow for the mix inside the GEM modules to change to the proper gas ratio. It was noted by Nilanga that for the future we should try to implement some sort of slow control that turns off the GEM HV or ensures it is off if the gas ratio goes above a threshold. Say 80% Ar.

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This is a plain text email for clients that cannot display HTML.  The full logentry can be found online at https://logbooks.jlab.org/entry/4017844
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