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Hi GEM folks,</div>
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I was able to talk to Jack today about the BigBite gas system and I will try to summarize the information he told me.</div>
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<b>Background:</b> Just so everyone is on the same page, last Wednesday (<a href="https://logbooks.jlab.org/entry/4017844" id="LPlnk882430">https://logbooks.jlab.org/entry/4017844</a>) the BigBite gas mixture reached 100% Argon and GEM folks were notified.
Fortunately, all BigBite GEM HV was off at the time. So no sparking or damaged occurred to the GEM layers. Since then Jack has been working on trying to figure out what caused the problem and to remedy the situation. Last Thursday Sean closed up the gas inputs
and outputs of the BB GEMs so that way the gas system could be manipulated by Jack. We will need to flow good mix through them for probably a couple days to make sure everything is correct.<br>
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<b>Update: </b>Yesterday Jack swapped out the original Mass Flow Controllers (MFC) and the Mixing Control System, both of which are fairly old (~20 years). The MFC for the CO2 line appears to have failed and the Mixing Control system had damage to the electronics
board, due to water and mineral oil leakage over the years. <br>
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Having replaced both MFCs and the Mixing Control System the Ar/CO2 ratio returned to ~78% Argon in CO2, which is what we expect and matches the equivalent ratio of gas for the premixed 75/25 cylinders. The main tool that saw this imbalance in gas ratio was
the Binary Gas Analyzer, as the voltage readback systems did not readback fast enough to notice this problem. The sensors readback every 2 seconds and the problem was only noticeable on scope on the order of half a second.
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<b>Moving Forward: </b>This is probably the most problematic scenario for the gas system, however, we were fortunate that the gas system had these failures when the HV was not running or during beam-on conditions, and we were able to recognize there was a problem
via the binary gas analyzer. In a sense I am grateful that it occurred now and the equipment could be replaced. I think Nilanga noted that it would be best if we had some sort of automatic shutoff of the GEM HV if this were to ever occur again. I will defer
to Sean on this, since he is working with Steve Wood to try and implement said automatic shutoff via EPICS.</div>
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Now the GEM folks just need to decide when to begin flushing the BB GEMs again, now that the gas mixture has become stable. What we could do is wait a couple of days and monitor the gas system behavior and then decide in a couple of days if we want to return
flow to the BB GEM layers.</div>
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Let me know if you have any questions or comments.<br>
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Best,</div>
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Zeke<br>
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