[Sbs_gems] [New Logentry] Calibration of Binary gas analyzer used for the GEM detectors in Hall A

[chandan at jlab.org]

Logentry Text:
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This morning after the Hall went to restricted access, we tried to calibrate the binary gas analyzers (one on the big-bite (BB) and another on the Super Big-Bite (SBS) side) after Holly confirmed that it was safe to proceed.

Fig 1, shows a very schematic of the gas system we have for both BB and SBS side. The mass flow controllers (MFCs) were set ( via applying proper voltages) to achieve an Ar ratio in the gas mixture of ~73% (for the SBS side) and ~78% (for the BB side).
During today's calibration process, those voltage settings were not touched - so, the gas mixing ratio for both the SBS and BB sides should remain the same (it did!).

At the beginning of the process, we closed the valve after the buffer tank to close the gas flow to the SBS side BGA. Then we connected the outlet of the BB side BGA to the inlet of the SBS BGA.

The expected result for SBS BGA should be 78% if the SBS side BGA has the same calibration coefficients. The ratio was (see Fig 2) increasing towards 77% (you have to multiply with the values from the strip chart to get the Ar percentage in the gas mixture). But, I noticed the waviness in the SBS BGA readout. I doubted the series connections of the two BGAs.
So next, I connected them in parallel and the BB side showed 77.% and the SBS side showed 77.1% and that was still on the increasing trend. However, the waviness is still present in the parallel connections. I am not 100% sure about the cause yet.

When we saw that both the BGAs are giving results within 1% - that sort of proved that the calibrations of both the BGAs are very similar. So, decided to connect a premixed 75-25 (Ar-CO2) gas bottle. We rolled the bottle on the ground before connecting it to the BGAs as experts suggested. BTW, the premixed bottles have an uncertainty of 2% in their mixing ratios.
Both the BGAs gave very similar readings (77.5%) for the premixed 75-25% bottle. If I believe that the bottle was exactly 75-25 mixture, then we have about 2.5% offset. However, we need a very accurately calibrated mixed bottle to establish that offset. 

There are flowmeters in the inlet of the BGA (see Fig 1). The SBS and BB sides have flow meters of different ranges and their flow settings were different. So, to study the inlet flow dependence on the, I tripled the flow through the SBS BGA (compared to its normal settings; See Fig 2 (a) region), and the change in the reported percentage was 0.5. Similarly, in the (b) region of Fig 2, I closed the BB flow by half, and the reported ratio was reduced by 0.5.

In conclusion, both the BGAs have very similar calibrations.

I undid all I did for the calibration, and the gas system is ready to use.


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