[Dsg-mvt] [New Comment] second isobutane MFC running

[mdefurne at jlab.org]

Comments:
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 - by mdefurne on Fri, 01/12/2018 - 00:12.

Mixture requirements - I do not worry about the pressure in the tank but about the quality of the mixture. Currently, the pressure remains stable at 16psi, but the flow is multiplied or divided by the same factor every second to maintain the pressure at 16 psi. Because of that, we cannot read anymore the flow in the mixing GUI . Now, what can prove me that we are still mixing 90-10 and not something different by changing every second the set point of the MFC? Maybe the Argon MFC adjust faster than the C4H10 MFC to the setpoint, and consequently it will change the ratios of the two gas in the mixture. Or maybe it is the opposite. 
Unfortunately I won t try tomorrow to turn on the detectors because it could kill the detector if the argon fraction is unexpectedly too high. The stability and absolute control of the mixture is a crucial aspect.

Also, maybe I am completely wrong in my comparison, but switching on and off the light bulb is not good for it. Can we have the same effect if we change every second the setpoint of the MFC?

 - by beng on Thu, 01/11/2018 - 22:53.

Mixing Flows - I think just keeping an eye on the tank pressures to see that if it gets over 16 psi and the system tries to reduce the pressure, if all the flows are above the minimum MFC flows the current logic can be used (at the current demanded flows).

If we change the logic we should first try to use something similar to the DC mixing system where it bounces between the pressures, but by reducing and increasing the input flow by a fraction of the output flow instead of completely stopping the input flow. This seems to work well, but also they are much higher flow rates and much bigger tanks.

I'm not sure that requesting zero flow will actually result in no flow which in turn would throw off the mixing ratios when attempting to reduce the pressure. Maybe solenoids would need to be installed to actually stop the flow to reduce the pressure?

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Logentry Text:
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Since C4H10 MFC Mix1 is going very well, we switched on the MFC Mix2. So far so good.

We should maybe find another way to scale the required flow to maintain a reasonable pressure in the tanks. It might be better to fill at full flow up to 16psi, completely stop and restart when pressure drops under 7psi. 

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