[Hallb-engineering] [Revised Logentry] Follow-up Re: Hall B cryo investigation and changes to allow operation of Hall B magnets

[kashy at jlab.org]

Logentry Text:
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We are in a belt and suspenders mode to get things going again.
The vacuum in the return U-tube for the Buffer dewar was checked and it seemed pretty good  ~3E-5 torr, but we are leaving a turbo on it and the pressure is reading ~2E-6.
We are also adding a pump to the Torus return U-tube, but again the VJ is not particularly cold and there is no frost or even condensate on it.

A strange thing I've noticed is that the flow to Hall B is indicating suspiciously low, averaging about 6g/s. While this should be a good thing, I'm not sure it is real. if the flow is too low then the system will get unstable. This is because the return flow will warm and then as it flows back to ESR it warms the supply flow and causes pressure pulses.

A second order problem is the helium level probe in the 500liter buffer dewar. It has dead bands in it read back and tends to flat line a different values then jump. This can be an issue in that the supply valve may end up at its min or maximum position for a long period and if at it minimum the flow through the dewar and the return U-tube does get hot which causes the problem.

I've updated the controls for the Buffer dewar PID that controls the inlet JT valves (EV8210A) minimum position. It has always had this control, but it was set up to control on the Buffer Dewar outlet temperature TD8111 with a setpoint of 5.5K as noted in my previous entry. The critical temperature is the temperature of the return gas after it passes through the long flex return U-tube to the Hall B distribution box. The Buffer Dewar return flow temperature is measured by TD8522DR. Its been oscillating from ~8K to 20+K for a while. I've made the setpoint of the  loop 7.5K and the input TD8522SR. The Dewar level is filling and I expect it to overfill as some point. Hopefully this will not effect dewar pressure. If it does it may upset the cryotarget and we will need to look elsewhere for a solution. 

Note: The min position cascade PID on the Dewar JT now mirrors that of min position cascade PID of the Solenoid. 

So far we have not had any bumps in TD8522DR for an hour. We can adjust the setpoing fo the EV8210A loop and change it up or down from the present value of 7.5 to attempt a fine tune if needed.

Hall B techs Dontre and Steve are putting a pump on the return U-tube from the Torus and we will check the vacuum in it and leave the pump pumping on it, if it is needed. 


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