This page documents the problems we are experiencing with the Leiden cryofree fridge since installing the new Firmware (version 2.48.1) in July 2013
See the attached files for the firmware v2.48.1 flowcharts of different modes (He3 condensation
, He4 condensation
, Normal Circulation
, and Recovery
Also the former firmware flowchart is attached V2.07
Gas leaking into the He3 dump
Immediately after installing the new firmware, we noticed that the pressure on the He3 dump seems to be creeping up while the fridge is in normal operation. The odd thing is it rises in discrete steps.
The fridge was running normally, so the electronic valves to the dump are closed.
The strange thing is that the pressure P7 increases in steps.
One possibility is that we have suddenly, since installing the new firmware, developed a leak into the He3 dump. But then why would P7 increase in steps?
Another possibility is that the firmware is briefly opening two valves to let He3 into the dumps. Hard to imagine.
After pulling mixture out, gas disappears from back of He3 pump
28 July 2013. Oleh noticed that after pulling all the mixture out into the dumps and closing the manual dump valves M1 and M2, the next day the pressure at the back of the pump, which should be the same as the pressure in the He4 dump (P6) had dropped to 173 mbar. Note that the system was not in Auto mode, so shouldn't be opening and closing valves unless in an emergency.
To check how much gas had leaked through to the front of the He3 scroll pump, he restarted the pump. But P5 only rose to 310mBar.
Fridge status and temperatures (cryocooler is off):
Here is the pressure log. Look how P5, at the back of the helium pump, suddenly drops at 13:00 when nobody is present -
P5 rises at 15:30 when Oleh turns the He3 pump back on - but it only gets back up to 310mB, not 1000mB as P6. So where has all that gas gone??
Now let's look what happened inside the fridge. The Still pressure gauge is maxxed out. But look at P4:
P4 jumps from 0 to 8 mBar at exactly the same time as P5 drops from 1000mBar to 0. Where did all that gas go? It's such a large volume of gas that it can't have gone into the space infront of the P4 - the only place it can have gone is into the still! But to do that the firmware would have had to open several electronic valves, and maybe even the gate valve, when it was not in Auto mode.
Oleh is going to pump the still out and see how much comes out (via the N2 traps so as not to get too much air out from the fridge, which has blocked condensor lines, into the He4 dumps).
Oleh discovered that gas didnt go into the still but in fact was in one of the nitrogen traps (the other trap was isolated by manual speedy valve M4). The were no gasinside the turbo pum space behind valve 0. It looks like the system has opened valves 4, 5 , 8, 2, 3 allowing gas from the back of the S3 pump.
All gas has been recovered to its initial value P5=1156 mBar. 1200 mBar of gas in the still is most likely air which blocked condenser lines.
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