Pump and Flush Main Bath and 1K Pot

To prepare the main bath (MB) for transferring LN2, you must first evacuate it so there is no air within. Before cooling, you must also pump out the 1K pot, including the 1K pot needle valve shaft volume. This is a new procedure to pump out all of these volumes in parallel and pressurize them with He-4 gas. With the MB over-pressurized with He4 gas, you prevent air from rushing in when you open the exhaust port to do the LN2 transfer. Also, we want to let mixture into the fridge through a cold trap and leave it there during cooldown. We have found that this reduces the chance of a blockage in the fridge lines when trying to circulate.

1. Check the He-4 gas tank to make sure there's enough gas to flush the MB.
2. Pump out fridge lines
   1. Make sure mixture is isolated in the dump.
   2. Connect scroll pump to He-4 pump line, turn it on.
   3. Open pump line valve, V5A, V2A, V7, V2, V1, V3.
   4. Open condenser speedivalve and still gate valve.
   5. You're now pumping on the fridge lines. Wait until P1 and P2 flatten out.
   6. Close V2, V7, V2A, V5A and close and turn off the scroll pump
3. Let mixture into fridge through a cold trap
   1. Open V9, V13A(or V13B), V12A+speedivalve(or V12B+speedivalve)
   2. This lets mix through a cold trap and into both condenser and still sides of fridge
4. Make sure all openings to the MB (fill ports) and 1K pot are closed
   1. Make sure main bath hose to recovery is connected.
   2. Close all He recovery lines on the wall and ensure the yellow valve is switched to exhaust.
   3. Make sure that the 1K pot needle valve and spedivalve are closed.
   4. Make sure all Auxiliary valves (V1A, V2A, V4A, V5A) and the vent spedivalve are closed.
5. Pump out 1K pot and main bath
   1. Turn on He-4 pump, or scroll pump hooked up to He-4 line.
   2. Open V4A, V1A to pump on the MB until P2 reads < 100 mbar. This may take a few minutes to register on the Pirani gauge.
   3. Open the 1K pot speedivalve. This pumps out the 1K pot lines up to the (closed) needle valve. You are now pumping out the MB and 1K pot lines in parallel.
   4. Open the 1K pot needle valve by 5 complete turns. This should expose any trapped volume in the 1K pot needle valve shaft volume.
   5. Continue to pump until P2 reads < 1 mbar. This will take a few minutes to register on the Pirani gauge.
   6. Close V1A and 1K pot speedivalve (blocks off the MB and 1K pot).
6. Flush the 1K pot and main bath with He-4 gas
   1. Open V5A, V2A, and the vent speedivalve evacuate the clear hose leading to the He gas line.
   2. Once P2 falls to <1 mbar, close the He-4 pump line valve and turn off the He-4 pump or scroll pump.
   3. Open the blue valve that connects gas line to IGH - gas is filling the clear hose now.
   4. When G3 starts to rise (> 100 mbar), open V1A and 1K pot speedivalve - at this point, He-4 gas will be flowing through to the MB directly and through the 1K pot.
7. G3 will decrease in pressure when you open V1A - when it rises again above 100 mbar, close V2A, V5A, V4A, V1A, the 1K pot speedivalve, the vent speedivalve, and the blue gas valve.
8. Close 1K pot needle valve to prevent N2 entering the pot during the initial LN2 transfer. |