Now you are going to pump out the LN2 that was used to cool the fridge to 77 K. The LN2 is going to get pushed out of the MB with He gas, and while this is happening you will monitor the helium leak rate.
Turn on the leak checker and let it warm up for 20 minutes. Once warmed up, calibrate the leak checker.
Connect leak checker to IVC. First, evacuate the line connecting the leak checker to pumping manifold first (using the scroll pump), and then open the valve connecting to IVC. Take a background reading. Note it in the run diary.
Remove the rubber stopper from the transfer stick and attach the rubber hose to the transfer stick (as you did previously when preparing for the LN2 transfer). Make sure that the transfer stick hasn't moved vertically at all. We want it to be < 1/8“ from where it hit the transfer cone. If the transfer stick is not within the cone, you won't be able to remove all of the LN2.
Attach a short metal transfer stick to the other end of the long rubber hose and insert this into a bomb dewar (holds 25 L).
Make sure there is room for the hose to wiggle (and contract) when it's become frozen by the LN2.
Make sure there is an extra bomb dewar close by in case the first one fills up.
Seal the end of the He recovery line (where it connects to the top of the fridge) with a KF-25 blank.
Make sure the He recovery valves on the wall are closed.
Make sure all the valves on the right-hand side of the IGH are closed. Open the He gas cylinder. Open the He gas valve on the wall near the IGH - there should be a hose from this port to the vent inlet.
Make sure that the manual valve on the 4He pumping line is closed and that the 1K Pot speedivalve is closed.
Open the vent speedivalve, then open V2a, V5a, V4a, and V1a.
Check - is the leak checker reading the background of the IVC? Did you record the background rate?
Remove the blank from the end of the He recovery line. Allow helium to flow through for several seconds to purge the line. Then connect the recovery line to the main bath port at the top of the fridge.
You are now pressurizing the main bath with helium gas. This should push LN2 out of the main bath. The rubber hose will get cold and stiff.
Monitor G3 and make sure it doesn't go above 350 mbar. If it does, slightly close the regulator valve on the He gas tank and monitor the change.
The reason you closed the large speedivalve on the He-4 pump is to protect it during this process. If the valve is not closed and the pressure of the gas goes above 400 mbar, the pressure from the gas in the line will cause oil to be pushed out of the He4 pump. This is a MESS to clean up and recover from. (outdated: we now isolate the pump with a manual valve.)
LN2 will begin to flow out of the
MB and into the bomb dewar. This process should take about 1.5 hours; keep monitoring the leak rate throughout this time. Be ready to switch bomb dewars.
When the pressure reading in G3 drops significantly, the LN2 is almost completely out. Check the leak rate and record this value in the run diary.
Close V2A, V5A, V4A, and V1A. Close the He gas supply and the valve on the gas line. Wait for the rubber hose to warm up to remove the tubing from the transfer stick, and then remove the transfer stick and plug the fridge port. Now you will check to see if the
MB is empty. You are going to pump on the
MB with the scroll pump.
Open V4A and V1A.
Turn on the scroll pump and open its inlet valve a small amount until it starts making noise. You are now evacuating the
MB, continue to slowly open the scroll pump until P2 reads less than 1 mbar.
If the pressure of P2 is not less than 1 mbar, you have LN2 left in the main bath. The reason for this could be that the transfer stick is too high or that you actually ran out of He-4 gas to push out the LN2. Check the positioning of the transfer stick and the amount of He-4 gas left in the gas tank. Once you have found the cause for the LN2 not being pushed out and corrected it, you will need to re-start the flow of He-4 gas to the
MB.
Isolate the main bath (close V4A and V1A).
Open the valve on the He-4 gas tank. Make sure the valve on the copper gas line is closed.
Evacuate the clear hose leading to the valve on the gas line by opening V2A and V5A (as you've done before).
Close V4A and V1A.
Open the He-4 gas valve on the gas line.
Turn off the He-4 pump.
Go to the pump room and close the large speedivalve on the He-4 pump.
Open V2A, V5A, V4A.
As before, open V1A when the pressure goes above 70 mbar, but make sure it doesn't rise too high.
Hopefully you will see the hose start to cool, which means more LN2 is being blown out. When the hose becomes limp again, you can stop the He gas flow and stopper the transfer stick. Repeat the test of evacuating the
MB with the He-4 pump (described in previous steps) to make sure all of the LN2 is out. Make certain that when you are ready to pump with the He-4 pump that the large speedivalve on the pump is opened. When conrmed that there is no LN2 left in the
MB, continue to the next step.
At this point the
MB is being pumped on by the He-4 pump and you've confirmed that all of the LN2 is out. Isolate the He-4 pump and flush the
MB with He-4 gas. This step is necessary to remove the LN2 transfer stick from the
MB because we don't want air to rush in and freeze when you unplug the exhaust port. If you need to remind yourself how to do this, review section in appendix on how to pump and flush the
MB.
Confirm that the He-4 gas supply is off. The main bath should be full of He-4 gas.
Remove the LN2 transfer stick and replace the plug into that exhaust port.
Evacuate the
MB again. This must be done before the LHe transfer (review explicit instructions to do so in the `Pump and Flush
MB'section in the appendix).
If necessary, you can use the recovered LN2 to fill the LN2 cold trap. See the appendix on how to do so.