2016/09/08 (Thursday)

Previous Day

	Status: fridge at room temperature, cryostat Dewar is down, IVC is entirely removed. Pumping on still with leak detector, attached at top of fridge. Mixture is in dump and 3He pump, manual dump valves are closed.
	Yesterday a slight room temperature leak from IVC to fridge lines could be produced by bagging the IVC and filling with helium, but the signal could not be brought down again by flushing with nitrogen. Results inconclusive.
0815	Leak = 3.1 pbarL/s
0820	Squirting helium at various parts of the fridge plumbing produces no significant increase in leak signal
0835	Form a bag around the dilution unit using plastic sheeting and packing tape. Form cylinder and tape to IVC top flange, then tape over screw holes at top. Tape along the seam. At bottom pull into a cone with a small opening at the bottom.
0840	Begin feeding helium into IVC through top port. (Not using turbo pump manifold, instead attached hose barb to end of metal hose leading to top port.) Can feel air coming out of bottom of bag.
0900	Over the first 20 minutes the leak signal has increased from 3.2 to 6.6 pbarL/s and continues to increase about 0.25 pbarL/s per minute. Note that helium is flowing faster than yesterday because it's not going through the needle valve at the turbo station.
0910	Leak has risen to 9.1 pbarL/s after 30 minutes of flowing helium into bag. Still increasing at about 0.2 pbarL/s per minute
0915	Stop flowing helium into top port of IVC. Signal continues to increase but more slowly, about 0.1 pbarL/s per minute
0920	Begin flowing nitrogen into bag from the bottom, leaving top port open. Signal stops increasing, steady at 10.8 pbarL/s
0935	Stop flowing nitrogen. Leak has fallen slightly, to 10.6 pbarL/s.
0945	Leak signal has fallen to 10.0 pbarL/s. Start flowing helium out of hose near bottom of bag, but not into bag. This is to see whether just blowing helium into the room increases the leak signal. Over 10 minutes, leak signal increases from 9.7 to 10.6 pbarL/s. So, maybe it's just helium in the room.
0955	Stop helium flow. Over 15 minutes leak signal continued to increase, from 10.6 to 11.7 pbarL/s. Then over the next 10 minutes it fell from 11.7 to 11.0 pbarL/s.
1055	Leak signal has been decreasing slowly, -0.07 pbarL/s per minute. Currently 8.6 pbarL/s.
1115	Leak signal is 7.5 pbarL/s. With still gate valve closed, leak signal falls to 5.3 pbarL/s.
1122	Begin flowing helium into room again. Over 15 minutes leak signal begins to increase about as fast as the first test this morning when filling the bag with helium.
1135	Leak signal 8.3 pbarL/s, 6.4 pbarL/s with gate valve closed. So the increase in rate in the leak detector tubing seems to be as high as the overall increase.
1140	Stop flowing helium. Signal increases for 5 minutes then seems to level off. Gate-valve-closed signal continues to track with gate-valve-open signal, with gate-valved-closed always close to 2.0 pbarL/s smaller.

The increase from filling the bag with helium is about the same as when you just blow helium into the room

1300	Preparing to open mixing chamber and change seal. Unscrew from top of bottom plate of MC, from left to right, cylindrical thermometer, heat sink post, small thermometer (probably), cylindrical thermometer, heat sink post, larger spool (MC heater probably). This should disconnect everything from bottom plate so it can be removed.

1330	Let N2 gas into still. First set regulator on cylinder so that pressure is about 1.5 PSI. Then let gas in through the tubing that was leading to the leak detector (there was a valve at the end so it could be closed off) with a hose barb and rubber hose.
1350	Remove bottom plate of mixing chamber, after removing screws and tightening “pusher” screws. No signs of oil present!
	Take several pictures of the state of the inside of the mixing chamber for documentation. Begin removing indium seal. Cover bottom of fridge-side mixing chamber with plastic sheeting attached by screws to walls of mixing chamber.
1430	Polish external surfaces of mixing chamber base with Scotch-Brite. Clean off indium surfaces on mixing chamber base and on the upper part of the mixing chamber.
1503	Reseal and tighten mixing chamber bottom plate using 100mm indium. Pump on fridge lines through still-side port using scroll pump.
1535	Switch to pumping on condenser side of fridge. Pressure in lines leading up to condenser = 2.2E-2 mbar. Open condenser line speedivalve. Pressure remains at this level.
1545	Retighten mixing chamber screws now that it is under vacuum. I've tightened them about as tight as I can - it's steel screws in steel holes, so we shouldn't worry about damaging threads. Tighten to the point that the mixing chamber itself seemed like it was rotating slightly from the torque. I think it's tighter than they were before based on how easy it was to remove the screws.
1615	Attach leak detector to still, pump down to background of 4.5 pbarL/s. Squirting helium at the MC does not indicate any leak.
1730	Scotch brite the surfaces of the detached thermometers/heaters and mixing chamber, then reattach thermometers/heaters to mixing chamber.

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