2012/09/14

1020	Fed Cricket 3 pellets.
1030	Filled N2 cold trap full.
1638	Fed Cricket 4 pellets.
1800	Mixing chamber temperature 57 K. Helium leak rate in pipes leading to IVC 2E-9 mbar l/s. Open up IVC and helium leak rate increases to approximately 2E-3 mbar l/s before falling.
	Dump pressure is 713 mbar. It's around 10 mbar shy of where it should be. Where has our mixture gone? Open up valve 12B to pump on nitrogen cold trap 2: there may be some mixture trapped in there.
1820	Helium leak rate has reached 2E-8 mbar l/s and is still falling, showing little sign of slowing down. Residual helium exchange gas? Isolate leak checker and IVC. Open valve to turbo pump to evacuate lines leading to IVC.
1825	Switch path for 1 K pot pump exhaust from helium recovery to atmosphere.

2100	Since performing the leak check, the rate at which the mixing chamber temperature has increased has slowed. Coupled with the fact that the leak rate fell, it would indicate that there is some residual helium gas in the IVC, which was pumped out. Hypothetically, there may have been a pocket of trapped helium gas in the IVC. If this leak was coming from the main bath, the leak rate should not have decreased to <2E-8 mbar l/s as the main bath is still open to the recovery system and should still be filled with helium gas.
	Open up IVC and evacuate it with turbo pump. Mixing chamber temperature 63 mK. We can perform a series of leak checks tomorrow.
	One possibility is that we are simply pumping out the He-4 exchange gas too late. Perhaps we should aim to pump it out at a higher temperature, such as 25 mK, and let the mixture do the rest of the work. We should also consider filling the IVC with an order of magnitude less of He-4 exchange gas. Finally, we should think about sacrificing copper tape for plugging IR leaks for holes to aid evacuation.
2210	Fill N2 cold trap - not full: it's about a litre shy, but this shouldn't matter, especially as we have no mixture in the fridge.
	Pressure in dump is 713 mbar. Not sure if the difference can be explained by its temperature.