2019/04/11 (Thursday)

Previous Day

Status	Warming. All mixture in dump. LN2 traps warm. 1K Pot needle valve open. Some helium exchange gas in IVC. Helium exchange gas in cryostat vacuum jacket for fast warmup.
	By 20:00 last night, Recovery rate had dropped below 1 cuft/200 seconds (around 400 cuft/day, or 15 liq L/day) and continued to fall. At that time, MC = 37 K. This seems to be the point where the last LHe boils away.

Blue is the recovery meter reading converted to liters of LHe (then multiplied by 10 for scaling in the plot)
Reco meter continues to rise slowly, presumably just from expansion of gas in bath.

0940	MC(Cernox)/Sorb = 125/235 K
1120	MC(Cernox)/Sorb = 130/249.8 K. Sorb almost at max.
1325	MC(Cernox) = 140.6 K
1235	Let 200 mbar N2 gas into vacuum manifold. Then open cryostat vacuum jacket valve to let this into the vacuum jacket. Vacuum jacket → 5.4 mbar
1320	Recovery = 2.8 cuft/day = 0.1 liq L/day
1340	IVC = 0.54 mbar
1340	at close to this time, the recovery meter ran backwards briefly, by only about 1 cuft, then recovered. Odd. Looking closely, recovery speed in general increased a bit after adding exchange gas. Quite slow though.
1345	Let about 0.45 atm nitrogen gas into IVC
1400	Recovery = 12 cuft/day = 0.46 liq L/day
1410	Tower = 144.3 K Currently the warming time constant is about 27 hours, and the fridge won't reach dew point (12 celsius) till Sunday.
1530	Recovery = 4 cuft/day = 0.14 liq L/day
	Should think about when is a good time to close the recovery system, especially considering that at low pressure on our side there's a tendency for the meter to run backward. Currently we are still slowly recovering due to gas expansion as we warm. But consider:

Liquid helium is 700 times more dense than STP helium gas
Let's suppose the effective cryostat volume that's at an average temperature equal to the MC temperature is 50 L
If MC = 20 K, the gas will expand 15 times, and 14 times the cryostat volume will be pushed through recovery. That's 700 STP L, equivalent to 1 liquid L
If MC = 125 K, the gas will expand 2.4 times, and 1.4 times the cryostat volume will be pushed through recovery. That's 70 STP L, equivalent to 0.1 liquid L
0.1 L is the measurement error of a single Dewar weighing
4 L is the measurement error of the monthly meter reading (nearest 100 cuft).
So - certainly it makes sense to close the recovery when we are above 125 K - at that point the amount still to recover is truly negligible.

1550	MC(Cernox) = 155.6 K Now the warming time constant is about 18 hours. At this rate the fridge will reach dew point (12 celsius) mid-day Saturday.
1617	Recovery meter = 246401.5 cuft. Close main bath recovery valve.

Checked the following morning: the recovery meter hasn't moved now that the
main bath and 4He pump recovery valves are closed

1720	MC(Cernox) = 167.0 K Now the warming time constant is about 17 hours. At this rate the fridge will reach dew point (285 K) Saturday 10:25 AM.

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