| 0920 | Check dump pressure: 797.8 mbar on G2. Pbar = 30.09“, so corrected dump pressure = 803.6 mbar. |
|---|---|
| 0945 | Begin pumping out IVC. |
| note: scroll pump is very noisy. Sounds like it needs to be repaired. | |
| 1125 | Turbo pump pressure 6.3e-6 mbar |
| 1126 | Switch from turbo pump to leak checker to conduct leak check simultaneously with throughput test through cold trap 2. Throughput test yielded flow rate of 57.5 mbar/min. (G1 allowed to rise to about 667 mbar and fell to minimum of 104 mbar.) During throughput test leak rate dropped slowly from admittedly high background rate of 187E-9 mbarL/s to 186E-9 mbarL/s. Pump mixture back to dump. |
| 1135 | Begin blowing out LN2 from main bath with helium gas. Leak background 140e-9 mbarL/s |
| 1210 | Fill LN2 cold trap full. |
| 1245 | Cease LN2 blow out. Leak rate drops to 90E-9 mbarL/s, then gradually rises to 100E-9 mbarL/s as main bath fills with He gas. Commence pumping out main bath. |
| 1255 | Commence cryocleaning through cold trap 2. |
| 1320 | IVC leak rate falls to 70E-9 mbarL/s as main bath is pumped out. Once P2<0.1 mbar, isolate scroll pump and fill main bath with He-4 gas. While filling main bath, IVC leak rate rises to a stable 570E-9 mbarL/s, indicating a leak. Panic. |
| 1400 | Pump all He-4 gas from main bath, while monitoring leak rate. Leak rate falls to 66E-9 mbarL/s as main bath is emptied. Commence refilling main bath with He-4 gas, and leak rate climbs steadily to 545E-9 mbarL/s. Quick spray test shows no change in leak rate when He is sprayed directly at stripline tube elbow flange. Historically, largest successful leak rate was 140E-9 mbarL/s, occurring on 2013/11/21. However, it is stated on 2012/07/01 that a leak of 780E-9 mbarL/s would still be somewhat acceptable. Given this, we have decided to press on with the LHe transfer. |
| 1420 | Add 4.6E-2 mbar He-4 exchange gas to IVC. Isolate IVC. |
| 1430 | Commence LHe transfer (-1.0%). |
| 1703 | End LHe transfer (91.3%). Current mixing chamber temperature at 43 K. |
| 1715 | Cease cryocleaning. Pump to dump. |
Strange observation: the U02844 thermometer (Tower thermometer) is currently mounted on the still plate, and the Cernox is on the MC as always. The Cernox is our standard during cooling, but the Tower thermometer should be calibrated correctly up to 45 K. I expect these thermometers to agree during the initial transfer because both fridge elements are being cooled by the exchange gas. However the Tower thermometer reads much lower than the Cernox. More worryingly the Tower thermometer bottoms out at 4.4 K when the Cernox is still at 40 K (well before we start pumping out exchange gas - we generally do this when the Cernox reads 20 K). The Sorb thermometer bottoms out at the same time as the Tower thermometer. Is the fridge much cooler than we think it is? Does the Cernox thermometer have a “delay”? Or is the MC actually significantly warmer than the still during initial transfer, for reasons I don't understand?
| Cernox | U02844 |
|---|---|
| 50 K | 32 K |
| 47 K | 25 K |
| 42 K | 4.6 K |
| 40 K | 4.4 K, flat |
Update: As of 17:45, after we started pumping out the IVC, the Cernox thermometer is still cooling. I checked the other MC thermometer, the H35, and found that it is still cooling as well (and its calibration agrees reasonably well with the Cernox - Cernox at 14 K, H35 at 13 K … generally we don't use the H35 in this range, but it looks about right). This suggests that the temperatures we are seeing are correct… and that the MC cools more slowly than the Still and wherever the Sorb thermometer is. The Sorb thermometer is suspected to be near the 1K Pot. The drop to 4K seen in both the Sorb and the Tower thermometers is quite sudden. Perhaps it occurs when the level reaches the top of the IVC, so that LHe is in contact with the tubing leading directly to the Pot and the Still…
We also took a look at the pressure in the IVC before pumping it out when Cernox = 25K. It read 1.3e-3 mbar. Compared to the initial 4.6e-2 mbar at 77K, an ideal gas calculation suggests the gas was at 2.2 K at this time. However we had to let IVC gas into the long bellows leading to the turbo pump, which was previously evacuated, so that would make the pressure lower. Still I would not be surprised if the gas was near 4K at this time. It would be interesting to monitor the IVC pressure at the external pressure sensor during the next cooldown. I don't see much reason not to. (MF)
| 1727 | With mixing chamber temperature at 25 K, Sorb temperature at 5.2 K, open IVC to turbo pump and begin pumping out. |
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