| 0220 | Mixing chamber cooled to 820 mK and then started rising again to 920 mK. Then it started cooling again to 740 mK before rising once more. Pressure and temperature of 1 K pot indicate that it isn't struggling. Close valve V6 on IGH to interrupt circulation. Let's try to recondense the mixture as pressures in G1 & G2 are still at around 1 bar. |
| 0225 | Open valve V3 to aid recondensation. It is possible that a bockage has formed. If so, with any luck it will be in the helium cold trap. Continue to monitor the situation. |
| 0255 | Pressure in G1 280 mbar. A little high, but 1 K pot shows no signs of strain. Close valve V12A on IGH just in case. Helium did appear to be boiling off at a faster rate since starting condensation, but has since slowed down. |
| 0505 | The more I think about it, the more it looks as though there is a blockage. The mixture is simply not condensing. Closed valve V3 to separate nitrogen cold trap 1 from pumping line. Closed valve V13A to stop mixture from entering nitrogen cold trap 1. Open valve V12A to feed mixture from nitrogen cold trap 1 to 1 K pot. Open valves V9 & V6 to pump mixture into dump. During last half-hour (ie. since process started) pressure in P1 has dropped to 0.012 mbar, but G1 remains high at 395 mbar. A blockage is almost a certainty. |
| | Just noticed that incorrect gasket has been used during installation of speedivalve for nitrogen cold trap 1. A high pressure gasket should have been used. This is a little worrying as pressure of mixture did rise to 1 bar for an extended period of time, implying that some may have escaped. Must remember to change this gasket once we've warmed up. |
| | A blockage would explain the loss of base temperature. However, warning signs such as an increasing G1 pressure usually indicate that a blockage is imminent. There were no warning signs. Assume that we will have to clean the cold traps, but will monitor the situation for a little longer. Hopefully, this can all be explained by a blockage and it exists in the helium cold trap. Otherwise, we will have no choice but to warm up and try our luck again after pumping out the mixing chamber lines. |
| 0530 | Pressures in P1 rose to 0.014 mbar and G1 increased to 420 mbar. Stop 1 K pot, but continue to pump mixture into dump through pumping line only. |
| 0550 | Pressures in P1 rose to 0.018 mbar and G1 increased to 430 mbar. Mixing chamber temperature 1.7 K. Turn on mixing chamber heater, power 5 mW. Continue to pump mixture into dump through pumping line only. |
| 0610 | Pressures in P1 rose to 0.026 mbar and G1 increased to 435 mbar. Open valve V5 on IGH to commence pumping from both sides of mixing chamber. |
| 0725 | There is definitely a leak somewhere. Dump pressure is 880 mbar, and increase of 38 mbar in the last 25 minutes. Pressure in G1 is 6 mbar. Pressure in P1 is 0.065 mbar. It's difficult to pump out mixing chamber lines. There is a pressure relief valve on dump that I would like to avoid using. |
| 0730 | Suspect that leak is on aforemtioned gasket on speedivalve for nitrogen cold trap 1. Cannot continue to pump into dump. |
| | Indication of leak could be given by isolating sections of the fridge and leaving for a few hours. Leak appears to be rather large, so may show up on one of the pressure sensors. Would have been nice to have 1 K pot warm up to >7 K to ensure that mixing chamber contains no mixture, but it's current temperature of 4.3 K will have to suffice. |
| 0750 | Close all valves on IGH and isolate fridge. Pressures in P1, G1 & G2 are 0.022 mbar, 2 mbar and 907 mbar respectively. Interestingly, pressure in G1 dropped once valve V1 on IGH was closed. This may simply be mixture coming out of the fridge or signs of a leak. |
| 0810 | Pressure in G1 at 1 mbar. Open nitrogen cold trap 1 speedivalve and valve V12A on IGH. Pressure in G1 rises to 12 mbar. Some of this may be mixture left in the trap. If pressure rises further, then gasket at speedivalve is cause of leak. |
| 0825 | If the gasket on the speedivalve is not the culprit, it may be a good idea to cryoclean whatever mixture has been recovered through nitrogen cold trap 2 before continuing to pump on the fridge. The dump pressure is rather high at around 900 mbar. We can then clean all the cold traps at the same time. It may require multiple cryocleaning cycles. |
| | Pressure in G2 has risen to 913 mbar. This may be due to a combination of mixture that still remains in intake line and the reduced volume since isolating the dump by closing valve V9 on IGH. Alternatively, it may be evidence of a leak - there are some flanges with low-pressure gaskets around the He-3 pump, although they are larger in diameter so may be fine. |
| 0840 | Fill N2 cold trap full. |
| | Turn off mixing chamber heater. |
| 1505 | Filled LN2 cold trap. |
| 1535 | Currently, p1=0.028mbar, G1=13.1mbar, and G2=911.6mbar. Begin cryocleaning mixture procedure.Topped off LN2 cold trap after opening V14 and V4. |
| 1600 | Cryocleaning started. |
| 1731 | Stop cryocleaning mixture and begin pumping mixture to dump. |
| 1810 | G2 was around 600mbar, when condenser line was opened and mixing chamber heater was turned on (5mW), resulting in a slight increase in G2. Opening the gate valve resulted in a large jump of roughly 100mbar in G2. Currently, G1=6.1mbar, P1=0.03mbar, and G2=787.3mbar and is still rising, though at a reduced rate. |
| 1835 | Turn off DCRC boards and remove from ebox. |
| | Close condenser line speedivalve and gate valve to isolate fridge. Open valve V3 on IGH in effort to increase pumping speed on G1 line. |
| 1855 | Commence helium transfer. Helium liquid level 12.0%. |
| 1915 | Turn mixing chamber heater off. We shan't pump on mixing chamber until tomorrow, so no need to increase its temperature prematurely in order to recover mixture. |
| 1940 | End LHe transfer (90.9%). Close V3, G2=811mbar of mixture recovered, with G1 around 4 mbar. |
| 2000 | Closed all valves, including manual dump valves, removed He and N cold traps and began pumping on them. |