| 0205 | The cold plate temperature initially fell when the still heater was turned on. However, the mixing chamber temperature rose with still temperature. We have seen similar behaviour before, and may imply thermal contact between the still and mixing chamber. However, we haven't been able to locate it during maintenance. Perhaps we are looking in the wrong places. |
| | With the still pressure being so low at 55 μbar, we could attempt to add more mixture. It is possible that the still is running dry and we do not have enough mixture to provide the fridge with sufficient cooling power to reach a lower temperature. Having said that, we have added mixture in the past and found that it increases the base temperature. We should try again. |
| | Turn off still heater and wait for fridge to return to base temperature of around 87 mK. |
| 0300 | Open valve V10 on IGH briefly to bleed some mixture from dump into fridge. Pressure in G2 reached 136 mbar before starting to fall. As expected, mixing chamber temperature started to rise due to introduction of warm mixture. Leave fridge alone for a while to reach new base temperature. |
| 0305 | Fill N2 cold trap full. |
| 1035 | Fill N2 cold trap full. |
| 1050 | Mixing chamber temperature appears to have been fluctuating between 87.5 mK and 89 mK overnight. It shows no sign of calming down. Still pressure is 50 μbar. Still, cold plate and second mixing chamber thermometers read 4.59 kΩ, 9.25 kΩ and 23.3 kΩ respectively. Both still and cold plate are slightly warmer than they were yesterday when still heater was off. Rather confusingly the still pressure appears to have decreased. Perhaps the still is running dry. |
| | Turn on still heater and set power to 1 mW. Recommence battle with high base temperature. |
| 1420 | Mixing chamber temperature is rather erratic (between 95 mK and 97 mK), and therefore it has been difficult to really take an accurate measurement. Waiting for it to calm down has had no effect. Still and cold plate thermometers read 4.49 kΩ & 9.67 kΩ respectively, and appear to be fairly steady. Still pressure is 60 μbar. |
| | One rather worrying possibility is that the position of the phase boundary is changing, and lies close to the heat transfer lines that lead to either the 1 K pot or the still. |
| 1430 | No matter what, the low still pressure would imply that the still is running dry. Turn off still heater before attempting to add more mixture. Worst comes to worst, we'll simply pump it out and recondense it. It may be a blessing in disguise as we can measure the amount of He-3 in our mixture. |
| 1520 | Mixing chamber temperature 87.1 mK. Bleed some mixture from dump to fridge via valve V10 on IGH. Pressure in G2 rises to 146 mbar before falling. |
| 1830 | Mixing chamber temperature appears to have stabilised at 95.1 mK. Still pressure is still 50 μbar. Still and cold plate thermometers read 4.55 kΩ and 9.65 kΩ respectively. Adding extra mixture appears to have reduced the temperature of the cold plate but increased that of the still. |
| 1840 | The 1 K pot temperature has been varying by about 0.1 K all day, dropping as low as 1.47 K and reaching as high as 1.61 K. It just doesn't appear to want to hold steady. Try opening needle valve on 1 K pot a little more to warm it up a bit. It is possible that its erratic behaviour could be due to the new mixture that has been added. Having said that, I would expect it to warm up rather than cool down as it struggles to cope. |
| | Perhaps the best thing to do is to pump the mixture out of the fridge, attempt to recondense all of it, reach base temperature after circulation and if it isn't low enough, pump the mixture out again in an effort to determine the proportion of He-3 and whether the phase boundary indeed lies within the mixing chamber. |