2015/03/17
| 1010 | Fill N2 cold trap full. |
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| 1425 | Isolate turbo pump and open line to leak checker. Mixing chamber and sorb temperatures 18.3 K and 18.9 K respectively. Helium background leak rate 1.4-9 mbar l/s. |
| 1430 | Commence throughput test while monitoring IVC leak rate. G2:650mbar. After letting mixture into cold trap 1, G2 at 608mbar. Allow mixture to flow through cold trap 1 to G1 and P1 until G1 stabilizes at 528 mbar. Close V12A, cold trap 1 speedivalve and V3, open V1 until. G1 drops to 124 mbar before rising. Flowrate of 107 mbar/min found, in line with expectations. Pump mixture to the dump. Leak rate remains unchanged at 1.4E-9 mbarL/s. Isolate IVC. |
| 1445 | After Isolating IVC, leak rate in lines leading to IVC dropped to 7.9E-9 mbarL/s. Isolate leak checker and commence pumping on lines leading up to IVC with turbo pump. Stop pumping on fridge lines as well, closing all valves on IGH as well as condenser line speedivalve and gate valve. Turn off leak checker. |
| 1456 | Start the 1K Pot. |
| 1512 | 1K Pot stable at 1.72 K. With G2=655 mbar, 1K Pot temperature increase to 1.84 K with no explanation. Give time to see if temperature falls back down. |
| 1518 | G2=655 mbar and 1K Pot temperature still at 1.85 K. Commence manual condensation procedure for all mixture through condenser side only. Mixing chamber temperature is 18.3 K currently. |
| 1624 | Open V12A all the way. |
| 1630 | With G1 and G2 around 120 mbar, start circulation procedure. Mixing chamber temperature is 2.25 K and slowly dropping. |
| 1702 | Open V6 all the way, circulation procedure complete. Mixing chamber temperature reached a minimum of 686 mK before beginning to rise. 1K Pot temperature still at 1.8K. G1=143 mbar, P1=240 ubar, G2=133 mbar. Open V14 to allow pump out remaining mixture in dump. |
| 1830 | P1=340 ubar, and mixing chamber temperature is at 411 mK. Close V14. |
| 1845 | Mixing chamber temperature at 408 mK, G1=147 mbar, G2=138 mbar and P1=240 ubar, 1K Pot at 1.8 K. Switch to Still thermometer. |
| 1902 | After some technical difficulties, mixing chamber temperature at 408 mK, G1=153 mbar, G2=142 mbar and P1=240 ubar, 1K Pot at 1.8 K. Switch to Still thermometer and perform single shot test while monitoring still temperature and not using still heater. Still temperature results of single shot test shown below. |
| Figure 1: Single Shot Test Still Temperature |
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| 1939 | Stop single shot test and turn on mixing chamber heater to 10 mW. Drain and turn off 1K Pot (was at 1.8 K). |
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| 2130 | Turn on sorb heater, power 10 mW, to aid mixture recovery and warm up. |
| Open valve V5 on IGH to pump from both sides of the fridge. | |
| 2150 | Fill N2 cold trap full. |
| Rather worrying: G2 pressure has increased from 655 mbar at 2130 to 715 mbar now. Such a difference cannot be explained by a dodgy pressure sensor. Something weird is going on. It would be worth extracting the cold traps tomorrow and taking a look. | |
| 2205 | G2 pressure has increased to 735 mbar and shows no sign of stopping. |
| Increase sorb and mixing chamber heater power to 20 mW to aid warm up. | |
| 2210 | It seems as though opening valve V5 could have caused this. If there is a leak, could it be on the condenser side of the fridge? P1 seems to rise occasionally, and this seems to correspond with a rise in a later rise in G2. If this was due to the circulation pump's shaft seal, P1 shouldn't rise at all. This potentially exonerates the circulation pump. |
| 2215 | Moved around some plumbing on the exhaust and circulation sides, but wasn't able to trigger any rise in P1. The question to answer is when did the leak occur? Was it during condensation and circulation? In that case, there should be contamination in the cold traps. If not, it must have been during mixture extraction. |
| 2225 | Turn up sorb heater power to 30 mW. |
| 2235 | Turn still heater on, power 20 mW. |
| 2250 | Pressure in G2 741 mbar. Seems to have stabilised. Stop extracting mixture from fridge. Close all valves except V9 on IGH. Close gate valve and condenser line speedivalve. |
| Pressures in G1 & P1 are 1 mbar & 6 μbar respectively. | |
| 2300 | Temperature of 1 K pot >7 K. Turn off sorb, still and mixing chamber heaters. |
| 2335 | Mixing chamber and sorb temperatures 17.3 K and 16.9 K respectively. Good enough. Helium leak rate in lines leading to IVC 1.1E-9 mbar l/s. Open valve to IVC. Helium leak rate increases to 3.7E-9 mbar l/s. It will be interesting to see if the same increase occurs without condensation and circulation over a similar time period (approximately 9 hours). |
| 2340 | Isolate leak checker and evacuate IVC with turbo pump. Leave overnight. |