The fridge has crashed. If you are reading this for the first time and freaking out, open 9 and let the mixture into the dumps, take a deep breath and read below.

A crash is when previously the fridge was in circulation (where G1~75 mbar and G2~50 mbar) and the 1K pot has heated up and now G2 and G1 are both above 800 mbar OR they are rising. Depending on the situation depends on how what you can do

(a) you catch the fridge when it starts to crash:
  • G1 and G2 start increasing above 100 mbar
  • The pot resistor is warm (around 750 Ohm or less and dropping)
(b) the fridge has completely crashed and the mixing chamber is below 1K
  • G1=G2~800 mbar
  • Pot resistor will show ~700 Ohm
  • RuOx is in the range of hundreds of mK depending on how long it's had to warm
(c) the fridge has completely crashed and the mixing chamber is above 1K
  • G1~G2~1000
  • Pot resistor is warm, usually around 600 or 700 Ohm.
  • RuOx is above a kelvin or close
  • probably the He level is low in the fridge dewar.

(a) The fridge is starting to crash

  • G1 and G2 are still low (~100 mbar.. but at later stages could be 200 or more) and are starting to increase (above 600 mBar and valve #9 will have to be opened as mixture will start to go into the dumps via overpressure valve.)
  • the pot resistor, R6 shows a low value, 700-750 or less.
  • RuOx is below 1K and is usually in the tens of miliKelvin range.
Usually this is because the pot is taking on too much heat load.

What to do:
  • turn off roots pump at back of gas handling cabinet
  • turn off still heater, turn off cryoamp if not measuring (if you are, then leave it on)
  • change the n/v to obtain the desired pressure. The pot should hopefully recover.
If it doesn't recover...

  • close n/v to let pot empty
  • When the pot empties (indicated by low P2 values. P2=4,5 or less) open n/v a few turns to let the pot fill with liquid.
    • P2 will read > 10E+3
    • G3 will start to rise, then it will reduce as the pot fills with liquid.
    • When the pot is filled with liquid, G3 will start to rise again.
    • At this point close the n/v fully.
    • Again let the pot empty, G3 will drop to below zero and P2 will drop from >10E+3 to more reasonable values. Around 20mbar open the n/v to normal operating point. The pot should be cold.
  • Now use the pressure of the pot, P2 to determine the position of the n/v. Aim for 8-20 depending on running condition.
  • R6, the pot resistor should read around 780-800 Ohms Ohms.
  • Once R6 and P2 are stable, turn on the roots pump as well as the still heater (say after 5 mins)
When the R6 reaches 780-800 the pot is cold enough to cool the mixutre and circulation should be okay. If this doesn't work and G1 and G2 keep increasing to >600 mbar, you will have to put the mixture back into the dumps (see part b.).

(b) The fridge has crashed and is below 1K:

  • G1 and G2 are ~800 mbar
  • the pot resistor, R6 shows a low value, 700-750 or less.
  • RuOx is below 1K and is usually in the hundreds of miliKelvin range.
If the fridge is still cold the mixture can be condensed right back into the fridge.
  • Open 9 to let mixture back into the dumps. When G2 and G1 reach equilibrium (5secs), close 9.
  • Turn off roots pump
  • Turn of still heater
  • Pump and flush the pot to cool it. Do this twice.
    • close pot pump (4a)
    • open n/v wide until full (G3~800-1000)
    • close n/v
    • open 4a
  • crack open the n/v about an 1/8th of a turn. Adjust n/v to get desired pressure. Pot resistor should be greater than 780, hopefully around 800.
  • slowly start to crack open valve 10 to bleed mixture back into the still.
    • The still valve will indicate how fast you're putting the mixture in. I keep it between 1-2mBar
    • The pot pressure will indicate heat load. I try to keep it less than 20.
    • Don't let G1 and G2 go above 125.
  • When 10 is opened, give the system time to recover: let the pot pressure reduce (towards the end it may not, let the pot cool down as indicated by the resistor.
  • Repeat until 10 is fully opened.
  • Wait 5 mins, close ten, start roots pump, turn on still. R0 should drop fast as the mixing chamber cools.

(c) The fridge has crashed and is above 1K

  • G1 and G2 are ~1000 mbar
  • the pot resistor, R6 shows a low value, 700-750 or less.
  • RuOx is below 1K and is usually in the hundreds of miliKelvin range.
The mixture will have to be recondensed (instructions adapted from K400FridgeSOPRestarting)

. Open 9 to let mixture back into the dumps. Close 9 when mixture is in. Roots pump and still heater should be off.

1. cool the pot
  • Close 1a (close pot to main bath)
  • Open 4a to pump pot out
  • Open the n/v to allow liquid to flow into the pot
  • Watch the resistance of the pot increase as it cools. Ideally you are looking for around 700-740 Ohm.
  • Adjust n/v to get desired resistance (usually operate fridge at around 800 Ohm, however the pot with a heat load will have a different operating point than without a heat load).
2. Prepare cabinet for recondense
  • Close 1 (condenser line)
  • Close trap lines 12a and 13a
  • Close 6 (still line)
  • Open three slowly to allow mixture back in through the still (also to allow a path to the dumps in the event mixture is still coming out). Watch G1 and let it drop about as fast as the second0-hand of a clock.
3. Condense the mixture via the still
  • Open 13a to pass mixture into the trap (assuming trap one is being used). Allow the mixutre to sit for 2-5 mins to cool.
  • Let 3 shots from the back of the pump into fridge to condense by:
    • Close 13a.
    • Open 12a. See pressure on condenser gauge.
    • Let through valve 3. Throttle just fast enough to see gauge drop like the second hand of a clock.
    • The pot temperature may rise as indicated by rising resistance and the pressure should increase. Let the pot recover.
    • Condense it all in (Check condenser pressure gauge) and close 3.
    • Shut 12a.
    • Open 13a to let in the next shot.
  • When done, ensure 13a is closed.
  • Open valve 9 to let dumps into the lower pump line (slowly if 3He pump is on)
  • Open 13a to give this to the cold trap for 5 mins.
  • Open valve 3.
  • Throttle through 12a. 100mb max on condenser gauge. Be careful not to let this run away. Gauge has a big lag. Once line pressure is below 100mb open 12 fully.
  • Keep an eye on the pot. Make sure it doesn't run out (may need to run slow-continuous fill).
  • When pressure gets down to ~50mb it's time to circulate the mixture
4. Circulate mixture
  • When pressure gets down to ~50mb it's time to circulate the mixture.
    • Shut 13.
    • Note the 3He pump should be on if it isn't already.
    • Valve 3 shut.
    • Valve 1 open.
    • Close valve 9.
    • Open valve 13 to equalise.
    • Slowly open 6 to ~200mb and let settle a bit. (600mb max pressure to avoid retreat to dumps.)
    • Watch cooling on still and RuOx?. Keep an eye on the pot.
    • Watch top gauge and open 6 fully as it comes down.
    • Ensure pressures still keep dropping. (~450mb return line and ~3mb at top gauge.)
    • Open 10 to drain dumps. Wait until line pressures drop right down.
    • Shut 10.
    • Check 1,6,12a and 13a are properly open.
    • Roots pump on (may need to plug it in at the wall).
  • Needle valve set for continuous fill. Looking for 8-10mb.
  • Once line pressures are settled and low, set still heater to 3.5mW.
  • Do a helium transfer (standard end transfer: 20-25cm above IVC or 90-95%).


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Topic revision: r5 - 14 May 2014, AlexHamilton
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