Setup one of the liquefiers for Dewar Cooldown Mode:
1.) Incoming flow (FC501A/B) should be as large as it can be. Ideally this should be around 60 slm but whatever the greatest flow is that you can hold consistently.
2.) The outgoing flow (FCA/B) should also be large so that the flow runs into the Coldbox, over the cold heads, then down to the dewar and out. This can be as high as 40 or 50 slm.
3.) The needle valve (NV501A/B) should be open at least one full turn more than the standard position. The standard position is approximately 1/4 turn open.
The cold head temperatures can get warmer in this phase with that not being a problem. The pressure on the dewar can be 6-7 psi and the flow in and out must be consistent. If that is hard to achieve there is usually an issue with the dewar or flow controller. Sometimes the flow controllers will behave better at higher pressure. When cooling at a high rate with the needle valve open the dewar neck will likely cool down and frost up so it's good to have a fan pointing on it to keep the ice from forming too much. If the dewar does not cool down after a couple of days with this configuration it may be a sign of a poor vacuum in the dewar or Coldbox.
Setup one of the liquefiers for Dewar Cooldown Mode:
1.) On automatic Incoming flow (FC501A/B) should be getting close to an average of around 50 slm. 60 slm is even better but generally, we want as much gas coming in and condense with the least amount of gas going out. This can be sporadic at times when things are stabilizing but when everything is working normally we should have a nice steady flow in of around 50 slm.
2.) The outgoing flow (FCA/B) should be pretty small and ideally be around 2 slm. If the pressure in the dewar is not stable and makes the inlet close from pressure building you can open this to 5 slm or so but if you have to open up to 10 slm this will likely warm the cold heads up too much and slow down LHe production. If this is a persistent issue it may be a sign that the dewar has a poor vacuum or something else.
3.) The needle valve (NV501A/B) should be precisely positioned for production which is approximately 1/4 turn open, however each liquefier has a precise location that is a bit different. These are marked with tape but its still a bit tricky to find just the right place for optimal production so this much be fine-tuned over time by monitoring inlet flow (the more the better) and LHe production rate over time.
The cold head temperatures should be averaging about 4-5K. If they are warmer it may be due to a flow dynamic of warm gas and there may be an adjustment of the NV501 needed. There may also need to be some balancing of flow performed by closing some of the helium gas delivery ball valves down. We have not yet needed to do this. The pressure on the dewar can be 6-7 psi and the flow in and out should generally be continuous. Some hiccups and periodic fluctuations are normal but long times with the inlet closed are not good. When everything is tuned right we should expect 4L/hour on each liquefier.
Vacuum pump out:
The vacuum of the dewars should in principle shouldn't need to be pumped out for a long time but we have already seen that Liq A performs better if it's pumped out at least every 2 months. When doing this make sure to pump down to at least low 10^{-3} torr and then close the turbo valve and check how fast it climbs (leak rate test). After all is well with the vacuum perform a pump and purge on the helium space of the dewar without going down much below atmosphere (can cause problems, or close all connections and remove stinger) and also perform the level pressure sensors cleaning procedure before starting the dewar cooldown.
The regulators PRV501A/B:
These regulators should not generally be changed. If they are bumped or accidentally changed follow this to reset them. Lower the dewar pressure at least 0.5 psi from the setpoint so that the inlet flow control is open all the way (easier in manual mode). It is usually useful to open the outgoing flow all the way too (60 in and 60 out) If the pressure jumps up when the flow controller opens then close the regulator down (turning to the left). If the flow is not able to reach the max (60 slm) that the flow control wants then gradually open the regulator (turn to the right) until the flow just nearly reaches 60 slm. In principle if one wants to operate the liquifiers at a higher or lower pressure you would need to reset these regulators so that at the new pressure they just barely open to 60 slm when the MFC is wide open. One may want to change the pressure to achieve optimal production rate or other possibilities. But once set for a particular pressure they should be good to go.