Pumping on the LHe Bath

• The pumping manifold is located in the rack on the left side of the alidade; here is a picture.  Familiarize yourself with it -- there is a large line that comes in from the pumps, a regulator valve for throttling the flow from the pumping line, and a vent line with a small valve and a hose barb.
• A green flexible hose is usually used to connect to the dewar.  The hose is strain-relieved so that zenith motion of the telescope does not stress the line.  A picture of the dewar with the line attached is shown here.
  
• The large PVC pumping line terminates in two pumps at the front of the right Nasmyth platform, through one large common valve and two smaller valves on each pump.

• Make sure that fridge cycle has completed.
• Fill cryogens.
  
• Leave cap off of LHe bath, also remove any vent line extensions (they get in the way of the pumping line).
  
• Close the regulator valve on the pumping manifold and turn on both vacuum pumps.
• Open the vent valve on the pumping manifold.
• Attach the vacuum plumbing to the LHe bath.  Make sure that all o-rings are tight.  All O-rings near the bath, including the one making the seal to the bath, should be silicone.  Don't overtighten them, they will get damaged (there should be lots of spares).
  
• Close the vent valve on the LHe bath inlet
  
• Check that He gas is coming out of pumpout manifold vent valve.
  
• Close the pumping manifold vent valve, then SLOWLY open the regulator valve.
• Try to maintain a pumpdown rate of approximately .1"/5 sec (on the digital gauge), this will require opening the regulator valve a small amount every minute or so. Watch the pumpout very closely until the pressure is ~35 torr (on the analog gauge).  If you suddenly pump down to 0 torr, you may have an ice plug, though you may just be out of LHe (unlikely if you just filled LHe).  See Dealing with an Ice Plug.
  
• After approximately 10 minutes tighten both O-rings on the vacuum hose near Bolocam. They will have loosened as they cooled down. Make sure that you don't hear any sucking sounds in the plumbing.
• Continue to slowly open the regulator valve to maintain a pumping rate of  ~.1"/5 sec.  After approximately 45 minutes you should be at a pressure of ~35 torr (on the analog gauge) and the regulator should be fully opened.
• At this point the array GRT should be fairly stable and you can begin taking data.  The LHe bath should pump down to ~10 torr (after a couple hours).  Continue to check on the plumbing, if the pressure drops significantly lower than 10 torr you either have run out of LHe or you have an ice plug; see Dealing with an Ice Plug.
  

Shutdown Instructions

• Set the telescope to ZA = 5.
  
• Close the regulator valve on the pumping manifold.
  
• Using the vent valve on the pumping manifold to slowly let He gas (NOT AIR) into the LHe bath.
• When the bath reaches atmospheric pressure (~ -20" on digital gauge) stop backfilling with He gas and disconnect the pump line.
• Fill LHe bath.  Do not dawdle!  The bath is not yet safe: the He gas you backfilled with will cool and contract, creating a negative pressure and sucking air in.  You must start the fill to warm the bath and create a positive pressure.
• Close the valves at the vacuum pumps and turn off the pumps.
  
• Set the fridge cycle to start approximately 4 hours before afternoon observers will arrive.

Dealing with an Ice Plug

• If the bath suddenly pumps down to 0 torr, you may have an ice plug, though you may just be out of LHe.  The symptoms are similar -- zero pressure in vacuum line, bath starts to warm up.  If you think you may have a plug, grab a copper rod from the AOS lab (the wall with all the transfer lines) and do the following:
  
• In either case, first vent the line up with He gas as described above in Shutdown Instructions.
• If you think you may have a plug, probe into the bath with the copper rod.  If you feel resistance, heat up the copper rod with a heat gun and use the hot rod to try to work through the plug.
  
• If you have no luck with the plug, you should put your own safety first and get away from the dewar.  It may explode, and you don't want to be nearby when it happens!
  

• 2003/12/08 SG
  Extracted from Observer Manual