Cryogen Fills

All the heterodyne receivers at the CSO require periodic cryogen service.

The sidecab receivers and the 850 GHz receiver are housed in hybrid cryostats, with mechanical Gifford McMahon coolers and a liquid helium (LHe) reservoir. The CSO staff will service these receivers.

The wideband 345 GHz receiver (Barney) at the Cassegrain focus is housed in a simple (purple) cryostat cooled by liquid nitrogen (LN₂) and LHe. It requires daily cryogen service. When staff are not available, observers may be called upon to Cryogens should only be handled by trained personnel. CSO staff will train novice observers.

1. Wear gloves and eye protection/face shield while handling cryogens.
2. Do NOT let 24 hours lapse between fills. The hold time is just less than 24 hours.
3. Move the telescope to ZA 55.
   uip> za 55
4. Move the LHe storage dewar close to the instrument.
5. Fill the white pail with LN₂. Then put the funnel tube in the pail so it cools. There will be a fountain of LN₂ out of the funnel until the tube cools down. Point fountain away from personnel and equipment.
6. Fill the LN₂ pot first.
   • The LN₂ port, on the outside, has tubing on its fitting.
   • Insert the funnel into the port, then fill the funnel with LN₂.
   • Continue filling until the pot is full and liquid starts to fountain out.
   • Remove the funnel and drain it into the pail. Point the liquid away from personnel and equipment.
   • Reinstall the tubing on the LN₂ fill port.
7. Fill the LHe pot.
   • The LHe fill port, in the center, has tissue in it. Remove the tissue.
   • Use the transfer tube with the long extension hanging behind the spectrometer rack in the lab.
   • Move the brass fitting on the transfer tube down towards the extension on the transfer tube.
   • Close the two relief valves and slowly open the top valve on the storage dewar.
   • Slowly insert the transfer tube into the dewar.
   • Be sure to couple the brass fitting on the transfer tube to the brass fitting on the storage dewar.
   • Once the fittings have coupled, push the transfer tube down into the storage dewar.
   • Once the transfer tube is inserted into the LHe storage dewar, the pressure will it will rise and cold gas will vent from the transfer tube.
   • After a a few minutes LHe will come out of the transfer tube. There will be a white plume.
   • Once you see the plume, insert the transfer tube into the receiver.
   • It is a good idea to time the LHe fill. It should take 10 minutes, ± 5 minutes.
   • When the LHe pot is full, the plume will become quite dense and, if you listen carefully, you will be able to hear a crackling or puffing noise.
8. When the LHe pot is full, withdraw the transfer tube from the receiver.
   • Holding the upper brass fitting down so that it doesn't uncouple, pull the transfer tube up about two feet out of the storage dewar until it is above the liquid level. Wear glove because the metal is cold.
   • Slowly open both vent valves.
   • Let the storage dewar vent slowly before removing the transfer tube.
9. When the LHe pot is full, the depth is 3.75 inches.
   • Use the acoustic “thumper” tube to measure the liquid level.
   • Replace the tissue in the LHe port. This prevents foreign objects from getting into the cryostat.
10. Withdraw the transfer tube from the storage dewar after the dewar has vented.
    • Measure the amount of LHe left in the storage dewar and note on the log sheet.
    • The normal usage is 5–8 liters of LHe.
11. Move the storage dewar to the outer edge of the alidade platform near the control room wall.