9  The Liquid Nitrogen Storage Dewars

9.1) Pressurizing LN₂ Storage Dewars

9.2) Hints on Filling Instrument Dewars

9.1  Pressurizing LN₂ Storage Dewars

The liquid nitrogen storage dewars are normally filled every Friday or before holidays. When they have just been filled, the vapor pressure in the storage dewars will be insufficient to expel the liquid nitrogen for the first 1-2 days.  If a tank needs to be pressurized,  speak with MDM staff who can do it safely for you.  

A situation such as this is extremely unlilely, but if there is no staff available, follow these steps:  To pressurize the storage dewar, attach the long black hose from the dry nitrogen tank attached to the south pier of the telescope to the horizontal nipple of the storage dewar, and open its valve. Open the main valve on the dry nitrogen cylinder, and turn the small black knob on the transparent flow regulator until a little ball inside floats to the top. The resulting pressure will be enough to force liquid nitrogen up the fill tube. Remember to close both valves on the storage dewar, as well as at the main valve on the dry nitrogen cylinder, once the instrument dewar has been filled.

9.2  Hints on Filling Instrument Dewars

Safety gear--gloves, face shield and smock--can be found in the dome by the door to the observing room.

If you have never been responsible for keeping a CCD dewar cold, have a staff member or experienced observer show you the technique. In particular:

• Insert the fill tube carefully but completely into the instrument dewar in a straight motion, and hold it using the attached plastic tie until the hoze freezes in place.  In the event that the hose breaks, immediately shut off the nitrogen feed and use the reserve tank to fill the instrument.  Inform MDM staff so the failed transfer hose can be replaced.

• The instrument dewar is full only when you see a "shower" of LN₂ spraying out. Dribbling doesn't count. It can take anywhere from 4-10 minutes to fill an MDM instrument dewar.

• Don't walk away from a filling dewar. You are likely to forget what you were doing and deplete your entire supply of LN₂.

Inattention to any one of these principles can create extra work for the staff in the best case, or a premature end to your observing run in the worst case.

There are typically two 50l tanks filled and available.  One should be considered a reserve supply in case there are issues with the dewar-in-use.