Filling the camera with LN2
There is a porous plug filter deep inside the camera. It does not take much debris to fill up this plug (potentially permanently). Every measures should be taken to prevent any debris or water from entering the camera fill line.
We also know that ice can diffuse up the fill line and plug the fill line and inline filter. The procedures below are geared toward keeping ice and debris out of the fill line to the maximum extent possible.
NO TEFLON TAPE IS TO BE USED ON ANY PIPE JUNCTION.
If it LN2 leaks from a fitting, tighten it a little. If it still leaks, let it leak.
The plug we experienced most recently was probably due to teflon tape shards produced when taped junctions were disassembled and reassembled. Check that pipe junctions are clear of debris and teflon tape before assembling (there should be very little need to reconfigure the piping except to change storage dewars).
The fill line we are currently using is a piece of surgical hose 2-foot long attached to the storage dewar and folded shut at the open end (to keep water from diffusing up the fill line and condensing in the cold fitting at the storage dewar itself). A short piece of surgical hose attaches a flow filter to the camera input port. The other (lower) port is capped with a folded piece of surgical hose to prevent water from diffusing into the fill line.
UNDER NO CIRCUMSTANCES ARE THE EXTERNAL FILTERS TO BE REMOVED UNLESS DIRECTED BY UMASS.
To fill the camera
- Unfold the end of the storage dewar fill line.
- Barely crack the valve on the storage dewar to start a slow flow of N2 gas through the line -- not enough flow to significantly alter the temperature of the line (or freeze it) but just a small flow of gas to disperse any foreign material or water in the hose.
Always have the hose pointing away from you when changing the valve opening: the flow rate can increase unexpectedly.
- Uncap the lower port on the camera.
Attach the surgical hose line from the storage dewar (with small gas flow)to the flow filter on the camera.
- Start with a slow flow of liquid N2 until the hose freezes solid.
Be careful, the hose may come off the flow filter until it freezes.
- Increase the flow of liquid into the camera until the pressure relief valve starts to chatter.
- You want to regulate the flow of liquid into the camera to a point where the relief valve just quits chattering.
- Continue to fill the camera until liquid nitrogen comes out the exhust port.
During a fill it is common for the camera temperatures to fall to -193. As long as it returns to -192 after filling this is not a problem.
- After the camera is full, leave the storage tank lines attached to the camera until you are ready to start the telescope.
Leave the storage dewar lines connected until the beginning of the next night's observations. The lines should only be disconnected for telescope operations, telescope maintenance purposes or when changing storage dewars.
If the fill line plugs completely or the camera will not take LN2, call UMASS. DO NOT attempt to clear the line.
Removing the LN2 nurse tank from the camera
When detaching the storage dewar from the camera:
- Detach the storage dewar hose ONLY at the flow filter.
- Cap the lower port (exhaust during fill )to prevent water from diffusing into the camera.
- Attach the surgical extension tube to the flow filter.
- Fold the open end of the hose attached to the storage dewar and hold it with a rubber band. This will help prevent water from condensing inside the hose and cold fittings on the storage dewar.
When switching storage dewars always check for debris in the connections before attaching the 3/8'' pipe fitting. After attaching the fitting, a blast of LN2 at full pressure through the line (i.e. open the valve and close with two turns of the wrist quickly). Doing this will blow out any debris from inside the fittings and hose. Fold and tie off the open end of the fill line as usual.
Appendix E
Version 2.3 - October 4, 1998