4.2 Filling the Dewars

The two dewars have hold times of roughly 24 hours, and they should be filled at the beginning of every night to ensure that the detectors are maintained at nominal operating temperature. An added complication is that the nitrogen is pumped to obtain additional cooling. The following procedure should be followed to fill the dewars.

  1. Visually inspect each dewar to make sure the outer shell is free of frost. Frosting of the dewars indicates that the internal vacuum is not good. DO NOT FILL THE DEWARS UNDER THESE SITUATIONS - CONTACT CFHT PERSONNEL.

  2. Using an ohmmeter, measure the temperature of each dewar from the BNC connector on the side of each preamp box (Section 2.3). If the dewars are too hot then they may be difficult to fill.

  3. Completely close throttle valves on Cass environment vacuum manifold by turning them in a clockwise direction.

  4. Open atmospheric vent valves on the vacuum manifold. A hissing sound will occur as the pump lines are brought up to atmospheric pressure.

  5. When the hissing from the pump stops, remove the coupling flange which joins the vacuum pump line to the top of each dewar. There is also an o-ring between the pumpline and dewar spout; make sure you don't lose this! The best practice is to put one of the o-rings in each pocket to keep them warm.

  6. Fill each dewar using the long neck funnel and styrofoam LN container. Wear protective gloves for this procedure!

  7. Reconnect the pumpline to the top of the dewar after clearing away and frost buildup and reinstalling the o-ring between the pumpline and dewar spout. When properly installed, it should be possible to completely close the coupling clamp using the wing nut and screw.

  8. Close atmosphere vent valves on vacuum manifold.

  9. Slowly open throttle valves on the vacuum manifold. Each valve should be opened 2 or 3 full turns in a counter-clockwise direction.

  10. Check pump lines for signs of liquid nitrogen; if this is seen, it indicates that the dewars are being pumped too hard - turn down the throttle valve a turn and recheck.

  11. Check dewar temperatures after half an hour of pumping; adjust vacuum as necessary.

Please send comments and suggestions to: veillet@cfht.hawaii.edu