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.
- 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.
- 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.
- Completely close throttle valves on Cass environment vacuum manifold by
turning them in a clockwise direction.
- Open atmospheric vent valves on the vacuum manifold. A hissing sound
will occur as the pump lines are brought up to atmospheric pressure.
- 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.
- Fill each dewar using the long neck funnel and styrofoam LN
container. Wear protective gloves for this procedure!
- 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.
- Close atmosphere vent valves on vacuum manifold.
- Slowly open throttle valves on the vacuum manifold. Each valve should
be opened 2 or 3 full turns in a counter-clockwise direction.
- 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.
- Check dewar temperatures after
half an hour of pumping; adjust
vacuum as necessary.