[Fews] Sniffer detector Dewar frozen

[Steve Christo]

At 10 pm last night the detector Dewar on the sniffer attempted to fill 
but completely emptied the 50 l dewar while trying. The 50 l Dewar was 
half full. I tried to remove the top hat with sensors to manually fill 
it but it was stuck. There is a block of ice inside of the detector 
Dewar holding the sensors! The same problem  occurred last week but  I 
didn't have to remove the top hat at that time so didn't notice the ice. 
Has anyone had this problem before? The only thing different now is that 
the foam pad under the drip tray (below the top hat) is not installed. I 
assumed (perhaps wrongly) that this was a only used as a spacer to keep 
the old sensor away from the bottom of the Dewar. Could this be causing 
it to condense more and drain water into the detector Dewar?

I ran through the remaining GN2 that was down there in an unsuccessful 
attempt to thaw the ice and liberate the top hat. There is no more GN2 
in the racks upstairs. I've ordered 3 more bottles. It may take all 
weekend or longer to thaw the ice unless I'm successful chipping at it 
with a needle tool. In the mean time I'll try to think of a way to 
insulate the space between the bottom of the top hat and the top of the 
Dewar. Maybe a soft foam donut.
Steve