LN2 cryostat warming up (fwd)

[Willie Koorts]

> I guess most of us use Liquid Nitrogen Cryostats for our CCD cameras. At
> our La Palma observatory there are irregular, but inconvenient, instances
> of the dewar warming up because the LN2 has run out in the night.
>
> Can anyone say if they never get this sort of problem?
> Does anyone have any good practical solutions?
> Does anyone implement a reliable LN2-loss indicator?
> Any other comments?
>
> Regards, Paul Jorden

Hi Paul,

We have the same problem with the same "irregular but inconvenient" 
frequency of occurrences ( probably the same cryostat design as well. )

We cope it as follows ( "cope" as opposed to "solve") :

1. We have a filling routine with logsheets etc, and standard specific 
times of day for filling the cryostat.  ( Either 3 fills/24hr or 2 
fills/24hr depending on the type of cryostat and known LN2 usage pattern). 
Currently two of the fills are performed by daytime staff, when the 
telescope is not being swung around and therefore there is less wastage. 

2. We tend to "overfill" - eg an old cryostat that has a 12 hour hold 
time on the bench is filled approxiamtely every 8 eight hours on the 
telescope.

3. We monitor the temperature of the LN2 container, and trigger an alarm 
condition on the instrument control computer if the temperature rises 
above a threshold - currently about 95K. We are happy that this gives an 
alarm within 15min or so of the LN2 running out, with plenty of time to 
organise a refill.

4. If an alarm occurs, the observer/night assistant must obviously 
refill the cryostat.

5. If any temperature in the cryostat has risen above a threshold
( currently 240K ), the warming up process must be completed and the
cryostat re-pumped before being cooled again. The somewhat arbitrary
threshold of 240K is based on the assumption that no cold part in the
cryostat has warmed up through 273K - i.e. that water vapour is still
frozen out. 

6. Most times when the cryostat does warm up it tends to be the
observer/user at fault - often due to bad weather and the fact that the
observer doesn't go anywhere near the dome overnight, and thus forgets to
do the nightime fill. There seems to be no way of removing the human
factor from the equation. 

regards
	Dave Carter
	Willie Koorts
	Geoff Evans
South African Astronomical Observatory