From: kaike pan
Subject: Re: 2.5m obslog 20080827 (MJD 54706)
Date: Thu, 28 Aug 2008 14:04:47 -0600
>> .
>>
>> We are waiting for the rotator fiducial data.
>>
>> Problem: Spectrographs require a fill every 6 hours or so.
>
>    I'm a little puzzled by this last one.  From the dewar plots, it
> shows the last fill from the 180 L prior to the warming incident
> was at 2:15 PM.  The warming happened around 2:45 AM, followed
> around 3:00 AM by a secondary fill.  The lines had been
> disconnected since about 11:30 PM.  I suspect the 10 L ran dry on
> sp2 around the 1:05 AM (9:05Z) LN2 empty fault.  I didn't see a
> dewar pressure error though. In any case the hold time for the 10 L
> dewars is shorter than normal at around 11 or 12 hours, so that
> needs to be taken into consideration when disconnecting the lines.
>
>
> 				Cheers,
>
> 				  Dan
     Hi Dan,
       We should have done a forced fill before disconnecting the 180
L dewar. Sorry for missing that. But Here is my concern.
We did NOT got watcher warning message till 2:45 AM (did we missed
one?). After getting the watcher message, I connected
the line and did a forced fill. During the forced fill, I checked the
CCD temperatures, chips 1 & 4 of SP1 were good at -91 C but
chips 2 & 3 were off, at -20 C and -30 C. After forced fill from 180
L, I did a "cc stopfileseg" following by a "cc startfillseq" from
SP2. Then temperature started to cool down.
     From last night incident, we know that CCD chips will warn up in
12 hours after disconnecting lines. For the winter nights,
we definitely need longer time internal. Before the summer shutdown,
I recall that a fill (from 180 L dewar) was requested
every 10 (in summers) to 18 hours (in winters). Now a fill is
requested every 6 hours or so. I don't know what happens
here.  Two possibilities I can think of.  Vacuum is not perfect so
chips warm up in shorter time period than they did
before. Or fills from 180 L are not fully complete so that 10 L dewar
get less LN than they used to. The second possility
is more likely. I checked and found that a fill took 45 to 55 lbs
before the summer shutdown, not it only takes less than
40 lbs.
   cheers,
   -kaike
>
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