Subject: Re: [sloan-25mlog 3115] Re: 2.5m obslog 20160923 (SJD 57655)
From: Dan Long
Date: Sat, 24 Sep 2016 11:39:08 -0600
> On Sep 24, 2016, at 10:35 AM, jdowney@apo.nmsu.edu wrote:
>=20
>=20
>>>=20
>>> (3) At 07:59Z we had to interrupt the night to force a secondary =
fill
>>> because
>>> the SP2 10L dewar had run dry. The fill took 30 minutes to complete. =
See
>>> Log for
>>> details.
>>=20
>>=20
>>> 02:07Z - Opened. Recommenced the force-fill of the BOSS =
spectrographs.
>>>=20
>>> 02:45Z - After almost 40 minutes since restarting the secondary =
fill,
>>> the SP2
>>> secondary fill was still in progress. The TPM was reporting the SP2 =
180L
>>> LN2
>>> pressure at 10 psi (rather low). A total of ~25 lbs of LN2 =
transferred
>>> from the
>>> SP2 180L dewar during the two separate evening fills (the fill we
>>> started while
>>> we were on the phone with JohnD and DanL was the first fill which we
>>> eventually
>>> stopped to troubleshoot the enclosure, and the fill we started after =
we
>>> finally
>>> got the enclosure off was the second fill). Last night, the SP2
>>> secondary fill
>>> took 50 minutes to complete and 15-20 lbs of LN2 transferred, so =
since
>>> ~25 lbs
>>> of LN2 had transferred tonight and the fill was still going while we =
had
>>> now
>>> entered dark time, the decision was made to stop the ongoing =
secondary
>>> fill and
>>> start science since the sky was clear.
>>=20
>> Looks like that the low pressure on 180L dewar was the root of the
>> problem. If there
>> is no way to rise the pressure when a secondary fill is needed, we =
should
>> consider to
>> swap out the reward. At the mean, we will discuss with day crew on =
the
>> pressure
>> issue on these dewars.
>>=20
>=20
> Tracy said that it probably didn't have time to build to full pressure
> after he replaced the popoff valve.  Next time this happens we'll just
> replace the dewar instead, or use another to bump the pressure up.
>=20
> John
>=20
If the dewar gets below 15 PSI, fills will become inefficient and time =
consuming.  At around 10 PSI, one starts seeing problems like low dewar =
pressure warnings and primary fill faults (should a primary fill occur =
when the 10L is still at very low pressure).  The bottom line is a 180L =
dewar should be changed for another if it can=E2=80=99t maintain =
pressure above 15 PSI. =20
-Dan
> ____________________________________________________________
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> =
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