RE: AMMRL: The big OOPS
Silverman, Bob (BSilverman@mednet.ucla.edu)
Thu, 27 Dec 2007 12:21:32 -0800
Dear Jane and Group,
In giving these issues some thought, I don't believe that there should be any harm in using a low grade of helium gas to pressurize a liquid helium dewer. The typical impurities in helium gas are nitrogen and hydrocarbons. The tiny amount of either of these that contacts the surface of the liquid helium will solidify. Assuming that the helium dewer is not emptied in the fill process (I always arrange for this never to happen.), and knowing that any contaminants will freeze out of the liquid, then only ultra pure helium will transfer into the magnet. I guess it is possible that a "spec" of frozen material could get swept into the liquid stream but this should sit on the bottom of the magnet helium can and cause no problem. Most, if not all, of the miniscule amount of oil residue will go back to the liquid helium supplier.
The same could probaly be said about using gases other than helium for the transfer although I don't feel that this should knowingly be done since this might produce a significant amount of solids at the bottom of the delivery dewer. In all instances the liquid helium dewer acts like a big cold trap!
Happy New Year to all,
Bob Silverman
UCLA
-----Original Message-----
From: Jane Strouse [mailto:strousej@chem.ucla.edu]
Sent: Wed 12/26/2007 12:43 PM
To: ammrl@ammrl.org
Cc:
Subject: AMMRL: The big OOPS
Dear AMMRLers,
I indirectly received the message below from someone who was pretty
embarrassed about having used the wrong gas to pressurize the storage
dewar for helium fills. I thought I would relate my experience with
this sort of thing. Quite a number of years ago, I found that the
people working for me had been using nitrogen gas. I have no idea how
long this had been going on or how it happened that we ended up with
nitrogen rather than helium on the cart we use for fills. Amazingly, we
seemed to have not observed any ill effects from this. I thought about
it and decided that if you really did your fills correctly, you could
get away with this, because none of the gas should enter the transfer
line, and if the line is sufficiently above the bottom of the storage
dewar, any condensed nitrogen would not be pushed into the transfer
line. At any rate, we did later de-energize a couple of the solenoids
to move them to a new building. There was no problem whatsoever with
putting in the charging rods, so if there was icing from the nitrogen,
it was not substantial. (Of course, it is possible that the tank had
just been put on the cart when I noticed it and was never used, but I
always presumed it had been used.)
This leads me to another question. I once had a mass spec service
person tell us that we should use ultra-high purity helium (UHP,
99.999%) for the liquid helium transfer into an FT-MS solenoid. I
thought he was crazy, but I supppose it is possible that it would be
better for that particular magnet. Do any of you use UHP helium gas for
transfers? We certainly do not.
Jane Strouse
Message received:
________________________________________________________________________
_______
Dear Spinners,
First of all, happy holiday season!!!
I have been a passive observer and certainly taking advantages of the
fruitful discussions at AMMRL. Here is a story that I feel I can use to
contribute to the community. I am very much embarrassed in making such
mistake, as you can see I am even hiding my name, but I still want to
post it, so everyone could learn something from it.
I have been filling liquid helium to my 400 MHz magnet by myself and for
the past two years I thought that I have been using helium gas to
pressurize the tank (well, read the label before using it). Just a few
days ago, I found that it had been argon gas all along. Before using
this gas cylinder, there had been no problem doing it and after
switching to the tank, I started experiencing technical problems and I
blamed the liquid helium supplier and moisture in the air. Here were
the problems: (1) the liquid helium inlet (on the magnet side) seemed to
be "ice" blocked, (2) it took from one to two hours to fill it, and (3)
at the end of the filling process, it was extremely difficult to pull
out the transfer tube (somehow the tube got stuck somewhere in the
system on the magnet side and especially on the liquid helium tank side
- I even destroyed one of my transfer tubes to pull it out). Of course
the root of the problems is that the argon gas solidified in the system.
Just last week, I found the mistake and switched to helium gas;
everything goes back to normal now.
So, what can we benefit from such odd experience? Although some might
think that there are other factors to consider, from the experience I
believe that we don't need ultra high purity helium gas at all for the
refilling. A regular grade helium gas will do the job just fine. The
gas I used is almost 100% purity argon and it has been two years. If it
doesn't hurt the magnet this way (in terms of building up alarming level
of solids in the magnet or clogging the system permanently), why we need
to use ultra high purity helium gas? By the way, the boiling point of
argon is about -186 C, oxygen is about -183, and nitrogen is about -195
C.
Anyone cares to shed some more opinions?
>From someone too embarrassed to show his/her name.
________________________________________________________________________
_______
----------------------------------------------------------------------
Dr. Jane Strouse
Director, UCLA Molecular Instrumentation Center
1410 Molecular Sciences Building
Los Angeles, CA 90095-1569
(310)-825-9841 - voice
(310)-825-2280 - fax
strousej@chem.ucla.edu
http://www.mic.ucla.edu
----------------------------------------------------------------------
----------------------------------------------------------
IMPORTANT WARNING: This email (and any attachments) is only intended for the use of the person or entity to which it is addressed, and may contain information that is privileged and confidential. You, the recipient, are obligated to maintain it in a safe, secure and confidential manner. Unauthorized redisclosure or failure to maintain confidentiality may subject you to federal and state penalties. If you are not the recipient, please immediately notify us by return email, and delete this message from your computer.
----------------------------------------------------------