A few weeks ago we experienced an increased He loss. The long and the
short of it is that the rate has now slowed to normal. We should have
been using a 1 to 5 psi gauge, which we have now purchased and will try
on the next fill a few months down the road. Many thanks to all of you
and especially to George Furst and Walter Osgood, who had lots of good
questions and suggestions. If you are having this trouble in the future,
most of the factors to consider are in this summary.
Orig posting:
> Subject: Helium loss
> Date: Mon, 08 Oct 2001 14:37:57 -0400
> From: "Robert Honeychuck" <rhoneych_at_gmu.edu>
> To: ammrl_at_chemnmr.colorado.edu
We have a Spectrospin Bruker 300 MHz magnet which we (I and several
graduate students) have filled once every 6 months for the past 7 years.
It has been a carefree, robust magnet. After the last fill 19 days ago,
the He level is going down at twice the old rate. The He gas pushing
pressure was probably higher on this fill than ever because the transfer
time was the lowest ever. I don't know what this pressure is because I
have never measured it with a gauge. Did I cause the seals to go, or do
they generally give up after 7 years anyway? There is some information
in the archives but I thought I'd add this.
If you reply directly to me at
rhoneych_at_gmu.edu
I will summarize for the list. Thank you very much.
Bob Honeychuck
George Mason University
---------------------------------------------------------------
hi bob:
Sounds like you have broken lots of fill rules. A couple of
questions first
1. Is this a Bruker Swiss magnet or oxford, long hold or normal
!2. Do you have a boil off gauge attached to the magnet
3. How long does it normally take to fill you magnet and how
fast was this fill
4. What is the pressure used for your nitrogen fills and has
the nitrogen boil-off slowed down
since your last Helium fill.
5. Any changes in rm temperature shims or magnet homogenity
Seven years is not a long time for a supercon we have 13
systems here range from 200 -750MHZ the average
we have seen on the old lower field oxfords has been about 12
years with some at 17years we have one at field
for 22years. Bruker had a problem with the O-ring material
on some magnets a while back do to a vendor formulation change but
that was well after your magnet was installed .
Have you checked for any blockages in the nitrogen ports . If
one or two got iced up in the fast fill that could cause a shifts of
the cans and a slight touch causing the increase in boiloff . Make
sure you get a pressure gauge for your fills, not knowing the
pressure is a real danger.
---------------------------------------------------------------
Bob,
Has the lock frequency changed? Does it use more nitrogen?
Is any part of the dewar colder than normal? I think that it
is unlikely for a fast fill to have hurt anything. Its probably
just depressed like the rest of the country and will settle down
in a couple of weeks.
---------------------------------------------------------------
Bob,
7 years is definitely sooner than typical (I'd say 10-15 years).
Perhaps the o-ring got frozen (during LN2 or LHe fill; perhaps
air eject was not clear away enough from magnet?) or got
saturated with He gas. Either way it appears it's trying to
go. At a 3 month fill rate, I would try pumping down the
vacuum and seeing if it will stabilize at the old refill rate.
---------------------------------------------------------------
I have a Spectrospin 300 WB magnet that has been up since January 1985.
I
would say that the seals should not go after 7 years.
---------------------------------------------------------------
Bob,
A 15 year old Bruker 300 Magnet in my Laboratory suddenly started loosing He
at twice the normal rate late last year (after a He fill). I cannot say if
anything unusual happened during the fill as it was done by the He vendor. I
have been waiting for a suitable time to take the magnet down and replace
the seals but now that I am in a position finally to do this the He loss has
now returned to its normal rate. Looking at the data, the rate loss changed
back instantaneously (well a couple of days at most) after 9 months at double the rate.
No obvious trigger has been identified.
I remained puzzled, but happy.
---------------------------------------------------------------
Bob,
It could be a number of things.
1) the seals going like you suspect (though seals can last for 10 to 15
years sometimes),
2) the helium sensor heater stuck on or malfunctioning so it is boiling off
more helium, or
3) N2, O2, or water vapor "Ice" build up inside from not purging the helium
transfer line with He gas.
Have you checked the boiloff rate on the flowmeter? Is it above instrument
specs? Ask Bruker. Is the boiloff rate increasing more each day? If it
is, it's quench time. In the future, it may be good to invest in a pressure
gauge and not let the He gas pressure go over 1 psi. Filling too fast can
make the He dewar inside the magnet "swing" from the force of the fill or if
you run out of liquid He during the fill and pump gas into the dewar, the
gas pushes the liquid out. Both result in a quench.
If it does quench, try to keep the dewars slow purged with He and N2 gas
respectively to keep moisture out. This will keep the pump down time to a
minimum if Bruker or whomever energizes the magnet for you does not think it
is the seals. If a seal job is needed, it can't hurt to keep things dry
inside.
Good luck.
---------------------------------------------------------------
Hello Dr. Honeychuck,
I suppose it is possible that your vacuum on the magnet has gone soft on
you. You can tell by the amount of helium gas exhausting out of your
magnet
if you have a flow gauge on the exhaust. This is assuming that you knew
what it was prior to the fill. But it should be directly
proportional......double the flow=double the loss.
I have a 300/52 Zurich which is painted white it flows about 5 CCH....
Or, perhaps you filled to fast and did not actually transfer any liquid.
---------------------------------------------------------------
Dear Robert,
I have a Bruker AMX360 system which we had to move when we renovated
our Science Center. This was 5 years after setup. It was indicated to me
at that time that while some magnets last a very long time, generally
magnets needed maintenance at about 7 years. We replaced o-rings, seals, and
re-established high vacuum in the chamber.
---------------------------------------------------------------
Received on Tue Nov 13 2001 - 08:47:37 MST