1/9/97
We are considering of installing an Automatic Liquid Nitrogen
Filling System for our SEM. The only source we came across is VBS
Industries, Inc. I wonder if any of you would like to share with me your
experience with any automatic LN2 filling system. Are they dependable? Are
there other sources that sell similar type system? Any feed back will be
greatly appreciated.
Pearl W. Yip
Rome Lab
yip@maxwell.rl.plh.af.mil
Perhaps it resides in Nestor's or the Florida archives??
I put in my comment that I would never have auto LN fillers again.
We had them on TEMs and SEMs in the 70s and early 80s and they all
failed catastrophically. (I guess there is no simple, benign failure
mode when the possibility of emptying a large LN cow over an
instrument and into an unoccupied room exists)!
Long term reliability under extreme temperature excursions seemed
to be the problem.
However, to be fair, we are talking 20+ year old technology. This is
the wonderful 90s...
ron-anderson@vnet.ibm.comRon
moving parts or electronics is not dependable. If you must keep something
cold all the time (much better than temp cycling, IMHO), I recommend that
you design and build a large LN dewar that keeps the thing cold that you
periodically refill every few days or so - much like a dewar for an EDS
detector. There are companies out there that will work with you on this
if you don't have the facilities to build your own.
Good Luck!
John Posthill
jbp@es.rti.org
Our automatic LN2 controllers are from Torr Vacuum, Inc. We have
had several disasters: 1) The LN2 source--a very large tank about 200 m
from our lab--would occasionally run dry; then the solenoid would over-
heat. Usually it would open, but occasionally it would fuse and draw
current until someone noticed (see #3). 2) The filler shut-off failed
on the system attached to our EDS detector. LN2 poured into and over
the dewar until the outer bottle deformed and ice formed all over the in-
side and outside of the system. We had to warm up everything, dry every-
thing out and re-form the outer bottle--it had a concave bottom originally,
but was convex after the disaster. Luckily, everything worked out well,
and the detector is still functioning a decade later with its specified
resolution. 3) For some reason--not an empty LN2 tank--the solenoid on
the line on the 200 kV TEM fused and heated the plastic foam insulation
starting a fire. The fire was, fortunately self-limiting; the event
occurred after hours on a Friday before a holiday weekend and was not
discovered until the next Tuesday.
We have not lost any expensive equipment, but there was certainly
the potential for such losses. I don't think our problems were the fault
of Torr Vacuum; they seem to be inherent in automatic LN2 systems. If you
can avoid such systems, I would reccommend you do so. Good luck--especi-
ally if you must use them.
Yours,
Bill Tivol
tivol@wadsworth.org
As others have said, these systems do seem to unreliable and considering
the value of the equipment they are attached to, I would not trust them. I
would even be wary about LN2 level alarms - they seem to be reliable, but
do you really want to have the safety of perhaps 500k dollars or more of
equipment depending on one?
Anyway, what is the problem with a regular manual schedule of re-filling?
You don't actually say what you are filling with LN2.
If it is an EDX detector, I would suggest that if you are really concerned,
you should have a routine for users to disconnect the HT from the detector.
Then if the detector should run dry, there is no risk of damage (although,
to be honest, I have, more than once, had an EDX detector go dry while the
HT was on and survive apparently unharmed, but I don't recommend anyone
trying it). However, this might require that you allow the detector to
restabilise following reconnection of the HT.
If you are filling LN2 traps on the vacuum system, then I would caution you
about running them continuosly anyway - over a period of time this will
actually degrade your vacuum. All cold traps should be allowed to warm to
room temperature at least once a week and the vacuum stabilise. If not, you
will gradually get a build up of ice and other contaminants on the traps
which, eventually will outgas at a sufficient rate to contaminate your
system.
Regards,
Larry Stoter
LPS@teknesis.demon.co.uk
the microscope, without first talking to the manufacturer regarding
warming/cooling procedures. However, from personal experience of Be window
detectors, they can survive warming up while on a TEM column, so if it does
happen to you (and you are responsible), try cooling it down again, before
you cut your throat :)
With UTW detectors, you probably won'tt be so lucky.
Regards,
Larry Stoter
Dr L. P. Stoter Technical Editor, MICROSCOPY & ANALYSIS
Technesis
17, Rocks Park Road email: LPS@teknesis.demon.co.uk
Uckfield, E. Sussex Phone: +44 (0)1825 766911
TN22 2AT Fax: +44 (0)1825 766911
United Kingdom
although there may be others. However, you might want to design your own
system, using cryogenic solenoid valves and sensors, and providing LN2 from
cylinders. It is fairly inexpensive to do this. I can send you information
on a system we built and used in our lab for some time, if you are
interested. I presented a poster on this system at the 1994 MSA meeting and
the description in included in the Proceedings from that meeting.
One problem we ran into with our system is that we did not have a failsafe
mechanism to shut the system off when the cylinder ran out of liquid. The
result was that nitrogen gas would continue to fill the dewar and eventually
would blow out the remaining liquid. I think this can be remedied by making
some changes to the system.
Regards,
Melanie A. Behrens
Texaco, Inc.
P.O. Box 509
Beacon, NY 12508
914-838-7261
behrema @ Texaco.com or MelanieOwl @ aol.com
The filler for an LN trap over the DP on our JEOL 2000FX TEM malfunctioned,
and a tank of LN2 emptied out all over the diffusion pump. The microscope
shut down, luckily valving the column off from the DP. The water lines
froze, including the water in the baffle at the top of the pump. This
caused the baffle to crack, and when it warmed up again, the pump and
reservoir tank flooded with water. The next morning the microscope was
found in the OFF condition, and, no problem being evident, was simply
restarted. This caused an emulsion of DP oil and water to be sucked into
the rough pump, which *was* evident, and the machine was immediately turned
off. The damage was done however, and a complete teardown and cleaning was
necessary, taking a couple of weeks.
So if you use a filler system, be sure to provide a way to funnel LN away
from the microscope, in the event that the solenoid fails to close properly
and the tank empties.
allardlfjr@ornl.gov
Larry Allard