Subject: More Vt problems!
Author: lew cary <lcary@chem.unr.edu> at Internet
Date: 1/6/98 12:07 PM
When I walked in Monday, the unity500 had , the day before, toasted a
sample! Overnight, the temp had gone up enough to pop the cap on a cdcl3
sample, and evoporate the solvent, and cover the top of the spinner with
solid sample. With slight cleanup every thing seems normal.
We always run the vt on and at 25C. One probe has an intermittant
with the thermocouple. The cable is wonky, but with care works with all
other probes. The symptom is always the same: the appearent temp. goes to
max., but the sample is not heated.
My question is: how could a thermal runaway have happened? Over
temp shuts off the heated in all cases that I have seen. No one was there
Sunday, of course.
What a great way to start '98, Lew
Lew:
In my experience the heater element has a certain mass
which heats up and retains heat after the max temp switch
has activated. The gas continues to flow in some cases and
carries the heat to the sample. If the gas is cut off the
heat rises by convection until the mass of the heater cools
down. Either way the sample absorbs enough heat to pop off
the chloroform.
My recommendation is to run down to the Good Guys and buy
yourself a new.....
Happy New Year,
Jerry
Lew,
Our Unity/vxr-300 must have the brother of your vt cable. If I loop
it over the preamp box , everything is fine. If it just hangs, the
temperature display sometimes goes high until we wiggle it again.
We run with it always on at 20C but haven't had anything cook yet.
happy to hear that it didn't melt the probe,
dave scott
iowa state university
515-294-4057
We have an early Unity 500 (ca. late 1989) with Oxford VT
controller which, when it was only a year or two old, did
this to us at least once, and tried to several more times.
What would happen was that the indicated temperature would
suddenly change from 25C (or whatever, near room temp) to
~ - 200C (I forget the exact negative number, but it was
always the same.) Naturally, when the temperature controller
_thought_ it was at ~ - 200C, it would turn the probe heater
on full blast to try to get back up to 25C, but the increase
in temperature didn't register on the VT controller, which
continued to indicate ~ - 200C.
Eventually, I figured out that the Oxford VT controller was
using a couple of voltage-to-frequency converters to measure
the output of the probe thermocouple and of a junction
temperature compensating chip mounted on the back of the unit.
The register used to store the output of the temperature
compensation voltage-to-frequency converter was apparently
overflowing under some room temperature conditions in a way
that made the probe temperature look negative. I forget the
details, but changing the value of a resistor inside the VT
unit cured it, with the only side effect being that, when the
thermocouple becomes disconnected, the run up towards maximum
indicated temperature is slower than before.
I remember that for several months, before making the
modification, we did not dare leave the instument unsupervised
with VT running.
John M. Wright, Dept. of Chemistry & Biochemistry, M/S 0314, UCSD,
La Jolla, CA 92093-0314; email: jwright@ucsd.edu; phone: (619) 534-3049
Lew,
We haven't fried any samples, but we've had lots of trouble with the
thermocouple contacts staying well seated at the probe and giving a wrong
indication then of the actual sample temp (can get us off 10deg without
any clear indication that its off until someone does a check with
methanol!). I wonder if you might have had the same type problem with a
bad TC connection, and the TC indicates the sample temp is too low and
then the heater comes on.
Let me know what happens. As I said, we are still struggling to find a way
to get a more consistent TC contact.
----------------------------------------------------------
Charlie Fry Tel: (608)262-3182
Director, MR Facility Fax: (608)262-0381
Chem. Dept., Univ. Wisconsin
Madison, WI 53706 USA email: fry@chem.wisc.edu
----------------------------------------------------------
>Jim: I think my vttype is 2. If it is not set, then the vt won't go.
>Remember, ours is working all the time to keep 25 deg. I'll wait a week or
>so and then send a summary. Happy New Year, Lew
Lew..
I guess the upshot from my perspective is..
With vttype=2 you can ask for any temperature. Once the temperature is
set (ie. temp=30 su), you
can reset vttype=0, AND that then makes the VT controller transparent (vt
controller not present).
Therefore no matter what you do in software from this time, the temp is
"locked" to 30 deg. Even if
you ask for another temp (ie. temp=50 su) , the temperature will not
change. I do this routinely.
Jim.
==============================================================================
"Never try to outstubborn a cat."
loo@chemistry.ucsc.edu
http://www.nmr.ucsc.edu
(408) 459-2485
(408) 459-4923
==============================================================================
We just ran into a similar problem, our Inova was nice enough to pickle/boil our
peptide sample in H2O. The VT controller thought it was -65 C when in fact it
was 25 C, so it adjusted the actual temp to about 115 C. We tracked it down to
broken pin in our VT cable connector at the probe. I recommend pulling apart
the VT cable connector and carefully checking the pins, even if a continuity
check is OK. In addition, if the connection is really bad the VT will read
+999 C.
Good luck,
Andreas
*****************************************************
Andreas Kaerner
Department of Chemistry
University of California, Riverside
Riverside, CA 92521
andreas@chem2357.ucr.edu
andreas@habanero.ucr.edu
dlr@woodward.ucr.edu
(909) 787-3573
(909) 787-4713 (fax)
*****************************************************
* Gravity kills *
*****************************************************
Lew,
The probelm could be related to:
1) High Decoupling Power. Check if you was running a decoupled experiment like
CW decoupling and Composite pulse decoupling with a higher decoupling power
than the expected.
2) Not enough body air and/or VT air to maintain the system "cold" during the
experiment.
3) A combination of 1 & 2.
4) No air pressure during the experiment (compressor off).
In addition some samples becomes hotter than others during decoupling
experiments. It is a phenomenon for which I have not received a good
explanation yet.
Have a nice day,
JOE
We have had some VT problems with a Varian Unity + 400. The two major
causes of the problems are either in the probe (the temperature will
not regulate) or the air going into the probe. The connector to the
probe dewar is the wrong size and we have had to wrap Teflon tape to
the nipple of the probe dewar. This occasionally is a problem and
thus we periodically check to insure that the connecting is OK.
We had also had a problem with the VT unit itself and this seems to be
rectified by rebooting the unit. This has occurred only when the hose
has slipped off the probe during low temp (10C) work and the
temperature shoots up. This has happened maybe once every six months.
We have not had any other problems, in fact I completed 5 days of
unattended experiments at 20 C with out any problems.
Bob Dudley
rdudley@arserrc.gov
Hi,
I would simulate a failed thermocouple , loss of air flow if I was in
your shoes. I think a properly designed VT unit should protect the
system for at least these failures. Good luck.
Jim
We had a problem with an intermittant in our vt cable on a Unity system.
It looked and tested good but on pulling all the pins I found that one of
the thermocouple leads had a broken solder (or crimp, I forget which)
joint. Our cable had also become quite loose due to repeated
connection-disconnection and simply replacing the DB9 shell solved the
problem. I built a temporary cable out of regular copper wire and it
worked fine except that there is an offset of a couple degrees.
Kenner Christensen
Chemistry Department NMR Facility
University of Arizona
Tucson, Arizona 85721
(520) 621-2308
Somos pocos, pero estamos locos
Dear Lew;
A Happy New Year! Sorry to have a small mishap at the beginning of new
year.
Remember that the solvent, CDCl3, is very volatile (carcinogenic as
well, it says) and easily evaporate. If you keep the capped CDCl3
solution even in the refregirator the solvent disappears eventially. So
it you had set the temp, say at 30 or so, the cap may pop off due to the
build up the vapor pressure (it would take a while, though). If the
temp was set higher the chance of losing cap becomes higher. So I
wouldn't be surprised if the temperature didn't change during your last
run but cap just poped off.
If the temp at the monitor shows +999C or smoe such unbelievable number,
this is mostly caused by bad connection between the vt cable and a
probe. Are you using Highland VT controller? Their cable (metal
jacketed for shield-grounding) is very heavy and connection becomes
losen. I used a TMC leg to hold the cable so that the cable is held
horizontally and the connection is rather secured. Old Varian probes
have their VT connector with smooth surface and the cable connection may
not be secured.
Are you using a FTS system to supply chilled air? In that case if the
source air gets wet and FTS forms ice. Eventually no air comes out from
FTS although the source air never stoped. This causes unexpected but
nasty problems. Last autumn there was some work on the supply lines of
compressed air in this facility but we didn't know that the air become a
little wet -- bad enough to plug the FTS unit --- lost fully labelled
protein! (The quality of the air was/is extremely good here, so nobody
thought about such wet air supplied).
The 800 magnet has been up since mid-December and today the cryoshimming
commerced (Yes, we have a proton signal).
Best regards,
Toshi