Glenn, AMMRL;
You asked why. There's a lot to the why. I know there are tubings,
dewars glues and other important structures in the probe important to VT
but the main control parts are: Heater, overtemp sensor and
thermocouple.
There are only 2 controlled inputs: Air flow (manual) and
heater-power(VI) using the highland control (control loop). Varian
probes are sorely lacking a servo flow control like for the spinner air.
So we are part of the air flow control loop and we have to set it right
or it won't work. By using the FTS for hi-VT you are adding another
manually controlled input, inlet air temperature.
What the t-couple reads is virtually a real-time measure of the AIR (or
N2) temp at the bottom of the sample (not the sample). The over-temp
sensor is a slower device and is imbedded directly into the heater. The
heater energizes to a temp. that is high enough to produce the air temp.
needed to bring the sample up to proper temp. It is somewhat fast to
heat up and slow to cool as you have it set up because there is not
enough "cooling power" for lack of a better term to overcome thermal
inertia and overshoots. Varian VT systems are basically not very energy
efficient when set up correctly and working well so that they can be
quick, responsive and able to operate in the huge temp. range that they
do. This is only possible when you have a cooling power available which
does not make sense but is true. So if you want to be able to quickly
move from 50C then down to 25C then you need much less than 25 or you
can wait an 1/2 - 1 hour to truly equilibrate. Adequate high cooling
power can force equilibration and provide a means to quench the heater
before it gets too hot. (Did he say quench?)
The Overtemp sensor is both very accurate and rugged. It measures the
point (set in software) when the heater has exceeded its maximum design
temperature. The highland controller then shuts power down to the probe.
This is great and good but the software will not reset VT, check the
temp and resistor again and try a lower current to the heater so an
unattended experiment then gets ruined.
The probes are designed to be used with and without the preconditioning
units. An argument can be made that heated air is better for the probe
... Less stress ... But this is nothing compared to the extremes the
probes are designed to handle when LN2 cooled air is heated to 5 degrees
below room temperature. IMHO the preconditioners are only useful for
making low temp air so long as it is Varian's design practice to heat
air from within the probe.
There is no direct control relationship from any Varian parameter with
any FTS preconditioning unit and I can not think of a reason to use one
for Hi-VT unless you can get it to regulate better than the highland on
room air (doubtful). To use the FTS this way you would need to disable
VT in config and simply use the highland as a temp monitor. We have
done this before when the HW Error was problem and we could not quickly
resolve. It worked well.
Try this: Use recommended airflow settings and calibrate the
system through one probes full range using the nitrogen bucket 1st for
lo-VT then simply with the FTS off and no LN2 for Hi-VT. Save and post
these data for your clients. If through the calibration Hi and Low there
were no HW errors then you can expect the FTS to cause none for the same
flows unless you turn on the heater because the pressure drop is low
through the FTS.
What I'd like to see from Palo Alto: The best of all worlds (highland +
FTS + Varian) combined into a single unit that does away with the heated
air part and puts the money into the low VT, provides servo air cooling
control and elegant software tools. It would be nice for intance to
have a semi-automated calibration tool that stores temperature offsets
in the probefile.
Simple solution for you: Turn off the FTS during high VT and see what
happens on recommended air settings. Adjust air until the problem goes
away. Then after finding and recording stable settings with trial and
error you can likely find stable settings to use both the highland and
FTS heaters. I think this better for the probes but it is a delicate
balance.
There is one last remote possiblity too ... You might actually have a
hardware failure. The Highland controller lets you monitor all the
parameters. I believe the resistor cutout is 180-190 Ohms. But
something might be loose or touching that changes with temp. Check the
last error on the controller.
Good luck and good job on the new console acquisition!
I hope this helps.
Robert Harker
////(--)\\\\
Engineer
University of Florida
Department of Chemistry UF Box 117200
Nuclear Magnetic Resonance (NMR) Laboratories M/S 82
Gainesville, Florida 32611-7200
Contacts:
352-392-4650 Office
352-392-9886 Labs
352-392-8758 FAX
rharker_at_chem.ufl.edu <mailto:rharker_at_chem.ufl.edu>
www.chem.ufl.edu/NMR <
http://www.chem.ufl.edu/NMR>
-----Original Message-----
> From: Glenn Facey [mailto:GAFACEY_at_science.uottawa.ca]
> Sent: Tuesday, October 05, 2004 10:30 AM
> To: ammrl_at_ammrl.org
> Subject: Varian VT question
Dear members,
I have a Varian INOVA console (2004) with standard VT unit and an
FTS preconditioning unit. Both the high temp and low temp air lines
from the magnet leg are combined and fed into the FTS
preconditioning unit which in turn goes to the probe. As far as I can
tell this makes the parameter "VTC" irrelevent as it no longer
matters which air line the air comes through.
I can do both high and low temperatures ( -20C to >85C) when the
FTS is set at room temperature or below (I use dtry air)
When I set the FTS to 75C and set the VT unit to 85C (i.e.
temp=85) I get a hardware error in the VT unit and regulation is
impossible. Why?
Glenn
Glenn A. Facey, Ph.D.
NMR Facility Manager
University of Ottawa
mailto:gafacey_at_science.uottawa.ca
http://www.science.uottawa.ca/~nmr
Received on Thu Oct 07 2004 - 12:04:07 MST