Hi all,
I'd like to submit this item related to my recent posting about
compressed air systems. Many of you know about the problem with
Varian Inovas' problem of runaway heating in the probe when VT air
flow is too low, and the AMMRL archive contains various postings by
people seeking a solution to the problem. There is at least one
mechanism in the archive that relies on detecting a drop in VT air
pressure, but here we offer a solution based on air flow. It was
designed and built for us by the University of Chicago Engineering
Center, and they've agreed to let me share the plans and parts list
with you. (They'll even build one for you if you're willing to pay
them for parts and marked-up labor, but they cost kind of a lot.)
I've posted the plans for this safety device, including a parts list
and photos of an installed unit, on two of our websites:
Hi-res plans and summary with parts list:
http://homepage.mac.com/jkurutz/FileSharing18.html
Photos & lo-res diagrams:
http://homepage.mac.com/jkurutz/PhotoAlbum19.html
This device is more complicated than a simple relay triggered by a
pressure drop, but it is designed to keep probes safe in the event of
low flow in addition to absent flow. A simple flow/no-flow or hi-
pressure/ambient pressure sensor works well to protect against total
compressed air failure. However, we recently had an instance (well,
several instances, but let's look to the future, shall we?) where the
dew point rose so high our FTS chillers formed ice plugs, and I
imagine our cold probe's plastic VT/tuning knob assembly melted long
before the pressure dropped completely.
This device is designed to shut off power to the VT controller and to
the FTS chiller if VT air flow drops below a user-adjusted threshold.
We've set ours to go off at approximately 4-5 lpm.
We've tested the device and it works well. I cannot guarantee it'll
work for your system, though, so if you build it and it fails, please
don't blame us. This is a best-effort proposition to help reduce your
chances of a meltdown, not an insurance scheme.
If you're interested, please visit our websites and download the
plans and explanatory document.
Enjoy and sleep better!
- Josh
Josh Kurutz, Ph.D.
Technical Director, Biomolecular NMR Facility
University of Chicago
Gordon Center for Integrative Science, room W123C
929 E. 57th St.
Chicago, IL 60637
Office: (773) 834-9805
Spectrometer Room: (773) 702-4052
Cell: (773) 315-5732
Fax: (208) 978-2599
nmr.bsd.uchicago.edu
homepage.mac.com/jkurutz
Received on Thu Nov 02 2006 - 13:56:06 MST