Thank you to everyone that replied! The consensus is that we should try to
keep as far away as possible from arc welders to avoid any possible
interference. It appears that there may be both magnetic (due to large DC
currents) and Rf components to the interference. It was suggested that a
UPS or filters would help eliminate transients on the ground or common
lines, iron shielding might attenuate magnetic field fluctuations, and a
Faraday cage would be attenuate any RFI.
I've attached the responses below (slightly edited).
Steve
Steve Pitzenberger
Merck Research Labs
West Point, PA 19486
---------------------------------------------------
Initial question:
We are in the process of evaluating a site for installation of a 600.
In reviewing the plans we noticed that there will be a welding shop
approximately 150 feet from the proposed instrument location (in the
same building).
The welding slated for the shop is called TIG (tungsten inert gas).
I have been told that TIG is also know as GTAW (gas tungsten arc welding).
Apparently a stream of argon gas is used to shield a tungsten electrode
and the molten weld pool as an arc is formed between the electrode and
the metal being welded. I once had a bad experience with an older
style arc welder causing interference with a 500 MHz instrument from
an adjacent construction site (might have been 100 feet or more).
Two questions immediately come to mind -
1) Does anyone have direct experience with a TIG welder causing or
not causing interference with a spectrometer?
2) Has anyone found a way to eliminate interference from an arc welder
by special grounding, shielding, or other means?
--------------------------------------------------
Responses:
----------------- from Paul Cope ...
I've seen the effect of arc welding used in construction on an NMR site
and it's not pretty. I don't know the subtle differences between the
types of welding operation, but if the arc is generate with high
electric current, you'll have a problem. Only shielding the room with
grounded cage will solve the RF interference. If there is also a
magnetic field generated, then you're into lots of iron shielding, much
like is used for larger bore imaging systems. The problem with this iron
shielding is that it can distort the magnetic field sufficiently that
you can't shim the magnet to HR requirements. Imaging isn't so high in
resolution, so the homogeneity isn't as stringent.
Find another home for the NMR that's several hundreds of yards away or
move the welding operation. Any other solution is simply too painful and
costly!
----------------- from Ben Hamilton ...
I sometimes do TIG welding on magnets to seal leaks and such. Todays
TIG systems don't have the high current that the older ones have, thus
reducing the spikes on the ground or common lines. If your lab has UPS and
line filters your probably ok. Only the real result will tell though.
----------------- from Bob Santini ...
There may be another consideration in addition to the RF emission from the
arc itself. We have used welders at one time or another to run high
intensity lamps or sustain arcs to perform synthetic work.
This type of welder is essentially a large DC power supply. The DC current
through the load can be 100 amps or more with lots of transients to higher
peak levels. The result will be a fairly strong magnetic field, centered
at the welder. It will vary from zero to some significant strength with an
unpredictable time dependence when welding in taking place. I would expect
that there would be difficulties keeping the NMR system locked.
----------------- from Matt Rees ...
Tig's use a High Frequency Arc that can cause interference problems
depending on the length of the welding leads. You might build a
faraday cage to attenuate the effects of the HF.
----------------- from Greg Leo (via telephone) ...
Greg told me about problems he experienced recently with workmen welding
about 75 feet from his 600. Despite having a digital lock, he was unable
to maintain lock while they were welding. He said they had a 400 across
the hall from his 600 (also with digital lock; maybe another 10 to 20'
distant) that was not affected.
-----------------
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Received on Thu Aug 29 2002 - 16:09:55 MST