Note to Paul Driscoll:

Gerald Pearson (gpearson@blue.weeg.uiowa.edu)
Tue, 26 Apr 1994 12:47:36 -0500


I tried emailing you a note to driscoll@bioch.ox.ac.uk, the return address on
your note to ammrl@bloch.cchem.berkeley.edu Fri, sent on 15 Apr 1994 at
14:42:38. My note bounced:

> From Mailer-Daemon@bioch.ox.ac.uk Sun Apr 24 12:20:09 1994
> To: gpearson <gpearson@blue.weeg.uiowa.edu>
> Subject: Returned mail: User unknown

... so I'm trying again, sending it this time to
ammrl@bloch.cchem.berkeley.edu. Hope you get it.

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Date: Sun, 24 Apr 1994 12:04:53 -0500
From: Gerald Pearson <gpearson@blue.weeg.uiowa.edu>
To: driscoll@bioch.ox.ac.uk
Subject: Re: Can I join? [ammrl]

Paul Driscoll:

> I am just about to be become the proud 'owner' of two Varian UnityPLUS
> spectrometers in a new laboratory in London and we have a few problems to
> iron out, even before we have moved in. ...
In addition to ammrl, you would probably want to get your name on the list
server for varnet (Varian users). We're a Bruker shop, so I don't use varnet,
but I do use BUM (Bruker-Users-Mail). You'll probably be in contact with Vera
Mainz in connection with ammrl, & she has set up archives of old varnet and
ammrl email. If you have any trouble getting the email addresses, drop me a
note & I'll dig out the details and send them to you.

> Does anyone out there work in an NMR lab which has an automatic helium
> sensor or other safety device that is activated in the event of a magnet
> quench (that leads to some mechanism for automatically venting the helum to
> the atmosphere, or the evacuation of the building)? These sort of things
> are probably regarded by most as a bit OTT, but we are having to covince
> our Health and Safety people that, should a deaf (and blind) person happen
> to be in the vicinity of the NMR magnet when it quenched, there would be a
> failsafe mechanism for raising the alarm.
I think that's overkill. We have a small exhaust fan to the outside going
continuously in the room with our narrow-bore 360. For our wide-bore 300,
narrow-bore 300, and narrow-bore 600, we just rely on the normal air-handling
systems.
Before our first supercon magnet was installed in 1983, I was worried
about oxygen depletion in the case of a quench, so I calculated the total
volume of the helium gas _at_ _room_ _temperature_ which would be produced by
boiling off _all_ of the liquid He in the _full_ cryostat. I then assumed
that this much air would be displaced from the room, and the remaining air
would rapidly and thoroughly mix with the gaseous He. Even with these
extremely conservative (pessimistic) assumptions, (and for a rather small
room!) I calculated that the oxygen concentration would drop by only about 20%
-- no worse than increasing your altitude by a few kilometers. If you do this
calculation yourself, for your particular magnet and room, I expect that
you'll be surprised at how small a fraction of the total room volume would be
occupied by boil-off He gas in the worst-case scenerio.
The only _real_ concern would be severe frostbite in the event that the
deaf and blind person would be standing on a stool or ladder so that s/he is
directly in the stream of the blow-off helium gas during the quench.

I think that the really important safety precautions are:
1. As an absolute minimum, make sure that the 10-gauss surface of the
fringe fields do not penetrate into any "public" area, such as a hallway or
teaching room. Better yet, make sure the fringe fields in public areas are 5
gauss or below.
2. Warning signs on the door.
a. "DANGER -- POWERFUL MAGNET -- AUTHORIZED PERSONNEL ONLY -- KEEP
ALL IRON AND STEEL OUT." [Custodians should enter the room ONLY under the
supervision of one of the NMR staff.]
b. "ABSOLUTELY NO ADMISSION TO PERSONS WITH CARDIAC PACEMAKERS."
3. Warning sign on the magnet. "POWERFUL MAGNET -- CAN SUCK IN IRON AND
STEEL WITH GREAT FORCE -- THIS INCLUDES GAS CYLINDERS, BOLTS, PAPER CLIPS, MOP
BUCKETS, AND TOOLS. -- CAN DAMAGE MECHANICAL WATCHES AND MAGNETICALLY-
ENCODED BANK CARDS AND CREDIT CARDS, AND CAN CAUSE CARDIAC PACEMAKERS TO
FUNCTION ERRATICALLY."
4. Barrier around the magnet -- i.e., a "non-verbal sign". Either
plastic chain or brightly colored plastic rope -- a couple of meters from the
magnet.

> I have never seen a catastrophic quench. What actually happens?
1. He gas comes out of the safety valves on top of the stack with a
_very_ loud hissing sound -- as if someone had opened a _large_ steam valve.
2. The temperature of the room drops several degrees in a few seconds.
3. A dense white fog clings to the ceiling. Almost no helium gets down
to floor level, and not enough gets down to head level to make any noticable
"Donald Duck" change to the human voice.
4. It all happens in less than 20 seconds.
5. If you have a dropped ceiling, the helium-filled fog moves into the
space above the dropped ceiling within a few minutes, because the He diffuses
like a shot through a permiable ceiling.

> Also, can static magnetic fields (5-10 Gauss) affect in any way the
> integrity of data-transmisssion lines (telephone or computer network)? It
I haven't noticed any problems with phone lines, ethernet, ZZNET, or
FASTRAN.

HOWEVER: NMR spectrometers are particularly sensitive to grounding problems
and electrical noise pick-up, so we take extra precautions to ensure that
there's no chance in hell that we could run into ground-loop or noise-pickup
problems. Specifically, we:
1. Put any computer with an electrical connection to a spectrometer
computer on the _same_ mains circuit as the spectrometer.
2. Use fiber-optic transceivers (instead of thinnet transceivers) to go
between rooms. This extra "insurance" costs several thousand $US, plus maybe
one or two thousand $ for each additional spectrometer. I consider this to be
insurance money well spent.

Hope this helps.
-- Gerry
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Gerald A. Pearson | _P_L_E_A_S_E_ _N_O_T_E_ _A_D_D_R_E_S_S_:
Chem.Dept., Univ.of Iowa | INTERNET gerald-pearson@uiowa.edu
Iowa City, IA 52242, USA | VOICE 319-335-1336 FAX 319-335-1270
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