From: quest@iowegia.uucp (Steve J. Quest)
Newsgroups: sci.physics
Subject: Re: Q:  How to de-Gauss a room?
Date: Tue, 26 Jan 93 11:23:19 CST
Distribution: world
Organization: Iowegia Public Access Usenet/UUCP, Clive IA USA.

bmw@ncc.uky.edu (Brian Whiting) writes:

> I thought my problem might be of interest to this group.  I have recently
> moved into a suite of rooms which formerly housed a magnetic resonance
> imaging facility, with field strengths of ~70,000 Gauss.  Shielding was
> not perfect because they had painted lines on the floor indicating presence
> of 5 Gauss field.
> 
> The problem is that there is a remanant field in the room, first evidenced
> by skewed, blotchy computer monitors.  Compass readings show presence of
> a dipole which corresponds to former orientation of magnet in room.  I
> am waiting to borrow a Gaussmeter to quantify the field.
> 
> Naturally, I feel that an environment such as this is not appropriate
> for my computer lab.  In particular, I fear for the long-term integrity
> of my data on disk and tape.  In addition, there may be a pacemaker hazard
> if the field is strong enough.  Is there any information available on the
> field strength in which it is safe to operate disks?
> 
> Best of all would be to remove the field entirely from the room.  Aside
> from the obvious approaches of raising the room temp above the Curie
> point or placing a strong magnetic in the opposite orientation, is there
> any way to get rid of this field?  Please e-mail me, as I don't normally
> read this group.  If there is enough interest, I will summarize back to 
> the group.

 
Brian,
 
        You have a unique problem- one which I am sure others down 
the road will experience as MRI becomes more commonplace.  I have 
not heard of a site pulling out equipment to date, so therefore
have not heard of this problem.
 
        The problem you are experiencing is residual magetization 
of all ferrous structures built in and about the room.  This can 
be steel reinforcements in the floor, steel studs in the wall, 
etc.  There is no "easy" way to degauss a room.  I can speculate 
that it would take a powerful alternating magnetic field, that 
slowly drops in intensity.  Since they had magnetic resonance  
imaging equipment in the room, there should still be high current 
three phase AC power feed available (we hope).  Find a large 
magnet, maybe you can borrow a junkyard electromagnet for the job- 
and stick it in close proximity to where the superconducting 
magnet of the MRI sat.  Now comes the tricky part- feed this large 
magnet as much current at single phase AC (60 cycle is good- no 
need to change that) as it will take.  Heat it up if necessary, 
and then diminish the amplitude of the wave.  Speculatively, I 
would install in series with the feed wire to the magnet a bunch 
of high wattage lightbulbs (resistors).  Please bear with me, I 
think cheap and this will be cheap and effective.  Make sure that 
the lightbulbs will only barely light up at the current you are 
feeding.  Also make sure the bulbs are rated to drop the WHOLE of
the voltage you are using to feed the magnet.  Using insulated 
cutters, one by one clip out one leg of the lamps.  The remaining 
lamps in parallel with one another (forgot to mention that 
earlier that the lamps are to be installed in parallel) will begin 
to drop more and more of the voltage- thus reducing the magnetic 
wave from the magnet.  Do the clipping out of lightbulbs 
gradually, one by one, over a period of several minutes (how ever 
long the magnet can stand without overheating).  When you get down 
to the last lamp, clip it as well- disconnect power, and you are 
done.  This method should work- but it is all theory concocted 
just now in my head, but I do believe it will work............sq
 
Newsgroups: sci.physics