Dear Colleagues,
recently, our University Fire Dept. (yes we have our own ... due to the large neutron
research reactor on campus) did one of its routine "visitations" to the Chemistry Dept.
One of the points they complained about: while there are "magnetic hazard" warning signs
on the doors of the spectrometer rooms, they missed specific markings of the 5 Gauss line
around each magnet ...
This we had avoided so far for some good (?) reasons:
1. With our larger NMR magnets (unshielded 750 MHz & 900 MHz), the 5 Gauss line is actually
VERY far away from the magnet (7 m and 10 m, resp.), and still far away from the region
where you start to feel a force on any ferromagnetic objects.
2. Practically all of our equipment (spectrometer consoles, cryo-platforms, vacuum pumps,
additional computer clusters installed in the rooms) are located WITHIN the 5 Gauss line
and hence would be out of reach for fire fighters.
3. For our unshielded 600, 750 and 900 MHz magnets actually the 5 Gauss line fills the whole
room, and for the 900 the whole inner court yard (20 m x 20 m) where the magnet building is
located. Thus this whole area would be "off limits" for rescuers in case of a fire or other
emergency - and completely unnecessary so, since the 900 magnet itself is "protected" by its
concrete housing (with only ca. 8 m diameter), outside of which no significant attractive
forces can be measured.
(Unnecessary to say that our gardening dept. ignores any signs etc. and keeps mowing the
lawn around the 900 MHz housing with its motorized riding lawn mower ... i.e., almost up
to the 100 Gauss line!).
Therefore I would strongly prefer to mark a smaller area (50 G?) and make the outer
parts of the 5 G area accessible (not to the public, of course! - but at least to
fire fighters, rescuers etc.). In the latest IFA (the German OHS agency) report on
exposure limits other numbers are given for(static!) magnetic fields:
- "exposure zone 2": 212 Gauss
- "exposure zone 1": 679 Gauss,
here it explicitly states "also take into account forces on ferromagnetic objects"!
- "short term exposure" (< 2h / day): 1273 Gauss
So, in my interpretation, for (non-magnetic) humans > 1000 Gauss would be still "short-term safe",
while for areas > ca. 200 Gauss no risk whatsoever exists - including magnetic forces (?).
Is there any precedence of such an approach? For your BIG magnets, do you have additional warnings
WITHIN the 5 Gauss line for fire fighters etc., and where?
Ciao,
gg
--
PD Dr. Gerd Gemmecker
Bayerisches NMR-Zentrum
Dept. Chemie, TU München
Lichtenbergstr. 4
D-85747 Garching
Germany
Raum/Room 32 109
Tel. +49 (89) 289-13308
Fax +49 (89) 289-13869
e-mail: Gerd.Gemmecker_at_ch.tum.de
Internet: http://www.gemmecker.de
Received on Thu Aug 18 2011 - 03:12:03 MST