I have done some research and found the following. Please check my math
because colleagues may rely upon my conclusions and they need to be
correct. I will post this at
http://opie.nmr.siu.edu when I know that it's
correct.
All respondents agreed that helium fills should be done with assistance.
Many also suggested that I seek a new job at a university that would
provide enough staff help so that I can get away from the NMR lab now and
then.
Assessing risks of superconductive magnet quenches
A. Excerpts from:
http://www.osha-slc.gov/Preamble/ConSpaces_data/CON_SPACE2.html
(Occupational Safety and Health Administration)
"OSHA has determined that a variety of confined space hazards have caused
deaths and injuries. The following discussion describes the hazards
identified by OSHA. Where the Agency has obtained incident data subsequent
to the publication of the NPRM, the circumstances of some of those
incidents are summarized as 'examples'. The discussion also references the
portions of the NPRM where pertinent incidents were described.
1. Atmospheric Hazards.
OSHA's review of accident data indicates that most confined space deaths
and injuries are caused by atmospheric hazards. OSHA has classified those
hazards into three categories: toxic; asphyxiating; and flammable or
explosive atmospheres, in order to account for their differing effects....
a. Fatalities in asphyxiating atmospheres. In its analysis of these
confined space incidents, OSHA uses the term 'asphyxiating atmosphere' when
referring to an atmosphere which contains less than 19.5 percent oxygen.
Oxygen levels under 19.5 percent are inadequate for an entrant's
respiratory needs when performing physical work, even if the space contains
no toxic materials...the original atmosphere in the space may intentionally
have been wholly or partly inerted using such gases as helium, nitrogen,
argon, or carbon dioxide. Victims of asphyxiation often are unaware of
their predicament until they are incapable of saving themselves or even
calling for help."
B. Calculations of risk
The NMR Laboratory at SIUC is approximately 7m x 8m x 3m to the suspended
ceiling, thus having an approximate volume of 168,000 liters. To reduce the
oxygen content of the room from 21% to 19.5% would require displacing 2520
L air with gaseous helium. This would require vaporizing 350 L of liquid
helium. The helium dewar of my Oxford 500/51 magnet contains approximately
70 L liquid helium, thus, risk of asphyxiation seems small. These
calculations are based on density figures of 0.1248 g/mL for liquid and
0.0164 g/mL for gas.
Relative risks would increase (a) at higher altitudes where pre-quench
oxygen content is lower, (b) in smaller NMR laboratories, and (c) in
laboratories with larger magnets containing more helium.
Please check my work!
Bill
William C. Stevens, Ph.D. Nuclear Magnetic Resonance Facility
Director Southern Illinois University
Carbondale, IL 62901-4405
618-453-6498 voice
618-453-6408 fax wstevens_at_siu.edu
http://opie.nmr.siu.edu
Received on Thu Jan 10 2002 - 17:43:58 MST