Dear colleagues,
I would like to share with you an observation that may be of interest
and ask a couple of questions.
The NMR facility here has two magnets: a 15-year old wide bore 400 and a
6-year old narrow bore 500.
Last Wednesday I noticed that the cryogen boil-off rates of the 400 were
too high and off scale (He, normally 8-10 cc/hr, was above 28 cc/hr and
N2, normally around 150 cc/hr, was above 200 cc/hr).
Given the magnet's age and the lack of unusual activity around it, I
feared a softening of its vacuum. Then I noticed that the boil-off rates
of the 500 were also abnormally high: He, normally 2-4 cc/hr, was ca. 50
cc/hr and N2, normally 100-150 cc/hr, was around 300 cc/hr).
That day we had a strong storm in the area with heavy rain and wind. We
also had a barometric low (when I checked it was 29.33 in Hg and
falling). In the evening when the storm started moving away the magnet
boil-off rates started getting back to normal. Yesterday and today they
have been fine. I am inclined to think that the increased boil-off rates
were due to the barometric low ( making an NMR magnet a rather expensive
barometer).
I have been aware of the effect of atmospheric pressure on cryogen
boil-off rates but I have not witnessed it before, nor did I have a
feeling for its magnitude.
The possibility of vacuum loss and magnet quench brought up some safety
considerations: Namely to educate users about the danger of asphyxiation
and the importance of immediate exit from the NMR room ( Please have a
look at
http://opie.nmr.siu.edu/quenches.html ).
In addition to the good ventilation of NMR labs, the presence of oxygen
monitors would be helpful with this respect. What is your experience
with them? Can you recommend suppliers?
Past incidents also came up. Do you know of (any) deaths from magnet
quench-related asphyxiation?
Regards,
Lazaros Kakalis
Rutgers University at Newark
Department of Chemistry
73 Warren Street
Newark, NJ 07102-1811
Tel: (973) 353-5040 (office, lab)
Tel: (973) 353-5329 (secretary)
Fax: (973) 353-1264
Email: kakalis_at_andromeda.rutgers.edu
Received on Fri Oct 18 2002 - 18:41:37 MST