The history of the magnet:
I manage a Bruker WM-360 with a wide bore magnet (0 to 100% helium level is
23L, the nitrogen can holds 63L). The magnet was originally commissioned in
1981 and was recently (3yrs ago) purchased by us. Since then, the magnet has
quenched three times (each quench was attributable to a softening of the vacuum
in the dewar.)
The magnet is located on the fourth floor of the science building. The
chemistry research lab is located next door and has a bank (6) of hoods that
run on a timer (on from 8-5, off at nite). The blower for the hoods is located
directly above (10-15 ft.) the magnet on the maintenance floor. A room on the
other side of the magnet is the site for our new walk-in refridgerator. This
stainless steel fridge is well within the fringe of the magnetic field (10
gauss touches the side of the fridge; the 5 Gauss line runs down the middle of
the fridge.) The refridgerator has only recently been installed and has not
been turned on yet.
My problem:
I recently have noticed that the boil-off rate for helium cycles throughout the
day. In the morning the boil-off rate is less than 0.1scfh. By 5:30pm the
boiloff rate has peaked near 0.25scfh. The peak boiloff rate slowly gets
higher as the days go by and the dewar gets emptier. However, by 8pm the
boiloff rate is again falling and by morning (9am) is less than 0.1scfh
(typically so low that an accurate reading is impossible, ie 0.0scfh).
What could be the cause of the cycle I'm seeing? I initially thought it was a
change in pressure due to the hoods in the room next door. Accurate barometer
readings have discounted this fact.
Any suggestions will be appreciated, as elimination of the cycling could
lengthen my fill cycle thus saving us money.
I will compile the answers sent to me directly and post them if there is an
interest.
Sincerely,
Michael D. Mosher
Department of Chemistry
University of Nebraska
Kearney, NE 68849
email: mosherm@platte.unk.edu