For those of you that have not seen this unit it has a "3 position" valve
on the gas input. This valve (theoretically) allows you to manually select
the route of the VT gas thought the heat exchanger. With the valve in the
lower position the VT gas is sent through the heat exchanger and the
delivery temp of the VT gas to the probe can easily reach <-150C. The
upper position bypasses the heat exchanger and delivers room temp gas.
With the valve in the middle position the gas is split 50:50 between the
heat exchanger and bypass routes and the output temperature is about -70C.
My problem with the unit is that when the valve it in the middle position
the flow through the heat exchanger loop gets blocked and the VT gas (and
probe temp) starts rising to ambient temp. I believe that the coil of
copper tubing in the heat exchange is so long that the nitrogen gas can
condense thus collecting in the coil and blocking the gas flow. Since
there is still a path through the bypass route there is no pressure to
force the block out. When this happens the only way to clear the block is
to put the valve in the lower position When this is done you can hear a
rush of gas and the probe temp drops drastically (10-30 degrees with in ten
seconds). Obviously this is not good for the probe or sample.
We are using dry nitrogen gas and purging the system for 15 min before
adding the liquid nitrogen to the dewar so I do not think that the blockage
is anything but condensed nitrogen. While we fill the dewar the valve is
left in the lower position to keep a good flow rate through the heat
exchange. Raising the VT flow rate to 15-20 LPM helps a little but does
not eliminate the blockage when the valve is put into the middle position.
If anyone has an idea how to stop this problem please let me know before I
start chopping up the heat exchange coil to replace it with a smaller loop.
(In fact I am thinking about several different length loops to allow
selection of lower temperature limits. I saw this with Doty's unit and I
thought it worked well.)
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Perry J. Pellechia
Director of NMR Services Ph: (803) 777-2088
Department of Chemistry & Biochemistry Fax: (803) 777-9521
University of South Carolina email: pellechia@psc.sc.edu
Columbia, SC 29208 http://homer.chem.sc.edu/~perry
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