Hi Folks:
On an AV-500 here, we do low VT using an exchange coil immersed in a 25 L
liquid nitrogen dewar. This is the standard equipment that came with the
Bruker spectrometer. We have been using this system a lot lately and I have
not been able to achieve very tight temperature regulation, despite doing
self tunes in edte, using adequate heater power, etc.
I'll admit I am a bit of a neophyte with an exchange system as all my
previous low temperature experience involved an evaporator set-up. In terms
of my methodology, when setting up to do low temperature experiments with
this equipment, I will first convert the console to N2 gas (using boil-off
from a storage dewar). Also, before the coil is immersed in the liquid
nitrogen dewar, I flow nitrogen gas through it for about 5-10 minutes to
make sure it's very dry. I then increase the flow rate and slowly immerse
the coil into the dewar. What I have observed is that as the coil cools
down, the gas flow rate drops and then eventually liquid actually comes out
of the transfer line (the black dewared line that connects to the probe). It
comes out in bursts. When I first observed this 2 or 3 months ago, I
figured I had not purged the coil long enough with dry nitrogen gas.
However, a longer purging period as well as heating with a heat gun resulted
in the same thing! I wondered if somehow the system might have a leak but a
subsequent leak test under about 40 psi showed no evidence.
I am completely at a loss to explain this but obviously suspect herein lies
the reason for poor temperature regulation at low temperatures. Does anyone
reading this have any advice/explanation for me?
Thanks,
Mike
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Michael D. Lumsden, Ph.D.
NMR Facility Coordinator
Room 428, Atlantic Region Magnetic Resonance Centre
Department of Chemistry, Dalhousie University
6274 Coburg Road
Halifax, Nova Scotia, Canada
B3H 4J3
phone: 902-494-1635
FAX: 902-494-1310
http://armrc.chemistry.dal.ca
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