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'[OT] Vacuum Thermal Conductivity'
2004\10\18@190437 by Glenn Jones

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Hello,
This is a bit of a long shot, but does anyone know any good references for
information relating vacuum pressure to thermal conductivity in a Dewar? I
am building a small Dewar with a multistage thermoelectric cooler inside
and I need to calculate some theoretical values for the heat load at
different pressures. I have tried numerous google searches, but basically
all I have found is that people genearlly use pressures of 10 millitorr or
less
Thank you,
Glenn

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2004\10\19@102737 by Peter L. Peres

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On Mon, 18 Oct 2004, Glenn Jones wrote:

{Quote hidden}

Afaik at about that pressure loss by radiation is significantly larger
than loss by convection so vacuum counts less and less for heat loss.

Peter
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2004\10\20@134440 by hilip Stortz

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i believe the thermal conductivity of the remaining gas will be
proportional to the residual pressure and the thermal conductivity at
atmospheric pressure.  as others have noted, radiation is a far greater
contributor at even modest levels of vacuum.  omega.com has some
excellent information on calculating thermal loads, though i don't know
if they cover vacuum insulation.  they also sell heating/cooling
electronics and elements.  the thermal conductivity of the supports will
likely be much higher than that of the residual gas.  note also that
different gases have drastically different thermal conductivities so if
you are considering a limited evacuation filling with the appropriate
gas will also make a difference.

the kurt j. lesker company sells a lot of vacuum equipment and also has
a number of excellent tutorials and information like thermal
conductivity of gases and general facts about the atmosphere and various
levels of vacuum (for instance, at sea level an "air", i.e. nitrogen,
molecule will on average travel about 4" before striking another
molecule).  it will also depend on how much moisture is in the residual
air as i believe moisture greatly increases the thermal conductivity of
air and any condensation may bridge the space between the walls.
thermoelectric coolers don't generally go down to the range where a
dewar is really worth while, i'd suggest that you also look at more
conventional solid insulating materials which are also easier to work
with.  materials meant for high temperature insulation may still be
appropriate at low temperatures, though they'll need to be sealed to
prevent water absorption.

without knowing how cold you need to go or what size container you are
keeping cooled it's hard to say more.

Glenn Jones wrote:
>
> Hello,
> This is a bit of a long shot, but does anyone know any good references for
> information relating vacuum pressure to thermal conductivity in a Dewar? I
> am building a small Dewar with a multistage thermoelectric cooler inside
------

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