On 6/8/17 1:19 PM, Bob kb8tq wrote:
Hi
If you look at the thermal conductivity vs very low pressures, the conductivity
comes up pretty quickly from a hard vacuum. There is essentially no impact
on Q.
basically, when the mean free path gets to be shorter than the distance
to the wall, the thermal conductivity drops off.
MFP = 65 nm at 1013 hPa = 760 torr
So at 10-4 Pa/0.75E-3 micron (start of very high vacuum) the MFP is 65 cm
High vacuum usually starts around 0.1 Pa (close to 1 micron), where the
MFP is 65 mm - this is where the MFP is comparable to the size of the
stuff you're pumping down, and where you can't use a "pump", but rather
you need something that flings the air molecules toward the exit
(diffusion or turbo molecular pump) or something that is like flypaper
for molecules (sorption, cold finger, etc.)
If you've got a "refrigeration" vacuum pump, they pull down to about
30-40 microns - MFP is a few millimeters
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