[time-nuts] backfill

jimlux jimlux at earthlink.net
Fri Jun 9 09:13:04 EDT 2017

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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