[time-nuts] backfill

[Bob kb8tq]

Hi

Cold traps and vac-ion pumps were very common on precision crystal seal setups 50 years ago. 
They have gotten better since then….

Bob

> On Jun 9, 2017, at 9:13 AM, jimlux <jimlux at earthlink.net> wrote:
> 
> 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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