[Milsurplus] Re: Milsurplus Digest, Vol 36, Issue 3

[Jim Stewart]

milsurplus-request at mailman.qth.net wrote:

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>  
>
>Very interesting stuff about the hard suit's origin. I had never  
>heard that part of the story before.
>
>A few numbers:
>
>Sea level pressure 760 torr
>
>Pressure at 25,000 feet = 282 torr
>
>Pressure at 30,000 feet = 225 torr
>
>Water boils at body temperature at a pressure of 47 torr (wonderful  
>piece of data)
>
>Vapor pressure of Mercury at room temperature (mercury vapor  
>rectifier) 1.7x10-3 torr
>
>Pressure at 1,000 Km = 10-10 torr
>
>Pressure at 400 Km = 10 -13 torr
>
>The easy way to measure vacuum is to set up a diode or triode in the  
>environment and measure it's leakage. When used that way the tube is  
>called a vac-ion gauge. Regardless of the name, it's still a tube,  
>filament and all.
>  
>
Hello all,  long-time lurker with nothing to contribute until now.

In my youth I spent long hours getting scanning electron microscopes
and e-beam lithography instruments to work.  The key number here
is the mean free path of an electron for a given pressure.   It's
the distance, on average, that an electron will travel before slamming
into a gas molecule.  You want that distance to be greater, preferably
much greater, than the distance between the anode and cathode in
your tube. 

At 30,000 feet, my back-of-an-envelop calculations
show the mean free path to be about 1/100,000 of an inch.  So I
don't think that would work. 

 From "Building Scientific Apparatus" 3rd ed.

Pressure (torr)                      Mean Free Path (cm)
 760 (atm)                               7x10^-6
 10-3                                       5
 10-6                                       5x10^3
 10-9                                       5x10^6