[Milsurplus] How Much Vacuum in "Vacuum Tubes?"

[Bob Camp]

Hi

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.

CRT's came into being long before high vacuum was a reality. The  
electron beam looks a lot like a light saber flying around under  
magical control. It's a very cool demo.

So much fun ...

Bob





On Apr 1, 2007, at 3:39 PM, Gregory W. Moore wrote:

> GA, All
>
> While I have never made a "circuit usable" vacuum tube from  
> scratch, I did, in the early 1970's make a triode
> out of parts salvaged from several different tubes, blowing the  
> glass envelope myself, and using the services of
> a CRT rebuild house to pull the vacuum on said tube.  Now, the tube  
> did not have a getter, but I managed
> to make a octal base which was similar to another triode ( now that  
> I'm 59 I forget just what I used) but It
> tested "good" in a transconductance tester, and probably would have  
> worked in a circuit, except that the
> process of testing, I managed to burn the filament out testing  
> higher filament voltages.  SO, I would imagine
> the concept would have been viable back in the day, I wish I could  
> have used the homemade tube in an
> actual application to see if the gain, and current curves would  
> have matched a commercial manufactured
> tube.