Homebrew PCBs

The 30 Hg is the height in inches of a column of mercury supported by 
the vacuum (980 millimeters, for you metric buffs, , IIRC) -   NOT the 
weight in pounds. That corresponds to 14.7 lbs per square inch (sea 
level, normal temperature).  You can't do better than 1 atmosphere of 
vacuum, even in outer space!

On the other hand, for outgassing resin, you don't need near perfect 
vacuum; even a 5 pound vacuum will draw out lots of bubbles.  Just use a 
less viscous resin mix, or pump longer.  Also, periodically relieving 
the vacuum and then pumping some more seems to help.

Norm

rruss45826@... wrote:

> 
>In a message dated 5/6/2005 1:21:48 A.M. Eastern Daylight Time,  
>robert.hedan@... writes:
>
>How can  you pull the vacuum down 2 atmospheres?  Assuming I am at  1
>atmosphere, isn't 0 a vacuum?  Or did I miss something and you can a  bigger
>vacuum?  Huh?  How can you go to negative  atmosphere?  I thought atmospheres
>were mostly used for pressure  chambers.
>
>I often see reference to 0-30Hg for vacuum chambers,  something about mercury
>level.  I figure I'll see where the vacuum  dial sits at rest, and where it
>goes as air is pumped out is the vacuum  side.
>
>Robert
>:)
>
>
>
>Most vacuum is measured in HG which is mercury. being so heavy  you can use a 
>much smaller scale! Now if I remember my old school books normal  atmosphere 
>at sea level is about 15lb's per sq inch. Now 2 Ats. would be 30lbs  per sq 
>inch. No go to vacuum and the same applies but in the opposite. So 30HG  vacuum 
>would be 2 Ats. When I do A/C work this is the standard test we do. Draw  down 
>to as close to -30 hg and cap it off. wait 1 hour and see if we have any  
>leaks then charge the system.
>If I did not get any of the math right I am sure someone will  jump in and 
>correct it! High school was a long time ago! LOL
>Just putting the mold under 1 At or -15lbs  of vacuum  will do the job you 
>need to do!
> 
>Ray Russell
>NMRA-40609
>General Contractor
>Norfolk &  Western Railroad
>Pocahontas Division
>Circa 1958
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>
>OR_ Click here:  Pocahontas Home_ 
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>For the old website!  
>http://milliron.home.sprynet.com/Pocahontas/Pocahontas1.htm
>
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