[s-cars] Blown Hoses, MPG's & Cloaking Device Failure

[Djdawson2 at aol.com]

In a message dated 4/10/2005 2:44:24 PM Pacific Standard Time,  
manuelsanchez at starpower.net writes:

The  boost side is marked in psi, I seem to understand that
one.  The  vacuum side is measured in "in of Hg".  So is 20 "in
of Hg" the exact  opposite of 20 psi, or are they completely
different scales?  
Yes and no... here's the poop in its simplest form.  Atmospheric  pressure, 
at sea level, is about 14.7 psi.  If by some means you reduce  this pressure in 
a confined space, it is referred to as vacuum.  Vacuum is  a state of 
pressure in a confined space that is lower than the atmospheric  pressure outside the 
confined space.  Inches of Hg means inches of  mercury... Hg being the symbol 
for mercury in the periodic table  (chemistry).  Atmospheric pressure 
(14.7psi) is equal to about 29.92 inches  of mercury.  Since that is the highest 
"inches of mercury" reading you can  have in earth's atmosphere, the same remains 
true for vacuum.  A total  absence of pressure would equal a reading of 29.92 
in of hg on your  gauge.  Most high performance vacuum pumps (like ones used 
to service A/C)  can only achieve about 26 inches of Hg.
 
Another interesting tidbit... did you know that you can boil water at room  
temperature?  It won't even be hot... all you have to do is apply a vacuum  to 
a confined space that contains a glass of water.  The reverse is true as  
well.  If you raise the pressure in a system, you will also raise the  boiling 
point of water.  This is why our cooling systems are under  pressure... boiling 
point is higher.


Another oddity is that on the vacuum side my VDO gauge marks
30  in of HG while the boost side only reaches 25 psi.


Hopefully you know why now.
HTH,
Dave in Idaho (actually SLC this week)