Atmospheric
pressure
Updated November
16, 2005
New for December 2005!
This page is part of a larger discussion of power handling capability.
Atmospheric pressure is important because lower air pressure means
that voltage breakdown occurs at lower electric field strength.
There are many units of pressure,
including millimeters of mercury (same as a Torr), inches mercury,
inches of water, Torr, Pascals (and kiloPascals and megaPascals
because a Pascal is so tiny). Let's review some of the conversions:
1 atmosphere=760 Torr (inches
Hg)
1 atmosphere=14.69 pounds
per square inch (PSI)
1 atmosphere =101325 Pascals=101.325
kiloPascals (kPA)
1 atmosphere= 1.01325 bar=1013.25
millibar (mbar)
Now you can do your own conversions
if you want to convert mbar to Torr for example. Altitude can be
measured in feet, miles, meters, kilometers. One foot is 0.3048
meters.
The equations for pressure versus
altitude are complicated, and the best resource for this is NASA,
and we don't have them handy so they won't be presented here. Here
we have downloaded data from the atmos.com web site in millimeters
of mercury, then we converted the data to atmosphere and millibars.
Now we will show two plots of
atmospheric pressure versus altitude. The first one is for fans
of the metric system, in mbars versus meters:
The second plot is for fans
of American units, in atmospheres versus feet:
Here's a "bonus" plot,
where we've graphed the pressure on a log scale. You can see that
it is almost a straight line, which implies that it is a negative
exponential function. In reality it is a piecewise fifthorder polynomial
(maybe we'll put the equations up some day).
Now it's time for a Microwaves101
rule of thumb:
At 100,000 feet, pressure is about 1% of what
it is at sea level. For all intents and purposes, if your system
has to operate at 100,000 feet, you can consider that it must perform
in a vacuum (zero air pressure).
