Correct. Pressure and density are not the same thing. But if we are
talking about arcing, should we be more concerned with pressure altitude, or
density altitude? A higher than standard temperature can make a huge
difference in the two.
As an old pilot, I had to ask!
Dan Allen
KB4ZVM
K-2 S/N 1757
Air is a dielectric. While its dielectric *constant* is almost identical
to vaccum, its dielectric *strength* (volts per distance) decreases with
density (pressure) until you get into some really hard vacuum situations.
See <http://www.isi-seal.com/searchs/doc/DielectricStrength.htm> for a
chart.
For air at STP it's about 3000 volts per millimeter
In interpreting that chart, remember that standard atmospheric pressure
is 760 torr, and at 2000 meters altitude (the notorious 6500 ft) the
pressure is about 600 torr. Pressure and density aren't linearly related
because of the temperature drop as you go up, but its a decent
approximation for the first couple thousand meters or so. By the time you
reach 300 torr, at about 7500 meters, the dielctric strength of air is
degraded by about a factor of 2, and by 30,000 meters (10 torr) it's
degraded by about 90%. We routinely fly our balloons above 38,000 meters.
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