[Elecraft] Multimeter Altitude Limits

[Dan Allen]

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.
>