On Fri, 4 Aug 2006 07:58:37 -0700 Peter writes: >Hi Jim; >I understand arcing at standard Temp. and pressure. I thought that >the reduction of media would produce an increase of voltage Vs. distance
>(vacuum relays and Caps.) 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. See <http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=TEXT%3ALIST&YEAR=2006&MONTH=08&FROM=0412&TO=0412&STNM=72240> for a representative chart of pressure, altitude, and other meteorological parameters. The vacuum relays and caps incorporate some pretty hard vacuum, I'd surmise. Pretty bitter pill to swallow... paying premium bucks for a better grade of nothing :^)) So, if your working around higher voltages than most of our QRP gear, I guess it's best not to inhale :^)) 73 Jim N5IB _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
