HIGH VOLTAGE BREAKDOWN vs. ATMOSPHERIC PRESSURE
Ralph Wallio, WØRPK   W0RPK at  netINS.net

During a recent TVNSP high altitude balloon mission in Idaho ( Treasure Valley Near Space Project - http://www.tvnsp.org/ - Flight TV01F on 11Aug01 ) the conventional B&W CRT-based viewfinder on an 8mm camcorder was found to be defective when the payload was recovered. Maximum altitude for this flight was 85,400ft ASL. The remainder of the camcorder worked AOK so there was immediate conjecture that this failure was associated with CRT high voltage vs. high altitude. The following discussion investigates this possibility.

Standard spark-gap breakdown voltage is 75KV/inch at standard temperature and pressure, 25dC and 760mm hg (aka 760 torr).  Within the pressure and temperature ranges we operate, breakdown voltage is approximately proportional with pressure and, to a much lesser degree (pun intended), inversely proportional with absolute temperature (dK). Multiplying factors for temperatures and pressures other than standard are available in engineering handbooks such as Reference Data for Engineers: Radio, Electronics, Computer and Communications, (Sams) page 48-4.

As a high altitude balloon payload rises it encounters both lower pressures and lower temperatures.  These values are from U.S. Standard Atmosphere, 1976:

Changes in multiplying factors for temperatures are small, on the order of 0.01/10dK, so we will ignore them.  The impact of changes in pressure is much more significant as shown:


  At 44Kft ( approximate maximum altitude of a more recent TVNSP mission when the viewfinder did not fail ) breakdown voltage drops from 75KV/inch to 33KV/inch.  At 80Kft ( approximate altitude of viewfinder failure ) breakdown voltage drops to 10KV/inch.  At 100Kft breakdown voltage drops to roughly 4KV/inch. It is therefore possible that the viewfinder suffered high voltage arc-over when breakdown voltage was reduced by higher altitude and lower pressure from approximately 75KV/inch to 10KV/inch.

Viewfinder high voltage is only a few to several hundred volts but inter-trace gaps on circuit boards and internal wiring are substantially smaller than an inch. Assuming 500V, the breakdown gap at STP is roughly 6.6 mils ( thousandths of an inch ). At 44Kft and 117 torr the 500V gap is 15 mils. Given a maximum rated operating altitude of 44Kft and a 100% safety factor, the circuit board designer would have allowed 30 mils. At 80kft ( approximately where the viewfinder failed ) the 500V gap is 50 mils, almost twice what the designer would have supplied.

Further left on the chart are pressure ( vacuum ) ranges where spark gap breakdown voltage increases significantly. It might be interesting to compare pressures encountered by high altitude payloads with those associated with various grades of vacuum:

Note from the chart that breakdown voltage does not start to increase until roughly 4 x 10-1 torr that corresponds to approximately 180,000ft ASL. The vacuum variable capacitor in my DIY HF antenna tuner was manufactured with a vacuum rating of 10-7 torr which yields a breakdown voltage approaching 1000V/mil (1MV/inch). This is off our chart but see http://www.jenningstech.com/technotes/dielect.shtml for more information.

We are only scratching the surface of high voltage topics here. Much more indepth discussions of the physics of high voltage are available on the web. As an example, see information from Jim Lux, W6RMK, starting at http://home.earthlink.net/~jimlux/index.htm (home page) and then http://home.earthlink.net/~jimlux/hv/hvmain.htm (High Voltage Experimenter's Handbook) and then http://home.earthlink.net/~jimlux/hv/paschen.htm (Paschen's Law).

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