Re: Break-down voltage of gaps and humidity

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 07:54 AM 11/14/2003 -0700, you wrote:
>Original poster: Tesla729-at-cs-dot-com
>In a message dated 11/13/03 4:43:09 PM Pacific Standard Time, 
>tesla-at-pupman-dot-com writes:
>>Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com>
>>
>>Regarding humidity and the effect of it on break-down voltage:
>
>
>Dan, all,
>
>You bring up a good question that I have also pondered. It is
>commonly accepted amongst the list in general that cold, dry
>winter days are the best for maximum performance from a
>Tesla coil, so it would seem that Kuffel's statement would
>be true, if this is the case. Cold air has higher pressure
>since it's denser (and I thought increased pressure suppressed
>corona)

Cold air may result in a higher pressure, and definitely changes the change 
in pressure with altitude, but the pressure is the pressure, regardless of 
temperature.  Now, as for density, though... At a given pressure, lower 
temperatures will be denser, which would increase the breakdown strength. 
(that PV=nRT thing)




>and it definitely has a lower dew point since the dew
>point can only reach as high as the ambient air temperature,
>and that's only when the RH (relative humidity) is 100%. When the
>relative humidity is < 100%, then the dew point is less than the
>ambient temperature. It's obvious that 90* F summer air can
>hold much more latent moisture before saturation (100% RH)
>than 30* F winter air, but warm sticky air is supposed to be the
>worst conditions for coiling.

I don't know that warm and sticky is bad for coiling.. Certainly, if you're 
running an electrostatic machine, or shuffling feet across the carpet, low 
humidity is important, but that's a surface resistivity/leakage effect, not 
a air breakdown effect. Here in Southern California, we occasionally get 
very low humidity and high temperatures simultaneously, and my empirical 
observation is that electrostatic effects are quite pronounced.


>There does seem to be a contra-
>diction here, when you consider that moisture as well as lower
>pressure air (at least until a hard vacuum is reached) are sup-
>posed to lend theirselves to better conductance than dry, mois-
>ture starved air and air that is under greater pressure.

Don't confuse conductance/resistivity and breakdown voltage.  They're 
different effects.



For what it's worth, I'm not sure that the breakdown voltage of air has all 
that great an effect on the "performance" of a tesla coil.  There's a lot 
of other factors that probably dominate (surface roughness of the top load, 
surrounding objects that perturb the electric field, movement of the 
air).  The effect of humidity on breakdown is way down in the noise (< 1% 
off the top of my head).