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 05:24 PM 11/13/2003 -0700, you wrote:
>Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com>
>
>
>Regarding humidity and the effect of it on break-down voltage:
>
>In testing, I (as well as others) may have noticed that corona discharge
>increases from a high voltage point in the
>presence of increasing humidity.
>
>However, in Kuffel's High Voltage Engineering text, he states that the
>break-down voltage between two gaps will
>actually increase with increasing humidity.
>
>This seems to contradict what I have experienced.  Can anyone comment on
>this?
>
>Thanks
>Dan


Water vapor has a higher breakdown strength than air, so a mixture of water 
vapor and air (i.e. higher humidity) has a higher breakdown voltage, 
especially if you normalize for density (water vapor is 18 (g/22.4 l) vs 
air is 29 (g/22.4 l)), which is what the breakdown curves really are in 
(the Paschen relation is really in terms of the product of density and 
distance). H2O also recombines very quickly after dissociation, which 
increases it's breakdown strength (less likely that there are free ions 
floating around to support an avalanche)

However, surface resistivity decreases with increasing humidity, so leakage 
and flashovers are more common.  Also, if there are small salt or dust 
particles on the surface, as the humidity increases, they get bigger 
(because they absorb the water vapor) and provide sites from which 
breakdown can start.