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Re: [TCML] Terry Filter, Saftey Gap Distance?


-----Original Message-----
>From: ConorPerry@xxxxxxxxxxx
>Sent: Jun 14, 2008 9:04 AM
>To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>Subject: Re: [TCML] Terry Filter, Saftey Gap Distance?
>
>Thank you for your info.  I'm curious how you came up with your 11/64" 
>(0.172") gap distance?  How you mention the atmospheric effects I wonder if 
>it's more from empirical data than by calculations.  Are you setting the 
>gaps just larger than they will fire without the primary?

ALmost certainly empiricism (i.e. try it and see)

Breakdown voltages are very hard to accurately predict from the physics and geometry.  There's a lot of other factors involved.

>
>When I initially calculated for mine I used the 3,000,000 Volts/Meter as the 
>dielectric of air (
>76,200 Volts/Inch).  My 12/100 NST with a Variac is guessed to have an 
>output of 14kV.  So I set my gaps just above that voltage at around 8kV per 
>side, which is a distance of 0.11".



30 kV/cm is a nice approximation for the breakdown of air in a uniform field.  However, there are some gap length effects (very short gaps have higher breakdowns than you'd expect), and it's rare that you'd actually have a uniform field.

>
>Then I run your numbers with the same math, but with a 15kV NST on a Variac 
>to make 17.5kV.  Your gap of 0.172 calculates to 13kV per side, and 26kV 
>total.
>
>So am I set too conservative, or are you too high?  Do your safety gaps ever 
>fire?  Maybe my assumption of using the 3,000,000 V/m is unfounded (since 
>that's "standard air" at 1atm.)

Ah.. but he's not using a uniform field gap.  You'd have to look at the gap geometry and see what the actual field is. 

See, e.g., http://home.earthlink.net/~jimlux/hv/paschen.htm or http://home.earthlink.net/~jimlux/hv/sphgap.htm

For spheres that are large compared to the gap (e.g. 10cm diameter spheres, 1 cm gap) there is a large body of empirical data that can be used to make "calibrated" spark gaps with known breakdown voltages (to roughly 5% accuracy)

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