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Re: Spark length dependence on air pressure.

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 11:02 AM 6/14/2005, you wrote: 
Original poster: "Mike" <mikev@xxxxxxxxxxxxxxxx>

Hi Bob,
              I remember reading in the early days when they wanted to
determine this answer, there was a difference from sharp pointed electrodes
and round ball type, also the material made a small difference.
Not knowing how you were going to use or apply the answer, I opted to pass
this question to a friend of mine who specializes in atmospheric electricity
at MIT. I also needed the breakdown at 15 Km so this reply was a two fold
benefit. Here is the answer to your message I had forwarded to him.
Regards,
Mike
Hi Mike
The equation is

Eb   = 3 x 10^6 v/m    exp(-z/7)

where z is height in km
(7km is the density scale height of the atmosphere and density declines
exponentially with altitude.)

So at z=0   Eb = 3 x 10^6 v/m
Mike wrote:

>Hi Earle,
>                  For a DC model, can you answer this question? I need to
>know for myself the breakdown voltage at 15 Km  (I think that is ~103 Torr)
>anyway so this is not a wasted effort. Is there a link to reference on
this?
>He wants the equation.
>Regards,
>Mike


A handy way to remember this sort of thing is that the "halving" elevation is 18,000ft. At 18,000 ft you're at half sea level pressure. Another 18,000 and you're at 1/4th, etc.

7.7 km might be a bit closer to the real constant for the exp(-z/k) term.


The 3MV/m breakdown is for uniform field.  Spheres and needles are much lower.



----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, June 13, 2005 3:55 PM
Subject: Spark length dependence on air pressure.


> Original poster: Robert Clark <bobbygc2001@xxxxxxxxx>
>
>   I've seen for example the Freau Equation for
> predicting spark length:
>
> Thoughts on spark length and the "Freau Equation"
> http://www.pupman.com/listarchives/2001/June/msg00433.html
>
>   But since the breakdown voltage in air depends on air
> pressure (or is it really air density?), what is the
> equation showing the dependence on air pressure (or
> air density)?
>
>
>    Bob Clark
>
> _
>
>
>