Re: Spark length dependence on air pressure.

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Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Ed,

Yeh, I think your right.  That was too easy.  I'm looking at standard 
atmosphere tables and the data is represented in graph form.  It looks like 
one can use the following formula for absolute pressure vs density altitude:

Pressure(altitude) = Pressure(sealevel) * 10^(-density_altitude/55700 ft)

I took the slope off of a semilog (base 10) plot of pressure vs altutude in 
the CRC Handbook of Chemistry and Physics 49th edition.  This slope seems 
constant up to 100km of altitude.  The lapse rate seems standard up to 10km 
of altitude so I guess the equation should not be used above 10km.  The 
altitude in the graph is in geopotential units (corrected for variation of 
gravity vs altitude).  So unless you are wanting the pressure in the 
stratosphere, I guess one could use density altitude.  This equation 
predicts the pressure at ~18000 feet to be half sea level pressure.  I 
restated the exponent constant for feet (vs km).  If you prefer km then use 
17km in place of 55700 ft. Also use what ever units of sealevel pressure 
you wish and the pressure at altitude will be in the same units.

Gerry R.


> Original poster: Ed Phillips <evp@xxxxxxxxxxx>
>
> Hi Jim,
>
> Another thought.  Instead of using a look up table (probably the most
> accurate) one could use the following exponential equation using 18000
> feet
> as the half pressure point.  Note, I use "2^" cause this is the half
> pressure point and this equation would predict that at 36000 ft the
> pressure would be 1/4 of sea level and I dont know if this is true.  Im
> being somewhat lazy here cause I dont want to look it up.
>
> Gerry R.
>
> The problem with that approach is that the temperature isn't constant
> over the region, but instead goes through several different regions.
> Colder and colder until you get into the stratosphere, and then things
> can change a lot.  It's a lot better than nothing but the normal
> "standard atmosphere" charts include nominal temperature variation with
> altitude.
>
> Ed