Ok, here is my guess:

On 6/25/05, *John Slade* <sladerj@bellsouth.net <mailto:sladerj@bellsouth.net>> wrote:

    OK, Dave. If that makes sense, explain this.... :)
        After start-up - 29 psi (24 psi cap)

 
coolant is expanding rapidly.  The pressure rating of most caps is a rough estimate at best.  It is determined by the metal spring.  That metal spring will also change the force it applies based on it's temperature (though I am not sure which direction and how much).

        Run-up  25 PSI

 
Coolant is still expanding though possibly at a slower rate and/or the spring is now also warm and exerting less force.

        Two trips around the pattern - zero PSI the entire time

 
Now you are getting good air flow after climb out and the temp drops a little from its peak value, so the volume of coolant contracts and sucks a little back in from the overflow - zero psi.

        third downwind - 8 PSI.

 
Now you have reached steady state temperature, or perhaps a second slight rise in temp as the overall under cowl temp rises (all heat sinks under the cowl become saturated with heat).   Also, there is continuous vaporization of the coolant, which rapidly re condenses after leaving the hottest areas. The amount of this vaporization is a function of the pressure and temp.  So it will seek some equilibrium at a pressure that is greater than zero.  ie. more vaporization increases the pressure, which in turn limits amount of vaporization.

        taxi back to hangar - 10 PSI.

 
Perhaps slightly warmer now (especially in those backward flying airplanes) and the vaporization:pressure equilibrium has shifted a little.

        After cool down - coolant still full to the brim.

 
Yea, no reason for me to think you have a leak.   (I can convince myself of anything  :-)

-- Dave Leonard
Turbo Rotary RV-6 N4VY
http://members.aol.com/_ht_a/rotaryroster/index.html
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