You're bound and determined to design and install a coolant level
indicator/alarm.
I'm confident you'll come up with a successful design and installation.  I wish
you well.  I personally see no point in it.  I've read a couple of thousand
words of deathless prose to the effect that some of us don't understand your
scheme (and would get on board if we did?) and how air and coolant and foam
behave in the system and how you can overcome all the obstacles to implementing
a reliable system.  What I haven't heard is how, when the smoke clears and the
dust settles, your scheme is going to improve on the simple, reliable, proven,
inexpensive pressure and temperature indications already in use.  My original
question was "... what will a coolant level indicator, if/when finally
implemented, buy us that we don't already have... "  I still haven't heard an
answer.  This whole thread is IMO an exercise in P.V.O.R.T  (Pole Vaulting Over
Rat Turds) aka "... chasing foul balls ...".
More complex, needs development, adds no discernible value .... Jim S.

David Carter wrote:
This is a case where "what actually happens" after the "blowby gas leak

> starts", is dependent on the size coolant hose running out of the bottom of
> "pressurized coolant expansion/header" tank:  If the hose is "large", like
> in the Ford (1" or almost full "radiator hose dia"), then the blowby gas
> would come UP that "coolant normally DOWN" hose - and over-pressurize the
> expansion tank's pressure cap and release pressure.
>     -  Now, with no leak, just infusion of hot, high pressure gas, the first
> indication would probably be a forcing of coolant up the pipe and out past
> the pressure cap, and thence out the vent line routed to come out in pilots
> view at aft side of engine cowl, and he'd see the brief spurt of coolant and
> be warned.  That should be a nearly instantaneous indication.
>     - Next/nearly simultaneously, whatever qty of coolant was expelled out
> of engine block & thence out past pressure cap and windscreen, would be
> replenished out of the expansion/header tank (reason I keep calling it a
> header tank - it replenished and keeps head of pressure on pump).
> .. . . . . . . . .--  There might, or might NOT, be enough coolant expelled
> to cause the low level light to come on.  If only a slug of coolant was
> ejected and pilot missed that, then the leaking gas might mix with water and
> simply circulate and be, by design, trapped off by the 1 or 2 "vent lines"
> previous proposed, and wind up in the pressurized expansion tank and simply
> blow out as "gas", not "coolant".  Thus, there would be no more lose of
> coolant, only a continued infusion of leaking combustion gas mixed with