While my system is laid out somewhat differently (not withstanding
the EWP), I too have 2 pressure caps, one 22psi cap on the system and a 15 psi
cap on the overflow bottle. My overflow bottle is a new snowmobile coolant
bottle that I had surplus and it happened to have a 1bar cap one it. Now I had
expected to see compound pressures on my system (22+15=37), but have never seen
greater than 23 psi. (On my latest flight it appears that the coolant press reg
has failed as it read 0 psi even when I loosened the cap and found plenty of
pressure).
In your post you state that it is possible to reach 38 psi, then
you go on to say that on take-off your system is at least 23 psi. So my
question is have you ever seen any compound pressures in your system (greater
than 23 psi ???)
Yes; both during pressure test and
during operation. During a pressure test, the gauge on my little battery
powered air pump read 40 psi when air was escaping from the overflow bottle. Unless
something else is happening, the pressures are additive. The cap controls
the pressure difference between what is under the cap, and what is downstream
at the overflow. The overflow from my filler neck cap connects to the
overflow bottle. When there is no pressure in the overflow bottle the system
pressure goes up quite rapidly to about 23# as the engine warms. As more
coolant is forced into the overflow bottle, and that pressure builds, the
system pressure increases.
I haven’t bothered to observe; but
as the coolant temp drops, I expect the pressure drops quite rapidly to about
the pressure that is in the overflow bottle.
Al