Hi Georges;
I was going to try to draw a diagram, but I think Georges has
explained it pretty clearly. Let me know.

Todd

Subject: [FlyRotary] Re: Radiator Expansion Tank

Starting with the cap referenced to the atmosphere, it will relieve
itself at it's rated pressure (above the reference pressure).  Let's say
it's rated at 15 psi. At sea level, the absolute pressure would be
15 + 14.7 = 29.7 psi.  But A pressure gauge is typically referenced to
ambient and would read 15 psi.  This 15 psi is the reference for the
second cap. If it were a 15 psi cap, it would relieve itself at 15 psi
above it's reference pressure or 30 psi.  The absolute pressure at sea
level would be 30 + 14.7 = 44.7 psi.

The actual pressure in the system depends on the temperature of the
coolant (vapor pressure).  The vapor pressure of water at 230 F is
around 21 psi. That's absolute pressure, so a 22 psi pressure cap should
not open at 230 F even in outer space.

Bob W.