Measuring water level reliably in tanks under pressure

Can you help me understand physical meaning of measuring water level
by measuring differential pressure?

In open water tank it's fairly easy. One side of the transmitter is
open and exposed to athmospheric pressure. In such case, differential
pressure is directly proportional to water level: deltap = density * g
* level, and simply I can setup appropriate scale for instrument.

In case if tank is closed under high pressure and temperature such ase
steam boiler drum, level is measured with differential pressure
transmitter. Tankage is made of steam and water, in upper part is
steam and in lower part is water. One transmitter pipe (steam side -
upper) has some kind of vessel whose purpose is to condensate steam
int water, so practically both pipes to transmitter is filled with
water. Lower transmitter pipe has direct connection to the tank.
Principle is fairly easy, differential pressure that is difference in
pressures between both transmitter pipes is directly related to the
water level. If water level is higher, differential pressure is
lower.
Now in order to measure it reliably this deltaP information needs to
be corrected regarding pressure and temperature in the tanke, because
density of water (and steam) is changing.
Can you suggest me link where can I find these informations in order
to calculate level correctly?

I hope, I was clear and you'd understand my question.

Thanks