Pressure P(liq) on any surface 
and container walls at depth h,
by the liquid of desnity d, is:

P(liq) = d × g × h + P(air)

where P(air) is the air pressure
and g is the acceleration due to
gravity (constant for a given
place) at the place of liquid
container. Replacing constants
the equation becomes:

P(liq) - P(air) = K × h

in short: Pressure difference
represents liquid level. One
convenient unit that clubs
pressure with level is mHO
(pressure felt at depth inmeters
while being immersed in water)
Trumen hydrostatic pressure
transmitter utilizes pressure
exerted by liquid P(liq) and
substract it by air pressure
P(air) using a single pressure
sensitive diaphragm and
air-vent in connection cable.
As Trumen hydrostaticpressure
transducer is immersed deeper
in the liquid, the P(liq) becomes
higher than P(air) and the
diphragm minutely deforms.

Thus pressure exerted by liquid is sensed by Trumen hydrostatic sensor which is directly denotes the depth from the surface of liquid.