Hydrostatic "Head Pressure" measurement is achieved by one of two basic methods; Direct Reading, side mounted or submerged sensor or Pneumatic Signal Reading (Bubbler). Each method has application attributes that will determine which is more appropriate for a given liquid and site.
Direct Reading, as the name implies, uses a sensor in contact with the liquid. A Pneumatic Signal Reading (Bubbler) is a well proven, simple non-contact approach that provides equal precision.
Imagine a glass filled with water and a drinking straw inserted. The water level in the straw will be equal to the level in the glass. If we were to blow into the straw, pressure builds in the straw pushing the water out until excess air pressure bubbles into the glass. The pressure created in the straw is equal to the head pressure of the water in the glass. It is a simple matter to convert the created pressure to a desired unit of measure from wherever the water level may be in the glass.
The glass, of course, is your tank. The straw is called a "pressure pipe" and the pressure from the level of the liquid in your tank is called the "Head Pressure". This simple analogy outlines the basic operation concept.
When a pressure pipe is inserted into the liquid and air or inert gas (B line) is used to purge the pipe, pressure in the pipe will increase only until all the liquid is evacuated. Thereafter, any additional air will bubble out freely through the opening at the bottom of the pipe. The resulting head pressure of the liquid returns to the (A line) of a Type "D" Tank-O-Meter® and raises the indicating digits to the corresponding depth of the liquid.
Digital Tank-O-Meters® convert either the resulting (A line) head pressure or the directly sensed hydrostatic head pressure into an electrical signal. The electrical signal is then conditioned/scaled and, if desired, compensated for density and/or non-linear tank configuration. Any unit of measure may be used and displayed by an easy-to-read 4 1/2 digit display.