The basis of hydrostatic pressure measurement for the level is such that the measured pressure is proportional to the height of liquid in the tank, irrespective of volume. The pressure is related to the height by the following:
P = h.ρ.g
where: P = pressure
h = height
ρ = relative density of the fluid
g = acceleration due to gravity
For constant density, the only variable that changes is the height. In fact, any instrument that can measure pressure can be calibrated to read the height of a given liquid and can be used to measure liquid level in vessels under atmospheric conditions.
Hydrostatic pressure transducers always consist of a membrane that is mechanically or hydraulically connected to a transducer element. The transducer element can be based on technologies such as inductance, capacitance, strain gauge or even semiconductor.
A pressure sensor is exposed to the pressure of the system, and therefore needs to be mounted at or near the bottom of the vessel. In situations where it is not possible to mount the sensor directly in the side of the vessel at the appropriate depth, it can be mounted from the top of the vessel and lowered into the fluid on the end of a rod or cable. This method is commonly used for applications in reservoirs and deep wells.
If the use of extension nozzles or long pipes is unavoidable, precautions are required to ensure that the fluid does not harden or freeze in the pipeline. The sensor must not be mounted directly on the product stream, as the measured pressure will be too high and the reading of the level will be inaccurate.
- Level or volume measurement
- Simple to assemble and install
- Simple to adjust
- Reasonably accurate
- Dependent on the relative density of the material
- More expensive than simpler types
- Expensive for high accuracy applications