Certain fluid properties can be readily measured or used to determine the presence or extent of a known fluid. The presence or absence of a fluid can provide an on-off signal indicating whether a certain level has been reached or not, whereas the extent of a fluid can be used to determine the depth.
Conductivity:
If a liquid is a conductor of electricity, its presence can be detected by a pair of electrodes subject to a potential difference. When immersed on rising level, they can generate an off-on electrical signal.
Capacitance:
If a liquid is a dielectric, probes can be inserted into a tank and the capacitance between them measured. This will vary with the degree of immersion and can be converted to a measurement of level.
Acoustic:
Most liquids conduct sound waves readily, and these are reflected from any interfaces, including the liquid surface. If an acoustic transmitter-receiver located at the bottom of a tank directs sound waves vertically upward and senses their reflection from the surface, the depth, and hence level, can be determined by the time taken for the sound wave to travel up and be reflected down (Fig. 2d).
Nuclear:
Since gamma rays are absorbed by many liquids, the presence of liquid can be sensed by the attenuation of gamma rays emitted from a gamma-ray source and measured by a detector a short distance away.
Thermal:
If an electrically heated thermistor is subject to immersion in a liquid, it will be cooled more effectively. The resulting drop in temperature will be reflected as a change in resistance which will indicate the presence of the liquid.