Traditional liquid level sensors based on pressure (diaphragm), mechanical or magnetic floating devices have moving parts in them. These moving parts make them less rugged, dependable and reduce their useful life.
Conductivity probes have no moving parts but are limited to conducting liquids and cannot be used for hydrocarbons. But more recently with the advent of electro optic systems, level sensing technology has undergone a sea change.

These systems are simple, solid state devices with no moving parts and are economical, compact in size and lightweight.

The electro optic systems based on infrared technology have an infrared emitter and an optical receiver.

Infrared radiation is radiation lying between visible and radio waves with wavelength varying from 0.7 to 1000 microns.
When only air is present in the object to be sensed, all the light emitted is reflected internally by the dome of the probe and returned to the receiver. When there is liquid, a large amount of rays emitted is absorbed / refracted by the liquid.
As the amount of rays picked up by the receiver reduces when liquid is present, the IR sensor senses the presence of liquid.
The response time of these sensors is very fast, almost instantaneous and detection accuracy is +/- 2 mm.

IR sensors can be mounted in any angle and top, side or bottom fitted.
They can be used for a variety of liquids including water, oil, CO2 etc. They have long life and are reliable.
They can withstand extreme temperatures, pressures, vibration and shock.

They require very little maintenance except for occasional cleaning of the photosensitive dome for removing any solid deposits using mild detergents.
They can output 100mA DC or triac 200mA AC current depending on input.
They can be directly connected to a PLC system for further processing of the output signal.
They have a wide operating temperature range from - 40 deg C to +125 deg C.

The IR sensors are not affected by the liquid hardness or conductivity and hence can be used in any geographical area without any alteration.
They do not require recalibration.

They can sense the levels of clear, translucent, reflective, as well as opaque liquids.
These liquid level sensors have a programmable timer inside for delaying the output switching. It increases the reliability of the output and takes care of any transients in the input like waves, bubble, foams etc.