A Guide to Liquid Level Sensors

Liquid Level Sensors

Image Credit: GasLab

Liquid level sensors, also known as liquid level switches, have been developed to change state when submerged in a liquid. The sensors are used to verify if a liquid or oil exists at a specific level in a container.

What are the Various Types of Liquid Level Sensors?

The various types of liquid level sensors include:

  • Optical
  • Capacitive
  • Conductive
  • Diaphragm
  • Float

1. Optical Liquid Level Sensors

Optical sensors use an infra-red LED and phototransistor which are optically coupled when the sensor is in air. When the sensing tip is submerged in liquid, the infra-red-light escapes which makes the output change state. These sensors can distinguish the presence or absence of almost all liquids. They are not responsive to ambient light and are not affected by small bubbles when in liquid or by foam when in air.

This makes them beneficial where the state change must be rapidly and reliably recorded. They can also operate dependably for long periods without requiring maintenance.

Optical Liquid Level Sensors

Image Credit: GasLab

For example, the SST Optomax Digital Liquid Level Sensor is a widely used low-expense, low-maintenance infra-red LED sensor.

The major drawback of an optical liquid level sensor is that it can only ascertain if liquid is present or not present. If different levels are required, (25%, 50%, 100%, etc.) each would need an additional sensor.

2. Capacitive Liquid Level Sensors

Capacitive liquid level switches use two conductive electrodes (normally constructed from metal) in a circuit that are a small distance from each other. When the electrodes are submerged in a liquid the circuit is complete.

A benefit provided by a capacitive liquid level switch is that it can be used to establish the rising or falling of liquid in a container. By ensuring the electrodes are an identical height to the container, the capacitance between the electrodes can be measured.

If there is no capacitance there is no liquid either. Conversely, full capacitance means a full container. Both “empty” and “full” measurements must be noted, then a meter calibrated with 0% and 100% to illustrate the liquid level.

Capacitive Liquid Level Sensors

Image Credit: GasLab

Although capacitive liquid level sensors benefit from having no moving parts, one of their pitfalls is that electrode corrosion will alter the electrode’s capacitance necessitating either recalibration or cleaning. They are also more responsive to the kind of liquid used.

3. Conductive Liquid Level Sensors

Sensors with an electrical contact at a specific liquid level are known as Conductive liquid level switches. Two or more insulated electrodes with exposed tips are used inside a pipe lowered into the liquid. A long electrode carries a low voltage, while a shorter one completes the circuit when the liquid level rises to meet it.

Conductive liquid level switches rely on the conductivity of the liquid in the same way as capacitive liquid level switches. Therefore, they are only useful for measuring certain kinds of liquids. Additionally, in order to reduce fouling these sensors tips must be cleaned frequently.

4. Diaphragm Liquid Level Sensors

Diaphragm or pneumatic liquid level switches depend on air pressure to push a diaphragm which interacts with a micro-switch inside the body of the unit. As the liquid level increases, the internal pressure inside a detecting pipe increases until the micro-switch is activated. As the liquid level drops, the air pressure also drops and the switch is disengaged.

A benefit provided by a diaphragm-based liquid level switch is that it can be used with many types of liquids because a power source inside the tank is not needed, since the switch does not come in contact with the liquid. However, it should be noted that over time it will need maintenance because it is a mechanical device.

5. Float Liquid Level Sensors

Float switches are mechanical devices that are the original liquid level sensors. A hollow float is attached to an arm. As the float rises and falls in the liquid, the arm is pushed upwards and downwards. The arm may be connected to a mechanical or magnetic switch to determine on/off, or it may be connected to a gauge that rises from full to empty as the liquid levels drops.

Float Liquid Level Sensors

Image Credit: GasLab

Float switches are a cost-effective method used by Sump pumps to measure the water level in a basement sump pit.

Float switches can measure any kind of liquid and can be developed to not need power to function. The drawback of float switches is that they are bigger than other kinds of switches and due to the fact they are mechanical, must be serviced more frequently than the other types of liquid level switches.

This information has been sourced, reviewed and adapted from materials provided by GasLab.

For more information on this source, please visit GasLab.


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