Liquid level sensor types

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What’s the difference between a float switch and an ultrasonic level sensor?  Which sensor should I use for my level sensing application?

Let’s take a look at five common types of liquid level sensor and some common applications.

Float switch

The simplest and most reliable method of liquid level sensing, the float switch is one of the most common instruments in process control. The actual switching mechanism may vary, from a simple micro-switch to a magnetic Reed switch, but the output is generally a simple digital off/on signal. Due to their versatility and reliability, float switches can be used in virtually any level sensing application. One potential disadvantage is that a separate float switch is generally required for each level setpoint, which in a smaller tank can result in tangling. Float switches are generally the most cost-effective means of level sensing.

Conductivity probe

A conductivity liquid level sensor produces a digital on/off signal similar to a float switch, by detecting the conductivity of the liquid as measured between two probes. A separate relay module is required to translate the probe signal into a digital output. Some models are longer to include multiple sensing points for more granular control. Conductivity level sensors are ideal for hazardous, flammable, corrosive or high temperature liquids, as the probes are stainless steel and the electrical voltages used are very small.  In some applications the probe must be cleaned regularly to ensure accuracy, and the sensitivity must be correctly calibrated for the type of liquid in use. Level probes are generally more expensive than float switches.

Pressure level sensor

A pressure level sensor is a level sensing instrument which produces an analog (continuously variable) signal. An analog signal allows for an exact reading of the tank or well level in percent or litres as required. The sensor is typically mounted at the bottom of a tank or well; it calculates the level of liquid based on the pressure detected on the sensor. Pressure sensors are very commonly used in water and wastewater applications. They do, however, tend to get clogged in applications involving very dirty water. Pressure level sensors are one of the most cost-effective level sensors that provide an analog signal.

Ultrasonic level sensor

For applications involving wastewater or other dirty liquids, an ultrasonic sensor is sometimes used. This sensor is mounted at the top of the tank, above the water line. It emits an ultrasonic frequency which bounces off the top of the liquid and returns to the sensor. The sensor then calculates the level and transmits an analog (variable) signal. Ultrasonic level sensors tend to be more expensive than their pressure sensor cousins.

Radar level sensor

Also used for dirty liquid applications, a radar sensor uses the same method as an ultrasonic sensor, but with radar frequencies rather than ultrasonic.