Liquid level sensors are used to detect liquid levels or interfaces between liquids such as oil and water or liquids and solids. They can be defined as sensors or transducers, or as integrated systems with instrumentation and control capabilities.

Types of Sensors

Liquid level sensors provide measurement of the height or position of a fluid surface using a variety of different technologies and methods.

•  Air bubbler systems involve a fixed flow of air passed through a tube with an opening below the surface level of the measured liquid. Pressure is proportional to the depth and density of the liquid.
• Capacitance sensors apply radio frequency signals to a capacitance circuit to sense materials with dielectric constants ranging from 1.1 to 88 or more.
• Conductivity or resistance sensors use a low-voltage power source applied across separate electrodes. Level is interpreted based on the amount of resistance or conductance of the substance which carries the current.
• Float sensors involve the opening or closing of a mechanical switch through either direct contact or magnetic operation from a device which floats on the surface of the measured liquid.
• Optical sensors detect the decrease or change in transmission of infrared light emitted from a diode through a material.
• Pressure membrane and differential sensors measure the pressure or change in pressure in a vessel such as a holding or storage tank.
• Radar or microwavesensors measure using the emittance and detection of microwave pulses.
• Ultrasonic or sonic liquid level sensors measure the length of time it takes for a reflected sound wave to return to a transducer from a target surface.
• Vibrating or tuning fork liquid level sensors use a piezoelectric crystal or other technology to vibrate a probe and then monitor the presence, absence, increase or decrease of that vibration.

For a more detailed analysis of different sensor types and their applications, consult the chart below:

Table Credit: Omega Encyclopedia - Level Measurement

Selection Criteria

Selecting the proper liquid level sensor requires the consideration of a number of different factors.

Continuous or Point Sensing

Level sensors can operate either continuously or by point sensing. Point sensing involves taking measurements at certain instances. Point liquid level sensors often trigger an alarm or turn off the system based upon a specific limit. Continuous sensors provide a constant measuring and indication of the level of the liquid.

Direct or Indirect

Liquid level sensors measure position and level either by direct or indirect methods. Direct measurement includes methods that directly sense position and level. Indirect measurement involves the sensing of other factors such as pressure head, which can then be translated to height or level. Indirect methods are not suitable when other parameters involved, such as liquid density, are variable.

Contact or Non-contact

Liquid level sensors measure using either contact or non-contact methods. Contact methods involve physical contact between the device and the liquid, while non-contact methods measure without touching the media. Non-contact methods are the best choice for servicing corrosive media.

Internal or External

Liquid level sensors can be mounted either internally or externally. Internal sensors are inserted inside the tank or vessel that incorporates the measured media. External sensors are mounted externally outside the tank or vessel. External sensors are the best choice when internal access for sensor maintenance would interrupt the industrial process or operation.

Materials

Liquid level sensors can be made from a variety of different materials depending on the type of device and the media being addressed.

• Nylon - oils and fuels
• Polypropylene - hot water applications
• Polyphenylene sulfide (PPS) - aggressive acids
• Stainless steel - high temperature fluid applications

Applications

Level monitoring is important in a wide variety of applications. Any industrial process involving the filling or storing a liquid in a tank or vessel would benefit from the use of a liquid level sensor. They are also an important part of process control systems which manage flow rate into and out of a storage vat or reactor.

References

Newark - Selecting the correct floatswitch

Omega Encyclopedia - Level Measurement

Image credits:

ABB Measurement & Analytics | IEEEGlobalSpec | IEEEGlobalSpec