Liquid Level Switches

Liquid level switches detect liquid levels or interfaces between liquids such as oil and water or liquid and solid interfaces.  Liquid level switches are used in a number of liquid container monitoring applications including flow line monitoring, heaters and furnaces as well as other household appliances, automotive applications, and control technology. 

There are a range of liquid level measurement technologies available including differential pressure sensing, ultrasonic / sonic, radar / microwave, optical, pressure membrane, electrical conductivity / resistance, capacitive / RF admittance, air bubbler, mechanical / magnetic floats, rotation paddle, radio frequency, and vibrating / tuning fork.  These level measurements are indicated by deactuating or actuating an electromechanical switch such as a reed switch or relay, or a solid state switch. Electrical conductivity and resistivity liquid level switches use a low-voltage power source applied across separate electrodes.  A conductive liquid such as a chemical or water contacting both probes completes a conductive circuit.  Electrical conductivity or resistance is generated when the liquid contacting both probes completes a conductive circuit in most cases the tank or vessel is included in this circuit. The signal can be relayed to a process controller where set points have been programmed to open or close valves, and/or initiate other fluid processes. Mechanical and magnetic floats are often used with liquid level switches.  A stem with a permanent magnet may be installed in the float to activate a reed switch. As the float rises or falls, the switch if normally opened, will close.   Mechanical float liquid level switches can be placed at multiple points to identify the level or flow of the process liquid.  A suspended float or tank mounted float may be a consideration. Since this technology is dependent upon an accurate indication of the fluid surface, foam, turbulence, or solids may cause a false reading when activating the switch.  Another technology may be more compatible with the application. Air bubbler liquid level switches may fit applications where sludge, wastewater, or other highly viscous fluids will interfere with the continuous level measurement.  An air bubbler liquid level switch uses a constant volumetric rate of gas flow (usually air) fed through a bubble tube submerged in a liquid. The amount of pressure required to force the bubble gas out of the bottom of the bubble tube is equal to the hydrostatic pressure at that point. Another technology that may not be affected by high temperatures, condensation, and the presence of heavy solids are vibrating tuning fork liquid level switches.  These use an electromechanical technology derived from a piezoelectric source, which actuates in an oscillating system when there is a presence or absence of liquid. In the presence of the liquid, the resonant frequency is reduced. The vibration can also be used to identify a number of characteristics of the media that is being measured.

• The Science of Selecting a Liquid Level Float Switch
  When liquid level switches are used in highreliability applications, such as aircraft, submarines, nuclear power plants and medical equipment, it is essential that they operate with precision and...
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• Fluid-Trac Liquid Level Sensor For AdBlue Tanks
  This application note covers the Fluid-Trac® liquid level sensor used as a replacement for a standard 33-240 Ohm resistive float sender, reed-switch tube sensor, or capacitive tube sender in...
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• Assemble-It-Yourself (AIY) Series: Custom Liquid Level and Temperature Switches
  Temperature switch assembly, select a Temperature Switch Part Number, then specify a temperature set point (5°C-130 °C) and a Switch Operation will, Open = C, or Closed = CR on rising temperature....
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Part Numbers for Liquid Level Switches