Techincal Library

High/Low Level Switches Used for Automatic Filling of a Tank

This is a simple level control application using two level switches to control automatic filling of a tank. The tank could be an open hopper, a pressure vessel, or a sump. The material being controlled could be plastic pellets, grain, crude oil or boiling acid. The concept is the same for all.


When the level falls to point "A", the pump turns on. The pump continues to run until the level rises to point "B" and then turns off.

The pump motor starter is activated by 115VAC control voltage at the Level Switch "A" #1 relay contacts. Wiring terminations are made between Common (C1) and and Normally Closed (NC1). The switch is set at "Fail-Safe Low", which means that it will de-energize the relay output upon absence of material (liquid or bulk solid) at the probe, and energize the relay upon presence of material at the probe.

Level Switch "A" will energize (opening up the NC contact) as the level rises and touches the probe. To keep the pump running as the level rises, the Auxiliary Contact of the pump motor starter is used to seal-in the run command to itself.

Level Switch "B" is set for "Fail-Safe High" (energize upon absence of material at the probe) with the wiring terminations made between Common (C1) and Normally Open (NO1) on the #1 relay contacts. Level Switch "B" must energize the relay output to enable the pump to run. When the level rises to point "B", Level Switch "B" will de-energize, and shut off the pump motor.

This Fail-Safe logic is used to prevent accidental overfill if Level Switch B's wiring is cut or relay contacts are burned.


  • Simple, reliable operation - no moving parts.
  • Minimal time to install and set up.
  • Minimal maintenance. Very tolerant of product and sludge build-up on the probe.

Pricing: THB35,000 to 50,000 depending on probe length and material, for two switch system.

call 02 938 1059 for immediate respond


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