The switch detects the level of liquid or solid (powder or granules) in a vessel. One or more switch contacts are actuated by the motion of the level sensing element using float. When the switch senses a minimum level (an empty vessel), it will be in its “normal” status. Any fluid level below the trip threshold of the switch is referred to as the “normal” status of the level switch.
The electrical schematic symbol is shown below for a level switch based on the mechanism with a round ball float drawn as the actuating element.
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Instrumentation Basics 