How does a water diaphragm pressure switch level control work?

Written by pauline gill
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Diaphragm based level control switches are used to control water levels in domestic dishwashers, clothes washers; and sump pump systems. They are also used in industry. They work on the principle of static head or pressure produced by a given depth of a liquid of known specific gravity. They are usually located on or near the bottom of the vessel where the greatest head pressure is available, augmenting reliable performance.

Liquid head pressure is a direct function of the depth of a liquid of known specific gravity above the measuring point. It is an expression of the weight of the liquid per unit area at that depth. For water, 27.71 inches of depth yields a pressure of one pound per square inch (psi) pressure at the bottom.

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Liquid Head Pressure

Liquid head pressure is a direct function of the depth of a liquid of known specific gravity above the measuring point. It is an expression of the weight of the liquid per unit area at that depth. For water, 27.71 inches of depth yields a pressure of one pound per square inch (psi) pressure at the bottom.

Switch Operation

Diaphragm level switches have a flat round sensing diaphragm about two inches in diameter made from chemically inert synthetic rubber. Diaphragms have a concentric convolution surrounding a rigid centre section to increase sensitivity. They look like small audio speakers. The wet side of the diaphragm is in the liquid. Outside the vessel, a range spring presses against the back of a rigid disc behind the diaphragm. A miniature electrical switch the size of a pat of butter is also mechanically linked to the disc. An increase in level increases the pressure against the diaphragm until the force overcomes the range spring force behind it to actuate the switch. In the case of a dishwasher, this would shut off the electric solenoid water valve filling it. A small amount of mechanical detent is designed into these switches so they do not cycle on and off.

Advantages

Diaphragm and other static pressure switches work better than floats in most appliance applications because floats are subject to tossing and splashing at the surface, while static pressure stays relatively constant despite conditions at the top. Pressure switches also have limited motion, increasing precision and reliability.

Most top loading clothes washers have their diaphragm level switch located at the top of the machine in the control panel. In this case, there is a plastic tube between the bottom of the water tub and the top mounted switch. As water fills the machine, it traps the air in the tube and compresses it to a pressure equal to the increase in liquid head. This pressure is applied to the diaphragm in the switch and from there operation is the same as the bottom mounted switch.

Liquids with specific gravities higher than water at 1.0 will exert the same liquid head force with less liquid depth. A liquid with twice the specific gravity would turn off the fill with half the depth, and inversely, a lighter liquid such as ethanol at 0.785 specific gravity will require about a 27% higher level to create the same head pressure.

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