Machine Design

Piston/diaphragm pressure switches

The pressure switches normally used to turn shop air compressors on and off today typically employ a sensing element that consists of a plunger or piston working against a compression spring.

The surface area of the plunger and the degree of spring compression determine the amount of pressure necessary to actuate the switch. When pressure on the plunger exceeds that exerted by the spring, the plunger moves back and presses on a small electrical switch that closes or opens electrical contacts.

Typical operating specs include ±2% repeatability of full setpoint range, 3 to 100-psi range, with a proof pressure of 350 psi. Switches employed as more critical compressor controls may exhibit tighter repeatability.

Modern designs incorporate a flexible diaphragm that isolates the plunger and switch mechanism from external contaminants. The most widely used diaphragm materials include EPDM, neoprene, and Viton. The expected environment determines which is a candidate for specific uses. For example, EPDM is a typical choice when the diaphragm may see synthetic oils. It is also widely used outdoors in extreme cold because it stays flexible at low temperatures. On the other hand, Viton diaphragms are compatible with a wider variety of oils but are too stiff at low temperatures to work well in the cold.

Older pressure switches targeting shop air and similar low-pressure tasks sometimes used only a flexible diaphragm as a sensing element. These devices had problems handling overpressures that could sometimes arise momentarily in normal use. Similarly, some older pressure switches used only a plunger incorporating an O-ring rather than a separate diaphragm. These tended to eventually leak air. (However, such designs performed better in hydraulic applications.) Modern designs incorporating both a diaphragm and plunger assembly overcome such difficulties and have become the most common configuration.

Piston/diaphragm designs work best in situations characterized by low cycle rates and where cost is a consideration. Suppliers list most such devices at prices of $10 or less. This contrasts with solid-state pressure switches, basically solidstate pressure transducers that actuate a switch at specific set points. These devices excel where there is long-term repetitive cycling and carry price tags far exceeding those of electromechanical switches.

Gems Sensors ( provided information for this article.

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