A designer's primer on pneumatic sensors and switches

29 April, 2015

Pressure switches and pressure sensors are the unsung heroes of pneumatic actuation and control systems. Even though there are only a handful of basic types of sensors and switches, the selection process isn't always as easy as it might seem. Each switch type is available in a near-infinite combination of packages, ports, pressure ranges, electrical outputs and connector styles which can make selecting the best match for a particular job a challenging, time-consuming task.

Pneumatic systems

Whether they're helping to control a high-speed manufacturing system or monitoring the various inlet and outlet pressures within an air compressor to determine its performance and efficiency, pressure-sensing devices play a critical role in every pneumatic (and hydraulic) application.

Industrial machines and manufacturing systems rely on pneumatic switches and sensors for the information that enables them to move precisely and apply the exact amount of force required for a particular task. In large heating and air conditioning systems, they are part of the pneumatic signaling network that connects the building's thermostats and control elements with the fans, burners, pumps and compressors that do the work. Pressure switches and sensors will also be found providing control and safety functions in the braking systems of trains, trucks, aircraft, and other transportation systems.

While all the pressure sensing devices highlighted in these examples employ a nearly-identical set of operating principles, their sensing ranges, performance requirements and operating conditions vary as widely as the pressure-sensing solutions available to meet a particular need.

Pressure switches versus pressure sensors

Pressure sensing devices fall into two broad categories: pressure switches; and pressure sensors.

Pressure switches: Pressure switches are relatively simple devices that indicate whether the pressure they are sensing is either above or below a predetermined threshold. Their output is a change in the state of an ‘on/off’ switch or a two-state electrical signal.

A typical pressure switch consists of a contact that's driven by a diaphragm or piston that is pushed in one direction by the gas or fluid being sensed and a bias spring that pushes in the opposite direction. The switch's contact changes state when the pressure on the switch's inlet side rises above the pressure exerted by the bias spring on the other side. Conversely, the switch returns to its original state once the inlet pressure falls below a predetermined threshold. Due to a phenomenon known as hysteresis, that threshold may or may not be the same as its actuation level. In some cases, hysteresis can be used to one's advantage when attempting to stabilize pneumatic systems.