Examples of systems built around pneumatic technology include vehicle tires, air brakes (on buses, trucks and trains), air compressors, compressed-air engines, vacuum pumps and more.
Examples of hydraulic applications include vehicle braking systems, power steering systems, shock absorbers, utility vehicles such as excavators and aerial platforms, lifts and industrial machinery such as hydraulic presses.
The overarching application of pressure sensors in pneumatics and hydraulics is to ensure the pressure within the system is at the correct level, or within an optimum range.
This is particularly important for hydraulics, where the liquid in the system may be volatile or flammable (for example, mineral oil) and reach very high pressures and temperatures, making leaks and accidents potentially dangerous.
Pressure sensors feature as part of pressure regulators, or automatic valves designed to control the pressure in the system (as shown below).
Pressure regulators match the demand for gas or liquid to the demands of the system, while maintaining a constant output pressure. As the system demands more power, so the load flow increases, and the regulator flow must increase, or the controlled pressure will fall.
Like other types of pressure sensor, sensors used in pneumatics and hydraulics can measure differential pressure (the difference between two pressures) or absolute pressure (measured against zero or another absolute value). In pressure regulators, differential pressure sensors compare the pressure on either side of a valve, to determine whether the inlet flow is equal to the outlet flow.
Pneumatic and hydraulic pressure sensors are transducers, generating an electrical signal in proportion to the pressure they measure. This allows pressure to be monitored by a range of electronic devices.
The technology used most often in pneumatic and hydraulic pressure sensors uses a physical diaphragm, often made of silicon, which bends as pressure is applied to it. The diaphragm is a strain gauge, which varies its electrical resistance when force is applied-in this case, from pressure exerted by air, gas or hydraulic liquid on the sensor. This resistance is used to modify the output voltage of the sensor.