Pressure sensors have been in demand since the advent of steam age. Billions of such sensors are used daily to monitor the pressure of fluids in pipes, motors, hydraulics, or in nature. Specialized sensors are also used to determine the pressure of solids or gases. A typical pressure sensor is about a cubic inch in size, although some may be a hundred or more times smaller, such as those used in microelectromechanical systems.
Most modern pressure sensors work based on a principle called piezoresistance. Pressure causes a material to conduct current at a certain speed, which leads to a certain level of charge current associated with a certain pressure level. This charge is fed to a wire leading to a control panel and human analysis display.
Conventional pressure sensors use film resistors, tension flaps, metal alloys, or polycrystalline semiconductors as resistive media. These materials carry more or less electricity based on geometric deformations in their structure. Because a linear increase in pressure does not result in a linear order of deformation, calibration techniques must be used to determine the actual pressure. These are built into most systems.
In more sensitive pressure sensors based on monocrystalline semiconductors, fabricated using conventional semiconductor technology, small deformations can give rise to large changes in resistance. The change in resistance is not based on geometric deformations in conductive material, but in smaller, more delicate structural warping.
There are several different types of pressure sensors. One is an absolute pressure sensor that measures absolute pressure using a vacuum as a reference point. Another is a gage sensor, which measures the pressure by reference to the ambient atmospheric pressure. There are also differential pressure sensors, which measure the pressure difference between two contacts.
Usually, there is an intermediate medium between the fluid to be measured and the pressure sensor itself. A metallic membrane or hermetically sealed fluid chamber is often used. Especially when measuring the pressure of volatile or corrosive materials, such as hot oil in an engine, is an intermediate layer used to prevent damage to the fragile sensing device. In addition to its ubiquitous use in process equipment, pressure sensors are also used by natural scientists to determine variables such as the depth of an ice bag or the density of a rock layer.