Pressure sensors have been in demand since the advent of the steam age. Billions of such sensors are used daily to monitor the pressure of fluids in pipes, engines, 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, though some may be a hundred or more times smaller, for example those used in microelectromechanical systems.
Most modern pressure sensors work based on a principle called piezoresistance. Pressure causes a material to conduct electricity at a certain rate, leading to a specific level of charge flow associated with a specific level of pressure. This charge is fed to a wire which leads to a control panel and display for human analysis.
Conventional pressure sensors use film resistors, strain gauges, metal alloys, or polycrystalline semiconductors as the resistive media. These materials conduct more or less electricity based on geometric deformations in their structure. Because a linear increase in pressure does not lead to a linear magnitude in deformation, calibration techniques must be used to determine the true pressure. These are inbuilt in the vast majority of systems.
In more sensitive pressure sensors based on monocrystalline semiconductors, which are fabricated using conventional semiconductor technology, tiny deformations can give rise to large changes in resistance. The change in resistance is not based on geometric deformations in the conducting material, but on smaller, more delicate structural warpage.
There are several different types of pressure sensors. One is an absolute pressure sensor, which measures absolute pressure using a vacuum as a reference point. Another is a gage sensor, which measures 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 intervening medium between the fluid being measured and the pressure sensor itself. A metallic diaphragm or hermetically sealed fluid chamber is often used. Especially when measuring the pressure of volatile or corrosive materials, like hot oil in an engine, an intermediary layer is used to prevent damage to the delicate sensing device. Besides their near-ubiquitous usage in process equipment, pressure sensors are also used by naturalists to determine variables such as the depth of an ice pack or the density of a rock layer.