Pressure is defined as a measured quantity: it can be described as the force exerted by a liquid or gas on a surface, and it is usually measured in force per unit area. Common units are Pascal (Pa), Bar (bar), N/mm², or psi (pounds per square inch).
Sensor definition: A sensor is a device that measures a physical quantity and converts it into a signal. The measured quantity can be temperature, length, force, or pressure, etc. The signal is usually an electrical signal, but it can also be an optical signal.
Pressure sensor definition: A pressure sensor is an instrument composed of pressure-sensitive elements that determine the actual pressure applied to the sensor (using different operating principles) and convert the pressure information into an output signal.
Pressure Measurement Principles
Pressure sensors utilize a variety of techniques to provide accurate results.
Strain-based pressure sensors:** These use strain gauges as the pressure-sensitive element, typically achieved by bonding a metal foil strain gauge or a diaphragm to a cylindrical elastomer. The greatest advantage of strain-based pressure sensors is their extremely high stiffness, allowing the measurement of pressures up to 15,000 bar. Electrical connections are usually made using a Wheatstone bridge, resulting in highly accurate and consistent measurements.
Capacitive pressure sensors:** These use a pressure chamber and a diaphragm to generate variable capacitance. When pressure is applied, the diaphragm deforms, and the capacitance decreases accordingly. A voltage-dependent electrical signal is then output through the measuring circuit. These sensors are limited to low pressures of approximately 40 bar.
Piezoresistive pressure sensors:** These consist of a diaphragm primarily made of silicon, using an integrated strain gauge to detect the strain caused by applied pressure. A Wheatstone bridge is typically used to reduce sensitivity and increase output. Due to the materials used, pressure is limited to around 1000 bar.
Unlike the above techniques, resonant pressure sensors use changes in the resonant frequency of the measuring mechanism to measure the stress caused by applied pressure.** In this type of sensor design, the resonant element can be exposed to a medium, where the resonant frequency depends on the density of the medium. These sensors are typically sensitive to shocks and vibrations.
Other pressure sensors that do not use a measuring body include thermal or ionization pressure sensors, which utilize density changes due to the flow of charged particles to measure applied pressure.
Pressure Measurement Categories
Pressure sensors can be classified in several ways, including by the pressure range they measure, their operating temperature range, or the type of pressure they measure.
Absolute Pressure Sensor: Measures pressure relative to a reference chamber (near vacuum).
Gauge Pressure Sensor: Or relative pressure sensor, used to measure pressure relative to the current atmospheric pressure.
Sealed Gauge Pressure Sensor: Similar to a gauge pressure sensor, but it measures pressure relative to a fixed pressure rather than the current atmospheric pressure.
Differential Pressure Sensor: Determines the difference between two pressures and can be used to measure pressure drop, liquid level, and flow rate, etc.
A significant advantage of absolute pressure sensors is that they are always measured against the same reference pressure (vacuum), and therefore are unaffected by changes in atmospheric pressure and temperature.
