The need for pressure sensors came with the rise of the steam age and the development of machinery whose pressure was required to be measured. Pressure sensors are devices that detect and sense pressure and then generate analog electrical signals whose magnitude relates to the applied pressure. Although the signals produced by pressure sensors are usually electrical in nature, they may also generate optic, visual and/or auditory signals.
Pressure sensors convert the measured value of pressure of a gas or liquid, which is detected using a pressure-sensitive element against a diaphragm made of stainless steel, silicon, etc., into an electrical signal as an output. They work on the principle of bending a membrane due to the pressure exerted by a liquid or a gas. A thin conductive screened layer on the membrane follows the bending of the membrane which can be measured either by measuring the distance between the conductive layer on the membrane and a reference layer in the housing of the sensor showing a change in the capacitive value or by measuring the change of resistance of the conductive layers on bending the membrane.
There are five types of pressure sensors based on the pressure types that they sense:
- Absolute pressure sensor. It measures the absolute pressure or the pressure relative to perfect Vacuum pressure (0 PSI or no pressure). As perfect vacuum is a condition in which no matter is present in the atmosphere and nil pressure exists, such sensors require strict specifications for precise results and have limited usage. However these are used in barometric or altitude related pressure measurements.
- Gauge pressure sensor. This sensor is used to measure the pressure at a certain point relative to the atmospheric pressure at a given location. Gauge pressure measurements are confined to non-critical measurements are they are inconsistent because they are based on atmospheric pressure which varies with altitude. They are applied in tire pressure or blood pressure measurement.
- Vacuum pressure sensor. It may be used to measure pressure less than the atmospheric pressure at a given location.
- Differential pressure sensor. These sensors compare the pressures of two distinct positions used as inputs in the sensing unit. These sensors are usually applied in purification systems and are also used to measure flow or level in pressurized vessels.
- Sealed pressure sensor. These sensors need to be calibrated by manufacturers before use to measure pressure relative to a fixed level pressure (eg. sea level).
Pressure sensors are also classified on the basis of the technology that they use, into but not limited to the following types:
- Fiber optic sensors. These sensors use the properties of fiber optics to alter the propagation of light in a fiber rendering it suitable to be used as a sensor. Miniaturized fiber optic interferometers can be used as pressure sensors to sense nanometer scale displacement of membranes.
- Mechanical deflection sensors. This technology measures the pressure of a liquid by making use of its mechanical properties.
- Strain gauge sensors. These sensors utilize the changes in resistance experienced by some materials as a result of a change in their stretch or strain, to sense their pressure.
- Semiconductor Piezo-resistive sensors. This technology measures the pressure through the change in conductivity of semiconductors brought about by a change in pressure.
- Micro-electromechanical systems (MEMS). This technology measures the pressure by using nanotechnology to combine microelectronics with tiny mechanical systems such as resonators, valves and gears etc.
- Vibrating elements sensors. This technology measures the pressure by making use of the change in vibration of different elements like silicon at the molecular level due to change in pressure.
- Variable capacitance sensors. These sensors calculate the pressure on the basis of change of capacitance detected by change in the distance between the plates of a capacitor.
Pressure sensors are widely used in different applications from automobile industry to touch screen phones. The below are some examples of applications where pressure sensors have been applied.
Industrial Uses: Pressure sensors are used in industrial settings to monitor gases and their partial pressures.
Automobile Industry: Pressure sensors are widely used in the automobile industry as an integral part of the engine and its safety. These sensors sense the oil and coolant pressure and control the power to be delivered by the engine on pressing the accelerator or applying the brakes. They are also an important part of anti-lock braking system (ABS) and the air bag systems that are important safety systems for the vehicles.
Aviation: Pressure sensors are used in the airplanes to maintain a balance between the atmospheric pressure and the control systems of the airplanes.
Marine Industry: Pressure sensors are used in ships and submarines to estimate the depth at which they are operating and for regulating the oxygen requirements of under water projects.
Touch Screen Devices: One of the most widely used application of the pressure sensors are the touch screen devices and smart phones with touch screen displays which generate electrical signals to be transmitted to the processor based on slight pressure applied on the touch screen using a finger or the stylus.
Bio Medical Instrumentation: Digital blood pressure monitors and ventilators also make use of the pressure sensors for optimization to patient’s condition and his requirements.