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The majority of pressure sensors do not measure the pressure directly. Being a composite sensor, it consists of a sensing element that takes a primary reading (most commonly deformation followed by displacement, force or strain) and translates it to another non-electric physical quantity. Another sensor then translates this into an electrical signal that represents the output of the pressure sensor.
Figure 1. Working principle of a composite pressure sensor.
Types of pressure sensors
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In Figure 2A the applied pressure causes deformation of the sensing element (for example diaphragm), which is connected to a strain gauge that exhibits a change in resistance when mechanically strained. The value for pressure is then obtained by reading the variation of the output voltage which is proportional to the change in resistance of the strain gauge.
Figure 2A. Strain gauge pressure sensor.
Similarly, in Figure 2B a wiper (moveable electrical contact) is mechanically linked to the diaphragm and the applied pressure controls the wiper's position. This affects on the length of the wire (representing the resistance) between the wiper and the end of a potentiometer (variable resistance). The measurement for pressure is then obtained from the output voltage that reflects the resistance value.
Figure 2B. Potentiometric pressure sensor.
Pressure from capacitance variation
Here the sensing element, such as diaphragm, is connected to one of the electrodes of a capacitor. A change in the effective area of the distance between the plates, the effective area of the distance between the plates, or the relative permittivity of the dielectric creates a change in capacitance which reflects on the output voltage, from where the pressure can be derived.
Figure 3. Capacitive pressure sensors with a change in distance d, area A, or dielectric type.
Pressure from inductance variation
In magnetic circuits we talk about reluctance instead of resistance. A change in the distance between two magnetic devices causes a change in reluctance. In this type of sensor, the pressure acts on a part of the magnetic circuit (movable core) and changes the reluctance between the coils. The The amplitude of the displacement is proportional to the output voltage which is used to calculate the applied pressure.
Figure 4. Inductive pressure sensor.
Pressure from piezoelectric effect
These types of sensors are used for dynamic pressure measurements. There are about 40 crystalline materials that generate electric charge when strained. Using such piezoelectrics as a sensing element, the strain produced from a stress on the diaphragm is converted to electric charge which is proportional to the pressure.
Figure 5. Piezo-electric pressure sensor.
Pressure from oscillation
These sensors use a vibrating element which frequency of vibration depends on the force applied to it. This element is made from ferromagnetic material and induces voltage between two magnets, which is consecutively amplified and measured. This voltage reflects the frequency of vibrations which follow a mathematical model to derive the pressure. The sensing element can be the vibrating element itself or connected to it.
Figure 6. Vibrating wire pressure sensor.
Pressure from light intensity variation
The displacement of the sensing element (vane) gives a variation in light intensity between the source LED and the measuring LED. The diaphragm connected to the vane is moved by pressure, and therefore, the amount of infrared light received changes. This change in light intensity corresponds to the pressure applied.
Figure 7. Optical pressure sensor.
Pressure from ion variation
The pressure of a gas can be derived from measuring the ion current flow. Electrical current is supplied and when the electrons entering the gauge hit the gas molecules, they form positive ions causing ion current flow. The amount of that ion current is related to the gas density which is proportional to its pressure.
Figure 8. Ionization gauge pressure sensor.
Data reliability
All pressure sensors are intrusive. Therefore, an incorrect installation can disturb the measurand or compromise the reliability of the system.
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Smart phones and some computer devices come with pressure sensors. Sensors in those devices determine where the pressure has been applied and inform the processor by generating electric signal. There are two or more sensors fitted at the the corner of the screen to give precise location where actually the pressure has been applied.
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Figure 9. Pressure indication on a mobile phone with a touchscreen.
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Pressure sensors have a great application in automotive industry and security systems. They help to monitor oil and coolant pressure by regulating appropriate power pressure in accelerators and brakes. Pressure sensors are usually incorporated in the radiator's fill cap and the pressure inside the radiator ranges from 0.6 bar to 1.0 bar.
Figure 10. Pressure control in a relief valve.
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