Table of Contents
Introduction
The physical quantity
Working principle
Pressure sensors types
Sensor characteristics
Applications
References
The physical quantity
Static pressure is the ratio of the perpendicular element of force to the element of surface it is exerted on.
ps = dF/dA
For the pressure inside a fluid we have:
ps = patm+ ρgh , where patm is the atmospheric pressure, ρ is the density of the fluid, g is the acceleration at the place of measurement, h is the distance from the free surface
The Si unit for pressure is pascal (Pa) which is equal to one newton per square metre (N/m2).
For liquids the total pressure can be expressed as:
p = ps + 0.5ρν2 , where ν is the velocity of the fluid
For gases, using the ideal gas equation the pressure is:
p = nkBT/V , where n, T and V are respectfully the number of molecules, the temperature and the volume of the gas, and kB is the Boltzmann constant
Working principle
The majority of pressure sensors do not measure the pressure directly. The sensor 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.
- A change in pressure causes the diaphragm to deflect, a corresponding change in resistance is induced on the strain gauge.
- This changes can be measured by DAQ system.
- A metal is strongly bonded in the nearby surface where the strain is being measured.
- Bonded foils have quick 1000 Hz response times to changes in pressure as well as their large operating temperature.
Types of pressure sensors
Pressure from resistance variation
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.
Similary, 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 determined by the resistance value.
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 plates , 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.
Pressure from inductance variation
The magnetic properties of the sensing element create a variation of the reluctance of the magnetic circuit which displaces the circuit's core. The direction and amplitude of the displacement are sensed by a diaphragm and translated into a signal, consecutively converted to electrical signal for the pressure.
Pressure from piezoelectric effect
These types of sensors are used for dynamic pressure measurements. The change in stress of the diaphragm creates a strain and when a piezoelectric is used as a sensing element, this strain is converted to electric charge.
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 supplied by a detection coil, consecutively amplified and passed to an excitation coil. 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.
Pressure from light intensity variation
The displacement of the sensing element gives a variation in light intensity. Using a photodiode that senses the change, it can relate that to the change in the force exerted on the sensing element.
Pressure from heating variation
The temperature of the sensing element varies in accordance to the surrounding pressure. The pressure measurement corresponds to that change in temperature.
Pressure from ion variation
The electrical current through a gas is related to the number of ions that provide it. The number of gas molecules is the same as the number of ions created in the gas by the impact of electrons. Therefore, the amount of electrical current reflects the molecule count, which then is used to calculate the pressure in accordance to the gas law.
Data reliability
All pressure sensors are intrusive. Therefore, an incorrect installation can disturb the measurand or compromise the reliability of the system.
Range
Resolution
Accuracy
Signal Characteristics
Linearisation
Calibration
Noise
Other error conditions
Applications
Pressure sensors are widely applied in energy generation industry where a constant monitoring and control of pressures is crucial for the operation of the power plants. Another application is in robotics where a pressure measurement is required in controls or as a substitute for touch. The proper operation of machines can be related to data from pressures of compressed air, gas, vapor, oil or other fluids.
Some applications are
1. Touch screen devices
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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2. Automotive industry and security
Pressure sensors have great application in automotive industry and security system. They help to monitor oil and coolant pressure there by regulating appropriate power pressure in accelerators and brakes. Pressure sensors are usually incorporated in the radiators fill cap and the pressure inside the radiator ranges from 0.6 bar to 1.0 bar.
When the temperature of the coolant rises up then the pressured in the closed system also increases which is then notified by the pressure sensor which opens the valve and the excess fluid is dumped into the overflow container.
Besides, pressure sensors have very important role in security system such as anti-locking braking system (ABS) and air bag system. Pressure sensors fitted into the automobiles work under ECU. ECU detects uneven (fast/slow) rotation of wheels during driving which then inform the pressure sensor to detect the specific parts of the problem and apply more or less pressure to give even wheel rotation.
Similarly, when the pressure is applied to the airbag control unit, then the pressure sensor fitted into the system determines the direction of impact and the restraint device is activated within 15-30 millisecond of the crash. This quick response of the system helps to prevent the passenger from possible dangers.
3. Bio medical instrumentation
- Digital blood pressure monitors and ventilators
4. Industrial Uses
- monitor gases and their partial pressures
- determine the depth in oil industry while exploring
5. Aviation Industry
- Pressure sensors are used to balance the atmospheric pressure with the airplanes’ control system.
- gives appropriate external environmental situation to the system
- helps to create a breathing condition in the cockpit
6. Marine Industry
- Sensors in ships and submarines help to detect the actual depth creating a safe situation.
- Sensors are of great use in underwater projects. They help to study the oxygen level and requirement.
References
- Ripka, P. and Tipek, A., 2007. Modern sensors handbook. 1st ed. Newport Beach, CA: ISTE USA.
- Huddleston, C., 2007. Intelligent sensor design using the microchip dsPIC. 1st ed. Amsterdam: Elsevier/Newnes.
- Engineers Garage. Available from: http://www.engineersgarage.com/articles/pressure-sensors-types-working
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