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.
Types of pressure sensors
Pressure from resistance variation
The deformation of the sensing element (for example diaphragm) causes displacement of the wiper of a potentiometer (Figure 2A) or a change of resistance in a metal strain gauge (Figure 2B). The output voltage is then proportional to the measured pressure.
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 (Figure 3A), the distance between the plates (Figure 3B), or the relative permittivity of the dielectric (Figure 3C) 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.
2. Automotive industry
- monitor oil and coolant pressure
- regulate power pressure in accelerators and brakes
- Anti-locking braking system (ABS)
- Air bag system
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.
What is a pressure sensor?
Monitor the pressure of fluid, solid or gases.
Most work based on piezoresistance.
Level of electrical charges flow proportional to the level of pressure.
Convert data into a form that can be displayed and understood by the user of another device.
Principle of Operation
The pressure sensing element is formed by diffusing semi-conductor strain gauges directly into the silicon diaphragm. Strain gauges are elements which change their resistance when they are deformed. The strain gauges are connected together to form a Wheatstone bridge circuit. When pressure is applied to the diaphragm, it deforms and stretches or compresses the strain gauges, which causes an imbalance in the bridge circuit, and a change in the output voltage.
Selecting a Sensor
The easiest way to select a sensor is to choose the ones that will meet our technical requirements at the lowest cost.
- Pressure measurement range
- Media to be measured – the amplified pressure sensors are suitable for all media, whereas the modules are only suitable for use with non-corrosive gas
- Material – Titanium is lighter than stainless steel but is susceptible to attack by some cleaning solvents and by methanol.
- Operating Temperature – Make sure the operating temperature is appropriate for our installation.
- Connector – Certain models are available with integral connectors. Other types have a flying lead.
- Mounting – Flanged versions of certain models are available.
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
http://www.futek.com/pressure_sensor_selection.aspx