Air pressure sensors are devices used to measure and monitor air or gas pressure in a wide range of applications. They utilize different sensing principles to quantify pressure and convert it into an electrical signal that can be measured.
Types of air pressure sensors
There are various types of sensors used for measuring air and gas pressures. The common ones are:
Piezoresistive pressure sensors
- Uses piezoresistive material that changes resistance when deformed
- Applied pressure deforms diaphragm causing resistance change
- Very low cost, used from a few mbar to 300 bar pressures
Capacitive pressure sensors
- Uses a diaphragm and measures change in capacitance
- Applied pressure causes diaphragm to deflect changing gap between plates
- High sensitivity and stable output, range up to 100 bar
Strain gauge based sensors
- Uses metal strain gauges arranged in Wheatstone bridge configuration
- Applied pressure induces strain changing bridge resistance
- Amplified output proportional to pressure, range up to 500 bar
Piezoelectric pressure sensors
- Uses piezoelectric crystals that generate voltage when stressed
- Applied pressure deforms crystals and produces output voltage
- Fast response and self-generating output, up to 300 bar range
Optical pressure sensors
- Uses an optical measurement technique like interferometry
- Pressure deflects diaphragm changing interferometry pattern
- Very high precision, fast response, range up to 10 bar
How piezoresistive pressure sensors work?
Piezoresistive pressure sensors are one of the most commonly used types for measuring air and gas pressures. Below is an explanation of their working principle and construction:
Construction
- Uses a diaphragm made of silicon substrate with piezoresistive elements diffused into it.
- The diaphragm is etched to a required thickness to give desired sensitivity.
- Piezoresistors connected as a Wheatstone bridge to sense resistance change.
- Sealed reference vacuum chamber below diaphragm.
Working principle
- Applied pressure deflects the diaphragm and introduces stress in piezoresistors
- The stress causes change in resistivity of the piezoresistive material based on the piezoresistive effect.
- This resistance change unbalances the Wheatstone bridge circuit.
- The output voltage from the bridge provides a measure of the applied pressure.
- Silicon provides excellent sensitivity, linearity and hysteresis characteristics.
- Can detect pressure variations in the range of a few mbar up to 300 bars.
Signal conditioning
- The output from the Wheatstone bridge is small and requires amplification.
- Temperature compensation is added to cancel temperature induced resistance changes.
- Additional linearization can improve measurement accuracy.
- Provides standard voltage or current output proportional to pressure.
How capacitive pressure sensors work?
Capacitive pressure sensors use capacitance change to measure applied pressure. Their working principle is:
Construction
- Contains two parallel conducting plates with air/dielectric between them.
- Flexible diaphragm plate moves with applied pressure.
- Fixed reference plate remains stationary.
- Plates separated by small distance to allow movement.
Working principle
- Pressure deforms diaphragm changing distance between plates.
- This changes the capacitance between the plates based on formula:C = (εA)/d
Where,
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C = Capacitance ε = Dielectric constant A = Overlapping area d = Distance between plates
- C is inversely proportional to d. So increase in pressure reduces d, increasing capacitance.
- The change in capacitance is converted to equivalent voltage output.
- Provides very high resolution and sensitivity. Range up to 100 bar pressure.
- Temperature compensation required to account for thermal expansion effects.
How strain gauge pressure sensors work?
Strain gauge pressure sensors use the piezoresistive effect to measure pressure induced strain on the diaphragm.
Construction
- Contains a pressure sensitive diaphragm with strain gauges in a Wheatstone bridge configuration.
- Strain gauges attached to the diaphragm experience deformation proportional to applied pressure.
- Change in strain gauge resistance unbalances the Wheatstone bridge.
Working principle
- Applied pressure deforms the sensing diaphragm.
- The deformation causes the attached strain gauges to elongate.
- Elongation changes the resistance of the strain gauge proportional to the pressure induced strain.
- This resistance change produces an unbalanced output from the Wheatstone bridge.
- The output voltage is amplified and calibrated to indicate the pressure.
- Provides very good accuracy and stability. Measures up to 500 bar pressure range.
- Temperature compensation required as strain gauges are sensitive to temperature.
How piezoelectric pressure sensors work?
Piezoelectric pressure sensors utilize the piezoelectric effect to produce a voltage output proportional to applied pressure.
Construction
- Sensor contains a piezoelectric sensing element sandwiched between two electrodes.
- The element is usually made of quartz crystals structured to enhance the piezoelectric effect.
- When mechanical stress applied to the element, the molecular structure generates charge.
Working principle
- Applied pressure deforms/stresses the piezoelectric sensing element.
- The mechanical deformation of the crystalline structure generates electrical charge proportional to the applied force.
- The charge accumulation produces a voltage difference across the electrodes.
- This voltage signal is conditioned and calibrated to provide the pressure measurement.
- Provides very fast dynamic response to pressure changes.
- Measures up to 300 bar pressure range.
- Sensitive to temperature changes which affects output.
How optical pressure sensors work?
Optical pressure sensors use light intensity changes to measure pressure variations. Some common techniques used are:
Optical interferometry sensors
- Uses a pressure sensitive diaphragm forming one mirror of a Fabry-Perot interferometer.
- Applied pressure deforms the diaphragm changing the optical cavity length.
- This alters the interferometry pattern of reflected light.
- Photodetector measures the intensity changes which corresponds to pressure variation.
Fiber Bragg grating sensors
- Uses fiber Bragg grating (FBG) sensitive to strain induced by pressure on diaphragm.
- Pressure deforms diaphragm changing the grating period of FBG.
- This shifts the reflected optical wavelength detected by an interrogator.
- Wavelength shift indicates the applied pressure.
- Provides very high resolution, fast response and electrical isolation.
- Limited to lower pressure range up to 10 bar.
Key performance parameters
Some important specifications and parameters to consider for an air pressure sensor:
- Pressure range – Minimum and maximum pressures that can be measured.
- Accuracy – Maximum deviation between measured and actual pressure.
- Repeatability – Ability to consistently return the same output for same pressure.
- Resolution – Smallest change in pressure that can be detected.
- Response time – Time required to reach a certain percentage of fullscale output.
- Operating temperature – Temperature range over which sensor can function.
- Thermal effects – Degree of output drift with changes in temperature.
- Sensitivity – Ratio of output signal change to the input pressure change.
- Nonlinearity – Maximum deviation from output linearity over full scale.
- Hysteresis – Maximum difference between output readings for same pressure approached with increasing and decreasing pressure.
- Overload capacity – Maximum pressure beyond full scale that can be applied without permanent damage.
Common applications of air pressure sensors
Some major application areas utilizing air/gas pressure sensors include:
Weather monitoring
- Barometers for air pressure measurement
- Altimeters for altitude based on air pressure changes
- Weather stations forecasting system
Process industry
- Measuring pressure in pipelines, vessels, reactors
- Level, flow measurements using differential pressure
- Monitoring gas pressures for safety
Heating, ventilation and air-conditioning
- Duct pressure measurement
- Filter pressure drop monitoring
- Blower and burner air pressure control
Automotive
- Intake manifold pressure sensing
- Barometric pressure for engine control systems
- Tire pressure monitoring systems
Aviation
- Airspeed indicators using pitot tubes
- Altitude measurements
- Cabin pressure control
Medical
- Respirators, ventilators
- Blood pressure monitoring
- Anaesthesia machines
Others
- Gas flow and analysis instruments
- Leak detection systems
- Vacuum measurement
- Pneumatic control systems
Frequently Asked Questions
Q1. What are the different technologies used in air pressure sensors?
Some common sensing technologies are:
- Piezoresistive
- Capacitive
- Strain gauge
- Piezoelectric
- Optical (interferometric, fiber optic)
- Resonant
- Thermal
Piezoresistive and capacitive sensors are most widely used.
Q2. How are pressure sensors calibrated?
Pressure sensors are calibrated by applying known pressures and adjusting the output:
- Using pressure controllers or dead weight testers.
- Measuring output at zero (vacuum), 25%, 50%, 100% of full scale pressures.
- Adjusting gain and offset trim resistors to get desired output.
- Generating calibration table/curve for linearization.
Q3. How does temperature affect pressure sensor accuracy?
Temperature affects sensor accuracy in several ways:
- Thermal expansion of components causes span shifts
- Alters strain gauge resistances
- Affects dielectric properties in capacitive sensors
- Changes piezoelectric crystal properties
Proper temperature compensation is necessary to account for these effects.
Q4. What are techniques to communicate with a pressure transmitter?
Common communication interfaces used with smart pressure transmitters are:
- 4-20 mA analog current loops
- HART protocol over current loops
- Fieldbuses like Profibus, Foundation Fieldbus
- Industrial ethernet protocols
- Wireless protocols like WirelessHART
Q5. What are applications of differential pressure flow measurement?
Differential pressure across a primary flow element like an orifice plate is used to calculate flowrate. This technique is used in:
- Gas flow meters in pipelines
- Airflow measurement in ducts
- Liquid flow rates in process industry
- Fuel consumption measurement
- Pneumatic conveyor flow
It allows accurate flow rate measurement in large pipes and ducts.
