PI2 Pressure

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Measurements

Part 2d: Measurement of Pressure

1Process Instrumentation Lecture NotesMar-12


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PRESSURE SENSOR

Can be divided into three types:

1. Deflection type2. Strain gauge type

3. Piezoelectrictype


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Typical Pressure Detector System


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1. DEFLECTION TYPE PRESSURE SENSOR

This sensor uses an elastic material to convertpressure to displacement e.g. stainless steel, brass.

The displacement will be proportionate to the value

of pressure exerted. Suitable to be used in an automatic control system.

The main element used is in the shape of Bourdontube, bellow or diaphragm.

The secondary element is the element that willconvert the displacement to electrical signals wherethe displacement can be detected through resistivitychange, inductance or capacitance.


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The Main Typical Element Used In A Deflection Type Pressure Sensor


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Basic Form of Mechanical

Pressure Sensors


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Example :

i. Bellow-resistance pressure sensor

The pressure is proportionate to the resistivity.

The resistance change is detected by displacement ofsliding contact in the resistance element.

Bellows Calibrated spring Sliding contact

Resistance Output Signal


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Example :

ii. Bellow-inductance pressure sensor

The pressure is proportionate to the inductance changewhich is detected from the displacement of the core in thewire coil.

The core movement will produce AC signal output which willgive the value and direction of inductance.

LVDT (linear variable differential transformer) demodulatoris used to convert the AC output to DC.

Bellows Core Output Signal


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iii. Diaphragm-capacitance pressure sensor

The pressure is proportionate to the capacitancechange at the output through dielectric change.

Pressure from the sensor element causes thediaphragm to move towards the plate and produces

dielectric change.


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Capacitance Type

Resistance Type


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2. STRAIN GAUGE PRESSURE SENSOR

Strain gauge is a type of resistive transduction.

Pressure measurement is obtained fromdisplacement of elastic element.

Pressure is measured through force that isexerted on the diaphragm where the force willbe detected by the strain gauge and resistancechange will be produced.

Wheatstone Bridge circuit is used to detect thechange in pressure and an amplifier is used toamplify the small output signals.


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Strain Gauge Pressure Sensor


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3. PIZOELECTRIC PRESSURE SENSOR This sensor consists of a piezoelectric crystal

(made from quartz) which functions as a force-sensitive voltage source where the piezoelectric

will be in between two plates. Pressure exerted on the crystal surface is

proportionate to the voltage produced by thecrystal.

This sensor does not require any voltage supply.

This sensor is suitable for fast changingpressure measurement.


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Piezoelectric

Material

Mechanical Stress

OutputVoltage

Plate Mechanical Stress

Piezoelectric Pressure Sensor


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Piezoelectric Pressure Sensor

PI2 Pressure

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