measurement of pressure

8/13/2019 Measurement of Pressure

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Measurement ofPressure

GC Saeed Awan (199)

GC Waleed Tahir (202)

GC Saddam Hussain (204)

GC Talha Najeeb (205)

GC Tanzil ur Rehman (206)


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Static pressure is uniform in all directions

so pressure measurements areindependent of direction in an immovable

fluid , in simple words pressure measured

in static system is static pressure.

It can only be measured accurately by

measuring it in a manner such that the

velocity pressure has no influence on the

measurement at all. This is carried out by

measuring it through a small hole at the

wall of duct or a series of holes positioned

at right angles to the flow in a surfacelying parallel to the line of flow.

The pitot static tube is an example of it.

Measurement of Static Pressure


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Measurement of dynamic pressure

Dynamic pressure is associated with velocity or

flow of the fluid. They are more complex thanstatic pressures and are more difficult to

measure.

It is in fact used to measure flow rates and air

speeds.

It can be measured by taking the differential

pressure between instruments parallel and

perpendicular to the flow


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Introduction Of Transducers

Transducer is a device, usually electrical, electronic,

or electromechanical, that converts one type of

energy to another for various purposes including

measurement or information transfer.

It must produce the output which depends on some

way on the input quantity/measurand.

Transducers are divided into twotypes:

Active Transducer

Passive Transducer


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Active Transducers

An active transducer does not require an externalpower supply to provide an output signal.

These transducers usually rely upon magnetic

inductance or piezoelectric effects to produce their

output signals.

Ex. Piezoelectric crystal used for accelerationmeasurement


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Measurand is converted into outputwithout any other form of energyrequirements.

They are also called self generatingtype transducers.


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Piezoelectric sensor

A piezoelectric sensoris a device that uses the piezoelectriceffect to measure changes in pressure ,acceleration, strain

or force by converting them to an electrical charge

Piezoelectric Pressure Sensors measure dynamic pressures.

They are generally not suited for static pressuremeasurements. Dynamic pressure measurements including

turbulence, blast, ballistics and engine combustion under

varying conditions may require sensors with special

capabilities

Internal generation of electric charge resulting from an

applied mechanical force.


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CLASSIFICATION OF PASSIVE

TRANSDUCERS


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The variable resistance transducer elements work on

the principle that the resistance of the conductor is

directly proportional to the length of the conductor

and inversely proportional to the area of theconductor. Thus if L is the length of the conductor (in

m) and A is its area (in m square), its resistance (in

ohms) is given by:

R = L/A.

L is the length ,A is cross sectional area and p is

resistivity of the resistance material.

So resistance can be changed if any of these value p,L or A is changed.


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Some of the variable resistive transducersare:

Sliding wire(potentiometer)

Strain gauge

Load cell

RTD

Thermistor Hot wire anemometer

Carbon microphone

Humidity sensor


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Capacitive Transducers measure the

pressure by measuring the change in

capacitance.

Where capacitance is given by,

C=A/D.

Where C= Capacitance in Farads

A=Area of cross section of plates in square

meters

D=Distance between two plates in meters

=Dielectric Constant


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Two types: Single plate & Double plate

Accuracy up to 0.1 to 0.2% of span

Pressure range: 0.8kpa to 35kpa

Second to strain gauges

Elastic element: Ni, stainless steel

Exposed to process pressure on one side

and reference pressure on the other.


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o The variable capacitance used to convert

physical phenomena into electrical signals is

called a variable capacitance transducer.

o As there is no movement so there is no wear

and the life expectancy quoted is 200 years.

o The major problem with these transducers is

their high impedance which makes them

very suspectible to noise.


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Advantages Of Capacitive Transducer

Low Hysteresis

Good linearity

Highly sensitive

Stability

Repeatability

Resolution as small as 0.003mm can be obtained

Static pressure measurement capability


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Disadvantages Of Capacitive Transducer

High impedance output.

High temperature sensitivity.

Need for complex electronics.

Dirt may vary the output.


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Variable Inductance Type Transducer

Inductance is another property of electric circuits

which can be used in transducers. If a length of wireis wound into a coil and an electrical current passes

through it a magnetic field is generated.

If the current varies the magnetic field varies and this

induces other currents in the coil which tend tooppose the original.

Process pressure moves the magnetic iron core inside a coil

which alters self inductance of coil.

As the core moves into solenoid, the induction of coilincreases in proportion to metal inside coil.

Ration inductance transducers are better than variable


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THE END

measurement of pressure

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