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ELASTIC PRESSURE TRANSDUCER

WASEEM ZIA (2007-CHEM-15)


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Synopsis:

Types

Pressure Sensors Summary

Functional

Uses

Merits & Demerits

References


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Types:

Bellows

Bourdontube

Diaphragm


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Bellows

The bellows is a one-piece,collapsible, seamless metallic unitthat has deep folds formed from

very thin-walled tubing.


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Bourdon tube

The bourdon tube pressureinstrument is one of the oldestpressure sensing instruments in use

today. The bourdon tube (refer toFigure 3) consists of a thin-walledtube that is flattened diametricallyon contrary sides to produce a cross-sectional area elliptical in form,

having two long flat sides and twoshort round sides.


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Bourdontubesaregenerallyare

ofthreetypes;

1. C-type

2. Helicaltype

3. Spiraltype


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Diaphragm

Diaphragm elements are made of circular metal discsor flexible elements such as rubber, plastic orleather.


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Pressure Sensors Summary

System pressure is applied to the internal volume

of a bellows and mechanical linkage assembly. As pressure changes, the bellows and linkage

assembly move to cause an electrical signal to beproduced or to cause a gauge pointer to move.

In a bellows-type sensor:


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Contd

Inabourdontube-typesensor:

System pressure is applied to the inside of aslightly flattened arc formd tube. As pressureincreases, the tube tends to restore to its originalround cross-section. This change in cross-sectioncauses the tube to straighten.

Since the tube is permanently fastened at oneend, the tip of the tube traces a curve that is the

result of the change in angular position withrespect to the center. The tip movement can thenbe used to position a pointer or to develop anelectrical signal.


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Contd.

In Diaphragmpressuresensor:

They are used to measure gauge pressures over verylow ranges.

They may be made up ofMetallic diaphragms gauge(brass or bronze) Slack diaphragms gauge (Rubber).


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Functional Uses:

Pressuresensorsperformthefollowingbasicfunctions:

Indication Alarm Control

Ifapressuresensorbecomesinoperative:

A sparesensorelementmaybeused (ifinstalled). A localmechanicalpressuregaugecanbeused (if

available). A precisionpressuregaugemaybeinstalledinthe

system.


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Contd.

Environmental concerns:

Atmosphericpressure Ambienttemperature

Humidity


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Merits &Demerits

MeritsofFlexiblePressuretransducer

FlexiblePressuretransducercostislow.

FlexiblePressuretransducerhaseasystructure

FlexiblePressuretransducershavebeentimetestedinapplication.

FlexiblePressuretransducerallowshighaccuracy,spatiallyinrelationtocost.


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Contd.

DemeritsofFlexiblePressuretransducer

FlexiblePressuretransducerhasalowspringpitch.

FlexiblePressuretransducerisinclinedtoshockandthrob.

FlexiblePressuretransducerisinclinedtohysteresis.


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References

http://controls.engin.umich.edu/wiki/index.php/PressureSensors retrieved at 11/10/2010

Kirk, Franklin W. and Rimboi, Nicholas R.,Instrumentation, Third Edition, American TechnicalPublishers, ISBN 0-8269-3422-6. Wightman, E.J., Instrumentation in Process Control, CRC

Press, Cleveland, Ohio.

Rhodes, T.J. and Carroll, G.C., Industrial Instruments forMeasurement and Control, Second Edition, McGraw-HillBook Company.

Process Measurement Fundamentals, Volume I, GeneralPhysics Corporation, ISBN 0-87683-001-7, 1981

http://vpmt.com/transducer.htm retrieved at11/10/2010


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THANKS