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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