Elastic Pressure Transducer 15

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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