07 pressure sensors

7/26/2019 07 Pressure Sensors

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Mansoor Asif

Lecturer

Electronics, Power and ControlGroup

Electrical Engineering Department

SEECS, NUST

Scool of Electrical Engineeringand Computer Science, NUST

!


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Capter"!#


$

Alan S. Morris


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Pressure Measurements


%


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!"Pressure

Pressure &P ' e(presses te magnitude ofnormal force &)"N' per unit area &A"m*$'applied on a surface

Units+

Pa N-m$ .ar !/# Pa!// 0Pa

m.ar !// Pa

A

FPor

A

FP

==


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$"Pressure Measurement

Pressure measurement is a 1er2 common re3uirementfor most industrial process control s2stems4

A.solute Pressure Te di5erence .etween pressure of te 6uid and a.solute

7ero of te pressure

Gauge Pressure Te di5erence .etween pressure of te 6uid and

atmosperic pressure

A.solute Pressure Gauge Pressure 8 Atmosperic PressureDi5erential Pressure

Te di5erence .etween a.solute pressure 1alues at twodi5erent points in te same 6uid


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%"Pressure Sensors

Normal 9ange &!4/:% .ar ; :/// .ar' Diapragms


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%4!4!"Diapragms &elasticelement t2pe'

Applied pressure causes displacementof diapragms and tis displacementis measured .2 displacement sensors

Can measure

#/ .ar"a.solute

$/// .ar"gauge pressure

$4# .ar"di5erential


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Diapragms

Materials used

Plastic &Ceapest'

Metal Allo2 &Accurate'

Stainless steel &Suita.le for ig temperature-corrosi1e application'

Ceramic &9esistant towards strong acids Bal0ali'

Displacement

/4! mm can .e measured wit strain gageplaced in a eatstone .ridge


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%4!4$"


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%4!4%"


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C" t2pe Tu.e

Arc su.tended is less tan %/ deg4

Can measure pressure up to /// .ar4

A C"t2pe tu.e of $# mm radius as ama(imum displacement tra1el of F mm4

Measurement accurac2 of 8"!

Measurement resolution is moderate

Displacement sensor as to .e emplo2ed


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?elical B Spiral Tu.e

9otating pointer mo1es against tescale to gi1e te 1isual indication ofmeasurement

Measurement accurac2 of 8"! of fullscale de6ection

Measurement resolution is relati1el2

iger tan C"t2pe tu.e Ma(imum measura.le pressure is ://

.ar4


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Disad1antages of


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%4!4F" Manometers

Passi1e instrument tat gi1e 1isualindication of pressure 1alues

T2pes

U"tu.e

ell t2pe


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U"tu.e Manometer

Applied pressure creates te di5erence in le1elof 6uid in te tu.es

PHg

Ppressure

H specic gra1it2 of 6uid

Can measure atmosperic, gauge

and applied pressure

?uman in1ol1ement in measurem"

ent of li3uid le1els in .ot tu.es

can cause error


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ell"t2pe or Cisternmanometer

Te cange in le1el of well due tocange in pressure canges 1er2negligi.l2

Te le1el of li3uid in tu.e as to .emeasured onl2


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%4!4#" Dead"weigt gauge

eigts are added until te piston isadIacent to te (ed reference mar0

>er2 accurate

t is used for cali.ration of oterpressure measuring instrument

Gauge pressure up to ://

.ar can .e measured


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4 4 " ermocoup e gauge&termal conducti1it2

principal' At low pressure tere is a linearrelationsip .etween pressure andtermal conducti1it2" Kinematic

theory of gases Measurement of termal conducti1it2

can gi1e an indication of pressure

Termal conducti1it2 J Pressure Pressure range

!/*"F m.ar ; ! m.ar


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Termocouple gauge

Source of Error

Termal radiation

ndependent of pressure of gas

t can .e 3uanti7ed and correction can .emade

?eated element wit low emissi1it2

sould .e cosen for low radiationloss


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%4$4$" Pirani Gauge &termalconducti1it2 principle'

Pirani gauge

Two identical tu.es are connected ina .ridge circuit

Pressure range

!/*"# m.ar ; ! m.ar

erm stor auge


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4 4 " erm stor auge&termal conducti1it2

principle' Uses semiconductor materials as aeating element

Pressure range

!/*"F m.ar ; ! m.ar


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%4$4F" oni7ation Gauge

Used for measuring 1er2 lowpressures !/*"!% .ar to !/ *"% .ar

Gas pressure is determined .2determining te current 6owing.etween te anode and catode

witin te 1essel


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%4%" ?ig pressuremeasurement &@:/// .ar'

Materials a1ing 9esistance"pressurecaracteristic tat is suita.l2 linear

Manganin

Gold"cromium allo2s

Measurement of pressure ismonitored electricall2 .2 monitoring

te resistance cange


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F" ntelligent PressureSensors

Adding microprocessor to pressuresensors ma0e tem intelligent

Ad1antages

mpro1ed measurement sensiti1it2

E(tended Measurement 9ange

Compensation for nonlinearities

07 pressure sensors

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