micro flow meter

8/11/2019 Micro Flow Meter

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MICRO FLOW METER-PRINCIPLE OF OPERATION

During each cycle, an amount of liquid is displaced equal to the difference inthe volumes of the chambers minus the volumes of the pistons.

This process repeats itself in a continuous flow mode at the rates of 1 to

210 complete cycles per second proportionately to the fluid flow through the

meter. In the !TD" !icroflowmeter, each cycle displaces appro#imately .02cc. $ppro#imately 200,000 cycles will displace one gallon. The !TD20

!icroflowmeter has a flow rate capacity five times larger than the !TD".

%ignal detection is accomplished by light interruptions of a photoemitter&detector device. $ ferromagnetic wire trac's the magnet in the

nutator (through a pressure tight barrier) causing these interruptions. Theinterruptions are electronically manifested as sine waves which are then conditioned

by conventional electronic means to provide a square wave output.


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Technology of Water Pollution Continuous Monitoring in JAPANPrinciples of Analyzers for Water Pollution Continuous Monitoring

3. Automatic Analyzers for Water Pollution Continuous Monitoring onStationary Source

3.11 Flow Meter

(1) Electromagnetic Flow meter

The principle of flow measurement by Electromagnetic Flowmeter is based on the

Faraday's electromagnetic induction rule: when the conductor crosses a magnetic field,

the electromotive force is caused at both ends of conductor

!hen a magnetic field is added at right angles to the electric conductive fluid, the

electromotive force that is proportional to the flow is caused at right angles to a magneticfield and the electric conductive fluid The direction of the electromotive force is based

on Fleming's right"hand rule, and its magnitude is following:

E=kBDv

E: electromotive force#: constant

$: magnetic flu% density

&: diameter of pipeline

v: velocity of fluid

therefore,

Flow: Q=D2v / 4 =DE / 4kB

Fig.4.3.9 Principle of Electromagnetic Flowmeter

FETE*1 Flow of the electric conductive li+uid is measured without influences of temperature,

pressure, density or viscosity


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-ressure loss is small

. $ecause of no movable parts, it has a long life

/ esponse is fast0 easuring range is wide (wide range ability)

2 3t is able to measure the both direction (plus and minus) flows

4 3t is available for measurement of strong corrosive li+uid or slurry by selection oflining materials

() 5rifice, 6o77le and 8enturi Tube (&ifferential -ressure ethod)

The most popular flowmeter for industrial use is a head flowmeter which has orifices

no77les or venturi tubes as a sensing element This flowmeter consists of 9throttle device9to be inserted in the pipeline to produce differential pressure 3n most cases, the

differential pressure is converted into standard air pressure (;1#-a) or electric

current (/;m&


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This orifice is used when eynolds 6umber is small due to the small flow or high

viscosity of fluid, or when viscosity changes during its use

(.) *egmental 5rifice or Eccentric 5rifice

$oth of these orifices are used when fluid contains foreign matters such asprecipitation, air bubbles, etc

65??=E

s no77les have superior durability and larger flow coefficient in comparison with

orifice plates, they are suitable for measuring flow of high temperature, highpressure and high velocity steam and water

8E6T3 T$E

=i#e no77les, venturi tubes have superior durability and small pressure loss s its

e%cellent construction eliminates accumulation of precipitation, this venturi tube

is used for fluid containing foreign matters 3t can also be used when pressure lossis re+uired to minimi7e or when it is re+uired to bury in the ground for a long time

without servicing
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Search this

Database Last Updated: 02/11/04

Categories> Sensors, Transducers and Detectors>Flow Sensing>

Liquid Flow Meters

What do you want to do?Search for Liquid Flow eters !roducts and su!!liers "# s!ecification using S!ecSearch

Learn $ore a"out Liquid Flow eters

Show all su!!liers in Liquid Flow eters

List #our !roducts on %lo"alS!ec

Liquid Flow Meters Speciications

Flow Meter !ype

&our choicesare'''Mass FlowMeter

The flow sensor or $eter $easures flow rate in units of $ass flow, for e(a$!le,l"s/$in'

"olu#etricFlow Meter

The flow sensor or $eter $easures flow rate in units of )olu$etric flow, fore(a$!le, $L/$in'

"elocity Flow

Meter

The flow sensor or $eter $easures flow rate as in units of )elocit#, for

e(a$!le, ft/sec'

Search Logic* +ll !roducts with +& of the selected attri"utes will "e returned as $atches'Lea)ing all "o(es unchec-ed will not li$it the search criteria for this question.!roducts with all attri"ute o!tions will "e returned as $atches'

$hysical Speciications
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$ipe Dia#eter

se this field to identif# the !rocess !i!e dia$eter to "e $onitored'

Search Logic* ser $a# s!ecif# either, "oth, or neither of the +t Least and o ore

Than )alues' roducts returned as $atches will $eet all s!ecified criteria'

%peratin&$ressure 'an&e:

The $a(i$u$ head !ressure of the !rocess $edia the $eter can withstand'

Search Logic* ser $a# s!ecif# either, "oth, or neither of the +t Least and o oreThan )alues' roducts returned as $atches will $eet all s!ecified criteria'

Liquid!e#perature'an&e:

The $a(i$u$ te$!erature of the $edia that can "e $onitored, usuall#de!endent on construction and liner $aterials'

Search Logic* ser $a# s!ecif# either, "oth, or neither of the +t Least and o oreThan )alues' roducts returned as $atches will $eet all s!ecified criteria'

Mountin&%ptions:&our choicesare'''(nsertion !ype

The flow $eter is inserted !er!endicular to flow !ath' suall# requiresthreaded hole in !rocess !i!e or other $eans of access'

(n)line Flan&ed

The de)ice is inserted !arallel to the flow !ath, usuall# inserted "etween two!ieces of e(isting flanged !rocess !i!es'

(n)line !hreaded

The de)ice is inserted !arallel to the flow !ath, and threaded into twoe(isting !rocess !i!es' T is the $ost co$$on thread t#!e'

(n)line *la#p

The de)ice is inserted !arallel to the flow !ath, and cla$!ed "etween twoe(isting !rocess !i!es'

%ther (n)line!ype

ther unlisted, s!eciali3ed, or !ro!rietar# configuration'

Search Logic* +ll !roducts with +& of the selected attri"utes will "e returned as $atches'Lea)ing all "o(es unchec-ed will not li$it the search criteria for thisquestion. !roducts with all attri"ute o!tions will "e returned as $atches'

"olu#etric Flow $eror#ance

Liquid "olu#etric Flow 'ate 'an&e:

+ $eters !erfor$ance can "e deter$ined "#the $eters turndown ratio' Turndown ratio isthe effecti)e d#na$ic or o!erating range ofthe flow $eter' For e(a$!le* 5f the 600 %flow rate de)ice has a turndown ratio of 60*1,it will effecti)el# o!erate and resol)e flowdown to 10 %' 5f the sa$e de)ice has aturndown of 100*1, it will effecti)el# resol)e to6 %'


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Search Logic* ser $a# s!ecif# either, "oth, or neither ofthe li$its in a Fro$ 7 To range. when "othare s!ecified, $atching !roducts will co)erentire range' roducts returned as $atcheswill $eet all s!ecified criteria'

+ccuracy

The accurac# of the sensor / $eter in 89/7: ;'

Search Logic* +ll $atching !roducts will ha)e a )alue less

than or equal to the s!ecified )alue'

"elocity Flow $eror#ance+ $eters !erfor$ance can "e deter$ined "# the $eters turndown ratio' Turndown ratio is theeffecti)e d#na$ic or o!erating range of the flow $eter' For e(a$!le* 5f the 600 SCC flow ratede)ice has a turndown ratio of 60*1 it will effecti)e o!erate and resol)e flow down to 10 SCC' 5fthe sa$e de)ice has a turndown of 100*1 it will effecti)el# resol)e to 6scc$'

"elocity Flow 'ate 'an&e:

For )elocit# flow sensors or $eters, the rangeof flow in distance/ti$e'


+ccuracy


Search Logic* +ll $atching !roducts will ha)e a )alue lessthan or equal to the s!ecified )alue'

Mass Flow $eror#ance

MassFlow 'ate 'an&e:

For massflow sensors or $eters, the rangeof flow in $ass/ti$e'


+ccuracy


Search Logic* +ll $atching !roducts will ha)e a )alue less

than or equal to the s!ecified )alue'

+dditional $eror#ance Measures

Measures !e#perature

The sensor or $eter also $easureste$!erature'


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Search Logic*


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!ri$ar# and secondar# ele$ent' The !ri$ar#ele$ent causes a change in -inetic energ#,which creates the differential !ressure in the!i!e' The unit $ust "e !ro!erl# $atched tothe !i!e si3e, flow conditions, and the liquids!ro!erties' +nd, the $easure$ent accurac#of the ele$ent $ust "e good o)er a

reasona"le range' The secondar# ele$ent$easures the differential !ressure and!ro)ides the signal or read7out that iscon)erted to the actual flow )alue'

?(a$!les of D $eters include rificelates, @enturi Tu"es, Flow o33les, ConeT#!es, itot Tu"es, Target eters, ?l"ow Ta!eters and


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ta! outside of el"ow and low7!ressure ta!inside of el"ow''ota#eters:


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"orte 2Sheddin& Meters: The frequenc# of)ortices shed fro$ a "luff "od# !laced in theflow strea$ is !ro!ortional to $aterial)elocit#'Ultrasonic Meters: ltrasonic can "e eitherDo!!ler effect $eters or Ti$e7of7Flight$eters' Do!!ler $eters $easure the

frequenc# shifts caused "# liquid flow' Thefrequenc# shift is !ro!ortional to the liquids)elocit#' Ti$e of flight $eters use the s!eedof the signal tra)eling "etween twotransducers that increases or decreases withthe direction of trans$ission and the )elocit#of the liquid "eing $easured' The# do notwor- well with liquids with sus!ended solidsor air ga!s'

!rue Mass Flow Meters

True ass Flow $eters are de)ices that$easure $ass rate of flow directl#, such asTher$al eters, Coriolis eters etc'!her#al Meters: Ther$al $eters use aheated sensing ele$ent isolated fro$ the

$ediu$ flow !ath' The flow strea$ conductsheat fro$ the sensing ele$ent' Theconducted heat is directl# !ro!ortional to the$ass flow rate' T#!icall# used for gas flowrates'*oriolis Meters: Fluid is !assed through as$all )i"rating flow tu"e causing a deflectionof the flow tu"e !ro!ortional to the $ass flowrate of $aterial'

Search Logic* +ll !roducts with +& of the selectedattri"utes will "e returned as $atches'Lea)ing all "o(es unchec-ed will not li$it thesearch criteria for this question. !roducts withall attri"ute o!tions will "e returned as

$atches'

%utput %ptions

Meter %utput&our choices are'''+nalo& "olta&e

Flow rate infor$ation is out!ut as an analog)oltage signal, such as 0710 $@' The out!ut)oltage is si$!l# a 8usuall# linear: function ofthe $easure$ent' 5t is continuous, rather

than !ulsed or discrete'

+nalo& *urrent

Flow rate infor$ation is out!ut as an analogcurrent signal, t#!icall# 4720 $+' ftencalled a trans$itter' + current is i$!osed onthe out!ut circuit !ro!ortional to the$easure$ent' Feed"ac- is used to !ro)idethe a!!ro!riate current regardless of linenoise, i$!edance, etc' seful when sendingsignals long distances'


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Frequency 4 $ulse

The flow rate infor$ation is out!ut as a signalencoded )ia a$!litude $odulation 8+:,frequenc# $odulation 8F:, or so$e other$odulation sche$e. e(a$!les are sine wa)eand !ulse train'

Switch Flow triggers a switch out!ut "asedon !reset flow rates'

Search Logic* +ll !roducts with +& of the selectedattri"utes will "e returned as $atches'Lea)ing all "o(es unchec-ed will not li$it thesearch criteria for this question. !roducts withall attri"ute o!tions will "e returned as$atches'

(nterace %ptions:&our choices are'''Serial (nterace

+ standard digital out!ut !rotocol 8serial:such as


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An introduction to different types of fluid flowmeters - Orifices,enturies, !ozzles, "otameters, Pitot #u$es, Calorimetrics,#ur$ine, orte%, &lectromagnetic, 'oppler, (ltrasonic, #)ermal,

Coriolis.

The most common principals for fluid flow metering are:

Differential *ressure lowmeters

+elocity lowmeters

*ositive Displacement lowmeters

!ass lowmeters

pen -hannel lowmeters

Dierential $ressure Flow#eters

3n a differential pressure drop device the flow is calculated by measuring the pressure drop over

an obstructions inserted in the flow The differential pressure flowmeter is based on the

$ernoullis E+uation, where the pressure drop and the further measured signal is a function ofthe s+uare flow speed

The most common types of differential pressure flowmeters are:

rifice *lates

low o//les
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+enturi Tubes

+ariable $rea otameters

%riice $late

!ith an orifice plate, the fluid flow is measured through the difference in pressure from theupstream side to the downstream side of a partially obstructed pipe The plate obstructing the

flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid

to constrict

The orifice plates are simple, cheap and can be delivered for almost any application in anymaterial

The Turn&own atefor orifice plates are less than 0:1 Their accuracy are poor at low flow

rates high accuracy depend on an orifice plate in good shape, with a sharp edge to the

upstream side !ear reduces the accuracy

rifice, o//le and +enturi !eters

"enturi !ube

&ue to simplicity and dependability, the 8enturi tube flowmeter is often used in applications

where it's necessary with higher Turn&own ates, or lower pressure drops, than the orifice

plate can provide

3n the 8enturi Tube the fluid flowrate is measured by reducing the cross sectional flow area inthe flow path, generating a pressure difference fter the constricted area, the fluid is passes

through a pressure recovery e%it section, where up to @A of the differential pressure generated

at the constricted area, is recovered
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!ith proper instrumentation and flow calibrating, the 8enturi Tube flowratecan be reduced to

about 1A of its full scale range with proper accuracy This provides a Turn&own ate1:1

5rifice, 6o77le and 8enturi eters

Flow -oles

Flow no77les are often used as measuring elements for air and gas flow in industrialapplications

The flow no77le is relative simple and cheap, and available for many applications in many

materials

The Turn&own ateand accuracy can be compared with the orifice plate
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5rifice, 6o77le and 8enturi eters

Te !onic "o##le $ %ritical Flow "o##le

!hen a gas accelerate through a no77le, the velocity increase and the pressure and the gasdensity decrease The ma%imum velocity is achieved at the throat, the minimum area, where it

brea#s ach 1 or sonic t this point it's not possible to increase the flow by lowering thedownstream pressure

This situation is used in many control systems to maintain fi%ed, accurate, repeatable gas flow

rates unaffected by the downstream pressure

'eco,ery o $ressure Drop in %riices5 -oles and "enturi Meters

fter the pressure difference has been generated in the differential pressure flow meter, the

fluid pass through the pressure recovery e%it section, where the differential pressure generatedat the constricted area is partly recovered

s we can see, the pressure drop in orifice plates are significant higher than in the venturitubes

"ariable +rea Flow#eter or 'ota#eterThe rotameter consists of a vertically oriented glass or plastic3 tube with a larger end at the top, and a meteringfloat which is free to move within the tube. luid flow causes the float to rise in the tube as the upward pressuredifferential and buoyancy of the fluid overcome the effect of gravity.
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The float rises until the annular area between the float and tube increases sufficiently to allow astate of dynamic e+uilibrium between the upward differential pressure and buoyancy factors,

and downward gravity factors

The height of the float is an indication of the flow rate The tube can be calibrated andgraduated in appropriate flow units

The rotameter meter typically have a Turn&own atio up to 1:1 The accuracy may be as good

as 1A of full scale rating

agnetic floats can be used for alarm and signal transmission functions

"elocity Flow#eters

3n a velocity flowmeter the flow is calculated by measuring the speed in one or more points inthe flow, and integrating the flow speed over the flow area


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$itot !ubes

The pitot tube are one the most used (and cheapest) ways to measure fluid flow, especially in

air applications as ventilation and B8< systems, even used in airplanes for the speedmeasurent

The pitot tube measures the fluid flow velocity by converting the #inetic energy of the flow into

potential energy

The use of the pitot tube is restricted to point measuring !ith the 9annubar9, or multi"orifice

pitot probe, the dynamic pressure can be measured across the velocity profile, and the annubar

obtains an averaging effect

*alori#etric Flow#eter

The calorimetric principle for fluid flow measurement is based on two temperature sensors in

close contact with the fluid but thermal insulated from each other


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5ne of the two sensors is constantly heated and the cooling effect of the flowing fluid is used tomonitor the flowrate 3n a stationary (no flow) fluid condition there is a constant temperature

difference between the two temperature sensors !hen the fluid flow increases, heat energy is

drawn from the heated sensor and the temperature difference between the sensors are reduced

The reduction is proportional to the flow rate of the fluid

esponse times will vary due the thermal conductivity of the fluid 3n general lower thermalconductivity re+uire higher velocity for proper measurement

The calorimetric flowmeter can achieve relatively high accuracy at low flow rates

!urbine Flow#eter

There is many different manufacturing design of turbine flow meters, but in general they are all

based on the same simple principle:

3f a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin

and rotate The rate of spin is measured to calculate the flow

The turndown ratios may be more than 1:1 if the turbine meter is calibrated for a single fluid

and used at constant conditions ccuracy may be better than CD",1A

"orte Flow Meter

$n obstruction in a fluid flow creates vortices in a downstream flow. 4very obstruction has

a critical fluid flow speed at which vorte# shedding occurs. +orte# shedding is the instance

where alternating low pressure /ones are generated in the downstream.


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These alternating low pressure 7ones cause the obstruction to move towards the low pressure

7one !ith sensors gauging the vortices the strength of the flow can be measured

The +orte# lowmeter *rinciple $n introduction to the vorte# flowmeter principle.

/lectro#a&netic Flow#eter

n electromagnetic flowmeter operate on Faraday's law of electromagnetic induction that states

that a voltage will be induced when a conductor moves through a magnetic field The li+uid

serves as the conductor and the magnetic field is created by energi7ed coils outside the flowtube

The voltage produced is directly proportional to the flow rate Two electrodes mounted in the

pipe wall detect the voltage which is measured by a secondary element

Electromagnetic flowmeters can measure difficult and corrosive li+uids and slurries, and they

can measure flow in both directions with e+ual accuracy

Electromagnetic flowmeters have a relatively high power consumption and can only be used for

electrical conductive fluids as water

The Electromagnetic Flowmeter -rinciple" n introduction to the electromagnetic

flowmeter principle

Ultrasonic Doppler Flow#eter

The effect of motion of a sound source and its effect on the fre+uency of the sound wasobserved and described by


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and can be use to calculate the fluid flow speed

The 5ltrasonic Doppler and Time of light lowmeter

$n 5ltrasonic lowmeter Tutorial $ basic tutorial about ultrasonic flowmeters.

$ositi,e Displace#ent Flow#eter

The positive displacement flowmeter measures process fluid flow by precision"fitted rotors asflow measuring elements nown and fi%ed volumes are displaced between the rotors The

rotation of the rotors are proportional to the volume of the fluid being displaced

The number of rotations of the rotor is counted by an integral electronic pulse transmitter and

converted to volume and flow rate

The positive displacement rotor construction can be done in several ways:

!eciprocating piston meters are of single and multiple"piston types

5val"gear meters have two rotating, oval"shaped gears with synchroni7ed, close fitting

teeth fi%ed +uantity of li+uid passes through the meter for each revolution *haft

rotation can be monitored to obtain specific flow rates

Nutating dis"meters have moveable dis#s mounted on a concentric sphere located in

spherical side"walled chambers The pressure of the li+uid passing through the

measuring chamber causes the dis# to roc# in a circulating path without rotating aboutits own a%is 3t is the only moving part in the measuring chamber

!otary vanemeters consists of e+ually divided, rotating impellers, two or morecompartments, inside the meter's housings The impellers are in continuous contact with

the casing fi%ed volume of li+uid is swept to the meter's outlet from each

compartment as the impeller rotates The revolutions of the impeller are counted andregistered in volumetric units

The positive displacement flowmeter may be used for all relatively nonabrasive fluids such as

heating oils, lubrication oils, polymer additives, animal and vegetable fat, printing in#, freon,

and many more

ccuracy may be up to G1A of full rate with a Turn&own of 4:1 or more

Mass Flow#eters

ass meters measure the mass flow rate directly

!her#al Flow#eter

The thermal mass flowmeter operates independent of density, pressure, and viscosity Thermal
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meters use a heated sensing element isolated from the fluid flow path where the flow stream

conducts heat from the sensing element The conducted heat is directly proportional to the mass

flow rate and the he temperature difference is calculated to mass flow

The accuracy of the thermal mass flow device depends on the calibrations reliability of theactual process and variations in the temperature, pressure, flow rate, heat capacity and viscosity

of the fluid

*oriolis Flow#eter

&irect mass measurement sets


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flow measurement


26/40

A statement of t)e conser*ation of energy in a form useful for sol*ingpro$lems in*ol*ing fluids. +or a non-*iscous, incompressi$le fluid insteady flow, t)e sum of pressure, potential and inetic energies per u

*olume is constant at any point.

special form of the EulerHs e+uationderived along a fluid flow streamline is often called the &ernoE'uation:
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For steady state incompressible flow the Euler e+uationbecomes (1) 3f we integrate (1) along the stre

it becomes () () can further be modified to (.) by dividing by gravity

6ead o Flow

E+uation (.) is often referred to the ea(because all elements has the unit of length

Dyna#ic $ressure


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e%pressed as (e.), it's possible to transform (e1) to (e/)

"ented tan8

special case of interest for e+uation (e/) is when the orifice area is much lesser than the surface arewhen the pressure inside and outside the tan# is the same " when the tan# has an open surface or 9venthe atmosphere t this situation the (e/) can be transformed to (e0)

9The velocity out from the tan# is e+ual to speed of a freely body falling the distance h9 " also #nown

Torricelli*s Teorem.

E+ample $ outlet )elocity from a )ente( tan,

h I 1 m

8I J % K@1 % 1L1DI 1/ mDs

$ressuried !an8

3f the tan#s is pressuri7ed so that product of gravity and height (g h) is much lesser than the pressure

difference divided by the density, (e/) can be transformed to (e2)

The velocity out from the tan# depends mostly on the pressure difference

E+ample $ outlet )elocity from a pressuri#e( tan,

h I 1 mDs, p1I 6Dm, pI 1 6DmD1I 1, h I 1 m

8I J(D(1"(1)) ( ( " 1)%12 D1%1. C K@1 % 1)L1DI 1KK mDs

*oeicient o Dischar&e ) Friction *oeicient

&ue to friction the real velocity will be somewhat lower than this theoretic e%amples 3f we introduce

friction coefficientc (coefficient of discharge), (e0) can be e%pressed as (e0b)

The coefficient of discharge can be determined e%perimentally For a sharp edged opening it may be aas 2 For smooth orifices it may bee between K0 and 1

luid lowmeters

Common Misspellings: 9ernouli:s ;aw, 9ernulli:s ;aw, 9ernolli:s ;aw, 9ernoulis ;aw, 9ernullis ;aw, 9ernollis ;a


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#)e orifice, nozzle and *enturi flow rate meters use t)e ernoulli&uation to calculate t)e fluid flow rate $y using t)e

pressure difference $etween an o$struction in t)e flow.

3n flow metering devices based onthe $ernoulli E+uationthe downstream pressure after

an obstruction will be lower than the upstream pressure before the obstruction

To understand orifice, no77le and venturi meters it's necessary to e%plore the $ernoulli

E+uation:

!he 7ernoulli /quation and Flow Meters

ssuming a hori7ontal flow (or neglecting a minor elevation difference between the

measuring points) the $ernoulli E+uationcan be modified to:

p#$ #%& ' v#&( p&$ #%& ' v&

& (1)

where

p I pressure

'I density

vI flow velocity

8ertical flow can be adapted by adding elevation heights h# and h&in (1)

ssuming that the velocity profiles are uniform in the upstream and downstream section

the


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E+uation (/) can be modified to mass flow for fluids by simply multiplying with the

density:

m I cd1%2 3&&' + &,p#"p&- % ',# . d

&- /#%& (0)

!hen measuring the mass flow in gases, its necessary to considerate the pressurereduction and change in density of the fluid The formula above can be used with

limitations for applications with relatively small changes in pressure and density

!he %riice $late

The orifice meter consists of a flat orifice plate with a circular hole drilled in it There is a

pressure tap upstream from the orifice plate and another Must downstream There are ingeneral three methods of placing the taps The coefficient of the meter depends upon the

position of taps

lange location Tap location 1 inch upstream and 1 inch downstream from face of orifice

+ena contracta location Tap location 1 pipe diameter actual inside3 upstream and 0.< to 0.= pipe

diameter downstream from face of orifice

*ipe location Tap location 2.> times nominal pipe diameter upstream and = times nominal pipe

diameter downstream from face of orifice

The discharge coefficient " cd " varies considerably with changes in area ratio and the

eynolds number discharge coefficient " cd " of 2 may be ta#en as standard, but thevalue varies noticeably at low values of the eynolds number

Discharge-oefficient

cd

eynolds umber Re

Diameter 10" 10> 10? 10@


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atio

d ( 3&% 3#

0,2 0,?0 0,>A> 0,>A" 0,>A"

0," 0,?1 0,?0< 0,>A= 0,>A=

0,> 0,?2 0,?0= 0,?0< 0,?0

0," 0,A>@ 0,A=" 0,AA< 0,AA>

0,? 0,A> 0,A=1 0,AA2 0,AA>

0,= 0,A" 0,A@= 0,AA1 0,AA>

The flow no//le is recommended for both clean and dirty liquids

The angeability is " to 1

The relative pressure loss is medium

Typical accuracy is B1 to B2 of full range

equired upstream pipe length is 10 to 1?@18200< !easurement of fluid flow by means

of pressure differential devices, *art 18 rifice plates, no//les, and +enturi tubes inserted incircular crosssection conduits running full. eference number8 I% >1?@18200


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weight and specific gravity. ormulas with e#amples.

pstream and downstream area can be calculated as:

A#I .,1/ ((,1 m)D)I ,4@0 m

A&I .,1/ ((,2 m)D)I ,@2 m

Theoretical flow can be calculated from (.):

q IA&+ &,p#"p&- % ',# . ,A&%A#-- /#%&

q I ,@2 J (1 106Dm) D (@ #gDm.)(1 " (,@2 m)D(,4@0 m)) L1D

I ,00 m.Ds

For a pressure difference of 1 #-a (,1 106Dm) " the theoretical flow can becalculated:

q I ,@2 J (,1 106Dm) D (@ #gDm.)(1 " (,@2 m)D(,4@0 m)) L1D

I ,00 m.Ds

The mass flow can be calculated from (0) as:

m ( q '

m ( (,00m.Ds) (@ #gDm.) I /,01 #gDs

Flow 'ate and *han&e in $ressure Dierence

6oteN " The flow rate varies with the s+uare root of the pressure difference

From the e%ample above:

" a tenfold increase in the flow rate re+uires a one hundredfold increase in the pressure

differenceN

!rans#itters and *ontrol Syste#

The nonlinear relationship have impact on the pressure transmitters operating range andre+uires that the electronic pressure transmitters have the capability to lineari7ing the

signal before transmitting it to the control system

+ccuracy


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&ue to the non linearity the turn down rate is limited The accuracy strongly increases inte lower partof the operating range
http://www.engineeringtoolbox.com/49_494.htmlhttp://www.engineeringtoolbox.com/49_494.htmlhttp://www.engineeringtoolbox.com/49_494.html


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An introduction to #urn 'own "atio and flow measurementaccuracy.

Turn"down ratios is often used to compare the span of different flow measurement devices

The Turn &own ratio can be e%pressed as

Turn3on ( Ma5imum 6lo % Minimum 6lo

$oth ma%imum and minimum flow is stated within a specified accuracy and repeatability for

the device

E+ample $ Turnown -atios

n flow instrument with ma%imum 1 #gDs and a minimum flow at . #gDs have

Turn&own I 1 #gDs D . #gDs I / or normally e%presses as /:1

This is a typical value for an orifice plate which in general has turndown ratios between .:1and 0:1

!urnDown 'atio and Measured Si&nal


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3n flow meters as 5rifice and 8enturi meters, the differential pressure increase with the

s+uare of the flow speed

The larger the turndown, the more cramped the measurement signal will be at low flow rates

!he +ccuracy and !urnDown Flow 'atio

>uite often the process instrument manufacturer ma#es accuracy statements for linear scalesat full scale values

The accuracy at lower flow rates are significant higher and can be computed:

Accuracy ( 6ull 7pan Accuracy % 8 Measured 7pan 9 #::

E+ample $ Te Accuracy an( Turnown Flow -atio

3f a manufacturer of process instrumentation states a flow meter to CD" ,0A F*& ( Full *cale

&eflection), the accuracy will be CD" ,0A at ma%imum flow (1A)

3f the flow is reduced to 0A the accuracy would be

,0 A D ( 0 AO 1 ) I CD" 1A

3f the flow is reduced to 0A the accuracy would be


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,0 A D ( 0 AO 1 ) I CD" A

micro flow meter

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