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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'
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 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'
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'
+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
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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
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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