force weight flow measurement

7/24/2019 Force Weight Flow Measurement

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FORCE, WEIGHT &

FLOWMEASUREMENT

UNIT-6


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CONTENT

Introduction to force, weight and ow

measurement

Strain gauge and its function

Load cell, Principle and Operation

Flow measurement using Orice plate


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Introduction to force, ei!"#nd $o %e#ure%entForce Me#ure%ent

Force is dened as a cause that produces resis

obstruction to any moing body, or changes the mo

body, or tends to produce these e!ects"

F#$%

&here,

F#force

$# mass, in 'g

% # acceleration, in 'g(m)


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Force Me#ure%ent

Met"od Of Force Me#ure%ent Are'

*ydraulic force meter"

Pneumatic force meter"

+lectric force transducers"


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Force Me#ure%ent

Li%it#tion to t"e %et"od of %e#urin! force

Force must be either reasonably constant in

changing gradually and continuously in one direction

he force must act perpendicular to the platform

scale, otherwise only the cosine component is measu

he measurement re-uires correction for local a

graitational constant"


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Wei!"t Me#ure%ent

&eight is the force with which a body is attracte

earth"

he relation between weight and mass is"

&eight# mass . acceleration


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Wei!"t Me#ure%ent

he fundamental principles used in practically all met

measuring weights are/

a" 0omparison with 1nown weights"

b" $easuring deection of a body, using *oo1

according to which the strain is proportional to stress

c" $easuring the hydraulic or pneumatic pressure re

support the un1nown weight"


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Wei!"t Me#ure%ent

d" $easuring the electric current in a coil whose m

eld supports the un1nown weight"

e" $easuring displacement of a li-uid, using %rch

principle, that a oating body displaces its own w

a li-uid"

f" Load cells"


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F(o Me#ure%ent

It is made for determining the proportions amount of materials owing in or out of a proces

&ithout ow measurement, plant material ba

-uality control and een the operation continuous process would be impossible"

Flow measurement is made both for uids and s


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F(o Me#ure%ent

Met"od of F(o Me#ure%ent #re'

Inferential type ow meter"

2uantity ow meter"

$ass ow meter"


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Str#in !#u!e #nd it) functio

he most popular method for measuring force strain gauge"

Strain gauge is one of the most popular t

transducer" It has got a wide range of applications"


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Str#in !#u!e

It can be used for measurement of force, tor-ue, pacceleration and many other parameters"

he basic principle of operation of a strain gage is

&hen strain is applied to a thin metallic wdimension changes, thus changing the resistanc

wire"


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*e+nition

% strain gauge is an e3ample of passie transdu

conerts a mechanical displacement into a ch

resistance"

% strain gauge is a thin, wafer4li1e deice that

attached to a ariety of materials to measure

strain"


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#ic Str#in-G#u!e T"eor

he relationship between resistance change and strain in the foil o

in strain4gauge construction can be e3pressed as

where ' is dened as the gauge factor of the foil or wire,

56 is the resistance change due to strain,

6 is the initial resistance,

5L is the change in length,

L is the original length of the wire or foil, and


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Str#in G#u!e


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Str#in !#u!e

% strain gauge is attached to the diaphragm when the diaphragm e3es due to the process

applied on it, the strain gauge stretches or comp

his deformation of the strain gauge cau

ariation in its length 8 cross section area due

its resistance also changes"


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Str#in !#u!e

he resistance change of a strain gauge is conto oltage by connecting one, two, or four

gauges, as of a &heatstone bridge and

e3citation to the bridge"

he bridge output oltage is a measure of the

sensed by the strain gauge"


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G


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Contruction #nd or.in!

wo strain gauges 6sg9 and 6sg: are mounted in suthat increasing pressure increases their resistance"

he remaining two strain gauge, 6sg; and 6sg< are

so that increasing pressure decrease their resistance

%t balance, when there is no pressure, no curre

through the galanometer ="


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Contruction #nd or.in!

&hen the pressure is applied, the strain gauge stre

compresses accordingly and the bridge circuit becomes b

hus the current ows through the galanomete

unbalanced condition"

So the change in current indicates the change in

pressure"


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T/e

#ed on /rinci/(e of or.in! ' $echanical

+lectrical

Pie>oelectric

#ed on %ountin! ' ?onded strain gauge @nbonded strain gauge


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T/e

#ed on contruction 'Foil strain gauge

Semiconductor strain gauge

Photoelectric Strain gauge


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Un0onded Str#in G#u!e


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Un0onded Str#in G#u!e

@nbonded strain gauge has a resistance wire stretch

two frames"

he rigid pins of the two frames are insulated"

&hen the wire is stretched due to an applied force, t

a relatie motion between the two frames and thus

produced, causing a change in resistance alue"

his change of resistance alue will be e-ual to the st


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onded Str#in G#u!e


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onded Str#in G#u!e

% bonded strain gauge will be either a wire type or a fo

It is connected to a paper or a thic1 plastic lm suppor

he measuring leads are soldered or welded to the ga

he bonded strain gauge with the paper bac1ing is c

to the elastic member whose strain is to be measured


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Ad1#nt#!e

Sensitie to small pressure change" Small si>e"

=ood accuracy"

Fast response"


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*i#d1#nt#!e

6e-uire constant oltage supply" +lectrical readout is necessary"

emperature compensation re-uired"


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Lo#d Ce((

% load cell is a transducer that is used to conert a electrical signal"

he most common type is a strain gauge load cell"


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Lo#d Ce((

Load cell is a passie transducer or sensor whichconerts applied force into electrical signals" healso referred to as ALoad transducersB"

Load cells use di!erent operating principles, i>"

Load 0ells based on uid pressure

Load 0ells based on elasticity

Load 0ells based on magnetostriction e!ect orpie>oelectric e!ect


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MEASURMENT 2RINICI2LE

Load cell primarily consists of a spring material and strain gage" Spring material causes strain due to applied load and strain gage changes its resista

the change in strain"

34 S/rin! M#teri#(

he sensing or spring element is the main structural component of the load cell" he element is designed in such a way that it deelops a strain, direload applied"

&hen e3ternal force is applied, a molecular force wor1s between the molecules constituting the obDect, generating an internal force that tries to prethe applied force" &hen the e3ternal force is balanced with the internal force generated inside the obDect, the deformation of the obDect ceases" %t thiforce per unit area that is generated on the cross section of the obDect is called the AstressB and the change in dimensions per unit original dimensionspring material deelops the strain in proportion to applied force"

In order to enhance the performance of a load cell, the characteristics of the spring material are ery important"

E 0reep, the phenomenon that occurs when the deformation of an obDect caused by e3ternal force becomes larger with time, should be small"

E he material should hae a high proportional limit, which guarantees a wide range of linearity"

E Secular ariation, change in an obDect due to permanent stress with passage of time, should be small

E he resistance to impact should be high"

E It should hae good wor1ability"

7ic1el4chrome4molybdenum steel, stainless steel, and aluminium steel are some of the materials meeting these re-uirements"


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54 Str#in G#!e

Strain gages utilise the principle of change in resistance of many metals ware elongated or contracted"

Since resistance depends upon the resistiity, length and cross sectional amaterial, the same metallic wire will hae di!erent electrical resistance dwhether it is elongated or contracted" he longer the metallic string becolarger the resistance" he strain gauge utili>es this principle and is denedeice whose electrical resistance aries in proportion to the amount of stdeice"

he metallic strain gauge consists of a ery ne wire or, more commonly,arranged in a grid pattern" he grid pattern ma3imi>es the amount of metfoil subDect to strain in the parallel direction" he cross sectional area of thminimi>ed to reduce the e!ect of shear strain and Poisson Strain" he gridto a thin bac1ing, called the carrier, which is attached directly to the test herefore, the strain e3perienced by the test specimen is transferred direstrain gauge, which responds with a linear change in electrical resistancegauges are aailable commercially with nominal resistance alues from :with 9;, :H, and 9 G being the most common alues"


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% load cell is made by bonding strain gauges to material" o eciently detect the strain, strain gare bonded to the position on the spring materiathe strain is ma3imum" &hen the stress caused applied force to an obDect is below the proportiothe strain aries linearly with the stress and the

resistance alue of the strain gauge aries lineathe deformation"


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6elation of the change in strain to the change inresistance is goerned by the =age factor, denthe ratio of change in resistance per unit resistathe change in length per unit length of the strain

herefore,

0hange in gage resistance # =age 6esistance J Factor J Strain"

=age factor of metallic strain gages is appro3" 9


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CLASSIFICATION' ASE* ON SHA2ES OF S2MATERIAL

Fie commonly used load cell shapes are

9" 0olumn ype

It uses a simple structure and uses ; strain gage

in longitudinal and other in transerse direction"be used for both tension and compressionmeasurements"


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9" 0olumn ype

; 6ober al pe MNouble be


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;" 6oberal ype MNouble4beype, Parallel4beam ypehese load cells use bending as the sensing

principle" &hen a force MF is applied to the 6obetype load cell, strain gauge 9 contracts while thegauge ; stretches" alue of strain depends uponwidth of the beam"

his structure is suitable for high4precision load

?ending beams o!er high strain leels at relatiforces, which ma1e them ideal for low capacity lcells" ypical measurement range is generally be91g and 9 , and are not suitable for large capac

; 6oberal ype MNouble be


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;" 6oberal ype MNouble4beype, Parallel4beam ype


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:" Shear ype

Strain gauges are bonded at a


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:" Shear ype


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H" Niaphragm ype

he diaphragm4type load cell has a round shapeprimary adantage of using a diaphragm4type lois that its height can be lowered and it is resistantranserse loading"


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H" Niaphragm ype


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Lo#d Ce((


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Lo#d Ce((

A load cell is a transducer which converts force

measurable electrical output.

There are many varieties of load cell, strain gauge b

cells are the most commonly used type.

d ((


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Lo#d ce((

The strain gauge load cell is an electromechanical t

which translates changes in force or weight in to ch

voltage.

The change in voltage calibrated directly in terms of for

applied to the cell.


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L d ((


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Lo#d ce((


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Contruction Strain gauge load cell is constructed of wire grids (know

gauge) bonded to precisely machined supporting colum

The grids are connected electrically to form a balanced W

bridge.

Additional compensation resistors are added to the

maintaining the accuracy of the bridge over a wid

temperature.


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Wor.in! The principle of operation of the strain gauge load c

upon the deflection of the column.

When a force or load to be measured is applied to the

column, the column is compressed causing the wires

bonded to sides ! and "! to decrease in length and

cross sectional area, thus decreasing their electrical re


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Wor.in!

The grid bonded to sides # and "# are unaffecte

comparison of the column.

These grids are attached to the column to minimi$e erro

temperature variations.

Wor.in!


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Wor.in!

When the column is stressed in tension, the measurin

(strain gauge) are lengthened and their resistance incre

The change in resistance of the strain gauge causes the

bridge, which is directly proportional to the force (load

the column, to become unbalanced, thus providing a

voltage signal with respect to the force .

Wor.in!


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Wor.in!

Strain gauge load cells are made for compression, t

universal loading of either type.

The output signals of strain gauges are in the range ! to

volt of e&citation, and are proportional to the e&citation.

The e&citation voltage can be A' or ' in the range of

Accuracy of the device is +.!- of full scale output.

CLASSIFICATION' ASE* ON


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C SS C O S O*IRECTION OF LOA*ING



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OUTER SHA2E



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TIGHTNESS

A*7ANTAGES


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A*7ANTAGES

They are small and compact in si$e.

They are well suited for measurements where an electri

signal is desired.

They are ine&pensive.

They respond rapidly to load variations.

They can measure up to the rated capacity, with deflection in

.!#mm to .#mm.

LIMITATIONS


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LIMITATIONS

verloads greatly in e&cess of their ratings should be av

LOA* CELL


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LOA* CELL

F(o %e#ure%ent uin! Or


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!/(#te

Ori+ce 2(#te

Ori+ce 2(#te


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Ori+ce 2(#te

%n ori+ce /(#teis deice used for measuring ow

It uses the same principle as a enturi no>>le,

?ernoulliQs principle which states that there is a rela

between the pressure of the uid and the elocit

uid"

&hen the elocity increases, the pressure decrea

ice ersa"

Ori+ce 2(#te


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Ori+ce 2(#te


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%n orice plate is a thin plate with a hole in it, which i

placed in a pipe" &hen a uid passes through the oripressure builds up slightly upstream of the orice but uid is forced to conerge to pass through the hole, thelocity increases and the uid pressure decreases" %downstream of the orice the ow reaches its point ofma3imum conergence, the vena contractawhere thereaches its ma3imum and the pressure reaches its mi

?eyond that, the ow e3pands, the elocity falls and tpressure increases" ?y measuring the di!erence in upressure across tapping upstream and downstream ofplate, the ow rate can be obtained from ?ernoulliQs eusing coecients established from e3tensie research


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Basic Operating Principle


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S/uare root relationship in the flow rate is proportions/uare of the differential pressure.

FLOW MEASUREMENT USINORIFICE PLATE.


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ORIFICE PLATE.

0t acts as a primary device.

The orifice plate restricts the flow of a fluid to produc

the differential pressure across the plate.

The result is high pressure upstream and low pressur

downstream i.e. proportional to the s/uare of flo

velocity.

rifice plate produces greater pressure loss than othe

primary devices.

ORIFICE PLATE


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*ariable head flow meters operate on the principle that

restriction or obstruction in the line or plane of a flowing

introduced by the orifice plate or venture tube, produces

differential pressure across the restriction element whic

proportional to the flow rate.

ORIFICE PLATE


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Two standard design of orifice plate are 1

Thin orifice plate.

Sharp s/uare edge orifice plate.

ORIFICE PLATE


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0mportant term used in orifice plate is diameter rati

2d 3 diameter of the orifice of the primary eleme

4pstream diameter of the pipe

T/e of ori+ce /(#te


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T/e of ori+ce /(#te

0oncentric orice plate

+ccentric orice plate

Segmental orice plate

2uadrant orice plate

Concentric ori+ce /(#te


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/

C t i i+


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Concentric ori+ce/(#te 0t is a simplest and largest e&pensive of the head meter 0t is made of stainless steel.

0ts thickness varies from %.!5mm and to !#.5mm dep

pipe line si$e and flow velocity.

0t has a circular hole in the middle.

0t is also made up of material like nickel, 'hromel,

bron$e etc to withstand corrosive effects of the fluid.

Eccentric ori+ce /(#te


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/

Eccentric ori+ce /(#te


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0t is similar to concentric plate e&cept for the offset hole

bored tangential to a circle.

6ocation of the bore prevents accumulation of solid ma

foreign particles and makes it useful for measuri

containing solids.

Se!%ent#( ori+ce /(#te


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Se!%ent#( ori+ce /(#te


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0t is used for the same type of services as the eccent

plate.

0t has a hole, which is a segment of a circle.

0t is installed in such a way that the curved sectio

opening coincide with the lower surface of the pipe.

8u#dr#nt ed!e ori+ce /(#te


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8u#dr#nt ed!e ori+ce /(#te


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0t is used for flow such as slurries and viscous flows.

0t is constructed in such that the edge is rounded t

/uarter circles.

The plate has a concentric opening with a rounded

edge rather than sharp.

7ent #nd dr#in "o(e in ori+c/(#te


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/(#te

7ent #nd dr#in "o(e in ori+c/(#te


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/(#te A drain hole is provided at the bottom when gases are

to allow the condensate to pass in order to prevent it

up at the orifice plate.

A vent hole is located at the top when li/uids are me

that gases can pass and gas pockets cannot build up.

A*7ANTAGES


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6ow cost.

0t offers the widest application coverage of any type of m

0t is accurate (78!29to78#-)

0t can be easily removed without shutting down the proc

0t is adaptable to any pipe si$e and flow rate

*ISA*7ANTAGES


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There is relatively high pressure loss in it.

0t is difficult to use for slurry services.

0t e&hibits a s/uare root relationship between head :

rather than linear characteristics.

0t is difficult to remove pulsating flow with this type of m


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Flow hrough an Orice $eter


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;low Through an rifice


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P9 P;

P

P9

d N

$echanical ow meters
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+lectromagnetic4F$
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coriolis ow meter


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orte3


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ariable area


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Ni!erential pressure


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force weight flow measurement

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