Balances

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Weighing scale

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Emperor

Emperor Jahangir Jahangir (reign !"# $ !%&' weighing his son (reign !"# $ !%&' weighing his son hah Jahanhah Jahan on a weighing scale )y on a weighing scale )y artist *anohar (+

artist *anohar (+ !#, *ughal dynasty, -ndia'.!#, *ughal dynasty, -ndia'. +

+weighing scaleweighing scale (usually ust /scales/ in (usually ust /scales/ in 01 and +ustralian English, /weighing machine/ in01 and +ustralian English, /weighing machine/ in

south +sia

south +sian English or n English or /scale/ in 0 English' is /scale/ in 0 English' is aa measuring instrumentmeasuring instrument for determining the for determining the weight

weight or or massmass of an o)ect. of an o)ect. +

+ spring scalespring scale measures weight )y the distance a measures weight )y the distance a springspring deflects under its deflects under its load. +load. +balancebalance

compares the

compares the tor2uetor2ue on the arm due to the on the arm due to the sample weight to thesample weight to the tor2uetor2ueon the arm due to aon the arm due to a standard reference weight using a hori3ontal

standard reference weight using a hori3ontal lever lever . 4alances are different from scales, in that. 4alances are different from scales, in that a )alance measures

a )alance measures massmass (or more specifically (or more specifically gravitational massgravitational mass', where as a scale measures', where as a scale measures weight

weight (or more (or more specificallyspecifically, either the, either the tensiontension or or compressioncompressionforceforce of constraint provided of constraint provided )y the scale'. W

)y the scale'. Weigheighing scales are uing scales are used in many indused in many industrial and commercstrial and commercial applications, andial applications, and products from feathe

products from feathers to loaded tractor$rs to loaded tractor$trailers are sold )y wetrailers are sold )y weight. peciali3ed might. peciali3ed medicaledical scales and )athroom scales are used to measure the

scales and )athroom scales are used to measure the )ody weight )ody weight of human )eings. of human )eings.

History

4alance scale in the Egyptian

4alance scale in the Egyptian Book of the Dead Book of the Dead The )alance scale is such a

The )alance scale is such a simple device that its usage lsimple device that its usage likely far predates the evidence. Whatikely far predates the evidence. What has allowed archaeologists to link artifacts

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a)solute weight. The )alance scale itself was pro)a)ly used to determine relative weight long )efore a)solute weight.=>

The oldest evidence for the e<istence of weighing scales dates to c. %8""$"" 4.?.E. in the -ndus @iver valley (modern$day 7akistan'. 4efore then no )anking was performed due to lack of scales. 0niform, polished stone cu)es discovered in early settlements were pro)a)ly used as weight$setting stones in )alance scales. +lthough the cu)es )ear no markings, their

weights are multiples of a common denominator. The cu)es are made of many different kinds of stones with varying densities. ?learly their weight, not their si3e or other characteristics, was a factor in sculpting these cu)es.=%>_{-n Egypt, scales can )e traced to around & 4.?.E.,}

)ut their usage pro)a)ly e<tends much earlier. ?arved stones )earing marks denoting weight and the Egyptian hieroglyphic sym)ol for gold have )een discovered, which suggests that Egyptian merchants had )een using an esta)lished system of weight measurement to catalog gold shipments and9or gold mine yields. +lthough no actual scales from this era have

survived, many sets of weighing stones as well as murals depicting the use of )alance scales suggest widespread usage.=6>

Aariations on the )alance scale, including devices like the cheap and inaccurate )ismar,=8>

)egan to see common usage )y c. 8"" 4.?.E. )y many small merchants and their customers. + plethora of scale varieties each )oasting advantages and improvements over one another appear throughout recorded history, with such great inventors as Beonardo da Ainci lending a personal hand in their development.=#>

Even with all the advances in weighing scale design and development, all scales until the seventeenth century ?.E. were variations on the )alance scale. +lthough records dating to the !""s refer to spring scales for measuring weight, the earliest design for such a device dates to &&" and credits @ichard alter, an early scale$maker.=!>_{pring scales came into common}

usage in 8" when @. W. Winfield developed the candlestick scale for use in measuring letters and packages.=&>_{7ostal workers could work more 2uickly with spring scales than}

)alance scales )ecause they could )e read instantaneously and did not have to )e carefully )alanced with each measurement.

4y the ;8"s various electronic devices were )eing attached to these designs to make

readings more accurate. These were not true digital scales as the actual measuring of weight still relied on springs and )alances.=>=;>_{Boad cells, small nodes that convert pressure to a}

digital signal, have their )eginnings as early as the late$nineteenth century, )ut it was not until the late$twentieth century that they )ecame accurate enough for widespread usage.=">

Balance

Thebalance (also balance scale,beam balance and laboratory balance' was the first mass

measuring instrument invented.=>_{-n its traditional form, it consists of a pivoted hori3ontal}

lever of e2ual length arms, called the )eam, with a weighing pan, also called scale, scalepan,

or bason (o)solete=%>', suspended from each arm (which is the origin of the originally plural

term /scales/ for a weighing instrument'. The unknown mass is placed in one pan, and

standard masses are added to the other pan until the )eam is as close to e2uili)rium as possi)le. -n precision )alances, a slider mass is moved along a graduated scale. The slider position gives a fine correction to the mass value. +lthough a )alance technically compares

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weights, not masses, the weight of an o)ect is proportional to its mass, and the standard weights used with )alances are usually la)eled in mass units.

Two "$decagram masses

*asses of #", %", , %, # and " gram

4alances are used for precision mass measurement, )ecause unlike spring scales their

accuracy is not affected )y differences in the local gravity, which can vary )y almost ".#C=6>

at different locations on Earth. + change in the strength of the gravitational field caused )y moving the )alance will not change the measured mass, )ecause the moments of force on either side of the )alance )eam are affected e2ually. -n fact, a )alance will measure the correct mass even on other planets or moons, or any location that e<periences a constant gravity or acceleration.

Aery precise measurements are achieved )y ensuring that the )alanceDs fulcrum is essentially friction$free (a knife edge is the traditional solution', )y attaching a pointer to the )eam which amplifies any deviation from a )alance position and finally )y using the lever

principle, which allows fractional masses to )e applied )y movement of a small mass along the measuring arm of the )eam, as descri)ed a)ove. For greatest accuracy, there needs to )e an allowance for the )uoyancy in air, whose effect depends on the densities of the masses involved.

The original form of a )alance consisted of a )eam with a fulcrum at its center. For highest accuracy, the fulcrum would consist of a sharp A$shaped pivot seated in a shallower A$shaped )earing. To determine the mass of the o)ect, a com)ination of reference masses was hung on

one end of the )eam while the o)ect of unknown mass was hung on the other end (see

)alance and steelyard )alance'. For high precision work, the center )eam )alance is still one of the most accurate technologies availa)le=citation needed >_{, and is commonly used for cali)rating}

test weights.

To reduce the need for large reference masses, an off$center )eam can )e used. + )alance with an off$center )eam can )e almost as accurate as a scale with a center )eam, )ut the off$

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center )eam re2uires special reference masses and cannot )e intrinsically checked for accuracy )y simply swapping the contents of the pans as a center$)eam )alance can. To reduce the need for small graduated reference masses, a sliding weight called a poise can )e installed so that it can )e positioned along a cali)rated scale. + poise adds further intricacies to the cali)ration procedure, since the e<act mass of the poise must )e adusted to the e<act lever ratio of the )eam.

+n aluminum, mass$produced )alance scale sold and used throughout ?hina. ote the larger ring )eneath the userDs right hand. The scale can )e inverted, and held )y this ring. This

produces greater leverage, and is used for heavier loads. This scale is sold with an aluminum pan, )ut this is rarely used. The cost appro<imately % yuan (G% 0' in %". 7hoto taken in

5ainan 7rovince, ?hina.

For greater convenience in placing large and awkward loads, a platform can )e floated on a

cantilever )eam system which )rings the proportional force to a noseiron )earing this pulls

on a stilyard rod to transmit the reduced force to a conveniently si3ed )eam. Hne still sees

this design in porta)le )eam )alances of #"" kg capacity which are commonly used in harsh environments without electricity, as well as in the lighter duty mechanical )athroom scale (which actually uses a spring scale, internally'. The additional pivots and )earings all reduce the accuracy and complicate cali)ration the float system must )e corrected for corner errors )efore the span is corrected )y adusting the )alance )eam and poise. uch systems are

typically accurate to at )est 9",""" of their capacity, unless they are e<pensively engineered.=citation needed >

ome mechanical )alances also use dials (with counter)alancing masses instead of springs', a hy)rid design with some of the accuracy advantages of the poise and )eam )ut the

convenience of a dial reading.

Analytical balance

+n analytical balance is a class of )alance designed to measure small mass in the su)$

milligram range. The measuring pan of an analytical )alance (". mg or )etter' is inside a transparent enclosure with doors so that dust does not collect and so any air currents in the room do not affect the )alanceDs operation. This enclosure is often called a draft shield. The use of a mechanically vented )alance safety enclosure, which has uni2uely designed acrylic airfoils, allows a smooth tur)ulence$free airflow that prevents )alance fluctuation and the measure of mass down to  Ig without fluctuations or loss of product.=citation needed >_{+lso, the}

sample must )e at room temperature to prevent natural convection from forming air currents inside the enclosure from causing an error in reading. ingle pan mechanical su)stitution )alance maintains consistent response throughout the useful capacity is achieved )y

maintaining a constant load on the )alance )eam, thus the fulcrum, )y su)tracting mass on the same side of the )eam to which the sample is added.=citation needed >

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Scales

Spring scales

-n a spring scale, the spring stretches (as in a hanging scale in the produce department of a grocery store' or compresses (as in a simple )athroom scale' in proportion to how hard the Earth pulls down on the o)ect. -t is therefore affected )y the local gravity. 4y 5ookeDs law, every spring has a proportionality constant that relates how hard it is pulled to how far it stretches. Weighing scales use a spring with a known spring constant (see 5ookeDs law' and measure the displacement of the spring )y any variety of mechanisms to produce an estimate of the gravitational force applied )y the o)ect, which can )e simply hung from the spring or set on a pivot and )earing platform. @ack and pinion mechanisms are often used to convert the linear spring motion to a dial reading.

pring scales measure force, which is the tension force of constraint acting on an o)ect, opposing the force of gravity. They are usually cali)rated so that measured force translates to mass at earthDs gravity. They have two sources of error that )alances do not the measured weight varies with the strength of the local gravitational force, )y as much as ".#C at different locations on Earth, and the elasticity of the measurement spring can vary slightly with temperature. pring scales which are legal for commerce either have temperature compensated springs or are used at a fairly constant temperature, and must )e cali)rated at the location in which they are used, to eliminate the effect of gravity variations.

Pendulum balance scales

7endulum type scales do not use springs. This design uses pendulums and operates as a )alance and is unaffected )y differences in gravity. +n e<ample of application of this design

are scales made )y the Toledo cale ?ompany=8> Electronic analytical "balance" scale

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Electronic analytical scales measure the force needed to counter the mass )eing measured rather than using actual masses. +s such they must have cali)ration adustments made to compensate for gravitational differences.=#>_{They use an electromagnet to generate a force to}

counter the sample )eing measured and outputs the result )y measuring the force needed to achieve )alance. uch measurement device is called electromagnetic force restoration sensor.

=!>_{This makes calling it an /analytical )alance/ a misnomer, )ecause it should actually )e}

called an /analytical scale/, due to it measuring force, rather than gravitational mass.=citation needed >

Strain gauge scale

igital kitchen scale, a strain gauge scale

-n electronic versions of spring scales, the deflection of a )eam supporting the unknown weight is measured using a strain gauge, which is a length$sensitive electrical resistance. The capacity of such devices is only limited )y the resistance of the )eam to deflection. The

results from several supporting locations may )e added electronically, so this techni2ue is suita)le for determining the weight of very heavy o)ects, such as trucks and rail cars, and is used in a modern weigh)ridge.

Hydraulic or pneumatic scale

-t is also common in high$capacity applications such as crane scales to use hydraulic force to sense weight. The test force is applied to a piston or diaphragm and transmitted through hydraulic lines to a dial indicator )ased on a 4ourdon tu)e or electronic sensor.

esting and certi!ication

*ain article: Aerification and validation

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cales used for trade purposes in the tate of Florida, as this scale at the checkout in a cafeteria, are inspected for accuracy )y the F+?Ds 4ureau of Weights and *easures. *ost countries regulate the design and servicing of scales used for commerce. This has tended to cause scale technology to lag )ehind other technologies )ecause e<pensive

regulatory hurdles are involved in introducing new designs. evertheless, there has )een a recent trend to /digital load cells/ which are actually strain$gauge cells with dedicated analog converters and networking )uilt into the cell itself. uch designs have reduced the service pro)lems inherent with com)ining and transmitting a num)er of %" millivolt signals in hostile

environments.

overnment regulation generally re2uires periodic inspections )y licensed technicians using weights whose cali)ration is tracea)le to an approved la)oratory. cales intended for non$ trade use such as those used in )athrooms, doctorDs offices, kitchens (portion control', and price estimation ()ut not official price determination' may )e produced, )ut must )y law )e

la)elled /ot Begal for Trade/ to ensure that they are not repurposed in a way that

eopardi3es commercial interest.=citation needed > -n the 0nited tates, the document descri)ing how

scales must )e designed, installed, and used for commercial purposes is -T 5and)ook 88. Begal For Trade certification usually approve the reada)ility as repeata)ility9" to ensure a ma<imum margin of error of "C.

4ecause gravity varies )y over ".#C over the surface of the earth, the distinction )etween force due to gravity and mass is relevant for accurate cali)ration of scales for commercial purposes. 0sually the goal is to measure the mass of the sample rather than its force due to

gravity at that particular location.

Traditional mechanical )alance$)eam scales intrinsically measured mass. 4ut ordinary electronic scales intrinsically measure the gravitational force )etween the sample and the earth, i.e. the weight of the sample, which varies with location. o such a scale has to )e re$ cali)rated after installation, for that specific location, in order to o)tain an accurate indication of mass.

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These scales are used in the )akery, delicatessen, seafood, meat, produce, and other perisha)le departments. upermarket scales can print la)els and receipts (in )akery

specially', marks weight9count, unit price, total price and in some cases tare, a supermarket la)el prints weight9count, unit price and total price. ome modern supermarket scales print an @F- tag that can )e used to track the item for tampering or returns. -n most cases these type of scales have a sealed cali)ration so that the reading on the display is correct and cannot )e tampered with $ in the 0+ the approval is TE7, for outh +frica it is +4, in the 01 it is H-*B.

Sources o! error

+ two$pan )alance.

ome of the sources of error in weighing are:

• 4uoyancy, H)ects in air develops )uoyancy force that is directly proportional to the

volume of air displaced. The difference in density of air due to )arometric pressure and temperature creates errors.=&>

• Error in mass of reference weight

• +ir gusts, even small ones, which push the scale up or down

• Friction in the moving components that cause the scale to reach e2uili)rium at a

different configuration than a frictionless e2uili)rium should occur.

• ettling air)orne dust contri)uting to the weight

• *is$cali)ration over time, due to drift in the circuitDs accuracy, or temperature change • *is$aligned mechanical components due to thermal e<pansion9contraction of

components

• *agnetic fields acting on ferrous components

• Forces from electrostatic fields, for e<ample, from feet shuffled on carpets on a dry

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• ?hemical reactivity )etween air and the su)stance )eing weighed (or the )alance

itself, in the form of corrosion'

• ?ondensation of atmospheric water on cold items • Evaporation of water from wet items

• ?onvection of air from hot or cold items

• ravitational differences for a scale which measures force, )ut not for a )alance.=> • Ai)ration and seismic distur)ances

Symbolism

The scales (specifically, a two pan, )eam )alance' are one of the traditional sym)ols of ustice, as wielded )y statues of Bady Justice. This corresponds to the use in metaphor of

matters )eing /held in the )alance/. -t has its origins in ancient Egypt.=citation needed >

cales are also the sym)ol for the astrological sign Bi)ra.

#ootnotes

. $ anders, B. + hort 5istory of Weighing. 4irmingham, England: W. L T.

+very, Btd. (;8&, revised ;!"' (http:99www.averyweigh$troni<.com9download.asp<M didN!%8;

%. $ 7etruso, 1arl *. OEarly Weights and Weighing in Egypt and the -ndus

AalleyP, * 4ulletin (4oston *useum of Fine +rts,', Aol. &;, (;', pp. 88$# (http:99www.stor.org9sta)le98&!68 '

6. $ 7etruso, 1arl *. OEarly Weights and Weighing in Egypt and the -ndus

8. $ -nternational ociety of +nti2ue cale ?ollectors: cale Types @etrieved

%"%$"$"#.

#. $ +very Weigh$Troni<. The 5istory of Weighing. (http:99www.averyweigh$

troni<.com9main.asp<MpN..6.8'

!. $ 7etruso, 1arl *. OEarly Weights and Weighing in Egypt and the -ndus

&. $ 4rass, 4rian (%""!': O?andlesticks, 7art P, E2uili)rium, o. , 7p. 6";;$

6"; (http:99www.isasc.org9E2uili)rium94ackQ-ssues9%""!$.pdf '

. $ 7etruso, 1arl *. OEarly Weights and Weighing in Egypt and the -ndus

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;. $ +very Weigh$Troni<. The 5istory of Weighing. (http:99www.averyweigh$

troni<.com9main.asp<MpN..6.8'

". $ Hmega Engineering: O-ntroduction to Boad ?ellsP, Engineering Technical

@eference. (http:99www.omega.com9prodinfo9loadcells.html'

. $ anders, B. + hort 5istory of Weighing. 4irmingham, England: W. L T.

+very, Btd. (;8&, revised ;!"' (http:99www.averyweigh$troni<.com9download.asp<M didN!%8;

%. $ /+ 7ractical ictionary of the English and erman Banguages/ (!;'

p."!;

6. $ 5odgeman, ?harles, Ed. (;!'. Handbook of Chemistry and Physics, 44th Ed.. ?leveland, 0+: ?hemical @u))er 7u)lishing ?o.. p.68"$68#

8. $ => #. $ +L training material !. $ ensors *ag &. $ 0? H7% hand)ook . $ http:99www.nist.gov9cali)rations9upload9n)s;6$8.pdf

E%ternal links

• ational ?onference on Weights and *easures, -T 5and)ook 88,Specifications,

Tolerances, And ther Technical !e"#irements for $ei%hin% and &eas#rin% De'ices,

%""6

• +nalytical 4alance article at ?hemBa) • 5owtuffWorks:-nside a )athroom scale

&hide' v t e ypes o! tools Cleaning • )room

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• )rush

• feather duster • floor )uffer • hataki

• mop

• mop )ucket cart • needlegun scaler • pipe cleaner • pressure washer • sand)laster • sponge • s2ueegee • steam mop • ta(ashi • vacuum cleaner Cutting and abrasive • )lade • )olt cutter • )roach

• ceramic tile cutter • chisel

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• countersink

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• ra3or • reamer • sandpaper • saw • scalpel • scissors • snips • steel wool • surform • switch)lade • utility knife • water et cutter • wire )rush • wire stripper (arden • ad3e • auger • a<e • )illhook • )ow saw • )roadcast spreader • )rush hook

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• punch

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metalworking • automatic lathe • )all$peen hammer • )roaching machine • drill press

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)easuring and

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• )eam compass • caliper

• chalk )o<

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• ig

• laser level • laser line level

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• flashlight • funnel • 5alligan )ar • hand truck • helping hand • kelly tool • ladder • pencil • tool)o< • sawhorse • vise • wheel)arrow • work)ench

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Balances

Weighing scale

Weighing scale

Contents

Contents

History

History

Balance

Scales

esting and certi!ication

Sources o! error

Symbolism

See also

#ootnotes

E%ternal links

+avigation menu