Weighing scale
1
Weighing scale
A weighing scale (usually just "scales" in UK and Australian English,
"weighing machine" in south Asian English or "scale" in US English)
is a measuring instrument for determining the weight or mass of an
object. A spring scale measures weight by the distance a spring
deflects under its load. A balance compares the torque on the arm due
to the sample weight to the torque on the arm due to a standard
reference weight using a horizontal lever. Balances are different from
scales, in that a balance measures mass (or more specifically
gravitational mass), whereas a scale measures weight (or more
specifically, either the tension or compression force of constraint
provided by the scale). Weighing scales are used in many industrial
and commercial applications, and products from feathers to loaded
tractor-trailers are sold by weight. Specialized medical scales and
bathroom scales are used to measure the body weight of human beings.
History
Emperor Jahangir (reign 1605 - 1627) weighing
his son Shah Jahan on a weighing scale by artist
The balance scale is such a simple device that its usage likely far
Manohar (AD 1615, Mughal dynasty, India).
predates the evidence. What has allowed archaeologists to link artifacts
to weighing scales are the stones for determining absolute weight. The
balance scale itself was probably used to determine relative weight long before absolute weight.[1]
The oldest evidence for the existence of weighing scales dates to c. 2400-1800 B.C.E. in the Indus River valley
(modern-day Pakistan). Uniform, polished stone cubes discovered in early settlements were probably used as
weight-setting stones in balance scales. Although the cubes bear no markings, their weights are multiples of a
common denominator. The cubes are made of many different kinds of stones with varying densities. Clearly their
weight, not their size or other characteristics, was a factor in sculpting these cubes.[2] In Egypt, scales can be traced
to around 1878 B.C.E., but their usage probably extends much earlier. Carved stones bearing marks denoting weight
and the Egyptian hieroglyphic symbol for gold have been discovered, which suggests that Egyptian merchants had
been using an established system of weight measurement to catalog gold shipments and/or gold mine yields.
Although no actual scales from this era have survived, many sets of weighing stones as well as murals depicting the
use of balance scales suggest widespread usage.[3]
Variations on the balance scale, including devices like the cheap and inaccurate bismar began to see common usage
by c. 400 B.C.E. by many small merchants and their customers. A plethora of scale varieties each boasting
advantages and improvements over one another appear throughout recorded history, with such great inventors as
Leonardo Da Vinci lending a personal hand in their development.[4]
Even with all the advances in weighing scale design and development, all scales until the seventeenth century C.E.
were variations on the balance scale. Although records dating to the 1600s refer to spring scales for measuring
weight, the earliest design for such a device dates to 1770 and credits Richard Salter, an early scale-maker.[5] Spring
scales came into common usage in 1840 when R. W. Winfield developed the candlestick scale for use in measuring
letters and packages.[6] Postal workers could work more quickly with spring scales than balance scales because they
could be read instantaneously and did not have to be carefully balanced with each measurement.
By the 1940s various electronic devices were being 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 balances.[7] [8] Load cells,
Weighing scale
2
small nodes that convert pressure to a digital signal, have their beginnings as early as the late-nineteenth century, but
it was not until the late-twentieth century that they became accurate enough for widespread usage.[9]
Balance
The balance (also balance scale, beam balance and laboratory
balance) was the first mass measuring instrument invented.[10] In its
traditional form, it consists of a pivoted horizontal lever of equal length
arms, called the beam, with a weighing pan, also called scale,
scalepan, or bason (obsolete[11] ), 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 beam is as close to
equilibrium as possible. In precision balances, a slider mass is moved
A precision balance scale for weighing silver and
along a graduated scale. The slider position gives a fine correction to
gold located on display at the Historic Archive
the
mass value. Although a balance technically compares weights, not
and Museum of Mining in Pachuca, Mexico.
masses, the weight of an object is proportional to its mass, and the
standard weights used with balances are usually labeled in mass units.
Balances are used for precision mass measurement, because unlike
spring scales their accuracy is not affected by differences in the local
gravity, which can vary by almost 0.5%[12] at different locations on
Earth. A change in the strength of the gravitational field caused by
moving the balance will not change the measured mass, because the
moments of force on either side of the balance beam are affected
equally.
Very precise measurements are achieved by ensuring that the balance's
fulcrum is essentially friction-free (a knife edge is the traditional
solution), by attaching a pointer to the beam which amplifies any
deviation from a balance position; and finally by using the lever
principle, which allows fractional masses to be applied by movement
of a small mass along the measuring arm of the beam, as described
above. For greatest accuracy, there needs to be an allowance for the
buoyancy in air, whose effect depends on the densities of the masses
involved.
Two 10-decagram masses
The original form of a balance consisted of a beam with a fulcrum at
its center. For highest accuracy, the fulcrum would consist of a sharp
Masses of 50, 20, 1, 2, 5 en 10 gram
V-shaped pivot seated in a shallower V-shaped bearing. To determine
the mass of the object, a combination of reference masses was hung on one end of the beam while the object of
unknown mass was hung on the other end (see balance and steelyard balance). For high precision work, the center
beam balance is still one of the most accurate technologies available, and is commonly used for calibrating test
weights.
To reduce the need for large reference masses, an off-center beam can be used. A balance with an off-center beam
can be almost as accurate as a scale with a center beam, but the off-center beam requires special reference masses
and cannot be intrinsically checked for accuracy by simply swapping the contents of the pans as a center-beam
balance can. To reduce the need for small graduated reference masses, a sliding weight called a poise can be installed
so that it can be positioned along a calibrated scale. A poise adds further intricacies to the calibration procedure,
Weighing scale
since the exact mass of the poise must be adjusted to the exact lever ratio of the beam.
For greater convenience in placing large and awkward loads, a platform can be floated on a cantilever beam system
which brings the proportional force to a noseiron bearing; this pulls on a stilyard rod to transmit the reduced force to
a conveniently sized beam. One still sees this design in portable beam balances of 500 kg capacity which are
commonly used in harsh environments without electricity, as well as in the lighter duty mechanical bathroom scale
(which actually uses a spring scale, internally). The additional pivots and bearings all reduce the accuracy and
complicate calibration; the float system must be corrected for corner errors before the span is corrected by adjusting
the balance beam and poise. Such systems are typically accurate to at best 1/10,000 of their capacity, unless they are
expensively engineered.
Some high-end mechanical balances also use dials (with counterbalancing masses instead of springs), a hybrid
design with some of the accuracy advantages of the poise and beam but the convenience of a dial reading. These
designs are expensive to produce and have become largely obsolete due to the advent of electronic balances.
Milligram scale
Milligram scales are typical used for industrial, research, chemical,
pharmaceutical, electronics, precious metals, jewelry (commonly sold
as "carat scale", "diamond scale", and "jewelry scale"), and educational
applications.
Analytical balance
An analytical balance is used to measure mass to a very high degree
of precision and accuracy. The measuring pan(s) of a high precision
A&D Orion series. Model HR-60 is one of very
(0.1 mg or better) analytical balance are inside a transparent enclosure
few Legal for Trade (NTEP in USA) certified
with doors so that dust does not collect and so any air currents in the
scales approved for 1 milligram readability.
room do not affect the balance's operation. The use of a vented balance
safety enclosure, which has uniquely designed acrylic airfoils, allows a
smooth turbulence-free airflow that prevents balance fluctuation and the measure of mass down to 1 μg without
fluctuations or loss of product. Also, the sample must be at room temperature to prevent natural convection from
forming air currents inside the enclosure, affecting the measure of mass.
Analytical precision is achieved by maintaining a constant load on the balance beam, by subtracting mass on the
same side of the beam to which the sample is added. The final balance is achieved by using a small spring force
rather than subtracting fixed masses. This makes calling it an "analytical balance" a misnomer, because it should
actually be called an "analytical scale", due to it measuring force, rather than gravitational mass.
3
Weighing scale
4
Spring scale
In a typical spring scale, the spring stretches (as in a hanging scale in
the produce department of a grocery store) or compresses (as in a
simple bathroom scale) in proportion to how hard the Earth pulls down
on the object. It is therefore affected by the local gravity. Every spring
has a proportionality constant that relates how hard it is pulled to how
far it stretches. Some weighing scales such as a Jolly balance (named
after Philipp von Jolly who invented the balance about 1874) use a
spring with a known spring constant (see Hooke's law) and measure the
displacement of the spring by any variety of mechanisms to produce an
estimate of the gravitational force applied by the object, which can be
simply hung from the spring or set on a pivot and bearing platform.
Rack and pinion mechanisms are often used to convert the linear spring
motion to a dial reading.
Spring scales measure weight, or more precisely, the tension force of
constraint acting on an object, opposing the force of gravity. They are
usually calibrated in units of force such as newtons or pounds-force.
They have two sources of error that balances do not; the measured
weight varies with the strength of the local gravitational force, by as
much as 0.5% at different locations on Earth, and the elasticity of the
measurement spring can vary slightly with temperature. Spring scales
A spring weighing scale can measure forces
which are legal for commerce either have temperature compensated
transmitted through the scale in any direction.
springs or are used at a fairly constant temperature, and must be
calibrated at the location in which they are used, to eliminate the effect of gravity variations.
Strain gauge scale
In electronic versions of spring scales, the deflection of a beam
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 by the resistance of the beam to deflection. The
results from several supporting locations may be added electronically,
so this technique is suitable for determining the weight of very heavy
objects, such as trucks and rail cars, and is used in a modern
weighbridge.
Hydraulic or pneumatic scale
Digital kitchen scale, a strain gauge scale which
works by the spring principle.
It 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 based on a Bourdon tube or electronic sensor.
Weighing scale
5
Testing and certification
Most countries regulate the design and servicing of scales used for
commerce. This has tended to cause scale technology to lag behind
other technologies because expensive regulatory hurdles are involved
in introducing new designs. Nevertheless, there has been a recent trend
to "digital load cells" which are actually strain-gauge cells with
dedicated analog converters and networking built into the cell itself.
Such designs have reduced the service problems inherent with
combining and transmitting a number of 20 millivolt signals in hostile
environments.
Scales used for trade purposes in the State of
Florida, as this scale at the checkout in a
cafeteria, are inspected for accuracy by the
FDACS's Bureau of Weights and Measures.
Government regulation generally requires periodic inspections by
licensed technicians using weights whose calibration is traceable to an
approved laboratory. Scales intended for casual use such as bathroom
or diet scales may be produced, but must by law be labelled "Not Legal
for Trade" to ensure that they are not repurposed in a way that
jeopardizes commercial interest. In the United States, the document
describing how scales must be designed, installed, and used for
commercial purposes is NIST Handbook 44. Legal For Trade
certification usually approve the readability as repeatability/10 to
ensure a maximum margin of error of 10%.
Because gravity varies by over 0.5% over the surface of the earth, the distinction between force due to gravity and
mass is relevant for accurate calibration of scales for commercial purposes. Usually the goal is to measure the mass
of the sample rather than its force due to gravity at that particular location.
Traditional mechanical balance-beam scales intrinsically measured mass. But ordinary electronic scales intrinsically
measure the gravitational force between the sample and the earth, i.e. the weight of the sample, which varies with
location. So such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an
accurate indication of mass.
See Verification and validation for further information
Supermarket/retail scale
These scales are used in the bakery, delicatessen, seafood, meat, produce, and other perishable departments.
Supermarket scales can print labels and receipts (in bakery specially), marks weight/count, unit price, total price and
in some cases tare, a supermarket label prints weight/count, unit price and total price. Some modern supermarket
scales print an RFID tag that can be used to track the item for tampering or returns. In most cases these type of scales
have a sealed calibration so that the reading on the display is correct and cannot be tampered with - in the USA the
approval is NTEP, for South Africa it is SABS, the UK it is OIML.
Weighing scale
6
Sources of error
Some of the sources of error in high-precision balances or scales are:
• Buoyancy, because the object being weighed displaces a certain
amount of air, which must be accounted for. Some high-precision
balances may be operated in a vacuum.
• Error in mass of reference weight
• Air gusts, even small ones, which push the scale up or down
• Friction in the moving components that cause the scale to reach
equilibrium at a different configuration than a frictionless
equilibrium should occur.
An old two pan balance.
• Settling airborne dust contributing to the weight
• Mis-calibration over time, due to drift in the circuit's accuracy, or
temperature change
• Mis-aligned mechanical components due to thermal
expansion/contraction of components
• Magnetic fields acting on ferrous components
• Forces from electrostatic fields, for example, from feet shuffled on
carpets on a dry day
• Chemical reactivity between air and the substance being weighed
(or the balance itself, in the form of corrosion)
• Condensation of atmospheric water on cold items
• Evaporation of water from wet items
• Convection of air from hot or cold items
• Gravitational anomalies for a scale, but not for a balance. I.e. using
the scale near a mountain; failing to level and recalibrate the scale
after moving it from one geographical location to another)
Balance with a counterweight to small objects
weighing 0-100 grams.
• Vibration and seismic disturbances; for example, the rumbling from a passing truck
• cables used for connecting the main machine to the display may damage due to heavy weights.
Software
An ongoing problem within the free software community is the lack of free software for analytical balances which is
currently is debated by some manufacturers.
Symbolism
The scales (specifically, a two pan, beam balance) are one of the traditional symbols of justice, as wielded by statues
of Lady Justice. This corresponds to the use in metaphor of matters being "held in the balance". It has its origins in
ancient Egypt.
Weighing scale
Footnotes
[1] Sanders, L. A Short History of Weighing. Birmingham, England: W. & T. Avery, Ltd. (1947, revised 1960) (http:/ / www. averyweigh-tronix.
com/ download. aspx?did=6249
[2] Petruso, Karl M. “Early Weights and Weighing in Egypt and the Indus Valley”, M Bulletin (Boston Museum of Fine Arts,), Vol. 79, (1981),
pp. 44-51 (http:/ / www. jstor. org/ stable/ 4171634)
[3] Petruso, Karl M. “Early Weights and Weighing in Egypt and the Indus Valley”, M Bulletin (Boston Museum of Fine Arts,), Vol. 79, (1981),
pp. 44-51 (http:/ / www. jstor. org/ stable/ 4171634)
[4] Avery Weigh-Tronix. The History of Weighing. (http:/ / www. averyweigh-tronix. com/ main. aspx?p=1. 1. 3. 4)
[5] Petruso, Karl M. “Early Weights and Weighing in Egypt and the Indus Valley”, M Bulletin (Boston Museum of Fine Arts,), Vol. 79, (1981),
pp. 44-51 (http:/ / www. jstor. org/ stable/ 4171634)
[6] Brass, Brian (2006): “Candlesticks, Part 1”, Equilibrium, No. 1, Pp. 3099-3109 (http:/ / www. isasc. org/ Equilibrium/ Back_Issues/ 2006-1.
pdf)
[7] Petruso, Karl M. “Early Weights and Weighing in Egypt and the Indus Valley”, M Bulletin (Boston Museum of Fine Arts,), Vol. 79, (1981),
pp. 44-51 (http:/ / www. jstor. org/ stable/ 4171634)
[8] Avery Weigh-Tronix. The History of Weighing. (http:/ / www. averyweigh-tronix. com/ main. aspx?p=1. 1. 3. 4)
[9] Omega Engineering: “Introduction to Load Cells”, Engineering Technical Reference. (http:/ / www. omega. com/ prodinfo/ loadcells. html)
[10] Sanders, L. A Short History of Weighing. Birmingham, England: W. & T. Avery, Ltd. (1947, revised 1960) (http:/ / www.
averyweigh-tronix. com/ download. aspx?did=6249
[11] "A Practical Dictionary of the English and German Languages" (1869) p.1069 (http:/ / books. google. com/ books?id=FuAwAAAAIAAJ&
pg=PA1069& dq=bason+ balance+ scale& lr=)
[12] Hodgeman, Charles, Ed. (1961). Handbook of Chemistry and Physics, 44th Ed.. Cleveland, USA: Chemical Rubber Publishing Co..
p.3480-3485
External links
• National Conference on Weights and Measures, NIST Handbook 44, Specifications, Tolerances, And Other
Technical Requirements for Weighing and Measuring Devices (http://ts.nist.gov/WeightsAndMeasures/
h44-03.cfm), 2003
• Analytical Balance article at ChemLab (http://www.dartmouth.edu/~chemlab/techniques/a_balance.html)
• HowStuffWorks:Inside a bathroom scale (http://home.howstuffworks.com/inside-scale.htm)
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Article Sources and Contributors
Article Sources and Contributors
Weighing scale Source: http://en.wikipedia.org/w/index.php?oldid=445904644 Contributors: Adrian 1001, Aij, Akaniji, Allisonh12, Altenmann, Alx 91, Amalas, Anthony Appleyard,
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