Weighing scale
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[[Image:Digi-keukenweegschaal1284.JPG|thumb|right|Digital kitchen scale, a strain gauge scale which works by the spring principle.]]
A '''weighing scale''' (usually just "scale" in common usage; except in [[Australian English]] where "scales" is more common) is a [[measuring instrument]] for measuring the [[weight]] or [[mass]] of an object. They use one of two techniques. A ''spring scale'' measures weight by the distance a [[spring]] deflects under its load. A ''balance'', described below, compares the unknown weight to a standard weight using a horizontal [[lever]]. 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 scale]]s are used to measure the [[Human weight|body weight]] of human beings.
==Balance==
<!--NOTE TO EDITORS: This section is linked from [[Kilogram]]. Please do not delete or rename without changing the referencing link.-->
A '''balance''' (also '''balance scale''', '''beam balance''' or '''laboratory balance''') is used to measure the [[mass]] of an object. It was the first mass measuring instrument invented. 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''' (hence the term "scales") '''scalepan''', or '''bason''' (obsolete <ref>"A Practical Dictionary of the English and German Languages" (1869) [http://books.google.com/books?id=FuAwAAAAIAAJ&pg=PA1069&dq=bason+balance+scale&lr= p.1069]</ref>) suspended from each arm. The unknown mass is placed in one pan, and standard masses are added to the other pan until the beam is as close to [[mechanical equilibrium|equilibrium]] as possible. In precision balances, a slider weight is moved along a graduated scale. The slider position gives a fine correction to the weight value. Although a balance technically compares weights, not masses, the [[weight]] of an object is proportional to its mass, and the standard weights used with balances are usually labeled in mass units.
[[Image:Balance à tabac 1850.JPG|thumb|left|Traditional balance]]
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%<ref>{{cite book
| last = Hodgeman
| first = Charles, Ed.
| authorlink =
| coauthors =
| title = Handbook of Chemistry and Physics, 44th Ed.
| publisher = Chemical Rubber Publishing Co.
| date = 1961
| location = Cleveland, USA
| pages =
| url =
| doi =
| id =
| isbn = }} p.3480-3485</ref> 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 [[moment (physics)|moments of force]] on either side of the balance beam are affected equally.
Very [[accuracy and precision|precise]] measurements are achieved by ensuring that the [[fulcrum]] of the beam is essentially [[friction]]-free (a [[knife]] edge is the traditional solution), by attaching a [[pointer]] to the beam which [[Amplifier|amplifies]] any [[deviation]] from a balance position; and finally by using the [[lever]] principle, which allows [[fraction (mathematics)|fraction]]al masses to be applied by [[Motion (physics)|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.
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 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, 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" (misnamed, since it is actually a balance). 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 are largely obsolete thanks to electronics.
==Analytical balance==
[[Image:Analytical balance mettler ae-260.jpg|thumb|left|Mettler digital analytical balance with 0.1 mg precision.]]
An '''analytical balance''' is an instrument used to measure [[mass]] to a very high degree of precision. The weighing pan(s) of a high precision (.01 [[milligram|mg]] or better) analytical balance are inside a transparent enclosure with doors so [[dust]] does not collect and so any air currents in the room do not affect the delicate balance. The use of a [[vented balance safety enclosure]] that has uniquely designed acrylic airfoils allows a smooth turbulence-free airflow that prevents balance fluctuation and the weighing of mass down to 1μg 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 weighing.
Analytical precision is achieved by maintaining a constant load on the balance beam, by subtracting mass on the same side of the beam that the sample is added. The final balance is achieved by using a small spring force rather than subtracting fixed weights.
==Spring scale==
[[Image:Spring weighing scale.jpg|thumb|right|A spring weighing scale can measure forces transmitted through the scale in any direction.]]
{{main|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. Every spring has a proportionality constant that relates how hard you pull it to how far it stretches. Some weighing scales such as a ''Jolly balance'' (named after [[Philipp von Jolly]] <!-- (1809-1884) --> who invented the balance about [[1874]])<!-- He was a professor at the University of Munich, --> use a [[spring (device)|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 [[gravity|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]], the local [[force]] of gravity on an object, and are usually calibrated in units of force such as [[Newton]]s or [[pound-force|pounds-force]]. They have two sources of error that balance scales do not; the measured weight varies with the strength of the local gravitational force, as much as 0.5% at different locations, and the elasticity of the measurement spring can vary slightly with temperature. Spring scales which are legal for commerce either have temperature compensated 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 especially suitable for determining the weight of very heavy objects, such as trucks and railcars, and is used in a modern [[weigh bridge]].
==Hydraulic or pneumatic scale==
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 [[pressure gauge|Bourdon tube]] or electronic sensor.
==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-gage 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.
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 [[National Institute of Standards and Technology|NIST]] Handbook 44.
Because gravity varies by over 0.5% over the surface of the earth, the [[weight#Weight and mass|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.
==Supermarket/retail scale==
These scales are used in [[bakery]], [[deli]], [[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 manufacturers are [[Adam Equipment]], [[AEW Delford]], [[Hobart Corporation]], [[Bizerba]], [[DIGI/Teraoka]], '''[http://www.averyindia.co.in Avery India]''', [[Mettler Toledo]], [[CAS Corp.|CAS]], [[Avery Berkel]], [[Ishida]] and [[ATP-Instrumentation]]. Some modern supermarket scales print an [[RFD]] 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]].
==Sources of error==
[[Image:Bascula 9.jpg|thumb|An old two pan balance.]]
Some of the sources of potential [[errors and residuals in statistics|error]] in high-precision balances or scales include the following:
* [[Buoyancy]], because the object being weighed displaces a certain amount of air, which must be accounted for. High-precision balances are often operated in a vacuum.
* Error in reference weight
* Air gusts, even small ones, which push the scale up or down
* [[Friction]] in the moving components that prevents the scale from reaching equilibrium
* Settling airborne dust contributing to the weight
* Mis-calibratation
** The calibration of electronic circuits may drift over time, or due to temperature changes.
* Mis-aligned mechanical components
** Due to [[thermal expansion]]/contraction of components of the balance.
* [[Magnetic fields]] acting on iron components
** The Earth's magnetic field
** Fields from nearby electrical wiring
** Magnetic disturbances to electronic [[pick-up coil]]s or other sensors
* Forces from [[electrostatic field]]s, 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
* The [[Coriolis force]] from Earth's rotation
* Gravitational anomalies (i.e. using the balance near a mountain; failing to level and recalibrate the balance after moving it from one geographical location to another)
* Vibration and seismic disturbances; for example, the rumbling from a passing truck
== Symbolism ==
<!--The article "Probative" links to this section-->
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".
==See also==
*[[Ampere balance]]
*[[Apparent weight]]
*[[Mass versus weight]]
*[[Nutrition scale]]
*[[Roberval Balance]]
*[[Steelyard balance]]
*[[Watt balance]]
*[[Weigh lock]] - for weighing canal barges
*[[Weighbridge]] - for weighing vehicles (such as trucks) and railcars
*[[Weigh house]] - historic public building for the weighing of goods
==Footnotes==
{{reflist}}
==External links==
{{Commonscat-inline|Weighing scales}}
* National Conference on Weights and Measures, NIST Handbook 44, ''[http://ts.nist.gov/WeightsAndMeasures/h44-03.cfm Specifications, Tolerances, And Other Technical Requirements for Weighing and Measuring Devices]'', 2003
* [http://www.dartmouth.edu/~chemlab/techniques/a_balance.html Analytical Balance article at ChemLab]
* [http://www.newscientisttech.com/article.ns?id=dn9270&feedId=online-news_rss20 Laser beams pluck nano-strings]
* [http://home.howstuffworks.com/inside-scale.htm HowStuffWorks:Inside a bathroom scale]
* [http://www.digitalscaledepot.com Where to get your own Digital Scale]
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