Editing Weighing scale (section)

==Force-measuring (weight) scales==
===History===
[[File:A Simple Balance.jpg|thumb|upright|A simple balance from the 19th century]]
Although records dating to the 1700s refer to spring scales for measuring mass, the earliest design for such a device dates to 1770 and credits Richard Salter, an early scale-maker.<ref name="jstor44"/> Spring scales came into wide usage in the United Kingdom after 1840 when R. W. Winfield developed the candlestick scale for weighing letters and packages, required after the introduction of the [[Uniform Penny Post]].<ref>{{cite journal|last=Brass |first=Brian|year=2006|title=Candlesticks, Part 1 |journal=Equilibrium |issue=1|pages=3099–3109|url=http://www.isasc.org/Equilibrium/Back_Issues/2006-1.pdf|access-date=2014-02-26}}</ref> 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.<ref name="jstor44"/><ref name="averyweigh-tronix2"/> [[Load cell]]s – transducers that convert force to an electrical signal – have their beginnings as early as the late nineteenth century, but it was not until the late twentieth century that their widespread usage became economically and technologically viable.<ref>{{cite web|url=http://www.omega.com/prodinfo/loadcells.html |title=Load Cells |publisher=Omega.com |access-date=2014-02-26}}</ref>

===Mechanical scales===
A mechanical scale or balance is used to describe a weighing device that is used to measure the mass, force [[exertion]], tension, and resistance of an object without the need of a power supply. Types of mechanical scales include [[decimal balance]]s, [[spring scale]]s, hanging scales, [[triple beam balance]]s, and [[force gauge]]s.

====Spring scales====
{{main|Spring scale}}
A [[spring scale]] measures mass by reporting the distance that a [[spring (device)|spring]] deflects under a load. This contrasts to a ''balance'', which compares the [[torque]] on the arm due to a sample weight to the [[torque]] on the arm due to a standard reference mass using a horizontal [[lever]]. Spring scales measure [[force]], which is the [[tension (physics)|tension]] [[force]] of constraint acting on an object, opposing the local [[gravity|force of gravity.]]<ref>{{cite web|url=https://www.inscale-scales.co.uk/blog/guide-to-choosing-best-mechanical-scale/|title=A Guide to Choosing the Best Mechanical Scale – Inscale|website=Inscale Scales|language=en|archive-url=https://web.archive.org/web/20171206203111/https://www.inscale-scales.co.uk/blog/guide-to-choosing-best-mechanical-scale/|archive-date=2017-12-06|url-status=dead|access-date=2017-12-06}}</ref> They are usually [[Calibration|calibrated]] so that measured force translates to mass at earth's gravity. The object to be weighed can be simply hung from the spring or set on a pivot and bearing platform.

In a spring scale, the spring either stretches (as in a hanging scale in the produce department of a [[grocery store]]) or compresses (as in a simple bathroom scale). By [[Hooke's law]], every spring has a proportionality constant that relates how hard it is pulled to how far it stretches. Weighing scales 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.<ref>{{Cite news|url=https://phys.org/news/2015-02-law.html|title=What is Hooke's Law?|access-date=2017-12-06}}</ref> Rack and [[pinion]] mechanisms are often used to convert the linear spring motion to a dial reading.

Spring scales have two sources of error that balances do not: the measured mass 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. With proper manufacturing and setup, however, spring scales can be rated as legal for commerce.  To remove the temperature error, a commerce-legal spring scale must either have temperature-compensated springs or be used at a fairly constant temperature. To eliminate the effect of gravity variations, a commerce-legal spring scale must be calibrated where it is used.

====Hydraulic or pneumatic scale====
It is also common in high-capacity applications such as crane scales to use hydraulic force to sense mass. 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.<ref>{{cite web|publisher=California Department of Food and Agriculture Division of Measurement Standards|title=A brief history of weights and measures|url=https://www.cdfa.ca.gov/dms/trainingmodules/Module_07.pdf}}</ref>

===Domestic weighing scales===
[[File:Feet on scale.jpg|thumb|right|A mechanical bathroom scale. Pressure on the internal springs rotates a disc displaying the user's weight in pounds.]]
Electronic digital scales display weight as a number, usually on a [[liquid crystal display]] (LCD).  They are versatile because they may perform calculations on the measurement and transmit it to other digital devices.  On a digital scale, the force of the weight causes a spring to deform, and the amount of deformation is measured by one or more [[transducer]]s called [[strain gauge]]s.  A strain gauge is a [[electrical conductor|conductor]] whose [[electrical resistance]] changes when its length changes.  Strain gauges have limited capacity and larger digital scales may use a [[hydraulic]] transducer called a [[load cell]] instead. A voltage is applied to the device, and the weight causes the current through it to change.  The current is converted to a digital number by an [[analog-to-digital converter]], translated by digital logic to the correct units, and displayed on the display. Usually, the device is run by a [[microprocessor]] chip.

====Digital bathroom scale====
A digital bathroom scale is a scale on the floor which a person stands on.  The weight is shown on an LED or LCD display. The digital electronics may do more than just display weight, it may calculate body fat, [[body mass index|BMI]], lean mass, muscle mass, and water ratio.  Some modern bathroom scales are wirelessly or cellularly connected and have features like smartphone integration, cloud storage, and fitness tracking. They are usually powered by a button cell, or battery of AA or AAA size.

====Digital kitchen scale====
Digital kitchen scales are used for weighing food in a kitchen during cooking. These are usually lightweight and compact.

====Strain gauge scale====
In electronic versions of spring scales, the deflection of a beam supporting the unknown mass 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 mass of very heavy objects, such as trucks and rail cars, and is used in a modern [[weighbridge]].

====Supermarket and other retail scale====
These scales are used in the modern [[bakery]], [[grocery]], [[delicatessen]], [[seafood]], [[meat]], [[produce]] and other perishable goods departments. Supermarket scales can print labels and receipts, mark mass and count, unit price, total price and in some cases [[Tare weight|tare]]. Some modern supermarket scales print an [[RFID]] tag that can be used to track the item for tampering or returns. In most cases, these types of scales have a sealed calibration so that the reading on the display is correct and cannot be tampered with. In the US, the scales are certified by the [[National Type Evaluation Program]] (NTEP), in South Africa by the [[South African Bureau of Standards]], in Australia, they are certified by the [[National Measurement Institute, Australia|National Measurement Institute (NMI)]] and in the UK by the [[International Organization of Legal Metrology]].

=== Industrial weighing scale ===
An industrial weighing scale is a device that measures the weight or mass of objects in various industries. It can range from small bench scales to large weighbridges, and it can have different features and capacities. Industrial weighing scales are used for quality control, inventory management, and trade purposes.

There are many kinds of industrial weighing scales that are used for different purposes and applications. Some of the common types are:

[[Truck scale|Weighbridges]] : A large scale that can weigh trucks, lorries, containers, and other heavy-duty vehicles. They are used in industries like manufacturing, shipping, mining, agriculture, etc.

Container Stacker Scale : A [https://www.hirsglobal.com/portfolio/container-stacker-scale/ container stacker scale] is a specialized weighing system designed for accurately measuring the weight of shipping containers. It is typically integrated into the equipment used for loading and unloading containers, such as container handlers or stacker cranes. Container stacker scales provide real-time weight measurements, allowing logistics professionals to ensure that each container is loaded within the specified weight limits. Container stacker scales are used in industries like ports, shipping, and logistics

Forklift scale : A forklift scale is a weighing system that is built into a forklift truck. It allows for the weighing of loads while they are being lifted and transported by the forklift. This eliminates the need for separate weighing operations and reduces the time and labor required for material handling operations. Forklift scales are used in various industries, such as manufacturing, logistics, and shipping.

Material Handler Scale : A Material Handler Scale is a weighing system that is integrated into a material handler machine, such as a grapple or a magnet. It allows for the accurate and efficient weighing of materials while they are being moved, unloaded, or loaded. A Material Handler Scale can be used in various industries, such as scrap, recycling, waste, and port and harbor. A Material Handler Scale can also transfer the weighing information to a cloud service or an ERP system for real-time monitoring and management of material flow.

A pallet jack scale is a device that combines a pallet jack and a weighing scale. It allows you to weigh and move pallets at the same time, saving time and labor. Pallet jack scales are used in various industries, such as manufacturing, logistics, and shipping.

Crane Scale : A crane scale is a device that measures the weight or mass of objects that are suspended from a crane. It has a hook at the bottom and a large display that allows distant viewing. Crane scales are used for various industrial applications, such as manufacturing, shipping, mining, recycling, and more

Wheel Loader Scale : A wheel loader scale is a system that measures the weight of the materials lifted by a wheel loader, a type of heavy machinery used for moving large amounts of earth, sand, gravel, or other materials. A wheel loader scale can help improve the efficiency and accuracy of loading operations, as well as the inventory management and safety of the industries that use them. A wheel loader scale typically consists of a hydraulic sensor, a display unit, and a data management system. The hydraulic sensor is installed in the wheel loader and detects the pressure changes caused by the load. The display unit shows the weight information to the operator and allows them to set target loads, select products and customers, and export data. The data management system can store, analyze, and transmit the weight data to other devices or platforms.

===Testing and certification===
{{See also|Verification and validation}}
[[File:Inspected scale 2.jpg|thumb|upright|Scales used for trade purposes in the [[United States]], as this scale at the checkout in a [[cafeteria]], are inspected for accuracy by the [[FDACS|FDACS's Bureau of Weights and Measures]].]]

Most countries regulate the design and servicing of scales used for commerce. For example, in the European Union weighing instruments are subject to 2014/31/EU and 2014/32/EU directives. A conformity assessment procedure is carried out before placing the instrument on the market, andv the instruments are verified after a given period of time in member states of the European Union. 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{{When|date=September 2015}} a 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.

Government regulation generally requires periodic inspections by licensed technicians, using masses whose calibration is traceable to an approved laboratory. Scales intended for non-trade use, such as those used in bathrooms, doctor's offices, kitchens (portion control), and price estimation (but not official price determination) may be produced, but must by law be labelled "Not Legal for Trade" to ensure that they are not re-purposed 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''. Legal For Trade (LFT) certification usually approve the readability by testing repeatability of measurements to ensure a maximum margin of error of 10%.{{citation needed|date=August 2014}}

Because gravity varies by over 0.5% over the surface of the earth, the [[Units of weight|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.

====Sources of error====
Some of the sources of [[errors and residuals in statistics|error]] in weighing are:
* [[Buoyancy]] – Objects in air develop a buoyancy force that is directly proportional to the volume of air displaced. The difference in density of air due to [[barometric pressure]] and temperature creates errors.<ref>{{cite web|url=http://andrew.ucsd.edu/co2qc/handbook/sop21.pdf |title=Applying air buoyancy corrections |publisher=Andrew.ucsd.edu |date=September 29, 1997 |access-date=2014-03-05 |url-status=dead |archive-url=https://web.archive.org/web/20060907221014/http://andrew.ucsd.edu/co2qc/handbook/sop21.pdf |archive-date=September 7, 2006 }}</ref>
* Error in the mass of reference weight
* Air gusts, even small ones, which push the scale up or down
* [[Friction]] in the moving components that causes the scale to reach equilibrium at a different configuration than a frictionless equilibrium should occur.
* 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]] or contraction of components
* [[Magnetic fields]] acting on ferrous components
* 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
* Gravitational differences for a scale which measures force, but not for a balance.<ref>{{cite journal|url=https://www.nist.gov/calibrations/upload/nbs93-4.pdf |title=Practical Uncertainty Limits to the Mass Determination of a Piston-Gage Weight |journal=Journal of Research of the National Bureau of Standards |volume=93 |issue=4 |pages=565–571 |date=1988 |access-date=2014-02-26|doi=10.6028/jres.093.149 |last1=Davis |first1=R.S. |last2=Welch |first2=B.E. |doi-access=free }}</ref>
* Vibration and seismic disturbances