Editing Weighing scale (section)

===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