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Overview of weighing

Weighing instrument measure weights and more

Weighing instruments are used for more than 4000 years to determine the mass of an object, but are now also used to measure volume and density. The determination of mass is important to many aspects of modern life, especially to the producing industry and to trade. For example does the value of all products, sold according their weights, exceeds a half of a billion Euro every year in Germany alone.

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Precision and maximal load

The variety of weighing instruments is astonishing. Besides a classification according to their construction, functionality or usage, weighing instruments can be grouped into different accuracy classes with different maximal load. High precision scales have a resolution of 0,001 mg and a maximal load of 5 grams, and some weighing bridges are designed for a load of more than 100 tons with a resolution up to 3000 verification intervals.

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Weighing instruments in need of an operator

Nonautomatic weighing instruments (NAWIs) are widely used in trade und domestic applications . For these instruments, the operator plays an integral part in the measurement.

Automatic weighing instruments

In general, automatic weighing instruments do not need the intervention of an operator, but repetitiously initiate weighing cycles. Industrial automatisation , especially in the producing sector, depends heavily on these kind of instruments. Often, automatic weighing instruments are complex, and even unique, systems.

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Usage of weighing instruments

Nonautomatic weighing instruments

  • analytical balances (accuracy class I and II);
  • point of sale and counter top scales, probably price computing (III);
  • platform scales (accuracy class II and III);
  • determination of a toll or tariff, for example in post offices (III);
  • weighing of road and rail vehicles (III) ;
  • mobile instruments, for example in forklifts (III);
  • determination of body weight with bathroom scales or weighing beds, for example (III);
  • bulk weighing of building material and garbage collection/recycling, etc; (IIII)
  • save load checking, for example the load on axles of vehicles. (IIII)

Automatic weighing instruments

  • automatic determination of the actual weight of goods in the production flow;
  • gravimetic filling up to a nominal value, for example of prepacked goods;
  • continuous weighing of bulk material using conveyor belts;
  • check weighing for good/bad selection or bucket sorting of goods;

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Accurate weighing instruments, fair trade

To protect the consumer and foster fair trading practices, every european country requires a certain standard for weighing instruments by law. The most importent requirement is the accuracy, for which Europe has established 4 accuracy classes for nonautomatic weighing instruments. The standards apply to the following categories of useage as spelled out in the Council Directive 90/384/EEC :

  • for commercial transaction;
  • determination of mass for the calculation of a toll or tarif;
  • determination of mass for the application of laws or regulations;
  • weighing for medical purposes, for example of patients and medicines;
  • determination of price on the basis of mass in the direct sale to the public in in prepackaging.

All other instruments, for example bathroom scales, do not need to meet the standard. It is the responsibility of the verification authorities (initial and subsequent verification of each individual instrument) and of the Physikalisch-Technischen Bundesanstalt (in the form of pattern approval ) to enforce the conformity to the standard.

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References

Manfred Kochsiek (Hrsg.), Handbuch des Wägens , 2. Edition, Vieweg (1989).