Weighing System Accuracy

Main keywords for this article are Weighing System Accuracy. Static Weighing Accuracy. Dynamic Weighing Accuracy. 

Weighing Accuracy

It is important to establish the requirement of weighing accuracy when either the static or the in-motion scale system is specified. For each type of weighing system, the method of stating accuracy is different. In addition, the accuracy of any weighing system, while obviously a function of the weight modules and information systems, also is dependent upon the equipment selection, support structure, piping attachments, environmental conditions, and proper installation of weighing equipment. In short, weighing system accuracy is closely tied to the degree of attention given to the mechanical details.

Static Weighing Accuracy

  • The NIST Handbook 44 provides complete statements of accuracy for static scales. Basically, these accuracies are related back to certified weights located in the NIST Handbook 44. Static weighing eliminates all external influences from the determination of the weight. The only influences on the scale are the time being weighed and the weighing device itself; therefore, the weights taken statically can be considered absolute within the accuracy limits specified.
  • The accuracy for static scales is defined as the ability of the scale to perform weighing functions within an acceptable or desirable tolerance; usually stated as a percentage of either full scale reading, or plus or minus units of weight (lb. or kg) referred to the total net weight to the measured by the static scale. 
  • Repeatability is a very important factor in static weighing or batching operations.
  • Repeatability is defined as the ability of a scale to produce results which tally one with the other for the same load deposit several times on the load receptor under reasonably constant test conditions. The repeatability of the result, the result obtained by adding or subtracting weight on the weight sensing device, and the influences of temperature and pressure all must be considered when vigorously stating total scale accuracy. High accuracy static weighing systems exhibit system errors under 0.05 percent and low accuracy static weighing systems are those with system errors greater than 0.5
    percent.

Dynamic Weighing Accuracy

  • Belt conveyor scales provide a means of weighing bulk materials while in motion. When specifying a belt conveyor scale, it is important to understand the definition of weighing accuracy for belt conveyor scales because of the dynamic effects of weighing in-motion. Consider the meaning of accuracy as it applies to a hopper scale, platform or truck scale.
  • The scale is checked for accuracy against a traceable standard weight. In the case of the belt conveyor, this procedure cannot be followed. The only way to test the accuracy of the belt conveyor scale is to compare the weight of material weighed over the belt conveyor scale to a reference static scale. NIST Handbook 44 contains a section that defines the requirements of this comparison.
  • Regulatory agencies, such as the weighing and inspection bureaus, require a belt conveyor scale to perform as follows:
  •  The scale’s accuracy must be plus or minus ¼ of 1 percent maximum error of totalized weight when the scale is operated over a range of 50 to 100 percent of rated capacity.
  • Governmental agencies, such as State Department of Weights and Measures, use NIST Handbook 44. This requires a scale to perform as follows:
  • The scales’ accuracy must be plus of minus ½ of 1 percent maximum error of totalized weight when the scale is operated over a range of 50 to 100 percent of rated capacity.
  • Regardless of any installation, it is important to know what factors contribute to accuracy in a belt conveyor scale system and that, due to the dynamic effects of weighing in-motion, a weighed material load test is the only way to establish traceable accuracy on a belt conveyor scale.
  • The accuracy statements published on the product specification literature of each belt conveyor company are different and it is necessary to make sure that you understand the meaning of the accuracy statement in the process of selecting a belt conveyor scale. 

Weighing System Accuracy. Static Weighing Accuracy. Dynamic Weighing Accuracy.

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