Auxiliary Equipment
Investing in a measuring device This article by Dr. Hilmar Bolte, R&D/ Head of Analysis at German firm Sikora takes a look at the meaning of “measuring rate”, “averaging” and “accuracy” when investing in a measuring device. Founded in 1973, Sikora is a global manufacturer and supplier of measuring and control technology for the wire and cable, hose and tube, sheet and metals as well as optical fibre and plastics industry.
Figure 1: Absolute accuracy and repeatability based on the example of a shooter
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Choosing the optimal measuring device for an extrusion line When deciding about investing in a measuring device, one of the main factors – besides the costs – usually is which device is the “best”. Characteristics where “more” or “less” is considered as “better” are seemingly easy to compare. This simplification, however, bears risks. In digital photography, for instance, the size of the sensors and, thus, of the individual pixel in general, is more important than the total number of pixels. The pixel count however is commonly the relevant sales argument. For that reason, it makes sense to question the characteristics of a measuring device, as well as their definition and interaction. Often further information about the conditions under which these characteristics are valid such as temperature, position dependency etc, are missing. Specifications usually contain the following characteristics: “measuring range”, “absolute accuracy” (also “correctness”), “repeatability” (also “precision”) and “measuring rate”. “Measuring range” indicates minimum/maximum object sizes that are measurable. Sometimes, the visual range is specified instead, this means: the overall range in which the objects to be measured are allowed to move. Occasionally, information about the minimum and/or maximum measurable size is missing, too. The colloquial meaning of “accuracy” is the total of all measuring errors. However, for the evaluation of a measuring device, it has to be differentiated: “absolute accuracy” means the comparison of a mean measuring value with a certified standard value. “Repeatability” is defined as the scattering of the measuring values under the same conditions and, therefore, a characteristic of the measuring value noise of the device itself. The sole specification of only a numerical value for “repeatability” is not sufficient. It might be that one supplier indicates the standard deviation of single values, whereas another calculates those based on a sequence of averaged values. A common visualisation of the definitions “absolute accuracy” (also called “correctness”) and “repeatability” (also called “precision”) is shown in Figure 1. The “measurement rate” of a measuring device is the number of measurement values generated per second. This is a further important comparison criterion where “more” is seen as “better”. For an objective comparison, however, the knowledge of the interdependence between measurement rate and absolute a) Not repeatable, b) Repeatable, absolute inaccurate absolute accurate accuracy and repeatability of a single measurement is crucial. It may be the case that a measuring device with a higher measurement rate, but lower single value precision is less suitable for controlling or characterisation of a process than a device with a lower measuring rate but higher single value precision. For example, this is the case when a long averaging time is necessary c) Repeatable, d) Not repeatable, absolute inaccurate (on average) absolute accurate due to a lower single value precision.
NOVEMBER / DECEMBER 2018