Page 42 - ITA Journal 3-2018

Technical Papers
SIKORA AG
Meaning of “measuring rate”, “averaging”
and “accuracy” when investing in a measuring device
Figure 1: Absolute accuracy and repeatability based on the example of a shooter
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 simpli cation, 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 charac- teristics of a measuring device, as well as their de nition and interac- tion. Often further information about the conditions under which these characteristics are valid such as temperature, position dependency etc. are missing.
Speci cations usually contain the following characteristics: “measur- ing 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 speci ed instead, this means: the overall range in which the objects to be measured are allowed to move. Occasionally, informa- tion about the minimum and/or maximum measurable size is missing, too. The colloquial meaning of “accuracy” is the total of all measur- ing 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 certi ed standard value. “Repeatability” is de ned 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 speci cation of only a numerical value for “repeatability” is not suf cient. 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 visualization of the de nitions “absolute accuracy” (also called “correctness”) and “repeatability” (also called “precision”) is shown in gure 1.
The “measurement rate” of a measuring device is the number of meas- urement values generated per second. This is a further important com- parison criterion where “more” is seen as “better”. For an objective comparison, however, the knowledge of the interdependence between measurement rate and absolute 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 characterization 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 due to a lower single value precision. Then, there is a risk that actual product variations, which occur within this averaging time, are levelled
Figure 2: Temperature pro le as example
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