Extrusion International 5-2018

64 Extrusion International 5/2018 MEASURING AND CONTROLLING Meaning of “measuring rate”, “averaging” and “accuracy” when investing in a measuring device Chosing the optimal measuring device for the 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 be compared. 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”) • “Measuring rate” “Measuring range” indicates minimum/maximum object sizes that are measurable. Sometimes, the visual range is specified instead, meaning: the overall range in which the objects to be measured are allowed to move. Occasion- ally, 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 mea- suring device, it has to be differentiated: “absolute accu- racy” 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 measur- ing value noise of the device itself. The sole specification of only a numerical value for “re- peatability” 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 aver- aged values. A common visualization of the definitions “absolute ac- curacy” (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 be- tween measurement rate and absolute accuracy and re- peatability of a single measurement is crucial. It may be the case that a measuring device with a higher measure- ment rate, but lower single value precision is less suitable for controlling or characterization of a process than a de- vice with a lower measuring rate but higher single value precision. For example, this is the case when a long aver- aging time is necessary due to a lower single value preci- sion. Then, there is a risk that actual product variations, which occur within this averaging time are levelled out Figure 2: Temperature profile as example Figure 1: Absolute accuracy and repeatability based on the example of a shooter