TPT November 2018

T E CHNOLOG Y

Measuring rate, averaging and accuracy when investing in a measuring device Dr Hilmar Bolte, research and development/head of analysis at Sikora AG, looks at the issues surrounding the choosing of optimal measuring devices for extrusion lines.

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 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. The ‘measurement rate’ of a device is the number of measurement values per second. This is an important comparison criterion where more is seen as better. For an objective comparison, however, the knowledge of the interdependence betweenmeasurement rate and absolute accuracy and repeatability 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 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 due to a lower single-value precision. There is a risk that actual product variations that occur within this averaging time are levelled out while really being present. In the worst case, the specifications might even be violated without this being signalled by the measuring device. The displayed ‘real value’ results from single measurements taken in ten-minute intervals. Averaging over a period of one hour only smooths the extreme values. When averaging the varying temperature for more than 12 hours, the changes in temperaturearedisplayed lower than they actually are. Furthermore, if the mean value is generated over an entire day, the information about the daily temperature variations is completely lost. Adevice that needs the latter averaging depth will not be suitable for a process where an alarm has to be raised or an adjustment has to be made depending on the temperature range. A practical example taken from the hose and tube production process is

WHEN deciding about investing in a measuring device, one of the main factors – besides the costs – is usually 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: 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, meaning 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. The colloquial meaning of ‘accuracy’ is the total of all measuring errors. However, for the evaluation of a measuring device,

Laser Series 6000 from Sikora

the diameter measurement based on the shadow projection method with rotating mirrors. Often high measuring rates are indicated, which result from the rotation rate multiplied by the number of mirror facets (Zanoni, 1973), (Vossberg, 1981). The specification of accuracy, however, is usually based on mean values of up to one second due to a relatively poor single-value precision. This has various reasons. Each single measurement is done with a different mirror facet. Product movements during measurement increase or decrease the product diameter – depending on the direction of movement – as the measurement of both product edges is not done simultaneously but sequentially. Lastly, the diameter information is only derived from the very transition from dark to light and light to dark. Example of a temperature profile showing the extent to which averaging of a measuring value can influence the perception

Line sensor technology for diffraction analysis in a Sikora diameter gauge head

Sikora AG – Germany Fax: +49 421 48900 90 Email: sales@sikora.net Website: www.sikora.net

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NOVEMBER 2018