A laboratory optical probe was developed to simultaneously determine the following particle characteristics: circularity, particle projection area, equivalent diameter of a circle, length of the particle outline or perimeter, maximum chord length, aspect ratio, and particle velocity. Using the projection area and the perimeter, the particle shape factor circularity can be determined. The aspect ratio was approximated by the ratio of the equivalent diameter to the maximum chord length. The basic measuring principle is multi-point scanning of the particle shadow image by a line of optical fibers. In addition, the particle velocity can be measured by a differential spatial filter of optical fibers. These fibers are step index fibers with a core diameter of 64 mm and cladding of 70 mm. The shadow image of a single particle was generated by a parallel laser beam. The uncertainty of the measured circularity and aspect ratio was investigated by using metal wires with diameters of 0.12 to 0.5 mm as test particles with known circularity and aspect ratio. The standard deviations were 1.9% for the circularity and 15.5% for the approximated aspect ratio. In addition, the optical probe system was investigated by measurements of solid particles with different shapes. As an example, the results of sand, marjoram seed, and metallic oxide particles are shown. Using 1000 sand particles, the correlation between equivalent diameter and particle velocity could be demonstrated. The presented configuration of the optical probe is applicable in the size range of 0.1 to 0.9 mm and up to a particle velocity of 5 m=s.
Zusammenfassung
Es wird ein optischer Durchflussgeber für die Messung des Volumenstroms einer Flüssigkeit in einer Mikrobohrung vorgestellt, der bei Anströmung mittels Freistrahl erfolgreich verwendet wurde. Als Mikrobohrung wurde eine Glaskapillare mit einem Innendurchmesser von 0,15 mm benutzt, die in eine 2 mm dicke Messplatte eingesetzt wurde. Wesentliche Bauelemente des Durchflussgebers sind ein LED-Beleuchtungssystem und ein faseroptisches Ortsfilter. Das Messverfahren wurde an einem Laboraufbau mit kreisrundem turbulenten Freistrahl sowie in einer Technikumsanlage zur horizontalen Leiterplattenbeschichtung eingesetzt.
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