Evaluation errors with dust measurements of laminar minerals by scanning electron microscope following German guideline VDI 3492

A number of seemingly fibrous particles are found by evaluation of dust measurements of the industrial mineral vermiculite in accordance with German Guideline VDI 3492, part 2 for “measurements of inorganic fibrous particles‐ SEM method.” A more thorough investigation based upon an extension of the measuring method proves the nonfibrous morphology of practically all of these particles. The basic idea of this extension is to tilt the filter holder of the electron microscope and to check the exact orientation of the particles on the filter, an examination which is necessary to determine their true length and diameter. The result indicates that only very few fibrous particles actually exist. In the case of vermiculite, these fibrous particles are rolled up and detached flakes called “scrolls.” The systematic error inherent in VDI 3492, part 2, without extension can be quantified in the case of vermiculite to have a factor > 50, that is, fibre counts obtained are more than 50 times too high.