Sizing of particles
Several different ways can be defined to measure a particle. According the ISO 14644-3 the manufacturer is allowed to define his own system of measuring. The PDM system uses a method of measuring widely used in machine vision. Airborne particle counters have a way of measuring the size equivalent to the polystyrene latex sphere (PSL) particle. Airborne particle counters use this method as the PSL’s are used as well in their calibration method. This type of measuring fits the sensor type best.
The PDM system uses an optical measurement standard on a glass plate for calibration purposes. The measuring system therefore uses the flat projected particle in order to extract the dimensional information. ( Flat Surface Longest Side FSLS ) Particles falling onto a surface will rest in their mechanical stable state. Consequently the particle will show it’s largest surface area to the detection system and therefore also its longest dimension. Use is made of this property. Particles down to 5 micrometer can found and counted. This fits the standard.
In the vision industry the “fitting box” length and width are used. For this the smallest rectangular fitting box is determined. The orientation of the box is not restricted to the axes of the Cartesian system used to measure.
In the drawing below a circle is shown. This circle represents the equivalent PSL particle diameter. The Feret’s diameter as used in the microscopy is comparable to the Cartesian measurement system in the vision industry.
In the ISO 14644-3 the fitting box is mentioned to measure the particle size. The PDM uses the fitting box method and provides the longest dimension.
Optical microscopes often use the Feret’s diameter.