Parsum probes use an optical method to measure the shadows of particles moving through a measuring gap at the tip of the probe.
To this end, a collimated laser beam is shone through the measuring gap. Moving particles then create moving shadows on the opposite side of the measuring gap, on which there are two fibre-optic receivers for detecting the velocities and times of flight of the individual particles.
The velocity sensor is a spatial frequency filter consisting of a linear grating arrangement of optical fibres, and the time-of-flight detector is a single optical fibre located after it. The geometric length of each individual particle shadow is then determined by combining the individual particle velocity with the individual time of flight of the particle, and is then stored in a dynamic ring buffer on an ongoing basis. The content of this adjustable-length buffer forms the basis for calculating the particle size and velocity distributions as well as the particle attributes – continuously, online and directly within the process.