The particle deposition monitor is a system that uses witness plates to collect the deposition at a given point in space. For this purpose the witness plate,  that has the shape of CD and is made of glass,  will be put at the place where to measure.   During a period of time dust will be collected on the witness plate. The imaging system of the PDM will measure and count particles deposited at the disk.

In the continuous measurement mode the disk may stay on the PDM while getting dusted. The PDM can take i.e. every 60 seconds a snapshot thus providing a near realtime measurement of the deposition taking place.

As it is hard to clean a disk perfect the system is capable of making differential measurements.  This allows to measure the parts that were added on the surface since last measurement.