Sampling test using Isokinetic principle

Purpose is to capture particles that pass through a defined area for a defined time without disturbing their paths.
The velocity of the air going into the sampling probe nozzle is equal to the velocity of the undisturbed air at that point.
The air is disturbed as little as possible so that the same particles (no more and no less) go into the nozzle as would have passed the area of the nozzle had it not been there.