AbstractCascade impactors are still the most suitable devices with which to measure fine droplet mists in gas ducts. This report describes the measurement technique and underlying principles. It is based largely on experience gained with a type of impactor that has been specially developed for droplet measurements, and also on applications of impactors in the chemical industry. Most of the measurement work has been undertaken in connection with environmental protection. The cascade impactor gives the mass‐related droplet size distributions and concentrations within a range extending from about 0.2 to 10 microns. Larger droplets are collected quantitatively in the sampling probe, which serves as a precollecto