A technique is described for measuring particle size in pneumatically conveyed powders. Particle size is inferred from measurement of the peak compression of a specially designed ultrasonic transducer subject to impact of the particles. The technique is well adapted to measure the coarser fractions of the particle size distribution, which have the largest compressions. Monitoring coarse fractions of powder distributions is essential for control of dry grinding processes such as those used in the cement and coal industries, In the laboratory the impact size monitor (ISM) has demonstrated the ability to discriminate between powders with a difference in peak particle size of 2% at an approximate peak size of 150 microns. The ISM operated successfully in the laboratory at a loading of 0.5 kg/m(3) of powder and measured particles down to 50 microns in size. As well as size information the ISM can provide powder distribution across a pneumatic transport pipe.