In-process measurement of particle size and concentration distributions provides continuous analysis, quality control, and optimization of product yield. As process production rates continue to improve, the delay between laboratory analysis and process correction of the product stream becomes more significant and costly in many commercial applications. Elimination of sample handling and operator manipulation is now possible for most pneumatic flows using optical methods which are properly interfaced with the process stream. Insitec has developed such an instrument based on ensemble laser-diffraction that has been successfully applied to a wide range of research and industrial process applications. As part of the adaptation of instrumentation to the specific needs of different process applications, we have developed several different interface arrangements, including a direct inline system and an eductor bypass instrument. A specific window purge design has been developed and shown to operate for extended time periods (exceeding one month and more) without maintenance. To address the wide range of operating concentrations that occur in practice, a variable pathlength interface has been developed along with a deconvolution algorithm that corrects multiple scattering effects for light transmissions down to 2%. Distribution update rates are computed and displayed in less than 5 seconds. This methodology is described along with measurements in a non-steady pharmaceutical powder processing application. Results show uniform size measurement for ten-fold concentration variations. Real time measurements allowed diagnosis and correction of non-steady feed rates to a classifier-mill.