A method and apparatus is disclosed for measuring the mass concentration of particulates entrained in a process or work area. One or more sensors are employed which electro-optically and continuously weigh the effluent extracted from the process area. The effluent is caused to enter a sample volume area either by convection or through a preseparator. The sampling volume is exposed to near infrared radiation of a light emitting diode (LED). An information signal indicative of small and large particulates entrained in the effluent is developed. The signal is converted and classified to signals corresponding to the presence of small or large particulates and further sub-classified as to size. The electro-optical "weighing" signals are operated on to produce a mass concentration read-out via an empirical algorithm exercised by a microprocessor in a centrally-located control/read-out unit which serves a multiple of remote sensors. The microprocessor also permits convenient automatic compensation of each sensor's electro-optical transfer function, i.e., baseline zero and span. Traceable calibration is also provided.