The general operating characteristics and maintenance requirements of light particulate monitors in an industrial air recirculation system were studied. The tests took place for an 18 week period in a foundry using three scattered light particulate monitors: a RAM/S, a PCAM/TX, and an ATI/722. The monitors were placed at a height of 122 centimeters directly in front of the recirculation system's secondary filters. Air was sampled at approximately 762 meters per minute. Dust in the manifold was collected on a 5 micron filter, and the size particle distribution was measured in a six stage Marple impactor. The high and low dust concentration levels for each monitor were compared. The gravimetric concentration measurements taken at the beginning of the study were 7mg/m3 and decreased to 0.6mg/m3 for the rest of the study. The monitor response factors of RAM/S were the same at the beginning and end of the study but the factors fluctuated widely throughout the study period. Response factors of the other monitors remained relatively constant throughout. Other problems with the monitors were discovered. On the RAM/S and the ATI/722 the zero mark drifted. A new light source had to be installed three times in the ATI/722. Also, the PCAM pump failed at the end of the study. The author made five recommendations: a convenient mechanism for adjusting zero and span in these monitors is needed; a shock mounting of the sensor assembly or a more durable light source for the ATI is needed; a mechanism for detecting low flow is needed for all monitors; filter, pumps, light sources, and other such items should be readily accessible; and users should be trained in the operation and maintenance of the equipment. The author concludes that these five needs were not met by any one of the three monitors tested.