Portable Instantaneous Mass Monitor for Coal Mine Dust

A battery-operated portable coal mine dust mass monitor has been developed based on the measurement of beta-radiation absorption by the aerosol particles collected by inertial impaction on a thin polyester film substrate. The nonrespirable fraction of the dust is retained by a cyclone precollector, and the respirable particles, down to almost 0.4 µm in diameter (unit density), are collected by the impactor mass-sensing stage. Each measurement-sampling run is performed in 1 minute, at the end of which the mass concentration (milligrams per cubic meter) during that period is displayed automatically on a digital semiconductor readout. The displayed output is proportional to the ratio of the logarithms of the initial to the final beta-pulse rate resulting in a measurement that is independent of beta-source decay, variations of substrate thickness and barometric pressure, changes in detector sensitivity, etc. The mass concentration range of the instrument is 1 to 50 mg/m3, at a sampling flow rate of 2 liters/min. Timing and signal processing are performed by means of solid-state analog-digital circuitry. The instrument is designed to meet the U.S. Bureau of Mines intrinsic safety requirements for operation in coal mine environments.