This invention provides methods and devices to measure particle suspension concentrations in the presence of potential interferents. Particle back-scatter readings are taken at light wavelengths that are absorbed by the medium before interacting with surrounding objects. Source-detector spacings are minimized compared to the mean absorbance path length of light, thereby maximizing the range of sensitivity to particle concentration. Discrimination against potentially interfering particles, such as bubbles, is provided by mapping the signal distribution against the central signal value and/or by the use of statistical measures with reduced dependence on outliers. The methods and devices allow accurate particle concentration readings over a wide range of concentration in environments crowded with potentially interfering objects and in the presence of variable concentrations and sizes of potentially interfering particles.