as described in detail below in applications where impurities such as particles of extraneous material have different reflective tendencies in the different spectra mass flow sensor 10 also includes one or more photodiode detectors 56 58 of appropriate sensitivity i e the detectors need to be sensitive to various ranges of light mounted in housing unit 40 thus leds 50 52 54 and detectors 56 58 are contained in a single housing unit 40 mounted in a single location and facing in the same direction i e in one of side walls 26 28 30 32 at either of inlet end 22 or outlet end 24 or outside of conduit 20 generally speaking the mass of the entrained materials is determined by measuring the light reflected by the materials in reflectance measurement circuits such as used in the present invention only the light emitted by the light source and reflected by the entrained materials is measured a transparent window 42 is located a surface of housing unit 40 between the flow of materials and leds 50 52 54 and detectors 56 58 sensor window 42 protects leds 50 52 54 and detectors 56 58 from the entrained materials in conduit 20 as the materials pass by mass flow sensor 10 mass flow sensor 10 further includes in housing unit 40 a signal processing circuit not shown that processes a signal from detectors 56 58 in such a way that the signal can be acquired and stored by a data acquisition system stray light variations such as from sunlight inside conduit 20 and build up on sensor window 42 such as for example dirt are two factors that may contribute to sensor error to help alleviate the stray light frequency modulated led light may be used as.