Patent ID: 8284398

Claim:
A nephelometer, comprising: an optical sensor unit having a light source and a detector positioned to detect light scattered from a view volume; a flow path coupled to an inlet of the optical sensor, the flow path having a junction with a first inlet branch coupled to receive an aerosol sample and a second inlet branch coupled to receive a supply of a clean gas, at least one of the inlet branches coupled to a pump and adapted to periodically supply the aerosol sample or clean gas in that branch at a specified modulation rate, an outlet of the junction providing a concentration-modulated stream that is variable blend of the aerosol sample and clean gas to the view volume of the optical sensor; and the optical sensor unit further having a signal processor adapted, synchronous with the modulation rate for the concentration-modulated stream, to filter a signal from the detector to remove a background parasitic instrument scattering level and obtain a modulation amplitude of the signal representing a particulate volume in the aerosol sample.