A particle sensor for optically detecting an unconstrained particle suspended in a flowing gas includes a sample chamber having a gas inlet and a gas outlet; a gas flow system for flowing said gas from said gas inlet through said sample chamber to said gas outlet, a source of light; an optical system directing said light through said sample chamber, an optical collection system located to collect light scattered by said particles in the gas, and a detection system located to detect the collected light. The total pressure drop through said gas flow system is 3 inches of water or less. The gas flow system includes an axial fan, which may be a high static pressure fan or a counter-rotating fan. In a 1.0 CFM system, the gas inlet nozzle has an area of 25 square millimeters or more.