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.