In order to find the instrumental errors, a comparison of nine commercial optical particle counters (OPCs) of five different types was made. Data analyses were performed in the following three steps : (1) The comparison of the same type OPCs showed that counting ratios almost fell within the range from 0.70 to 1.40. These errors are caused not only by the individual difference during manufacture but also by the elapsed time after calibration. (2) The comparison of the different type but the same manufacturer's OPCs suggested that these errors were caused mainly by the differences of the detectable smallest particle size in each channel due to the differences of the optical system. (3) The comparison of the different type and manufacturer's OPCs showed that counting ratios ranged over one order of magnitude, because of the different systems and different calibration methods, and the size distributions obtained from several OPCs were compared with those by different principle instruments.

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The evaporation-reaction type aerosol generator of ultrafine tin oxide particles is developed, and the effect of the evaporating temperature on particle size distributions and number concentrations of generated ultrafine tin oxide particles is studied. Particles generated are introduced at high velocity into a deposition chamber pumped down to about 4000 Pa, and are deposited on a glass substrate by inertia to make a thin film. The electrical characteristics of the film thus made are evaluated. The conductance of the film increases with the decrease of film thickness and with the increase of the substrate temperature. The current -voltage relation of the film is Ohmic. It is shown that the ultrafine tin oxide particle film can selectively detect H₂, N₂ and O₂ gases and is most sensitive to H₂ gas.

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