Operating principle

SDI is a 12-stage cascade low pressure impactor. An impactor has two co-linear plates; the other one acts as a collection surface for the particles, the other one has small nozzle or nozzles in it to control the flow velocity. After the sample passes through the nozzles it is turned sharply before the collection plate; particles larger than stage cut diameter cannot follow the flow stream lines but are impacted on the collection plate. Particles smaller that the stage cut diameter continue to the following impactor stages where they are further size classified or collected. By changing the dimensions in each impactor stage, different sized particles can be collected on different impactor stages. The flow through the SDI impactor is controlled with the lowest stage jet plate which acts as a critical orifice when 135 mbar is adjusted under the first impactor stage.

The particle size distribution in SDI is determined by analyzing the collected particles gravimetrically or chemically after the sampling period. The substrates used on the collection plates as particle collection surface are 19 mm in diameter, the actual particle deposti area is 8 mm in diameter. Different material substrates can be used depending on the used particle analysis methods.