This is slightly modified sampler compared to the 2-stage Anderson sampler. In this sampler, the air after entering the circular orifice is drawn through a series of six circular plates each perforated with 400 holes. The spores are impacted through holes onto the sterile medium taken in Petri dishes. The plates in the series have progressively smaller holes. The largest particle gets deposited in the first while the smallest in the last Petri dish. In the Petri dishes different media are used for different size fractions. Air is sampled at the rate of 28.3 litres per minute. The sampler has proved useful for sampling particles less than 8 mm, which include the bacteria Actinomycetes and moulds. May (1945) modified this sampler by changing the pattern of holes to avoid wall losses occurring due to larger particles and recommended not to use the nose cone of the original model. Further, in the wind, the device for intake should be fitted with a large stagnation point shield (Figs. 13.7 a, b).
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