Casella's Slit Sampler description.

The english pilot and scientist R.B. Bourdillon has described in "A Slit sampler for collecting and counting air-borne bacteria", 1941 an impactor that became the Casella Slit Sampler. In R.B. Bourdillon slit sampler is adopted Owen's principle. A good description how Owens' jet dust-counter works is findlable in "History of Impactors - The first 110 years".

Bourdillon's sampler sucks air throught a narrow slit, deep on the surface of agar contained in a Petri dish placed below the slit. The Petri dish is slowly rotated during sampling it can be roated either by hand or mechanically, the slit is placed radial to the circular surface of the Petri dish.

 The rotation origin distinguishes two kind of sampler: hand-rotated model and motor-driven model. Both models are accurately described by Bourdillon in his article from "Journal of Hygiene", vol 41, no.2. 

The sampler's body consists of an airtight metal box (1), the Petri dish is covered by a rubber covered disk (2) that is rotated by the spindle (3) and it pass through the plain tubular bearing (14). In the roof of the box there is a removable metal tube (4) to the lower end where are soldered two metal jaws (5) forming the slit. There are two side outlets from the box: one (8) connected to a simple water manometer and the other (9) to a power-driven suction pump with an adjustable by-pass valve in the connecting tube. When in used the suction causes a pressure difference inside the box and it depends on the rate of air flow through the slit. The valve is adjusted till the water column reaches 11 inches. In these conditions the sampler gives 1 cu. ft/min, which is the recommended rate.