N.1 – Principle of microbial air sampler by impact method on agar plate


Air containing microbe-carrying particles is aspirated and accelerate through a hole and direct towards a nutrient agar surface of a plate. As the air turns away from the agar surface, the microbe-carrying particles that cannot follow the flow are impacted. The plate containing nutrient agar is then incubate at a suitable time and temperature, and the resulting Colony Forming Units (CFU) are counted to evaluate the number of microbe-containing particles collected from a specific volume of air.

How the microbe-carrying particles impact on agar surface

The aspirated air passes through an intake orifice of the sampler head at a velocity of “U” and, as it approaches the agar surface, it turns. The arc of the turning circle has a radius of “r” which is assumed to be the same as the radius of the intake nozzle. The velocity round the curve is assumed to be “U”.

The microbe-carrying particle travels along the streamline and experiences a centrifugal force that causes it to move toward the agar surface of the plate.