The Hirst spore trap, (invented by Hirst in 1952), was the first suction type sampler readily available for sampling pollen and other spores. The vane tail keeps the 2.14 mm intake orifice facing the wind, and a rain shield protects the orifice from precipitation. It must be provided with an external vacuum pump (1/6 HP Motor).
The efficiency though variable with wind speed and with particle size, is reasonably high. Inside the housing containing the orifice, a greased microscope slide is drawn upward by a clockwork at a rate of 2 mm/hour. Particles in the air sampled are deposited by impaction on the slide, which is changed each day.
The suction trap provides data on rapid changes in the composition of air-spora. The spores in a measured volume of air are drawn through an orifice and are impacted on a sticky surface on a slowly moving microscope slide. The air is sucked through at the rate of 10 litres per minute, impinges on the microscope slide coated with solvent and vaseline, which form a sticky surface. The spore-free air passes out through the instrument into the pump. Thus, it leaves a trace at the end of 24 hours.
In the Hirst spore trap, there is a possibility of overloading of air spora on the collecting surface within a short period and so it requires constant checking.
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