The entry of aerosols into sampling tubes and heads

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Abstract
No general solution exists of the very complicated problem of aerosol sampling. In this paper the problem is broken down into sampling with a thin-walled tube and sampling with a bulky head. Tube sampling can be classified into `small tubes', which give the same results independent of their orientation, and `large tubes' where there is a change of sampling efficiency when the direction of the tube is altered. The limits of radius of a `small tube' sampling with an efficiency near to 100% are laid down in terms of particle size and suction speed; tables are given for evaluating the sampling efficiency in a wind. Calm air is defined by the wind speed being less than a value which is dependent on the sampling speed and particle size. The sampling efficiency of a `large tube' may depend on its orientation owing to the fall of particles due to gravity or to a wind blowing past the tube. Isokinetic sampling is necessary to obtain correct samples in strong winds; the effect on efficiency of sampling due to departure from the isokinetic conditions is discussed. The conditions are deduced for accurate sampling with a bulky suction head containing a small orifice: if the orifice faces downwards a critical suction speed exists below which no sample is obtained; at higher speeds sampling is selective and depends on the falling speed of the particles. When a bulky head is used in a cross wind there is no effect due to wind when the orifice is in the top. For a downwards facing orifice the effect of wind is to cause sampling to start at a lower rate of suction and to be more efficient at lower rates of suction. At high rates of suction the wind has no effect.

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