ENTRY OF AEROSOLS INTO SAMPLING TUBES AND HEADS

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.