Nearly all process involving sprays or aerosols are strongly affected by droplet size. The measurement of droplet size has therefore been attempted by numerous methods, including direct high-speed photography, light absorptionor scattering, droplet cooling or freezing, and collection of droplets on coated slides or immersion cells. Although no method is entirely satisfactory, the immersion sampling technique has been successfully used both in research and industrial testing of atomizing devices. This method entails collection of dyed-water droplets in sampling cells containing solvent or a similar immersion fluid. Photomicrographs of the droplets are then obtained for manual or automatic counting.
Although immersion sampling has several recognized advantages, it is limited by the tendency of large, high-velocity droplets to shatter and by the failure of very small droplets to impact on the cell. Another problem is interference by the shutter used to expose the spray. To help overcome these limitations the mechanics of sampling hollow or solid-cone sprays was analyzed, and procedures were developed for selecting optimum cell and shutter configurations, sampling distance, and exposure time. Extension of the method to sprays comprised of two liquid streams and to sampling of oil droplets is also discussed.