PRINCIPLES OF OPERATION
The Sub-Sieve Sizer operates on the air-permeability principle for measuring the average particle sizeof powders. The principle is based upon the fact that particles in the path of a regulated airflow willaffect that airflow in relationship to their size. More specifically, however, the principle finds its basisin the fact that a current of air flows more readily through a bed of coarse powder than through anotherwise equal bed of fine powder that is equal in shape of bed, apparent volume, and percentage of voids; but by reason of difference in general coarseness of material (differing in average pore diameterand in total interstitial surface), measurements of average particle sizes are obtained.Actually, the basic operating principles of the instrument are relatively simple (see figure 1). The airpump builds up air pressure to a constant head in the pressure regulator standpipe. Under this pressurehead, the air is conducted to the packed powder sample contained in the sample tube. The flow of airthrough this packed powder bed is measured by means of a precision manometer, in which the level of the fluid indicates the average diameter of the powder particles directly on the calculator chart.Ernest L. Gooden and Charles M. Smith 1
derived mathematical formulas involved in calibrating theSub-Sieve Sizer from the earlier work of P. C. Carman 2
. Although the formulas are complex, as aresult of Gooden and Smith’s work on the standardization of conditions, the operator can obtainaverage particle sizes from the instrument without mathematical computation.