Some observations on determining the size of pores in paper

The various methods which have been used to determine the size of pores in paper are briefly reviewed.These are of three general types: (a) methods based on rate of efflux of fluids; (b) methods based on rate of rise of liquids in vertical strips; (c) methods based on the capillary pressure involved in penetration by liquids.A new method is outlined, which takes advantage of the fact that the coefficient of slip of air in viscous flow through paper is a function of pressure.From the Meyer equation, the average effective pore radius is evaluated in terms of the air permeability values at two pressures.It is only necessary to determine the air permeability at two different pressures of an identical area of paper.Experimental values for two such air-permeability determinations on each of five papers, and the resulting values calculated for the average effective pore radius, are tabulated.