The ceramic diffuser 30 is preferably made of an alumina bisque, but other porous ceramics and materials such as POREX foam and cellulose are acceptable for use as well.Mercury porosimetry and nitrogen adsorption measurements of alumina bisque pore size and pore volume indicate acceptable diffuser ranges have pore sizes of about 0.5 to 2.0 microns and pore volumes of about 0.15 ml/g to 0.30 ml/g.