The specific surface-area of TiO₂, SiO₂ and CaCO₃ powders are determined by 3 commonly used techniques. For rapid and inexpensive measurement of surface-area of a pigment such as titanium oxide, the photometric suspension method is more reliable than the air-permeability method, while the surface-area of some ceramic raw materials such as quartz and calcite powders may be determined in a better accuracy, 40 % more, by the latter than by the former.
In any case, the 2 methods mentioned above does not give any true surface-area, which must be determined only thru the BET method. For titanium oxide powder, the specific surface-area measured by the photometric method is 1/4 of that by the BET method, and for calcite powder, that measured by the airpermeability is 1/8 of that by the 1-point gas-adsorption method by areameter.