Why is surface area so important? Because surface area is the means by which a solid interacts with its surroundings, be it gas, liquid or other solids. As particle size decreases, the surface area per unit volume (or mass) increases. Furthermore, the generation of porosity, especially when due to small pores, can produce surface area far in excess of that produced by particle size reduction! On very coarse powders the specific surface area can be as low as a few square centimeters per gram, while on finer powders it might be a few square meters per gram. Porous materials having a significant volume of very small pores might exhibit a surface area larger than a football field - several thousand square meters per gram.
Specific surface area is often correlated with rates of dissolution and other rate-related phenomena such as catalyst activity, electrostatic properties of powders, light scattering, opacity, sintering properties, glazing, moisture retention, shelf-life and many other properties which can influence the processing and behavior of powders and porous solids.
Therefore, surface area measurement is probably the most widely used means to characterize porous materials. Since the surface area corresponds to the roughness of the particle exterior and its porous interior, gas sorption is the preferred technique. In contrast, particle sizing techniques, for example, calculate assumed area values based on smooth, non porous spheres!
So, for accurate surface area measurements,the temperature and pressure of an inert gas are adjusted to cause a single layer of gas molecules to be adsorbed over the entire surface of a solid, be it porous, no-porous or powdered.
Pressure transducers or other sensors respond quantitatively to the amount of gas adsorbed. Using these data, and by means of a simple well-known calculation (the B.E.T. equation), it is easy to compute the surface area of a sample which is usually reported as the specific surface area (i.e. surface area per unit mass, usually m2/g).
For more information on the science behind the measurement, please see: Gas Sorption - Principles of Measurement
Quantachrome Instruments For Surface Area Analysis by Gas Sorption