The region of validity must be linear for the BET theory. This is an intermediate region and the theory fails at low and high pressures. An alternative method to approach this adsorption problem is to use the Langmuir theory. The Langmuir theory differs from the BET theory slightly. Adsorption cannot proceed beyond monolayer coverage.
All sites are equivalent and the surface is uniform.
The ability of a molecule to adsorb at a given site is independent of the occupation of neighboring sites. Figure 10 - The Langmuir isotherm for Alumina is linear through low and moderate surface coverage. But as the pressure increases, and the coverage is high, the line deviates from linear. The Langmuir theory holds under low pressures where the BET theory may fail. The problem with the Langmuir theory is that at high pressures, there may be more than just monolayer coverage.