Dear Mehrdad, I write below the principle of functionality of a BET tool (from a manufacturer).  Gianluca

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Principle

The method specified involves the determination of the amount of adsorbate or adsorptive gas required to cover the external and the accessible internal pore surfaces of a solid with a complete monolayer of adsorbate. This monolayer capacity can be calculated fro the adsorption isotherm using the BET equation. Any gas may be used, provided it is physically adsorbed by weak bonds at the surface of the solid (van der Walls forces), and can be desorbed by a decrease in pressure at the same temperature.

Nitrogen at its boiling point (about 77 K) is usually the most suitable adsorptive. If the sensitivity of the instrument when using nitrogen is insufficient for low surface areas, adsorptives of heavier molecules or of vapour pressure lower than nitrogent, e.g. krypton, may be used. The results of measurements with different adsorptives may deviate from each other because of different molecular areas, different accessibilities to pores and different measuring temperatures.

The adsorptive gas is admitted to the sample container which is held at a constant temperature. The amounts adsorbed are measured in equilibrium with the adsorptive gas pressure p and plotted against relative pressure, p/p0, to give an adsorption isotherm. Adsorption isotherms may be obtained by volumetric, gravimetric, calorimetric or spectroscopic measurement or by the carrier gas method using continuous or discontinuous operation.

In order to determine the adsorption isotherm volumetrically by the discontinuous method, known amounts of adsorptive are admitted stepwise into the sample container. At each step, adsorption of the gas by the sample occurs and the pressure in the confied volume falls until the adsorbate and the adsorptive are in equilibrium. The adsorbed volume may be conpensated by the introduction of calibrated increments of gas so that the pressure remains constant. The amount of gas adsorbed is the difference between the amount of gas adsorbed is the difference between the amount of gas admitted and the amount of gas filling the dead volume (free space in the sample container, including connections), which is determined by application of the general gas equation. The various volumes of the apparatus and their temperatures should be taken into account.

The dead volume must be determined before or after the measurement of the adsorption isotherm. The calibration is done volumetrically using helium at the measuring temperature. It should be noted that some materials may absorb helium. In this case, corrections can be made after measuring the helium isotherms. During sample measurement and determination of the dead volume, it is recommended that the liquide level in the collinig bath be maintained, unless otherwise compensated, at least 50 mm above the sample and constandt to within 1 mm. The determination of the dead volume may be avoided using difference measurements, i.e. by means of reference and sample tubes connected by a differential transducer.

In the continuous volumetric measurement, the amount of admitted adsorptive may be calculated from the pressure difference and the duration of the gas flow through a calibrated capillary or metering valve.