Physical adsorption in micropores occurs at relative pressures substantially lower (very often down to a relative pressures of 10^-7) than in case of sorption phenomena in mesopores. Physical adsorption in microporous adsorbents can span a broad spectrum of pressures (up to seven orders of magnitude) hence special care is necessary for the pressure measurements. Consequently, more than one pressure transducer is necessary to measure all equilibrium pressures with sufficient accuracy.

In order to study the adsorption of gases like nitrogen and argon (at their boiling temperatures) within a relative pressure range of 10^-7 ≤ P/Po ≤ 1 with sufficiently high accuracy, it is desirable to use a combination of at least three transducers with maximum ranges of 1 torr, 10 torr and 1000 torr. In addition, one has to assure that the sample cell and the manifold can be evacuated to pressures as low as possible, which requires a suitable high vacuum pumping system. The achievable pressure over the sample should be lower than the pressure of the first experimental point and preferably the pumping system should be able to evacuate the manifold and sample cell to less than a relative pressure of 10-7. The desired low pressure can be achieved by using a turbomolecular pump.

One such experimental setup, which fulfills these requirements in order to assure accurate and precise pressure measurements and a high vacuum environment, is shown schematically as below image. In addition to the obligatory pressure transducers in the dosing volume (manifold) of the apparatus, the analysis station of the volumetric sorption apparatus is also equipped with high precision pressure transducers dedicated to read the pressure just in the sample cell. Hence, the sample cell is isolated during equilibration, which ensures a very small effective void volume and therefore a highly accurate determination of the adsorbed amount. The saturation pressure Po is measured throughout the entire analysis by means of a dedicated saturation pressure transducer, which allows the vapor pressure to be monitored for every datum point. This leads to high accuracy and precision in the determination of P/Po and thus in the determination of the pore size distribution. It is advantageous to use a diaphragm pump to back the turbomolecular pump in order to guarantee a completely oil-free environment for (i) the adsorption measurement and (ii) the outgassing of the sample prior to the analysis.

Schematic representation of a high precision volumetric apparatus suitable for pore size analysis of micro- and mesoporous materials as well as for samples with low surface areas.