At the low pressures, where micropore analysis is performed, the void volume correction is relatively small, but because adsorption isotherms are usually performed over the complete relative pressure range up to 1, the void volume needs to be determined carefully. A non-adsorbing gas such as helium is used prior (or in principle after every analysis) to measure the void (free space) volumes at room temperature and at the temperature of the coolant. However, the use of helium for the void volume calibrations may be problematic, and recent investigations have confirmed that some microporous solids may adsorb significant amounts of helium at liquid nitrogen temperature (socalled helium entrapment). Therefore, after exposure of the sample to helium during free space measurements, it is highly recommended to evacuate the sample cell and repeat degassing of the sample at elevated temperatures (this so-called He-removal procedure should be performed at least at room temperature) before continuing with analysis. Another possibility is to determine the void volume with helium after completion of sorption measurement.

Another potential problem is that at these low temperatures (e.g., at the temperatures of liquid nitrogen, 77.35 K, and liquid argon, 87.27 K) and low pressures nitrogen and argon molecules can (in contrast to helium) be restricted from entering the narrowest micropores of some adsorbent due to diffusion limitations. As a consequence, nitrogen and argon sorption data obtained on such adsorbents can be affected by a small systematic void volume error.