The adsorbed amount as a function of pressure can be obtained by volumetric (manometric) and gravimetric methods, carrier gas and calorimetric techniques, nuclear resonance as well as by a combination of calorimetric and impedance spectroscopic measurements. However, the most frequently used methods are the volumetric (manometric) and the gravimetric methods. The gravimetric method is based on a sensitive microbalance and a pressure gauge. The adsorbed amount can be measured directly, but a pressure dependent buoyancy correction is necessary. The gravimetric method is convenient to use for the study of adsorption not too far from room temperature. The adsorbent is not in direct contact with the thermostat and it is therefore more difficult to control and measure the exact temperature of the adsorbent at both high and cryogenic temperatures. Therefore, the volumetric method is recommended to measure the adsorption of nitrogen, argon and krypton at the temperatures of liquid nitrogen (77.35 K) and argon (87.27 K).

The volumetric method is based on calibrated volumes and pressure measurements by applying the general gas equation. The adsorbed amount is calculated by determining the difference of the total amount of gas admitted to the sample cell with the adsorbent and the amount of gas in the free space. The void volume needs to be known very accurately. 

Both volumetric and gravimetric methods allow adsorption to be measured under either static and quasi-equilibrium conditions. In quasi-equilibrium methods the adsorptive is continuously admitted to the sample at a certain, low rate. To obtain a scan of the desorption isotherm the pressure is continuously decreased. The most difficult point associated with the quasi-equilibrium procedure is that one needs to reach at any time of the experiment satisfactory equilibrium conditions. To check that equilibrium has been established the analysis should be repeated using slower gas rates (gas bleed rate). The validity of the analysis is strengthened if identical data are obtained at two different gas flows. If one can reach true equilibrium conditions, the main advantage of this method is that it provides isotherms of unsurpassed resolution. 

In contrast to this quasi-equilibrium method, the continuous flow method proposed by Nelson and Eggertson, gives rise to a discontinuous, point-by-point adsorption, as is the case for the volumetric static method. This flow method is based on a continuous flow of a mixture of a carrier gas (helium) and adsorptive (e.g., nitrogen) through the powder bed. The change in gas composition due to the adsorption of nitrogen is monitored by a thermal conductivity detector. The method is still frequently used for single point surface area measurements. The static volumetric and the dynamic flow two methods are the most frequently used for the surface area and pore size characterization of porous solids.