Volumetric method was used in the early days of adsorption studies by Langmuir, Dubinin and others. It basically comprises a gas expansion process from a storage vessel (reference cell) to an adsorption chamber including adsorbent (adsorption cell) through a controlling valve C, as schematically shown in Figure 1. The reference cell with volume  is kept at a constant temperature . The value of  includes the volume of the tube between the reference cell and valve C. The adsorption cell is kept at the specified equilibrium temperature . The volume of the connecting tube between the adsorption cell and valve is divided into two parts: one part with volume  with same temperature as the reference cell. The other part is buried in an atmosphere of temperature . Its volume is added to the volume of adsorption cell .
The amount adsorbed can be calculated from the pressure readings before and after opening valve C based on the p-V-T relationship of real gases. A dry and degassed adsorbent sample of known weight was enclosed in the adsorption cell. An amount of gas is let into  to maintain a pressure . The moles of gas confined in  are calculated as:
The pressure drops to  after opening valve C. The amount of gas maintained in , , and  are respectively:
The amount adsorbed or the excess adsorption N is then obtained:
where  and  are the moles of the gas remaining in  and  before opening valve C. All of the compressibility factor values are calculated by a proper equation of state, which can generate appropriate z values for temperatures not close to the critical zone.
The main advantages of this method are simplicity in procedure, commercial availability of instruments, and the large ranges of pressure and temperature in which this method can be realized. The disadvantage of volumetric measurements is the considerable amount of adsorbent sample needed to overcome adsorption effects on the walls of the vessels. However, this may be a positive aspect if the sample is adequate. A larger amount of sample results in considerable adsorption and usually provides a larger void space in the adsorption cell, rendering the effect of uncertainty in “dead space” to a minimum.