Measurement of Free Space or: what's the dead volume?

The meaning of free space (dead volume) is that it is not the geometrical volume of the system but it is the volume of convenience used for the calculation of adsorption amount. In case of nitrogen adsorption measurement, the sample is cooled down by liquid nitrogen. After cooling, there will be two different temperature zones in the sample cell. The gas density of the cooled zone is higher and it is influenced by the coolant temperature and by the change in the coolant level. The volumetric method counts the number of un-adsorbed gas molecules (density), and in the process the dead volume plays an important role. The conventional volumetric measurement procedure is to cool down the sample by liquid nitrogen, measure the dead volume by non-adsorbing He gas at the liquid nitrogen temperature, and then measure the nitrogen or other adsorptive gas isotherm(*). During the adsorption measurement the coolant level needs to be maintained.