How does a Brunauer–Emmett–Teller device function?
BET analysis on zeolithes represents a typical example.  Dependent on the size of the particles you have, you select a glass cell of about a centimetre; for a powder a smaller cell, for more coarse pellets you don't want to grind a bigger one.  You need to outgas the samples first, i.e. by applying vacuum and elevated temperatures already adsorbed gases and moisture is removed from the surface of the material to be analyzed as well as the inner walls of the sample cell. This already may take time to be accomplish -- because you do not want to destroy or alter the material in question by too harsh conditions.

The actual measurement requires the sample cell being kept at a constant temperature.  An initial determination takes place while the sample cell is immersed in a Dewar of liquid nitrogen.  This low temperature provides better conditions for the adsorption experiment.

A first blank is measured with Helium gas, entered via a graduated piston. Helium typically does not adsorb on surfaces (at this temperature).  This allows to determine the dead volume of the measurement cell.  In the second step, Nitrogen gas enters the measurement cell, again in a controlled manner (graduated piston, pressure control).  Again, this takes time as you want to establish the equilibrium between the adsorbent and the amount of gas adsorbed in function of gas pressure and sample temperature.
A controlled entry of Helium gas is necessary as a second determination of the dead volume to complete the calibration eventually leading to the read-out for our substance at given temperature.

Of course, dependent on what kind of isotherme (BET/Langmuir) is to be determined, Nitrogen gas as a load may be replaced by other gases, like methane.  Beside determination of the surface area of a solid, the adsorption properties may be determined in a series of more elevated temperatures, too.