Measuring the amounts of gas adsorbed under storage/separation process conditions often requires super-atmospheric pressures. Such measurements are done in static volumetric devices using manometric principles, that is determining the amount of gas from pressure changes over the sample in a known fixed volume. This type of device is also known as a Sieverts apparatus.
Gas is expanded from a thermostatted dosing manifold of known volume into the sample cell whose internal volume is determined automatically by the instrument in a helium-expansion calibration step (this takes into account the volume occupied by the sample). The volume of gas transferred into the cell is calculated from the change in pressure in the manifold. After a programmable equilibration period, the final pressure in the cell is recorded. This represents the gas not adsorbed by the sample. What is adsorbed is calculated by difference. This process is repeated automatically at higher and higher pressures, usually until the maximum achievable pressure is reached (100 bar or 200 bar depending on the instrument model).
A plot of adsorbed amount versus pressure is called an adsorption isotherm. A plot of pressure against percent uptake is referred to as a pressure-composition-temperature (PCT) curve. In a porous system those voids are filled with unadsorbed gas whose density, at supercritical conditions, continually increases with increasing pressure. Therefore the additional amount that constitutes association with the surface of the pores often goes through a maximum (unlike low pressure cryogenic adsorption used for surface area and pore size determinations). The amount adsorbed under high pressure conditions is therefore often referred to as surface excess adsorption.
Careful thermostatting essential for meaningful high pressure work, since small changes in temperature can mean quite large uncertainties in calculated amounts of gas (the higher pressure have a magnifying effect on volumetric errors). High quality seals are also necessary as a pressure differential to atmosphere of many tens of bar, up to 199bar, might be required during an analysis, so all tubing and sample cell fittings use metal-to-metal type seals.