Volumetric systems are designed with a dosing volume, typically at room temperature, and an uptake volume partially immerged in a liquid to keep the sample at the desired temperature throughout the experiment. This means that for any experiment at a temperature different from the environment the uptake cell will have a portion of the volume at room temperature (hot volume) and one (cold volume) at the target experimental temperature which needs to be determined for accurate measurements. The standard procedure consists in a He expansion in the sample cell, monitoring the total pressure while the uptake cell goes from room to the experimental temperature. Here the underlying assumption is that He does not adsorb on the sample at the temperatures explored. For temperature significantly below room temperature, such as measurements at 77K, He adsorption cannot be considered negligible anymore and alternative methods for the determination of the hot and cold volumes are required for accurate isotherm measurements.
Differential volumetric systems feature two symmetrical branches (sample and reference) connected by a differential pressure transducer. In this case, the design offers the advantage to use the reference side for the measurement of the hot and cold volumes therefore avoiding any contact of the He with the sample at cryogenic temperature.
With this contribution we present a novel methodology that allows the determination of the hot and cold volume determination using an Adsorption Differential Volumetric Apparatus (ADVA). The approach can be used for both the measurement of the excess isotherm as well as the direct measurement of the absolute adsorbed amount. The technique is demonstrated on the measurement of the H2 isotherm on the NIST reference NaY material at 77K
