To calculate a gas sorption isotherm from measurements made using the manometric (Sieverts) technique requires accurate knowledge of the sample cell void volume, the volume able to be occupied by the free gas. For many combinations of material and morphology, this volume changes as the sample expands or compresses, swells, undergoes chemical change or adsorbs gas. For gas adsorption measurements on porous materials, the volume of the adsorbate is often accepted as the pore volume, and the adsorbate density changes as the material adsorbs. For materials exhibiting IUPAC type I isotherms, including many carbons, the uptake saturates at high pressures and additional information about the volume (adsorption space) and density of the adsorbate can be extracted at high pressure, provided adsorption has finished. For hydrogen, this typically requires cryogenic temperatures at pressures up to 10 MPa. In this work, hydrogen isotherms are conducted at pressures up to 200 MPa, in order to investigate the properties of the adsorbate at ambient temperatures. In addition, the manometric method gas uptake equations for absolute, excess and net isotherms are revisited and extrapolated to high pressure in order to extract information on the adsorbate volume and density, and enable the construction of the absolute isotherm.