The most important measurement made on a new hydrogen storage material is the pressure–composition isotherm that captures the storage conditions and the amount of hydrogen stored. The most popular experimental approach for constructing such isotherms is the Sieverts technique, in which the amount of hydrogen absorbed or desorbed by the sample is inferred from measurements of the hydrogen pressure in a stepwise procedure. An in-depth discussion of the factors contributing to good performance is presented, based on a model of the sensitivity of the Sieverts manifold to changes in hydrogen uptake. Quantification of satisfactory performance may be achieved through a Figure of Merit for the apparatus in combination with the sample to be measured. The Figure of Merit is derived for the general case of a Sieverts apparatus in which the constituent volumes are not at the same temperature, and related to the supposed maximum amount of hydrogen taken up by the sample. The analysis confirms that (i) low sample temperature and high sample molar volume pose challenges for authoritative measurements and (ii) tests with a dense sample like LaNi5 do not validate a Sieverts manifold for accurate measurements with high-surface-area adsorbers such as carbons and metal-organic frameworks. The calculation of a Figure of Merit for a proposed experiment provides an indication of the expected accuracy of the results, as well as scope to vary quantities such as the amount of sample in order to improve the Figure of Merit. In addition, a procedure for devising an optimised experiment is proposed, based on the reference and sample volumes of the Sieverts manifold being configurable.