Common Errors Found in Volumetric Hydrogen Capacity Measurements and How to Avoid Them

Recent comparisons of hydrogen capacity measurements among different laboratories on identical samples show a wide disparity of results and begs the questions: "How can this be?" and "What are the root causes for this disparity?" The volumetric technique for capacity measurements is based on a mass balance assumption and entails keeping track of the number of moles in the gas phase before and after a change of pressure for the sample. Each instrument must use a mass-balance model and has accompanying assumptions. We examine some of the common errors that occur and lead to large systematic measurement errors. Issues to be discussed include: proper absolute and null calibrations, uniformity and constancy of the temperature profile, choosing the correct mass-balance model and verification of instrument performance. Specific examples will show the large systematic errors that can occur with improper application of the volumetric technique.