There have been several reports indicating higher accuracy of the differential volumetric technique for isotherm measurements. The basic premise is the fact that the differential pressure transducers are inherently more accurate at high pressure than absolute pressure transducers resulting in ten-fold increase in accuracy. This approach is especially promoted for hydrogen storage applications. In this project, we constructed a differential volumetric apparatus with great care to increase its accuracy. The apparatus also provide data for standard “absolute” volumetric measurements. The PT diagram of palladium hydride is measured in this system. (PT diagram of metal hydrides also characterize gas uptake by solids just like isotherms). By comparing the data calculated both ways, differential and absolute volumetric approaches, the accuracy issues are addressed directly.