face 142 may have a shape and dimensions that match the size of the aperture 116 in the wall of canister 112 thus a person desiring to gauge the capacity of hydrogen left in the canister 112 need simply to view the face 142 to see the needle indication of metal hydride material expansion which roughly correlates to the amount of remaining hydrogen capacity a single scale is shown on face 142 because the physical volumetric expansion of the contained metal hydride material depends to a much greater extent on the amount of hydrogen that is contained in the hydride material than it depends on the temperature of the hydride material the correlation of volumetric expansion is so much more dependent on the hydrogen absorption amount that the temperature expansion effect can be effectively disregarded however should a more accurate reading be desired so as to take into account the volumetric expansion of the hydride material which is contributed by an increase in temperature a calibrated scale can also be provided to the face 142 similar to those of the faces 32 132 shown respectively in fig3 or 4 the diaphragm 150 fig6 of the container 118 closes off fluid.