Should a pressure gauge be installed between the rupture disk and safety valve on the pressure relief system of an LH2 tank? If not, why?

A pressure gauge is not usually provided, as one set of devices (safety valve and rupture disc) is always online by design. If the diverter valve connecting the two sets of relief devices is in the center position, both sets are online although depending upon the design of the diverter valve, it might not allow full flow in either direction. For the secondary stack with a rupture disc and a locked open valve, a pressure gauge is usually installed since it also serves as an independent means to measure the pressure within the tank. This circuit provides both a secondary means to relieve pressure in the tank as well as monitor pressure. It’s intended to be redundant with the main safety circuit in case of a blockage.