Abstract To develop a sensor for measurements of hydrogen (and its isotopes) concentration in liquid Pb–16Li, two permeating capsules made with niobium and pure iron have been tested. The experimental results demonstrated that the Nb-sensor is not suitable for the foreseen application. The Fe-sensor cannot be operated in equilibrium mode because of the very long time necessary to reach the hydrogen pressure equilibrium. However, the equilibrium mode cannot be definitively excluded. In fact, by means of a substantial optimisation of the sensor geometry, the pressurisation rate could be fast enough to reach acceptable response times. In this context, different optimised solutions have been developed and good performance demonstrated by simulations. The present Fe-sensor can be successfully operated in dynamic mode, since it is possible to link reliably the permeation fluxes to the hydrogen concentration in the liquid metal.