Future magnetic-confinement devices which produce significant power by the fusion of D-T nuclei will require an adequate removal rate of helium ash from the plasma core, in order to maintain core purity and the level of power production. Hence, the role of the divertor in removing helium is of crucial importance. At JET, several divertor configurations have been studied to characterise their efficacy at this task, under various operating conditions. More recently, a number of plasmas have been produced with high concentrations of He, up to 100%, to study their characteristics. Such plasmas are seen as relevant to ITER during its early phase of operation, where there are concerns about keeping activation of the vessel and its components to a minimum. The partial pressures of the deuterium and helium in the sub-divertor volume are measured using a Penning gauge, which is a well-established technique. In this paper, a new approach to the measurement is described, which is particularly useful for plasmas with high and varying concentrations of helium.