Several devices aimed at allowing in-situ measurement of dimensional changes of HIP cans inside a HIP vessel, have been designed and tested during the last decade. Sensors were installed inside the pressure vessels making it possible to track changes occurring in the dimensions of powder filled cans in the course of a HIP cycle. The data retrieved is then related to the change in density of the specimens encapsulated in the cans. The use of these techniques yields no direct information concerning processes taking place at the contact areas of adjacent powder particles. Further research aimed at studying densification mechanisms and their dependence upon the process parameters is still required in order to establish the validity of 'HIP diagrams'.