In the past calorimetry has been developed as a powerful tool in radiometrology. Calorimetric methods have been applied for the determination of activities, half lives and mean energies released during the desintegration of radioactive isotopes. The fundamental factors and relations which determine the power output of radioactive samples are presented and some basic calorimeter principles are discussed. At the Kernforschungszentrum Karlsruhe (KfK) a family of 3 calorimeters has been developed to measure the energy release from radioactive waste products arising from reprocessing operations. With these calorimeters, radioactive samples with sizes from a few cm3 to 2·105 cm3 and heat ratings ranging from a few mW to kW can be measured. After modifications of its inner part the most sensitive calorimeter among the three calorimeters mentioned above would be best suited for measuring the tritium inventory in T-getters of the Amersham-type. Its new time constant and sensitivity is calculated with FE-methods.