The calorimeter allows the water absorbed dose, a measure of radiotherapy, to be measured according to its definition. Different detector types are used in the PTB calorimeter; this paper discusses heat conduction, convection and the radiolysis of the H-2/O-2 system for a 'seated' detector. The 'sealed' detector allows the calorimeter to be operated at room temperature in the case of Co-60-gamma-radiation.