A scaled ZrCo bed ( 25 g tritium capacity of design ) with gas flowing calorimetry system was fabricated to establish the “in-bed” tritium accounting technology to apply to the ITER tritium storage beds. The basic calorimetric characteristics, steady state temperature raise of He gas stream flowing through a secondary coil line fixed in the ZrCo tritide, was measured and correlated with power input by heater to simulate tritium decay heat or with actual tritium storage. The target accuracy is 1 % which means to measure +− 1 gram (0.32 watt) of tritium on 100 g storage. The results shows the good accounting function that the temperature increases of He stream of 4.7 and 96.8 degrees were measured under power input of 0.32 and 8.0 watts, respectively, with good reproducibility. These “in-bed” tritium accounting function was well demonstrated storing a gram level of tritium gas within a few days.