A calorimeter was integrated in KEPTL (KEPCO Research Institute Tritium Laboratory) and the various performance tests were performed. The inventory of tritium transport vessels delivered to the ITER tritium plant will be measured by calorimetry. For calorimetry measurement the tritium transport vessel will be inserted in an aluminum secondary container for the tritium leak prevention. The heat capacity and geometry of measuring objects, however, can affect the performance of the calorimeter such as measuring time, sensitivity, etc. In this study, the effect of the heat capacity of the tritium vessel on the performance of the twin cell calorimeter is studied by using JEC and aluminum container which are dummy vessels simulating the tritium decay heat with electric heaters. The average sensitivity in the test with aluminum containers is measured to be 96 V/mW which is similar that with JEC so it does not depend directly on the heat capacity of the tritium vessel. The aluminum container, however, makes the measuring time increase and the heat flow signal could be unstable in the range of low tritium and high heat capacity like a waste vessel after tritium loading out.