A calorimeter includes a bucket cover which is used to reconfigure an isothermal water reservoir to provide for temperature equilibration prior to sample analysis and subsequently define a fixed volume of water during analysis in which high precision temperature measurements can be recorded. The apparatus includes mechanisms for sealing and controlling the cover, and for coupling the combustion vessel to the cover while minimizing the thermal contact between them. Improved thermal isolation between the fixed volume of water and the surrounding environment is also achieved.