On the steady‐state drift conditions of a water calorimeter in clinical megavoltage photon and electron beams

We have investigated the characteristic behavior of a water calorimeter in a clinical environment where the temperature is not strictly controlled. Since the ambient temperature plays a crucial role in the successful operation of a calorimeter, its effects were closely monitored. The results were compared with theoretical predictions based on a simple model of heat transfer by conduction. They were found useful in analyzing the radiation dose measurements. In particular, a relationship between the absorbed dose and the pre‐ and postirradiation thermal drifts was examined. Finally, some suggestions are made concerning the desirable conditions under which a water calorimeter may be operated.