The bulk temperature at any location in the heat transfer section was calculated from the power supplied to the tube, assumed distributed uniformly per unit length over the heat transfer surface area, decreased by the heat losses through the insulation, while the outlet temperature was checked also by placing a thermocouple at the end of the heated section. The heat losses towards the environment were estimated in a preliminary calibration of the apparatus aimed to measure the overall thermal resistance between the tube wall and the environment. This procedure was performed as follows:in absence of fluid circulation, a known power rate was supplied to the tube wall and the wall to ambient temperature difference was measured.