Abstract: A temperature probe 3 determines the instantaneous ambient temperature of a multiphase fluid 2 flowing through a pipe 1, and at a position further downstream there is an inlet 4 for water at a constant temperature significantly different from that of the fluid 2 injected at timed intervals through a high pressure water jet nozzle 5. The effect of the jet is to mix the injected water intimately with the fluid 2 flowing through the pipe and create a localised pulse of heated (or cooled) fluid which then flows through the pipe 1 to be measured by a further temperature probe 10 and possibly by yet another probe 11 at positions downstream of the nozzle 5. The time taken for the localised heated pulse of fluid to travel along a fixed length of the pipe 1 can be measured, and will be a function of the rate of flow of the fluid through the pipe.