A fluid flow meter is described that uses thermal tracers to measure flow speed. For fluid flowing through a conduit, the fluid is heated at a heating location in the conduit with a time-dependent heating strength. A speed of sound in fluid flowing in the conduit is measured at multiple sensing locations downstream from said heating location. The flow speed of the fluid is determined from a delay with which the time dependence is detected in the sound speeds measured at the sensing locations. A frequency of the variation of heating strength that is used to determine the flow speed is selected automatically based on the flow speed and/or other circumstances.