The disclosed apparatus includes a heat meter and a flow meter. The heat meter includes means for delivering trains of pulses to a heat register, the pulses per train being proportional to the temperature difference between fluid inlet and outlet passages of heat exchange means, and the recurrence rate of the pulse trains being determined by a flow sensor. A visual indicator of the pulse trains facilitates zero-difference adjustment of the temperature sensors when they are both at the same temperature. Compensation for variations in the specific heat of the fluid and temperature-related inaccuracy of the flow sensor is effected by non-linear frequency adjustment of an oscillator that generates the pulses. In a flow meter (omitting the temperature-difference circuit) compensation for temperature-related inaccuracy of flow-sensors is effected correspondingly.