The flow sensor comprises a heater arranged between two sensing thermopiles. In addition, at least one monitoring thermocouple is provided for measuring the temperature of the heater. The signal from the monitoring thermocouple can be used to improve the accuracy of the device in several ways. In particular, the signal from the monitoring thermocouple depends on changes of the Seeback constant and other inherent properties of the thermopiles in the same way as the signal from the sensing thermopiles, which allows a compensation of such effects. The signal from the monitoring thermocouple can also be used as a reference voltage to an A/D converter for converting the signals from the sensing thermopiles.