A flow sensor configured to measure the flow of a fluid flowing through a tubular structure comprises a first detection unit that is configured to detect flows above a predefined lower flow level representing the lowest flow that can be measured by the first detection unit and a second detection unit that is configured to estimate the flow below the lower flow level (on the basis of the temperature difference between the surroundings and a fluid). The second detection unit is configured to estimate the flow below the lower flow level on the basis of one or more measurements made in a flow-calibration-area in which the flow sensor can detect the flow and in which the flow depends on the temperature difference.