Apparatus and method for measuring air flow

An apparatus for measuring air flow in a duct, e.g. in a ventilation duct, wherein

• the apparatus comprises a sensor fittable into connection with the duct, the sensor comprising an ultrasound transmitter and at least two ultrasound receivers, and a control unit to which the ultrasound transmitter and ultrasound receivers are connectable,
• wherein the apparatus further comprises a temperature sensor for measuring temperature, and wherein the apparatus is adapted to determine the temperature compensation data automatically at the installation site of the apparatus in conjunction with normal operation as follows:
• the apparatus is adapted to measure the temperature of the air and/or of the sensor in a duct with the temperature sensor,
• the apparatus is adapted to perform a sensor calibration measurement at a temperature in question by arranging the flow in the duct to be essentially zero if the sensor calibration measurement at the measured temperature and/or within a certain predefined temperature range has not been performed earlier from the environs of the measured temperature, and
• the apparatus is adapted to determine and/or to record in memory, temperature compensation data for the measurement result of air flow for the temperature in question based on the velocity of the air flow measured, the temperature compensation data being formed on the basis of the sensor calibration measurement.