a system or a method for measuring flow in a flow duct has at least two ultra sound transducers . the flow of air in a duct is measured by one or more transducers transmitting beams of ultra sound controlled by a microcontroller based electronic system in which the microcontroller stores a vector of data samples for each direction of transmission , which vector includes an appropriate number of n samples forming a frame , the microcontroller multiplying each value of the frame which a complex number . based on the result , the microcontroller calculates the flow in the duct . as a result , an efficient flow measurement of air flowing in a duct can be achieved .