During data collection from my project, I came across a very interesting paper, in which researchers described theory and construction of Altersson airflow measurement device-productive air. General principle is that sound waves propagate faster in the direction of airflow than against it. Based on the precisely measured transit times in both directions, it is possible to calculate speed of airflow. But is it possible to build such a device at home with chip components and a chip satisfactory precision? Researchers describe two different concepts. I have decided on a single path acoustic transit times low matter because it seems to me that getting correct angles on both side will be easier. One of the most accessible and easy to use ultrasonic sensor is 40 kilohertz SR-04. For only $2.50 we get integrated circuit, two sensors and the library for Arduino. However, it is reflective sensor and in my application I had to measure transit times in both directions. So my idea is to use two of these and make distance between transmitter and receiver a little longer to do this I have disoldered sensors and connected them to original PCBs by wires. First test in open environment generates pretty constant readings, with no air movement propagation time in both directions pretty much the same with around 1%. This probably can be compensated in the software. I expect that when airflow is introduced the difference between propagation times is rising, proving that the contact is working. To calculate airflow speed I will use this problem from Wikipedia. I only need to know pink readings distance between transmitter and airflow angle. In this case angle to airflow is 0 so 2x alpha is equal to 2. After adding this formula to Arduino sketch I get air speed in meters per second. When there is no airflow the value is close to 0 and it seems to rise with airflow speed increase. The questions how close are those readings to riddle values and if it is accurate enough for residential HVAC systems. In next video I will test this whole mid-contrapson with real ventilation system and compare readings with industrial great measure devices. If this concept will not be very correct I will try to implement other methods.