AK-­AV air velocity transmitter is a ideal option for accurate ventilation control. The innovation of the working theory is using the pitot tube (The pitot tube is for testing the pressure). Because of the simple structure, the convenient use, and the perfect theoretical study, this method is widely used. Pitot tube test according the pressure which caused by liquid flow. The standard pitot tube is composed by inner tube, outer tube and the HIP catheter. It can detect the higher pressure, medium pressure and the low pressure which insure the accuracy and the average the test. On the top of pilot tube, there is a small hole which is vertically with the direction of the fluid flow. The hole on the top and the side is connect with the two pips of each others. The air pressure which enter into the top of the pitot tube is called overall pressure. Except the HIP pressure of the flow, it also contains the pressure translated by the kinetic energy which caused by the fluid stagnation. The pressure enter the hole of the side is only the HIP pressure. According the overall pressure and the HIP pressure we can calculate the static pressure. Then calculate the velocity.