Because "Micro Cooling Fan" parameter is usually calibrated in free air transmission, that in the absence of pressure circumstances. In the practical application of the system, "Micro Cooling Fan" is often given to bear the system back pressure resistance. And this pressure is related to many factors, it is difficult to determine its size. For this reason, in order to compensate for this influence of the pressure difference, generally to selected "Micro Cooling Fan" parameter derating 60% to 80% use.