The figure on the left shows the performance curve of the CFM-120 series from Same Sky, a 120 mm by 120 mm frame axial fan with dual ball bearing construction. As shown, manufacturers characterize fans by plotting airflow (measured in either Cubic Feet per Minute, CFM, or Cubic Meters per Minute, CMM) against static pressure (measured in either inches or millimeters of water, often written as Inch H₂O or mm H₂O). Unfortunately, the result calculated in Equation 3 on the previous slide is only accurate for conditions with no back pressure or system impedance, but in reality there will always be some system impedance that needs to be calculated or estimated. Once system impedance is accounted for it can then be plotted on the fan’s performance curve shown on the right and the point at which they cross can be taken as the operating point for the fan. However, if the airflow through an enclosure cannot be measured, then an alternative would be to specify the operating point above the number gathered from Equation 3. For example, if the airflow calculated is 50 CFM with zero back pressure, over-specifying the fan at 100 CFM with the intention of operating it at 75 CFM provides a good margin of error, as well as the ability to increase airflow during operation. Furthermore, taking steps at the design stage to minimize system impedance can positively impact the size and power of fan needed. Best practices include keeping areas around air inlets and outlets clear, considering the system impedance a filter will introduce, and ensuring component placement directs airflow to and around critical components.