In a well designed product, this is a stable point not only well within the maximum airflow capability of the fan, but also low enough to minimize unwanted fan noise Very similarly to economics, where the price of a product is determined by where the supply and demand curves intersect, the product airflow operating point is determined by where the product impedance curve intersects with the fan performance curve. In the USA this is again typically in units of inches of water versus cfm. The airflow performance curve is also typically plotted as pressure versus airflow. As the impedance to airflow increases, the amount of airflow decreases. When the fan is operated in a free field, its airflow performance is high. Here we are measuring the performance of an air moving device such as a fan or blower. The airflow performance curve is also measured using a calibrated airflow test chamber. This curve typically increases faster than linearly at low airflow rates and in the typical intended operating range. One thereby obtains the product “impedance curve” showing the pressure difference increasing from zero at zero airflow and increasing with increasing cfm. This pressure difference ( typically measured in the USA in units of “inches of water”) increases as the quantity of air (typically measured in the USA in units of cfm, cubic feet per minute) increases. This impedance can be measured by forcing air to flow through the product, using a calibrated airflow test chamber, and measuring the pressure difference between the inlet and outlet of the product. The product therefore has an impedance to airflow. The movement of air through a complex electronic product, or through a simple duct, results in the air flow being restricted compared to air flow in free space. Airflow testing is the measurement of the movement of air through, into, or out of a component or product, or the measurement of the performance characteristics of an air moving device, such as a fan or blower.
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