7.1.5.2 Efficiency of other sizesSince larger fans in a geometrically similar range tend to perform better than smaller fans, it is preferable to test a number of fans in the series such that the performance of a fan is never calculated from a larger size.Where this cannot be achieved, then by agreement between the purchaser and the manufacturer, a formula such as the Ackeret formula may be used to calculate the efficiency of either a larger or smaller fan. For better safety, use half the increase, for increased size, or the full decrease for a smaller size. The following formula is only applicable in the vicinity of the best fan efficiency: