Sensor fans are used for air measurement, e.g. for air-conditioning systems in motor vehicles. These fans have, for example, an outside diameter of 30 mm, i.e. these are what is referred to in technical jargon as mini-fans.
Mini-fans of this kind also serve to cool processors in computers, for equipment cooling in small equipment, etc., and their dimensions are very small. For example:    fans of the ebm-papst 250 series have dimensions of 8×25×25 mm;    those of the ebm-papst 400F series have dimensions of 10×40×40 mm;    those of the ebm-papst 400 series have dimensions of 20×40×40 mm; and    fans of the ebm-papst 600 series have dimensions of 25×60×60 mm.
The power consumption of such fans is 0.4-0.6 W for the 250 series, 0.7 to 0.9 W for the 400F series, and 0.9-3.4 W for the 400 and 600 series. Their weight is, for example, approximately 5 g for the 250 series, between 17 g and 27 g for the 400/400F series, and approximately 85 g for the 600 series.
In fans of this miniature size that must be very inexpensive, it is important to make the production and assembly thereof extremely simple, so that a high degree of automation becomes possible and so that uniformly high quality and low noise in such fans are obtained.
A further complicating factor with such extremely small fans is that their components, entirely analogously to those of a mechanical clock mechanism, are very delicate and therefore not very robust. The rotor shaft, for example, is often only as thick as a knitting needle and therefore can easily be bent if handled carelessly, rendering the fan unusable.