Metrological features of the rotating low-speed anemometer calibration facility at INRIM

The INRIM standard facility for low airspeed (0.20 to 5.00 m/s), called Rotating Arm (RA), is presented. This is a dragging facility, i.e. it is based on the reciprocity principle. This means that the relative movement between air and measurement instrument is obtained through the translation of the latter through nominally still air. The instrument is mounted on the tip of an arm approximately 3.5~m long, which rotates about an axis; the tip velocity measurement is traceable to length and time primary standards measurements because it is obtained through measuring the instrument positioning radius and the rotation frequency. The working principle, the design and the measurement chain of the RA are described in detail. Because of the movement of arm, air movements arise in the measurement ambient; this can be seen as a systematic error connected to the functioning of the system; the methods for correcting this and other systematic errors are described in the paper, together with specific details and operating procedures aimed at reducing the uncertainty contributions. Finally, an evaluation of the uncertainty budget is discussed.