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

The INRiM primary standard facility for low airspeed called towing tank is here 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 achieved by the translation of the latter through nominally still air. The instrument is mounted on a carriage which is translated over a 30 m long tank; the velocity measurement is obtained through the measure of the distance covered and the time required to cover it. The velocity measurement is therefore traceable to length and time primary standards measurement. In the paper, the working principle, the design and the measurement chain of the towing tank are described in detail, including a critical discussion about the admissibility of the Galilean invariance hypothesis and its limits. Specific details and operating procedures aimed at reducing the uncertainty contributions are described. Finally, an evaluation of the uncertainty budget is discussed.