## 1 Introduction  
The measurement of the movement of air had its beginning in the very early days of aircraft development [1]. At present, air velocity measurement has become a very common and important activity within heating ventilation and air-conditioning, safe, comfortable and aerodynamic testing techniques [2], meteorology, aircraft building and flight, mining, precision chemical technology, medicine, ecology for monitoring harmful emissions into the atmosphere [3], dispersion of pollutants [4], and many other areas. The range of air velocities of interest is quite large even in incompressible flow regime, as they span from about 0.05 m/s, typical of natural air convection flows in a controlled room (e.g., clean rooms, the heat transfer in the confined space of a computer, ventilation, or indoor), to almost 50 m/s, typical of high speed winds in open external areas (meteorological or outdoor) [1; 4; 5]. There are many engineering applications in fluid mechanics that require air velocity to be measured at extremely low speeds (i.e., less than 1.0 m/s) [5]. The hot-wire anemometer (HWA) remains the first choice of researchers in measuring laminar, transitional, and turbulent flows, due to its accurate interpretation of the signal and simplicity in use [6], especially when high frequency response and temporal information on the flow are needed [7]. Traditionally, hot-wire anemometers have been used to measure mean velocity and turbulence intensity [8]. The output of HWA is obtained from a bridge-top voltage, which is determined by the convective heat transfer from the hot wire to the fluid flow around the wire [9]. Two kinds of HWAs are currently used: a constant-temperature anemometer (CTA) and a constant-current anemometer (CCA). The CTA is widely used for measurement of flow velocity and turbulence [10]. For a hot-wire probe a relationship between the magnitude, direction of the cooling fluid velocity and the voltage across the wire(s) on the probe is required to be established (i.e., to be calibrated) before the actual usage of the hot-wire probe [11]. That is, the air velocity sensors must be calibrated by means of a velocity standard (reference velocity) to know their metrological performances [2]. In addition, international (and/or national) standards agencies are defining more and more restrictive rules for the traceability and certification of instruments [4]. Thus, calibration must be performed before a hot-wire anemometer can be used in flow velocity measurement. If a low-speed wind tunnel or nozzle is used as a calibration facility, relatively high flow velocities (e.g., greater than 4 m/s for air flows) in the calibration facility can be easily calculated from the difference between total and static pressures obtained with pressure tubes or wall taps on the facility, whereas for relatively low flow velocities (especially when velocity is less than 1.5 m/s) the calibration of hot-wire anemometers cannot be achieved using this conventional calibration rig since the pressure difference in flow is so small that the sensitivity and accuracy of pressure measurement instrumentation are decreased and make it hard to read the pressure difference accurately with a nanometer, thus the measurements of airspeed from pressure difference are inaccurate [5; 6; 12; 13]. Furthermore, air flows generated in the test section of classical wind tunnels at low fan speeds may be affected by pulsations; therefore, they are usually not steady and uniform enough to be used as measurement standards below approximately 1 m/s [4]. Consequently, a wind tunnel or a nozzle is not appropriate enough to be used as a calibration facility in extremely low speed regime (i.e., velocity < 1.0 m/s). In such extremely low speed cases it may be tempting to extend calibration relationships obtained at higher velocity to lower velocities, but results have shown that such an approach may lead to unacceptable errors [14]. To avoid these shortcomings, several methods have been proposed to determine the reference velocity.