## 1 Introduction
An alternative method to calibrate anemometers at extremely low speeds is not to generate the air flow, but to mechanically translate the instrument (probe) with a velocity in nominally still air. In fact, this condition is the same as that of an anemometer immersed in a uniform flow at the same velocity, because of the reciprocity principle [4]. Thus, the calibration methods can be classified as static or dynamic, depending on whether or not the hot-wire is given a movement in addition to or instead of the fluid flow [5; 13]. When the hot-wire probe stands still in the air flow, the calibration method is called static, while if the hot-wire probe moves in stagnant air, the calibration mode is called dynamic. It seems that the calibration facilities can be classified into two types: the wind-tunnel-like rigs (including jet-type nozzles, laminar pipe flow rigs) and the probe-moving-type rigs. Dynamic methods include towing, rotating, oscillating and swinging the probe by appropriate devices in nominally stagnant air and require proper care to eliminate undesirable influences such as drafts and vibrations, while some of them are also based on certain assumptions and theoretical models [13]. These dynamic methods pay more attention to procedures for obtaining quiescent fluid and the development of control systems to ensure a constant velocity of the probe support during the calibration time period for each calibration point [14]. Besides, in dynamic methods the standard airspeed uncertainty is a function of the measurement conditions (average velocity and therefore output voltage) and the effects of secondary motions, which are the dominant factor. Indeed, even when the contributions from the other factors attain their maximum value, they are still two orders of magnitude lower than the contribution from secondary motions [4]. A major weakness of the hot-wire anemometer in application is that, even when the fluid velocity remains constant, the output of the hot-wire anemometer can vary with the fluid temperature [15]. The output signal of a CTA is related directly to the convective cooling effect of an air stream passing over the sensing element. In non-isothermal flows, the responses of a hotwire to changes in velocity and temperature are indistinguishable. As a result, temperature contamination of the sensor leads to large errors in the measured velocity [16]. Even in isothermal flow conditions in which a hot-wire anemometer is used to measure air velocity, the fluid temperature may still be different from the calibration temperature, this temperature discrepancy also brings errors in velocity measurement [10]. That is the restriction or disadvantage of a hot-wire anemometer that it is calibrated in a constant temperature flow and must be operated in a flow of identical temperature. If the temperature of the flow drifts or is intentionally changed, this is misinterpreted as a velocity change unless some allowance is made [8]. On the other hand, the heat transfer process itself is temperature dependent due to variations in the air properties with temperature--this also adds noise in the measured velocity data [17]. To overcome such a temperature dependency of the output of the hot-wire anemometers, various efforts have been made to compensate for the effect of temperature variation [15]. Therefore, whether the flow over the sensor is non-isothermal or the fluid temperature is different from the calibration temperature, to avoid large errors in the measured velocity and/or turbulence, the original calibration curve must be corrected to reflect the actual flow conditions, and compensate for the variations in the fluid temperature. For example, without temperature correction, error in the measured air velocity for each degree Celsius increase is about \(-1.9\%\)[10]; at 3 m/s the relative error introduced into the velocity measurement is approximately \(1.5\%\) per degree Celsius [16].