## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c} \hline
**Institution, Country** & **Facility Type** & **Velocity Range/m/s** & \begin{tabular}{c} **Velocity Control** \\ **Accuracy** \\ \end{tabular} & \
\begin{tabular}{c} **Velocity** \\ **Uncertainty** \\ \end{tabular} & **Calibration Error** \\ \hline NIST, USA & Wind tunnel (DISWT) & 0.15–75 & Overall error: \(\pm\)4\% & \\ \hline NIST, USA & Wind tunnel (LVF) & 0.05–11.2 & & Total uncertainty of \\  & & & mean speed: 1\% & \\ \hline CETIAT, France & Wind tunnel & 0.05–2 & & Global uncertainty: \\  & & & 0.002–0.014 m/s & \\ \hline UCL, Belgium & Wind tunnel & 0.3–60 &
\begin{tabular}{c} Velocity profile accuracy: \\ 1\% \\ \end{tabular} & \\ \hline Nottingham Trent University, UK & Thermal jet wind tunnel & 0.7–9 & 0.5\% & \\ \hline Universidad Nacional de La Plata, Argentina & Wind tunnel & 0.2–1.25 & & Expanded uncertainty: \\  & & & 0.06 m/s (\(<\)10\%) \\ \hline University of Gavle, Sweden & Thermal wind tunnel & 0.3–3 & \(\pm\)0.02 m/s & \\ \hline Institute of Atmospheric Physics, Chinese Academy of Sciences. & Meteorological wind tunnel & 0.15–22 & Standard error 2.6–7.4\% & \\  & China & & & \\ \hline Institute of Metrology, CAMS, China & Wind tunnel & 0.1–2. & & \\ \hline National Institute of Metrology, China & Wind tunnel & 0.1–1 & Uncertainty: 0.018 m/s. \\ \hline National Institute of Metrology, China & Jet wind tunnel & 0.13–1.43 & & Extended uncertainty: \\  & & & 0.0114 m/s-0.03377 m/s & (k = 2) \\ \hline IROST, Iran * & Thermal jet wind tunnel & 5–60 & \\ \hline University of Gaziantep, Turkey & Laminar flow pipe & 0.029–1.79 & Fitting curve error: \(\pm\)4\% \\ \hline University of Gaziantep, Turkey & Rotating disc & 0.05–1.05 & Fitting curve error: \(\pm\)5\% \\ \hline \end{tabular}
\end{table} 
Table 1: Performances of world-wide calibration facilities in extremely low speed regime. The probe-moving facilities for extremely low speed calibration can be found in the Italian National Institute for Metrological Research (INRIM) located at the Polytechnic University of Turin in Italy (moving carriage) [4], the Nanyang Technological University (NTU) in Singapore (moving carriage) [5], the Brighton Polytechnic in the UK (moving carriage) [26], the National Research Laboratory of Metrology (NRLM) (now National Metrology Institute of Japan, NMIJ) in Japan (moving carriage) [27], the National Institute of Metrology, China(NIMC) (moving carriage) [28], the University of Ottawa in Canada (pendulum arm) [13], the King Fahd University of Petroleum and Minerals in Saudi Arabia (horizontal swinging arm) [29], the INRIM (horizontal rotating arm) [30], the University of Bradford in United Kingdom (UK) (swinging arm) [14], the University of Gaziantep in Turkey (rotating disc) [6], and the Universitat Erlangen-Nurnberg in Germany (a climate chamber with air temperature and humidity controllable) [18]. The air velocity in these facilities varies from 0.02 m/s to 36.6 m/s with a velocity control accuracy of 0.001 m/s to 0.03 m/s (0.82-7%), a velocity uncertainty of 0.012 m/s (0.82-8%), and a calibration fitting error of 0.014 m/s (2.42-15.04%) (see Table 1). In addition, among the wind-tunnel-like calibration rigs, only the wind tunnel in CETIAT can control both air temperature and humidity simultaneously [2, 21], while the jet wind tunnel of Nottingham Trent University [16], the small wind tunnel at University of Gavle in Sweden [17], and the jet wind tunnel of the IROST [10] can only control air temperature, but not humidity (IROST's velocity range of 5 m/s to 60 m/s is beyond the extremely low speed purview of less than 1.0 m/s). Among the probe-moving calibration facilities within the extremely low velocity range of less than 1.0 m/s, no existing facility can control air temperature or air humidity.