## 1 Introduction
A small chamber (if it can be classified into the probe-moving facility) at Universitat Erlangen-Nurnberg in Germany can control air temperature and air humidity [18], but its velocity range of 12.75 m/s to 36.6 m/s is far away from the range 0.1 m/s to 1.0 m/s, which falls within the bounds of extremely low speed (see Table 2). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Institution, Country** & **Facility Type** & **Velocity Range/m/s** & **Velocity Control** & **Velocity** & **Calibration Error** \\  & & & **Accuracy** & **Uncertainty** & \\ \hline INRIM, Italy & Moving carriage & 0.1–1.0 & & & \\  & & & & (8.0–1.1\%) & \\ \hline Nanyang Technology & Moving carriage & 0–0.35 & \(\pm\)0.001 m/s & Fitting curve error: \(-\)1.98\% \\  & & & (\(-\)3.0\% to 7.0\%) & to 15.04\% \\ \hline Brighton Polytechnic, & & & Velocity repeatability: & \\  & UK & Moving carriage & 0.02–1 & \(\pm\)0.0005 m/s to & \\  & & & \(\pm\)0.0082 m/s (\(\pm\)0.31\% to & \\  & & & \(\pm\)0.82\%) & \\ \hline NRLM (NMIJ), Japan & Moving carriage & 0.05–1 & & Uncertainty \\  & & & & 0.00735–0.014 m/s \\ \hline National Institute of & & & Expanded uncertainty & Expanded uncertainty 2.42\% \\  & Metrology, China & Moving carriage & 0.1–1.05 & 0.82\% (k = 2) & (k = 2) \\ \hline University of Ottawa, & & & & \\ Canada & & & & \\ \hline King Fahd University & & & & Maximum uncertainty: \\ Petroleum and Minerals, & Horizontal swing arm & 0–0.15 & 4.1\% \\  & Saudi Arabia & & & Expanded uncertainty: \\ \hline INRIM, Italy & Horizontal rotating arm & 0.2–5 & 4.0–0.84\% \\ \hline University of Bradford, & & & \\ UK & Swinging arm & 1–6.5 & 0.03 m/s & \\ \hline Universitat & & & & \\ Erlangen-Nürmberg, & & & \\ Germany * & & & \\ \hline \hline \end{tabular} * Velocities exceed 1.0 m/s.  
\end{table} 
Table 1: Cont. So far, several problems such as the low velocity control accuracy, the unchangeableness and uncontrollability in air temperature and humidity leading to contaminations, and the secondary air motion (adverse disturbing flow, especially for the probe-moving facilities) in the design of calibration facilities for hot-wire anemometers in extremely low speed remain unsolved. More work is needed to verify the possibility in improving velocity control accuracy, and reducing temperature and humidity contamination. To promote advances in these aspects, this paper presents a probe-moving facility with a moving carriage and an enclosed chamber for the calibration of hot-wire anemometers in extremely low speed range of 0.10 m/s to 1.0 m/s with temperature and humidity being variable and controllable. It is believed that this facility is likely to enhance the velocity control accuracy since it adopts a mechanical system to produce a relative motion of the hot wire to air, leading to a more exact velocity control than the wind-tunnel-like rigs. Besides, in this facility the calibrations under any arbitrary user-specified temperature and humidity within their ranges can be performed, thus the contamination caused by temperature and humidity discrepancies would be thoroughly removed. The design requirements and targets of the calibration rig are: (1) wind velocity range: from 0.10 m/s to 1.0 m/s, velocity control accuracy: \(\pm\)0.003 m/s (or \(\pm\)0.4%), velocity calibration accuracy: \(\pm\)0.03 m/s; (2) air temperature range: ambient atmospheric temperature to 60 \({}^{\circ}\)C, temperature control accuracy: \(\pm\)1 \({}^{\circ}\)C, temperature measurement accuracy: \(\pm\)0.5 \({}^{\circ}\)C; (3) air humidity range: from 20%RH to 80%RH (between ambient atmospheric temperature to 40 \({}^{\circ}\)C), humidity control accuracy: \(\pm\)4.0%RH, humidity measurement accuracy: \(\pm\)2.0%RH. The extremely low speed calibration facility in this paper can be divided into several subsystems according to their functions: (1) mechanical structure system, (2) master control and motion control system; and (3) temperature and humidity control system.