## 6 Application in Hot Wire Calibration  
### HotWire Calibration  
The Dantec Streamline constant-temperature hot-wire anemometer (Figure 17) owned by the NF-6 Pressurized Continuous Transonic Wind Tunnel Laboratory at the Northwestern Poly technical University was selected as the anemometer to be calibrated in the extremely low speed calibration facility presented in this paper to appraise the calibration ability of the facility. The hot-wire anemometer is a multi-channel anemometer equipped with 55P11 probe. The straight probe with only one single wire with one channel was used in the calibration and the wire has a length of 1.25 mm and a diameter of 5 \(\upmu\)m. The calibration was conducted at each of the following 10 velocities: 0.1 m/s, 0.2 m/s, 0.3 m/s, 0.4 m/s, 0.5 m/s, 0.6 m/s, 0.7 m/s, 0.8 m/s, 0.9 m/s, and 1.0 m/s consecutively at a set humidity value under one of the 3 specifiedtemperatures: 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 60 \({}^{\circ}\)C.Undereach of 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, the humidity was set as 20%, 60%, and 80%, respectively, while under 60 \({}^{\circ}\)C, the humidity was specified as the ambient atmospheric humidity at the test site. A calibration is normally performed at a specified velocity under a specified temperature and a specified humidity. The general procedure of calibration was described in Section 3.2. Normally, the temperature control system is first started, and then after the temperature in the enclosed chamber reaches the set temperature, the humidity control system is started. After the temperature and humidity are controlled to set values, the humidity control system and the stirring fan are turned off but with the temperature control system still running to keep the enclosed chamber temperature constant at the set value, while the chamber humidity is supposed constant if the temperature remains constant and there is no air leak. The temperature and humidity are kept constant, and the operators wait for 12 min for the disturbed air to return to stagnation. In fact, a 12-min time interval between any 2 consecutive calibration runs is necessary to allow the disturbed air to calm down. When the air becomes quiescent under constant temperature and humidity, the motion velocity control is started. When the set velocity is reached, the condition for the calibration of hot-wire anemometers is ready. In the present calibration, the velocity data were acquired continuously for 6 s within the constant-speed stage at each set velocity (under a specified temperature and a specified humidity) with an acquisition frequency of 1000 Hz for a \begin{table}
\begin{tabular}{c c c c c c c c} \hline \(T_{\text{set}}\) (\({}^{\circ}\)C) & \(H_{\text{set}}\) & \(H_{\text{ini}}\) & \(H_{\text{avg}}\) & \(\sigma_{H}\) & \(\mid\!H_{i}-H_{\text{avg}}\!\mid\!_{\text{max}}\) & \(\mid\!H_{i}-H_{\text{set}}\!\mid\!_{\text{max}}\) & \(\mid\!H_{\text{avg}}-H_{\text{set}}\!\mid\) \\ \hline
21.5 & 20 & 42.7 & 20.9 & 0.8474 & 0.8 & 1.7 & 0.9 \\
21.5 & 60 & 24 & 60.1 & 0.0935 & 0.6 & 0.7 & 0.1 \\
21.5 & 80 & 59.6 & 79.8 & 0.1871 & 0.8 & 1.0 & 0.2 \\
30 & 20 & 58.5 & 20.6 & 0.5613 & 2.3 & 2.9 & 0.6 \\
30 & 60 & 19.2 & 60.3 & 0.2806 & 0.5 & 0.8 & 0.3 \\
30 & 80 & 60 & 80.2 & 0.1871 & 0.3 & 0.5 & 0.2 \\
40 & 20 & 77.9 & 19.9 & 0.0935 & 0.3 & 0.4 & 0.1 \\
40 & 60 & 35.5 & 59.7 & 0.2806 & 0.3 & 0.6 & 0.3 \\
40 & 80 & 57.1 & 79.8 & 0.1871 & 1.3 & 1.4 & 0.1 \\ \hline \end{tabular}
\end{table} 
Table 9: Spatial humidity distribution characteristics when the chamber humidity reaches the set value (humidity: %RH). total of 6000 times, and the velocity values were averaged every 60 ms to obtain 100 mean velocity data. The 100 velocity values \(V_{i}\) (\(i=1\) to 100) were averaged to obtain the average velocity \(V_{\text{avg}}\), and the average velocity value was used as the calibrated velocity value under each set velocity.