# Development of an Online Detection Setup for Dissolved Gas in Transformer Insulating Oil
## 4 Experiment and Discussion  
### Gas Detection Experiment  
#### 4.1.1 Calibration Experiment  
The amplitude of the second harmonic reflects only the relative change in gas concentration, not the exact value. Therefore, it is necessary to establish a relationship between a certain concentration and the corresponding amplitude of the second harmonic. A gas dividing instrument was used to mix different concentrations of gases in the gas cell. The second harmonic signals of various gas concentrations are shown in Figure 11. Figure 12 shows the linear relationship between the amplitudes of the second harmonics and the corresponding concentrations.  
The amplitudes of the second harmonic of the gases at different concentrations are different in Figure 11. Therefore, the gas concentration can be described by the magnitude of the second harmonic. The second harmonic signals are used for linear fitting. Figure 12 shows the linear relationship between the amplitudes of the second harmonics and the corresponding concentrations.  
The second harmonic has a good linear relation with the concentration in Figure 12. Therefore, detection sensitivity can be expressed by the slope of the fitting curve. This is consistent with the situation in the literature research [26; 27; 29]. The detection sensitivities of methane, ethylene, ethane, carbon monoxide, carbon dioxide, and acetylene are 5.1, 6.4, 4.6, 3.4, 1.6, and 12.8 mV/\(\mu\)L-L\({}^{-1}\), respectively.  
\begin{table}
\begin{tabular}{c c c c} \hline
**DFB LASER** & **Operating** & **Output Center** & **Type of Detected Gas** \\
**Devices** & **Temperature (°C)** & **Wavelength (nm)** & **Type of Detected Gas** \\ \hline
1579 nm laser & 29.6 & 1579.74 & CO \\
27.9 & 1579.57 & CO\({}_{2}\) \\ \hline
1684 nm laser & 17 & 1684.04 & CH\({}_{4}\) \\
1684 nm laser & 9.4 & 1683.1 & C\({}_{2}\)H\({}_{6}\) \\
12.9 & 1683.35 & C\({}_{2}\)H\({}_{4}\) \\ \hline
1532 nm laser & 21.3 & 1532.83 & C\({}_{2}\)H\({}_{2}\) \\ \hline \end{tabular}
\end{table}
Table 2: The center wavelength of laser devices output.  
Figure 10: Structure diagram of the gas detection system.  
random errors, the average value of 30 measurements was used as the evaluation standard. The measurement results of the TDLAS system are shown in Figure 13.  
According to Figure 13, the standard deviations of methane, ethylene, and ethane are 2.0, 2.5, and 2.3 \(\upmu\)L/L, respectively. The standard deviations of carbon monoxide and carbon dioxide are 2.4 and 2.8 \(\upmu\)L/L, respectively. The standard deviation of acetylene is 0.5 \(\upmu\)L/L. The average detection values of the TDLAS system and the data from the reference device are shown in Table 3.  
Table 3 shows the relative errors of the reference device, which are 1.67 \(\upmu\)L/L for CO, 13.47 \(\upmu\)L/L for CO\({}_{2}\), 0.05 \(\upmu\)L/L for CH\({}_{4}\), 1.96 \(\upmu\)L/L for C\({}_{2}\)H\({}_{6}\), 3.16 \(\upmu\)L/L for C\({}_{2}\)H\({}_{4}\), and 0.09 \(\upmu\)L/L for C\({}_{2}\)H\({}_{2}\). Therefore, the detection error of the TDLAS system meets the requirements of the power industry and it can be applied to the detection of dissolved gases in transformer insulating oil.