## 3 Results and Discussion  
### Comparison of Off-Gas Measurements Lsing TDLAS and NDIR Analyzers  
Figure 3 shows the concentration of carbon oxides in the off-gas measured in real-time during the RH operations using TDLAS and NDIR analyzers during two representative RH operations. Here, the concentration of carbon oxides means the sum of CO and CO\({}_{2}\) concentrations. The change in the concentration of carbon oxides during the operations showed similar values and trends in both analyzers. However, the TDLAS analyzer was installed in situ closer to the vacuum chamber and due to its fast response characteristics, it was able to detect the changes in carbon oxide concentration about one minute faster than the NDIR analyzer. This shows that the TDLAS analyzer can measure the changes in the off-gas composition quickly and accurately in the RH process with high temperature and severe pressure changes.  
Although the results of the measurements with the two types of analyzers showed similar trends overall, the graph of the measured values of the NDIR analyzer was wide and tended to decrease slowly toward the latter part of the operation. In general, the degassing reaction occurs intensively in the early stages of the operation and the reaction is significantly reduced after reaching a high vacuum [14,17,20]. Therefore, there should be a point at which the carbon oxide concentration in the off-gas converges rapidly to zero as in the graph measured by the TDLAS analyzer.  
We assumed that one of the reasons why the measured value of the NDIR analyzer decreased very slowly in the latter of the operations was the reduced flow rate of the off-gas. Once the pressure is sufficiently low, the pressure in the vacuum chamber and the exhaust line is similar, reducing the overall off-gas flow rate and speed. Accordingly, at the end of the exhaust line, it takes a long time to detect the change in the off-gas  
Figure 3: Changes in concentration of carbon oxides in the off-gas during two representative RH operations measured by TDLAS (solid lines) and NDIR (dotted lines) analyzers. Each graph represents the sum of the CO and CO\({}_{2}\) concentrations measured at each time point. (**a**) Secondary decarburization is clearly shown. (**b**) Secondary decarburization did not occur.  
composition occurring inside the vacuum chamber. Leakage in some of the long exhaust lines of the RH degassing unit due to equipment aging may also be another reason for the very slow decline of carbon oxide concentration measured by the NDIR analyzer late in the operations. On the other hand, by installing the TDLAS analyzers relatively close to the vacuum chamber, it was possible to reliably analyze the off-gas components regardless of the aforementioned reasons.  
The carbon oxides gradually decrease after a sudden occurrence at the beginning of the RH operation, but in some operations, they tend to increase briefly in the middle of operation (about 5 min after the start of the operation in Figure 3a). This is called secondary decarburization. Although the exact cause is not known, we estimated that the gas components trapped in the molten steel were rapidly discharged as the circulating flow reached the maximum value. During this period, the flow rate of the off-gas increases and the pressure in the vacuum chamber is kept flat or decreases slowly.