## 2 Materials and Methods
### Measurement of the Carbon Oxide Concentration  
As shown in Figure 1, we measured CO and CO\({}_{2}\) concentrations in the off-gases using two types of gas analyzers: Non-Dispersive Infra-Red (NDIR) and TDLAS. Both are IAS (Infrared Absorption Spectrograph), meaning they use the property that different gas species absorb different wavelengths of light. The concentration of a specific gas is measured by detecting the intensity of the light emitted by the analyzer being absorbed or transmitted by the gas component. Analyzers using IAS are classified according to the type of light source and signal processing method.  
The NDIR spectrograph [37, 38, 39] is the most commonly used analyzer for measuring the concentration of gas components in the RH process. It consists of an infrared source, a sample cell containing the gas of interest, a reference cell containing an inert gas and  
Figure 1: Schematic diagram of the RH vacuum degasser.It shows the installation locations of the TDLAS and NDIR analyzers (filled circles).  
a detector. The light emitted from the broadband light source is split into two branches and passes through the sample cell and the reference cell, respectively. We can calculate the concentration of the gas of interest using the difference in intensity between the two beams passing through the two cells. Even though this method is an easy and inexpensive method widely used for off-gas measurement in steelmaking processes, it is sensitive to disturbances such as dust, humidity, temperature changes and pressure changes and has limitations in harsh environments and in situ measurement. Therefore, the NDIR analyzers are typically installed at the end of the exhaust line of the RH process to minimize disturbances, which causes a time delay in detecting changes in the off-gas composition occurring in the vacuum chamber.  
The TDLAS analyzer has been expanding its applications in various industries such as petrochemicals, power plants and steelmaking processes due to its excellent wavelength selectivity and strong resistance to disturbances [40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52]. It emits a monochromatic laser beam of a specific wavelength absorbed by the sample gas from the diode laser (transmitter) and measures the intensity of light passing through the sample using a detector (receiver). By measuring the absorption spectrum while varying the wavelength of the laser, it is possible to reduce the influence of background noise and disturbance and improve measurement accuracy. In addition, the TDLAS analyzer measures the gas concentration within a volume determined by the laser beam width and Optical Path Length (OPL), which reduces the measurement error due to the uneven gas mixtures, resulting in more reliable gas composition. These features allow direct measurement of the off-gas concentrations inside the exhaust line in situ, even in harsh environments.  
In this work, we used two TDLAS analyzers to measure both CO and CO\({}_{2}\) concentrations and installed them in a cross duct configuration to measure the gas concentration in the center of the exhaust duct. We installed the TDLAS analyzers on the rear of the gas cooler close to the vacuum chamber; there it was less affected by dust and also used the existing NDIR analyzers installed near the stack at the end of the exhaust line.