# Spectroscopic Approach for the On-Line Monitoring of Welding of Tanker Trucks
## 3 Experimental Results and Discussion
### Field Trials
Ratio of Na I line (589 nm) to Al I line (396 nm) (thick blue line) and 670 nm (thin green line) associated with (**e**) Trial #1 and (**f**) Trial #2.  
level to ensure the protection of the pool, and the H I participation depicted in Figure 4c is significantly lower than the one discussed in Figure 3d. However, the H I profile does not indicate the occurrence of the defect provoked by the use of the dry compressed air at the end of Seam #7. On the contrary, the plasma RMS and color temperature (Figure 4a) and the ratio of the Ar I line (738 nm) to the Ar I line (763 nm) profiles give rise to the identification of a clear event at x \(\approx\) 8400 mm. (It is worth mentioning that the x-axis has been sequentially generated for all the weld trials, i.e., Trial #1 goes from 0 to 1000 mm, #2 from 1000 to 2400 mm, and so on.) Figure 4: Output monitoring profiles for Weld Trials #7 and #9 (final part) using different spectroscopic parameters. Plasma RMS signal (thick blue line) and color temperature (thin green line) associated with (**a**) Trial #7 and (**b**) Trial #9. Ratio of H I line (656 nm) to Al I lines at 396 nm (thick blue line) and 670 nm (thin green line) associated with (**c**) Trial #7 and (**d**) Trial #9. Ratio of Ar I line (738 nm) to Ar I (763 nm) lines associated with (**e**) Trial #7 and (**f**) Trial #9. This result is backed by the X-ray inspection (Figure 4, top-right), where the undercut close to the end of the seam is associated with the use of compressed air. Note that defects indicated by the radiographic analysis and their locations have been indicated over the seam under analysis. There are also two isolated porosities around x \(\approx\) 8200 mm whose identification is not so clear (although there is a peak in Figure 4e), but in this case the H I profile does not indicate contamination of the process. Figure 4d, however, shows high levels of H participation in the process, thus supporting the X-ray detection of several defects in the associated seam, leading to the conclusion that the employment of oil and water in the process backing gave rise to clear contamination and, consequently, a defective seam. In this case, the Ar profile (Figure 4f) also exhibits some peaks that can be correlated with the quality-related events presented in Figure 4d. Please note that the images of the seams at the top of Figure 4 (and also Figure 6) are presented at a scale that does not correspond to the associated graphs. These images, with the indications of the welding quality-related events, are intended to provide a better understanding of the trials and the associated results. Spectra captured during Weld Trial #9 are shown in Figure 5. In particular, a comparison is shown between a spectrum associated with a section of correct welding and a spectrum from a section where contamination occurred. It can be observed that the contamination gave rise to an increase in the H I line at 656.2 nm (see detail in the inlay), while clear decreases are shown in the intensities of the Ar I lines located between 695 and 811 nm. This behavior agrees with the results provided in Figure 4. It can be observed how some of the emission lines overlapped due to the limited resolution of the low-cost spectrometer chosen. However, this resolution does not compromise a suitable spectroscopic analysis for the required monitoring purposes. It is also interesting to analyse the performance of the electric sensors involved, i.e., those that acquire welding voltage and current information. In general terms, it might be concluded that, at least with respect to the studies performed in this work, these sensors exhibit a poorer performance than the optical plasma spectroscopy approach. Figure 6 shows the results of two weld trials in which the electrical signals showed some correlation  
Figure 5: Welding spectra captured during Weld Trial #9: spectra associated with correct welding (blue line) and process contamination (orange line). Inlay: detail of the variation in the intensity of the H I line at 656.2 nm. with quality-related events.