## 3 Experiment and Results
### Experiment Results  
Figure 11 shows the scan results of the MFLT module on the Tube-1. Magnetic field distribution around the damages could help to recognize the presence of the damages. All the taper-type wears (t11, t12, t13, and t14) could be detected, and the magnetic field intensity increases as the size of the wear increases. In addition, the slit-type damages on the membrane (m11, m12, and m13) could also be detected, but the damages on the welding line (w11, w12, and w13) were out of the sensing area. The smallest slit-type damage (m11) has a length of 20 mm, depth of 0.9 mm, and 7 mm width that could be detected. Similarly, the taper-type wears on Tube-4 (t41, t42, t43, and t44) could be detected, as shown in Figure 12. However, the smallest size of slit-type damage on Membrane-3 (m31) having a length of 60 mm, depth of 0.3 mm, and width of 7 mm could not be detected; this is because the damage has a smallest depth of 0.3 mm. The damages (m32 and m33) which have depths of 1.2 mm and 2.4 mm, could be detected. The slit-type damages on the Weld-4 (w41 and w42) were out of the sensing area, but a part of the w42 signal could be measured because the damage has the deepest depth of 1.8 mm. Figure 11: Distribution of magnetic field on the Tube-1, Membrane-1, and Weld-1 having artificial taper-type wear (single side) and slit-type damages. Figure 10: The prototype of the inspection system for water-cooled tubes. Figures 13 and 14 show the detection result of the slit-type damages. All defects (t21-t28, t31-t38) with a depth of 0.3 mm or more located in Tube-2 and Tube-3 could be detected. In addition, damages of 0.9 mm in depth and 30 mm in length (w11) or more were detected in Weld-1 could also be detected. However, damages (w21, w22, w31, w32, w33) located in Weld-2 and Weld-3 were difficult to detect. Nevertheless, damages with a depth of 0.9 mm or more in Weld-4 (w41, w42) and damages with a depth of 1.2 mm or more in Membrane-3 (m32, m33) could be detected. This is because that when the sensor for magnetic flux density measurement scans Tube-4, it is skewed toward Membrane-3, and the lift-offs of Membrane-2 and Membrane-3 are not the same. From the above results, the depths of damages detectable in the tube and membrane are 0.3 mm and 0.9 mm, respectively. In addition, some damages having a depth of 0.9 mm or more could be detected in the welded part due to the influence of the welding beads. Figure 12: Distribution of magnetic field on the Tube-4, Membrane-3, and Weld-4 having artificial taper-type wear (double side) and slit-type damages. Figure 13: Distribution of magnetic field on the Tube-2, Membrane-1 (Weld-1, 2) having slit-type damages. Figure 15 shows the graph showing the relationship measured data with the depth of the defect on Tube-3. The damages have the same width of 7 mm and length of 60 mm. The measured data is the minimum data points selected from the numbers of Hall sensor array that are on the damages during the scan. There are 30 sensors, and 15 sensors data plotted on Figure 15a,b. The data in Figure 15a has more noise than in Figure 15b because some sensors are located far from the damages. Then, data of 15 sensors is used for further evaluation of the damages' depth. The average data of the sensors are used to reduce noise that may occur rather than a single sensor. Also, the relationship between the measured data with the damages' depth is expressed in Equation (13). This form of the experimental equation is same as the theoretical analysis by the dipole model of the previous section (Equation (9)). The factors \(c_{1}\), \(c_{2}\), and \(a_{2}\) are 121.08, 6.65, and 2.52, respectively. \[d=\sqrt{121.08\ V_{H}+6.65}+2.52 \tag{13}\]  
Figure 14: Distribution of magnetic field on the Tube-3, Membrane-3, and Weld-4 having slit-type damages. Figure 15: Relationship between the depth of flaw and the measured data with magnetic flux density method: (**a**) data of 30 sensors and (**b**) data of 15 sensors.