## 3 Results and Discussion
### Relationship between the Position of the Fine Inspection Port and Detection Characteristics
flow rate \(Q_{2}\) and inlet volume \(Q_{1}\) for air and helium at \(p_{1}\) = 10 Pa. The relationship between \(Q_{2}\)/\(Q_{1}\) and the position of the fine inspection port is shown in Figure 8.  
As shown in Figure 8, the percentage of return flow of helium was higher than the percentage of return flow of air at the same fine inspection port position. The helium mass spectrometer leak detector had a high detection sensitivity when the fine inspection port was between the 6 th- and 8 th-stage blade rows, with a large difference in the return ratio of air and helium. 2.2 Relationship between the Position of the Fine Inspection Port and the Working Pressure of the Inspected Part  
In the prevacuum stage and at different fine inspection port positions, the pressure \(p_{2}\) was calculated when the leakage rate to air \(Q_{1}\) was 0.1 Pa\(\cdot\)m\({}^{3}\)/s, 0.01 Pa\(\cdot\)m\({}^{3}\)/s, 0.001 Pa\(\cdot\)m\({}^{3}\)/s, 0.0001 Pa\(\cdot\)m\({}^{3}\)/s. The calculation results are shown in Figure 9.  
According to Figure 9, when \(Q_{1}\) = 0.1 Pa\(\cdot\)m\({}^{3}\)/s, the opening position must be after the 8 th-stage blade row to meet the working pressure conditions of the mass spectrometer chamber. When \(Q_{1}\) = 0.01 Pa\(\cdot\)m\({}^{3}\)/s, the opening position can meet the conditions after the 5 th-stage blade row. When \(Q_{1}\) is 0.001 Pa\(\cdot\)m\({}^{3}\)/s and 0.0001 Pa\(\cdot\)m\({}^{3}\)/s, the opening position can meet the allowable pressure of the mass spectrometer chamber no matter which stage blade row is set. The model for the calculation of inspection conditions was applied to calculate the return flow rate of air \(Q_{2}\) and helium \(Q_{2}\)\({}^{\prime}\) when \(Q_{1}\) was \(10^{-6}\) Pa\(\cdot\)m\({}^{3}\)/s and \(10^{-9}\) Pa\(\cdot\)m\({}^{3}\)/s, respectively. The relationship between \(Q_{2}\)\({}^{\prime}\)/\(Q_{2}\) and the position of the fine inspection port is shown in Figure 10.  
Figure 8: Relationship between \(Q_{2}\)/\(Q_{1}\) and the fine inspection port. Figure 9: Relationship between \(Q_{2}\)/\(Q_{1}\) and the fine inspection port. From the results in Figure 10, when the fine inspection port was closer to the low vacuum side, greater advantages of the countercurrent inspection method could be exploited. After the position of the fine inspection port was below the 5 th-stage blade row, the helium backflow began to increase significantly. As the position of the fine inspection port was moved down, the helium return flow rate increased. 2.3 Relationship between the Position of the Fine Inspection Port and the Minimum Detectable Leakage Rate  
An important indicator of a helium leak detector is the minimum detectable leak rate, which refers to the smallest change that can be detected by a helium leak detector when pure helium is used as the leak detection medium at standard atmospheric pressure and the operating conditions of the leak detector are optimal. The minimum detectable rate represents the detection accuracy of the leak detector. When the helium flow back to the mass spectrometer chamber is equal to the minimum detectable leakage rate of the leak detector, changing the position of the fine inspection port results in different leakage rates; therefore, the minimum detectable leakage rate affects the position of the fine inspection port. Given the minimum detectable leakage rate \(Q_{2}{{}^{\prime}}=5\times 10^{-12}\) Pa-m\({}^{3}\)/s, the air leakage rate \(Q_{1}\) and air return flow \(Q_{2}\) were calculated by using the model of the fine inspection working condition. The relationship between \(Q_{1}\) and \(Q_{2}\) and the position of the fine inspection port is shown in Figure 11.  
According to Figure 11, as the opening position moved from the 1 st-stage blade row to the 12 th-stage blade row, the detectable leakage hole gradually increased. The closer the opening position was to the high vacuum side, the smaller the det  
Figure 11: The relationship between \(Q_{1}\) and \(Q_{2}\) and the position of the fine inspection port.