# An Experimental Study of the Leakage Mechanism in Static Seals
## 3 Experimental Design for Sealing Surface Leak-Rate Measurement  
### Experimental Devices  
This experiment focused on the study and investigation of the leakage mechanism of sealing interfaces. During the design of the experimental device, the replacement of test pieces and the control and measurement of the operating parameters were both considered. As illustrated in Figure 2, the experimental device consisted of a sealing cell, a measurement cell, and a control cell. (1) Sealing Cell  
The sealing cell mainly comprised a test piece and silicon wafers. As shown in Figure 2, the test piece was a cylindrical boss, with the small-radius (50 mm) round surface as the sealing surface butted to the silicon wafers to form a sealing interface. The large-radius round surface was used for fixing and clamping (a three-jaw caliper), the position at the center of the circle was connected to the leakage measuring tube through a tapered threaded hole, and the fluid in the measuring tube infiltrated the sealing surface through the through-hole at the center of the test piece. The connection position between the test piece and the leakage measuring tube was sealed (by tape wrapping and dispensing) to ensure that the gas leak rate at this position was less than \(5\times 10^{-8}\) Pa\(\cdot\)m\({}^{3}\)/s, thereby controlling the experimental measurement error. The silicon wafers were approximated as a smooth plane, and the elastic modulus of the silicon wafers and the test piece was equivalent to \(E=[(1-v_{1}^{2})/E_{1}+(1-v_{2}^{2})/E_{2}]^{-1}\) in the leak rate prediction model. The surface size of the silicon wafers was considered equal to the apparent contact area of the sealing surface. (2) Measurement Cell  
The measurement cell mainly consisted of a measuring tube (plexiglass tube). The amount of fluid leakage was calculated by observing the change in the fluid level in the measuring tube over a certain period. In this experiment, the inner diameter of the measuring tube was 100 mm, the length was 250 mm, and the initial position of the fluid level was 200 mm. In order to reduce the effect of fluid pressure variation caused by a change in the fluid level on the experimental data, in the seepage experiment, the upper limit of the leakage was set to 0.75 mL (i.e., a height change of about 10 mm), which could be extended correspondingly in the leakage experiment driven by gas pressure. For this device, a leakage amount of greater than 0.15 mL (i.e., a height change of about 2 mm) was considered observable, so the measurement range was 0.15 mL-0.75 mL. In view of this, it was necessary to adjust the time of each experiment according to the experimental conditions and the leakage amount. For a 120-min measurement, the precision in measurement time could reach \(1\times 10^{-11}\) m\({}^{3}\)/s. The lower end  
Figure 2: Schematic of the measurement setup. of the measuring tube was connected to the test piece, while the upper end was connected to the gas pressure control cell and the fluid supplementing cell. (3) Control Cell  
The control cell was composed of a load control cell, a fluid supplementing cell, and an gas pressure control cell. The pressure control cell consisted of a three-jaw caliper, a polytetrafluoroethylene (PTFE) cushion block, and a membrane pressure sensor, providing a uniform, stable and measurable contact load to the seal surface. As shown in Figure 2, the test piece was fixed by the three-jaw caliper, and the contact load was controlled by a rotating screw. The tapered head of the screw was placed on the cylindrical PTFE cushion block on the lower side of the silicon wafer, and the contact load was distributed through contact points to the entire contact surface, thereby providing a normal pressure no larger than \(1\times 10^{4}\) N for the contact surface. In addition, a membrane pressure sensor was installed between the cushion block and the silicon wafer to monitor the contact load in real time. The fluid level control cell was composed of a syringe, a plunger pump, and a motor controller.