## 3 Calibration Equipment and Tests
### Calibration Test  
The calibration test aimed to check and verify the measurement performance of the sensor. According to the applicable fields of the FBG sensor, the national standards of China (JJ6860-2015), and the common calibration methods for fiber optic sensors described in the literature, the calibration test program for the developed FBG sensor was formulated.  
(1) Sensor preloading: Temperature will have an influence on the sensor. Based on previous experience in field stress testing, we decided to set 20 \({}^{\circ}\)C as the calibration temperature. Firstly, put the sensor in the laboratory for more than 2 hours at room  
Figure 12: Diagram of the FBG sensor: (**a**), assembled sensing unit; (**b**), assembled hemispherical shell; (**c**), assembled connecting rod; (**d**), fiber splicing; (**e**), optical cable outlet.  
Figure 13: Schematic diagram of calibration device: 1, adjustable base; 2, bearing plate; 3, guide bar; 4, loading plat; 5, sensor base; 6, sensor; 7, loading bar; 8, weights.  
temperature, 20 \({}^{\circ}\)C, to eliminate the effect of temperature change. Then, place the sensor in the calibration device for preloading. Each sensing unit preload is divided into three levels: 80, 160, and 240 kg, respectively, which correspond to the loading pressures 10, 20, and 30 MPa, respectively. For each preloading level, the loading stability time is set as 5 min. After loading, unload the sensing unit to a zero pressure state, and adjust the sensor position for the next preloading of the sensing unit. The purpose of preloading is to make close contact between the sensor parts to ensure that the sensing units will be of strong working condition and that the wavelength output will be more stable. At the same time, check whether the calibration device and related test instruments are working properly, and whether the test data output are accurate.  
(2) Sensor cyclic loading: After preloading, the cyclic loading tests are carried out for each sensing unit. The specific steps are as follows:  
1 Place the sensor into the sensor base of the calibration device, adjust the senor unit and dowel bar to ensure they are coaxial, and record the initial output wavelength of the sensing unit.  
2 Put weights onto the loading plate of the calibration device with successive loading steps of 20 kg. After stabilizing for 2 mins, record the wavelength of the FBG demodulator under the corresponding load. Unload the weights when loading reaches 240 kg and the unloading path is consistent with the loading process.  
3 Repeat step 2 three times so that each sensing unit has three loading and unloading cycles.  
4 Repeat steps 1-3 until all sensing units are calibrated.  
5 Remove the sensor from the calibration device and end the calibration test.  
The process of the calibration test is shown in Figure 14. As FBG is a temperature-sensitive material, temperature changes will affect the accuracy of calibration data. Therefore, during the test, the temperature change should be kept within \(\pm\)5 \({}^{\circ}\)C, and the current temperature should be recorded every 30 mins. If the temperature change exceeds \(\pm\)5 \({}^{\circ}\)C, the test data will be invalid and will need to be re-calibrated.  
In order to check the test performance of the sensor under long-term working conditions, the stability test was carried out. The stability test can obtain the long-term stability of the sensor by applying long-term stability load to the sensing units and observing the change in the output wavelength of the sensing units with time. The test load was set to the full range, i.e., 30 MPa. The loading time was 24 days and the data recording time interval was 24 h.