Actively heated optical fiber method (AHFO) based on fiber Bragg grating (FBG) temperature measurement technology has been continuously applied in soil moisture monitoring due to its advantages such as good durability, quasi-distribution, and small size. The FBG alumina ceramic tube sensor with symmetrical internal heating is used to design a laboratory model. By changing the inner capillary tube, a continuous function was used to fit the calibration relationship between the temperature rise and the volumetric water content, and the influence of different heating times on the calibration relationship was analyzed. Laboratory test studies have shown that the capillary tube with a low thermal conductivity material inside can help improve the sensor's ability to resolve volumetric water content. The calibration relationship under short-time heating also has a high correlation, and it can also reduce the impact on the soil environment. The research results are of great significance for improving the measurement accuracy of the AHFO method and the improvement of the test method.
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