COMPARATIVE ANALYSIS OF DIFFERENT MEASURING METHODS OF LOESS MOISTURE CONTENT BASED ON AHFO-FBG METHOD

The Actively Heated Fiber Optic method is based on Fiber Bragg grating method(AHFO-FBG)technology and has become the latest research hotspots due to its advantages of small size, high temperature measurement accuracy, quasi-distributed measurement, anti-electromagnetic interference, and corrosion resistance. Research on the calibration method of this technology is very critical to improve the accuracy and applicability of moisture content measurement, but this content is not involved in the existing research. In this paper, the self-developed AHFO-FBG sensor is used to carry out a series of indoor calibration tests to determine the temperature change process of loess under various moisture contents. A comparative analysis of the moisture content measurement results of the ΔT_max method and the ΔT_cum method is carried out in this paper. The influence of different heating time and different heating power on the calibration results of the ΔT_max method and the ΔT_cum method is further explored. The results show that both ΔT_max method and the ΔT_cum method can get good moisture content calibration results. The RMSE of the ΔT_max method is 0.001 m³·m^-3 higher than that of the ΔT_cum method, so the advantage of the ΔT_cum method is not very significant. The ΔT_max and the ΔT_cum method show that the longer the heating time, the error of the two methods to measure the moisture content can gradually increase. Under the same heating time, the ΔT_cum method calculates the moisture content better than the ΔT_max method, and the shorter the heating time, the more obvious the phenomenon. Under the same heating power, the RMSE of ΔT_max method is small in the low power(5~10 W·m^-1), and the RMSE of ΔT_cum method is small in the high power(15~35 W·m^-1). An appropriate increase in the heating power can help reduce the moisture content calibration error. The research results provide a basis for the AHFO-FBG technology to achieve accurate measurement and further application of soil moisture content.