A Dual-Probe Heat Pulse Approach using Heated Fiber-Optic Temperature Sensing

Abstract

The distributed actively heated fiber-optic method is widely used to measure soil thermal conductivity and water content. However, the significant limitations of this technique are that prior soil specific calibration is required, and the measurement of soil volumetric heat capacity, a critical soil thermal parameter, is restricted. This study investigates a novel fiber-optic dual-probe heat pulse method, which utilizes fiber-optics to monitor the temperature response at a certain distance from the heat source to obtain the thermal conductivity and the volumetric heat capacity. Subsequently, the water content can be estimated from its linear relationship with soil volumetric heat capacity. The fiber-optic dual-probe heat pulse method is aimed to overcome the spatial-scale measurement limitations considering the distributed sensing capability of fiber-optic. This requires the sensor to be more robust than the traditional dual-probe method, hence a completely different ratio of the probe diameter to the distance between adjacent cables. Therefore, the applicability of the existing theory needs further verification, and the heating strategies need careful consideration. This study uses a 6 mm diameter fiber Bragg grating (FBG) sensor encapsulated by the alundum tube to conduct laboratory feasibility tests. Two adjacent sensors are fixed in the soil at a distance of 14.60 mm. Different heating powers (30 W/m, 40 W/m, and 50 W/m) and heating durations (10 s, 30 s, 50 s, and 120 s) are implemented. The results showed that the novel approach allows accurate soil volumetric heat capacity and water content estimation with appropriate heating strategies and data interpretation methods. The heating duration below 30 s is a poor fit because of the low-temperature rise, requiring a longer heating duration. For the 120 s heating duration, the measurement error in soil volumetric capacity is less than 2%. On the other hand, the instantaneous line heat source theory overestimates the soil volumetric heat capacity by 5%, while the short-duration line heat source theory performs well in the method.