Design and numerical analysis of a button heat pulse probe for soil water content measurement

Evaluation of a newly designed button heat pulse probe (BHPP) demonstrated the potential for improved soil water content measurements. Compared with the conventional heat pulse probe (HPP), the BHPP does not use needles, but instead consists of a ring-shaped 6-mm-radius heating element and a central thermistor, both embedded in the face of a plastic disk (6 mm thick and 16 mm in diameter). After calibration of the effective heater-thermistor spacing from temperature response measurements in agar, a sensitivity analysis was conducted to demonstrate the potential advantages of the BHPP. Measurement accuracy and robustness of the BHPP were significantly improved compared with the conventional HPP. Errors associated with needle deflection were eliminated by the design, whereas the water content measurement sensitivity was increased by the central placement of the thermistor. We used inverse modeling with a finite-element heat transport model to illustrate the increased sensitivity of the BHPP to soil water content measurements.