Matric Potential Sensor Employing the Principle of Dual Probe Heat Pulse.

Soil matric potential determines water availability for plants and water transfer in soil so that in-situ measurement of matric potential is important for field water management. A heat dissipation sensor (HDS), which indirectly measures in-situ matric potential, was developed in 1990s. The HDS is composed of a porous medium and a single stainless needle in which a heater wire and a temperature sensor are embedded. The stainless needle is enclosed in the porous medium. The HDS determines matric potential from temperature increase associated with a heat pulse application in the porous medium. This temperature increase is correlated to thermal conductivity that is a function of matric potential. The HDS measures the wide range of matric potential but not for > -100 cm H₂O. In addition, matric potential measured with the HDS is temperature-dependent because the thermal conductivity is affected by temperature change. The principle of dual probe heat pulse (DPHP) techniques is similar to the system employed in the HDS. The DPHP requires two stainless needles, one for heating and the other for temperature sensing. The DPHP measures volumetric heat capacity, thermal diffusivity, and thermal conductivity with the two needles. Since the DPHP measures three thermal properties simultaneously, the HDS is supposed to be improved by employing the DHPH principle. Therefore, we aimed to develop a matric potential sensor employing the principle of the DPHP in this study. A cylindrical porous medium (r = 14 mm, h = 40 mm) was developed by sintering a mixture of kaolinite and carbon powder. A pair of 40 mm long stainless needles (o.d. = 2.1 mm, i.d. = 1.5 mm) were inserted into the porous medium in parallel. A heater wire (1120 ohm m^-1) was embedded in one needle and a thermistor temperature sensor (NTC thermistor 10kΩ, Murata Manufacturing co. Ltd.) in the other. The needle spacing was 8 mm. The readings of the developed sensor were compared with the readings of tensiometer, and MPS-6 (Decagon Devices Inc.), a capacitive matric potential sensor. The relationship between matric potential and each thermal property of the porous medium was evaluated and the most suited way to determine matric potential from the sensor output is discussed.