Response Time Characterization of Heat Dissipation Sensors for Soil Matric Potential.

Heat dissipation sensors are used extensively to inexpensively and reliably measure soil matric potential in the range -2.5 MPa to -0.01 MPa.  Water flux between the adjacent soil and the sensor changes the thermal conductivity of the ceramic which results in a change in the temperature increase near the heating element.  Water content changes in the ceramic or soil affects hydraulic properties and thus the response time of the sensor to changing soil matric potential.  This work was conducted to characterize the hydraulic properties of the sensor-soil system and use the results to estimate the response time of the sensor for a range of initial water potentials and changes in soil water potential.  Coupled heat and mass transport in the sensor from large thermal gradients is also considered as it relates to temperature sensitivity and equilibration after the measurement.  The system was modeled using Hydrus and laboratory measurements collected to validate the results.  The results show the importance of the ceramic pore size distribution and identify conditions of high soil water flux, such as near surface, when the sensor has significant lag with respect to changing soil water potential.