Heat pulse moisture sensors have been used widely to determine moisture levels in substrates. Existing theoretical models of the sensors neglect temperature distribution inside of the probe. This leads to inaccurate solution for small time approximation.
An improved model of a heat pulse moisture sensor, which takes into account thermal conductivity of the probe material, is discussed in this paper. Solution for the system of two differential equations, with corresponding initial and boundary conditions, are found. The solutions depend on three dimensionless parameters, two time scales calculated for the probe and the substrate correspondingly, and criteria Bio. Analytical expressions for small time approximations were obtained.
The solution includes what is known as “contact resistance” between the probe and substrate. Contact resistance is responsible for data scattering during calibration of heat pulse moisture sensors. How to use the collected experimental data from small and large time scales to determine contact resistance is shown. The theoretical results are in good agreement with experimental data.