Assembly and Evaluation of a Multi-Functional Heat Pulse Probe for Measurement of Soil Properties

A device capable of estimating soil properties quickly and accurately is of great worth to individuals in many disciplines. The multi-functional heat pulse probe (MFHPP) is an instrument which allows for simultaneous in situ measurements of soil heat capacity, thermal conductivity, thermal diffusivity, water content, water flux density, and electrical conductivity. Previous studies showed this device exhibits instrumentation and model limitations which reduce its measurement accuracy. It is important for the future use of the MFHPP to fully investigate sources of error, increase the range of testing and develop improvements to alleviate these issues. The main objectives of this study were to: (1) construct a functioning MFHPP, (2) investigate sources of error which impede measurement accuracy in various soils, and (3) estimate thermal and hydraulic properties of different soils. A MFHPP was reconstructed with modifications which helped reduce potential sources of error. The modified MFHPP was implemented to estimate the properties of sand and five soils using a traditional calibration. While the sand estimates showed accuracy similar to previous studies, there was a poor fit between estimated and measured values for the soils. Results of a sensitivity analysis indicated deviations in sensor spacing (r) and reductions in heater output (q’) (up to 80%) can cause measurement errors. Due to the difficulty of completely alleviating sensor spacing errors, methods were developed to reduce inaccuracies by accounting for heat loss. Three new calibration techniques are presented in addition to the conventional agar, full heat approach. These are the agar-reduced heat, physical and quasi-empirical calibration techniques. In addition to accounting for heat loss, these calibrations: (1) improve estimates of soil properties by 50% on average, (2) repetition is not necessary prior to each use, and (3) they account for error causing artifacts of probe construction. Additional findings indicate the MFHPP estimates are susceptible to error from the length of heating, duration between cycles, and soil texture, which suggests an inability of the model to explain the complex process of heat transfer in soils. The results of this study provide an in depth analysis of this technique from which further improvements can be made