Evaluation of a self-correcting dual probe heat pulse sensor

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The dual probe heat pulse method (DPHP) can be used to measure soil thermal properties. However, in situ probe spacing is prone to change resulting in large errors of measured soil heat capacity c. A newly designed DPHP sensor has two thermistors in the temperature probe which can be used to calculate in situ changes in probe spacing. This new sensor has been tested under the limited condition of linear coplanar probe deflection. i.e., probes were deflected in-line but were not bent in different directions. However, actual probe deflections may not always be coplanar linear. In this study the applicability of the new DPHP sensor is evaluated in fine sand and in silica sand for a variety of probe deflections. Probes are deflected at the bottom end (bottom) or top end (top) of probes. Results indicate that the new sensor can significantly eliminate errors in c caused by coplanar and non-coplanar probe deflections. Compared with bottom deflections, top deflections have larger errors in c. Thus, theory used to correct for deflections is more applicable to bottom deflections than to top deflections. The errors in c for saturated soil materials are larger than the errors in c for dry soil materials, because of the finite response times of the thermistors used in the probes. Clearly, the newly designed DPHP sensors have the potential for reducing errors caused by various kinds of probe deflections that may be encountered in the field.