A general in situ probe spacing correction method for dual probe heat pulse sensor

Abstract

The dual probe heat pulse (DPHP) method is gaining popularity for measuring soil thermal properties. However, because of the fact that the DPHP measured heat capacity (c) of soil is hyper-sensitive to probe spacing variation, probe deflection causes large error in the measured c. To deal with probe deflection, recently, Liu et al. (2013) has proposed an in situ probe spacing correction method, but, their method was based on the inline deflection assumption. Wen et al. (2015) found that for non-inline deflected DPHP sensors, the method of Liu et al. (2013) provided poor reduction of the error in c. To cope with the non-inline deflection, in this study, we introduce a new DPHP sensor design by using three thermistors within the same temperature probe. A related probe spacing correction method for non-inline deflected probes was also presented. Both numerical simulation and experiment were conducted to test the new model. We define θ as the inclination angle of the probe deviation from the vertical direction. For experiment of the outward deflection (5.8°<θ<6.8°), the error in c is in the range from −23% to −76%; after inline in situ correction, the error decreases to the range of 12% to 44%; after using our non-inline in situ correction, the error is between 1% and 8%. Our three-dimensional finite element numerical simulation also demonstrates that, compared with the method of Liu et al. (2013), the new sensor and correcting method can significantly eliminate errors in c caused by probe deflections. The new DPHP sensor design and the non-inline deflection model have the potential to replace the method of Liu et al. (2013), and become the standard method for correcting probe spacing in situ.