A Self-Calibrated Dual Probe Heat Pulse Sensor for In Situ Calibrating the Probe Spacing

Dual probe heat pulse method (DPHP) is a popular method for measuring soil thermal properties. However when this method is applied in the field, the distance between the temperature probe and the heating probe (probe spacing) may change due to probe deflection, and thus cause error in measured heat capacity (c) of soil. We developed a self-calibrated DPHP for in situ calibration of probe spacing, using a temperature probe with two imbedded thermistors (10 mm apart in the longitudinal direction). Experimental results for sand and fine sand samples indicate that this method can significantly reduce error caused by the probe deflection. If we define theta as the inclination angle of probe deviation from the vertical direction, for inward deflection with 0.8 degrees < theta < 0.9 degrees, the error in c is between -8.1 and -14.3%; after in situ calibration, it is between -0.1 and -4.3%. For outward deflection with 3.2 degrees < theta < 3.5 degrees, the error in c is in the range from 22.9 to 38.8%; after in situ calibration, the error reduces to from -3.8 to -7.8%. This new DPHP design may be applied in the field for reducing error caused by probe deflection.