Agricultural and Forest Meteorology 200 (2015) 203–208 Contents lists available at ScienceDirect Agricultural and Forest Meteorology j our na l ho me page: www.elsevier.com/locate/agrformet Short communication Evaluation of a self-correcting dual probe heat pulse sensor Minmin Wen a , Gang Liu a,∗ , Baoguo Li a , Bing C. Si b , Robert Horton c a Department of Soil and Water, College of Resources and Environment, China Agricultural University, Beijing 100193, PR China b Department of Soil Science, University of Saskatchewan, Saskatoon, SK, S7N5A8 Canada c Department of Agronomy, Iowa State University Ames, IA 50011, USA a r t i c l e i n f o Article history: Received 25 December 2013 Received in revised form 11 September 2014 Accepted 28 September 2014 Keywords: DPHP Coplanar deflection Non-coplanar deflection Probe spacing Heat capacity a b s t r a c t 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Campbell et al. (1991) introduced a dual probe heat pulse (DPHP) method for measuring soil thermal properties (Bristow et al., 1994; Ren et al., 2003), and many others have used heat pulse probes for laboratory and field measurements (Tarara and Ham, 1997; Bremer and Ham, 1999; Ochsner et al., 2003). The DPHP method uses two probes, one as a heater, and the other one as a temper- ature probe. The probe spacing (≈6 mm) is the distance between the heating and the temperature probe. In most cases the probe spacing of DPHP sensors is calibrated (Mori et al., 2003) before a sensor is used in situ. Usually, calibrations are carried out by using agar stabilized water. However, there is no guarantee that the probe spacing will remain constant in situ. The probes are made of stain- less steel tubing with typical diameter of 1 mm and length between 28 mm and 40 mm (Campbell et al., 1991; Mori et al., 2003; Liu et al., 2013). Probe deflection can occur during probe insertion into soil, and natural and human-induced soil shifting can also occur ∗ Corresponding author. Tel.: +86 10 62732217; fax: +86 10 62733596. . E-mail address: liug@cau.edu.cn (G. Liu). (Kluitenberg et al., 2010; Liu et al., 2013). Probe spacing of sen- sors change due to probe deflection. If the effective probe spacing is not accurately calibrated, there can be large errors in the mea- sured heat capacity c (Campbell et al., 1991; Kluitenberg et al., 2010). Liu et al. (2008) showed that if the new probe spacing of the deflected probes is known, we could estimate c accurately. Liu et al. (2013) proposed a self-calibrated DPHP sensor for in situ mea- surement of c. However, the method of Liu et al (2013) received only limited testing for the condition of coplanar, in-line deflected but not bent heating probe or the temperature probe. Additional testing is needed for bent and non-coplanar deflections (deflec- tions that cause probes to be located in different planes). In this study, experiments for various probe deflections are performed to evaluate the ability of the Liu et al. (2013) method to provide accurate estimates of in situ probe spacing and soil thermal prop- erties. 1.1. Theory The prevalent model for the DPHP method assumes that the heating probe can be approximated by an infinite line source (ILS) embedded in an infinite soil mass (Campbell et al., 1991). When http://dx.doi.org/10.1016/j.agrformet.2014.09.022 0168-1923/© 2014 Elsevier B.V. All rights reserved.