On the Construction and Calibration of Dual-Probe Heat Capacity Sensors

parallel hypodermic-tubing probes extending from a small sensor body (Fig. 1). Hereafter, probe will be used Dual-probe heat-capacity (DPHC) sensors can be used to measure to describe the hypodermic tubing components while soil heat capacity (C ), water content, and temperature. Research sensor will refer to the entire assembly. One probe conwas conducted to test design factors that affect sensor calibration, including: (i) calibration media, (ii) diameter and length of the needle tains a heater made of resistance wire and the other probes, (iii) sensor body material, and (vi) duration and total power probe houses a thermistor. Once installed, the heat caof the applied heat pulse. All sensors were calibrated in media with pacity of the soil surrounding the heater is determined known C, including: agar (water), water-saturated glass beads, and by applying a short heat pulse (e.g., 8 s) and then meadry glass beads. Calibration consisted of collecting heat pulse data in suring the subsequent temperature increase at the thera given media and then calculating the apparent probe spacing (rapp, mometer probe for about 60 s. As shown by Campbell distance between heater and detector needles) that yielded correct et al. (1991), the heat capacity of the soil can be estivalue of C. An ideal sensor would have the same rapp regardless of mated as media type. The rapp for all sensor designs increased as C decreased, on average changing by 6.7% between agar and dry beads. This undeC q/( erTm) [1] sirable result was consistent with previous studies that showed DPHC sensors calibrated in agar overestimated C in drier soils. Needle diamewhere C is heat capacity (J m 3 K 1), q is applied power ter (1.27 vs. 1.65 mm), sensor body material (urethane vs. high-conducper length of probe (J m 1), r is the distance between tivity epoxy), and shortening of the detector probe had a small effect the hypodermic probes (m), and Tm is the maximum on rapp. Sensors made with urethane bodies, 1.27-mm diam. needle increase in temperature at the detector probe following probes, and shortened temperature probes showed less sensitivity to the application of the heat pulse ( C). Equation [1] is calibration media and are therefore recommended. The rapp for this a solution to the heat equation assuming an infinite linedesign only increased by 2.6% between dry and water-saturated wet source heater and an instantaneous heat pulse. Knight beads. Apparent probe spacing was not affected by changes in total and Kluitenberg (2004) presented an improved heat applied power (400–1600 J m 1) or heat pulse duration (2–16 s) when capacity equation that considers the time interval of the correct analytical model was used to compute C. heating