On the construction and calibration of dual-probe heat capacity sensors

Dual-probe heat-capacity (DPHC) sensors can be used to measure soil heat capacity (C), water content, and temperature. Research was conducted to test design factors that affect sensor calibration, including: (i) calibration media, (ii) diameter and length of the needle probes, (iii) sensor body material, and (vi) duration and total power of the applied heat pulse. All sensors were calibrated in media with known C, including: agar (water), water-saturated glass beads, and dry glass beads. Calibration consisted of collecting heat pulse data in a given media and then calculating the apparent probe spacing (r(app), distance between heater and detector needles) that yielded correct value of C. An ideal sensor would have the same r(app) regardless of media type. The r(app) for all sensor designs increased as C decreased, on average changing by 6.7% between agar and dry beads. This undesirable result was consistent with previous studies that showed DPHC sensors calibrated in agar overestimated C in drier soils. Needle diameter (1.27 vs. 1.65 mm), sensor body material (urethane vs. high-conductivity epoxy), and shortening of the detector probe had a small effect on r(app). Sensors made with urethane bodies, 1.27-mm diam. needle probes, and shortened temperature probes showed less sensitivity to calibration media and are therefore recommended. The r(app) for this design only increased by 2.6% between dry and water-saturated wet beads. Apparent probe spacing was not affected by changes in total applied power (400-1600 J m(-1)) or heat pulse duration (2-16 s) when the correct analytical model was used to compute C.