Standardizing Heat Pulse Probe measurements for thermal property determination using ice and water

Abstract The growing demand for Heat Pulse Probes (HPPs) to estimate thermal properties and surrogate soil processes comes with a need for improved standards for calibration and validation in reference materials. This study proposed air-free ice as a calibration material to determine the apparent rod spacing (rc) for HPPs. The advantage of using air-free ice as a calibration standard over agar-stabilized water stems from the fact that at 0 °C ice provides a reference volumetric heat capacity (Cv = 1.93 MJ m−3 °C−1), less than half that of water (Cv = 4.22 MJ m−3 °C−1), while ice provides a four times larger reference thermal conductivity value (λ = 2.16 W m−1 oC−1) relative to that of water (λ = 0.56 W m−1 °C−1), which leads to a sharper peak temperature rise curve that may improve thermal property estimates in ice compared to the elongated peak obtained in water. We performed rc calibrations with air-free ice in a freezer at −21 °C and agar-stabilized water at 21 °C using both Infinite-Line Source (ILS) and Identical-Cylinders Perfect-Conductors (ICPC) models to fit λ and Cv to temperature rise data and compute thermal diffusivity (κ) from the fitted λ and Cv (i.e., κ = λ/Cv). Results showed that the ICPC model yielded similar rc values in both air-free ice and agar-stabilized water with reduced Total Error (TE) in estimating λ, Cv, and κ compared to results with the conventional ILS model. We suggest air-free ice as another standard for Heat Pulse Probe (HPP) rod spacing calibration and sensor-based thermal property validation.