Three prototype Stevens HydraProbes were modified to accurately measure soil temperatures from -30°C to +40°C. The three prototype sensors were placed in an environmental chamber capable of maintaining constant temperatures in the range of interest. Temperature measurements were obtain by the three Hydra Probes at various temperatures within the range and compared to that of a standard thermistor. The Pearson correlation coefficient (R2) was used to quantify the performance of the HydraProbe measurements in the selected range. The coefficient of variation (CV) and a 99% confidence interval were the statistical tools used to quantify the precision of the measurements. The R2 values were found to be 0.99943 and the precision under a normal 99% confidence interval (1<CV ) was +/- 0.26°C.

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