Investigation of calibration equations for NTC thermistors utilized in the deep-ocean temperature range

Deep-ocean temperature measurement demands sub-millikelvin precision over the range of 1–3 °C. Performance of a thermometry system using negative temperature coefficient thermistors is highly dependent on calibration methods and equations. Six pre-aged thermistors were calibrated in a water bath utilizing heat pipe technology. The primary standard triple point of water validated the thermistor stability. The performance of five calibration equations was evaluated by comparing fitting curves and interpolation residuals. The fitting curve deviation of all the equations was within 0.23 mK, excluding the Basic equation that reached 0.65 mK. The average interpolation residuals for the Hoge-1 and Basic equations were 0.15 mK and 0.35 mK, respectively. The Hoge-1 and Basic equations were shown to be sufficient and the third (or fourth) order equations were unnecessary for deep-ocean temperature measurements. Both the Hoge-1 and Basic equations demonstrated capacity to extrapolate by 0.5 K within 1 mK error.