Investigation of Calibration Equations for Ntc Thermistors Utilized in the Deep-Ocean Temperature Range

Abstract

The deep-ocean temperature measurement demands sub-millikelvin precision of approximately 1–3 °C. Performance of negative temperature coefficient (NTC) thermistors is significantly dependent on calibration methods and equations. In this study, six pre-aged NTC thermistors were calibrated in a water bath using a comparison method. The primary standard triple point of water validated thermistor stability. Performances of six calibration equations using 11 combinations of calibration points were evaluated. Fitting lines and interpolation residuals of the calibration equations were compared. The fitting line deviation was within 0.23 mK, excluding the Basic equation that reached 0.65 mK. The average interpolation residuals for Hoge-1 and Basic equations are 0.15 mK and 0.35 mK, respectively. It was demonstrated that Hoge-1 and Basic equations are sufficient and that the third (fourth) order equations are unnecessary in deep-ocean temperature measurements. Both Hoge-1 and Basic equations demonstrated capacity to extrapolate 0.5 K within 1 mK error.

Keywords: deep-ocean temperature measurement, negative temperature coefficient (NTC) thermistor, calibration equation, uncertainty