Deep Ocean Temperature Measurement with an Uncertainty of 0.7 mK

The uncertainty of deep ocean temperature (~1°C) measurement was evaluated. The time drifts of six deep ocean standards thermometers were examined based on laboratory calibrations as performed by the manufacturer in triple point of water (TPW) cells and gallium-melting-point (GaMP) cells. The time drifts ranged from −0.11 to 0.14 mK yr−1. Three of the six thermometers were evaluated at the National Metrology Institute of Japan in five TPW cells and a GaMP cell, and the temperature readings agreed with the realized temperature of the national standard cells of Japan within ±0.14 and ±0.41 mK for TPW and GaMP, respectively. The pressure sensitivities of the deep ocean standards thermometers were estimated by comparison with conductivity–temperature–depth (CTD) thermometers in the deep ocean, and no notable difference was detected. Pressure sensitivities of the two CTD thermometers were examined by laboratory tests, and the results suggest that the deep ocean standards thermometers have no pressure s...