Hydrostatic Pressure Correction Coefficient of the Triple Point Cell of Water

The hydrostatic pressure correction coefficient of the traditional triple point cell of water is determined from the measurements of temperature distribution along the thermometer well. The hydrostatic pressure coefficient obtained from these measurements is −0.855(30) mK/m, which is slightly different from the recommended value, −0.73 mK/m, of the International Temperature Scale of 1990. The latter is the value calculated from the liquidus curve of ice. This discrepancy is estimated to be caused by the difference between free-floating ice and the ice mantle in the cell which is compulsorily immersed around the thermometer well. The temperatures near the bottom of the thermometer well are also confirmed to be cooler than the other part of the thermometer well. It is recommended that these new uncertainty sources must be included in calibrating thermometers at the triple point of water.