Evaluating Temperature Coefficient Wander in Thin Film Platinum Resistance Elements
Evaluating temperature coefficient wander in thin film platinum RTDs requires tracking substrate strain, glass passivation diffusion, and multi-point calibration.
Triple Point of Water
Equilibrium Reference
Thermodynamic equilibrium between the three phases of pure water establishes the most fundamental defining fixed point of the International Temperature Scale of 1990. This unique state, known as the triple point of water, is defined as exactly 273.16 kelvin or 0.01 degrees Celsius. It serves as the primary reference for defining the kelvin.
Coexistence Phase
Maintenance of the three phases is achieved inside a sealed glass cell containing high-purity water of a specific isotopic composition. To establish the triple point of water, an ice mantle is formed around the central well of the cell by introducing a cooling rod. This mantle is then freed from the glass tube to create a thin layer of liquid water at the interface.
Metrological Standard
Resistance measurements of standard platinum resistance thermometers are referred to the value obtained at this fixed point to calculate resistance ratios. The triple point of water can be reproduced in a dedicated metrology laboratory with an uncertainty of less than one ten-thousandth of a degree Celsius. This high reproducibility makes it the baseline measurement for tracking the drift of standards over time, and any deviation in the thermometer resistance at this point indicates a change in the physical state of the platinum sensor.
Interference Source
Isotopic variations and hydrostatic pressure effects within the water column can introduce small temperature deviations. If the water contains elevated levels of heavy isotopes, the transition temperature shifts. Correcting for the depth of the thermometer coil below the liquid surface is necessary to achieve the highest level of accuracy during calibration.