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.
Gallium Melting Point
Thermodynamic Reference
Fixed points of the International Temperature Scale of 1990 provide highly reproducible thermal states for the calibration of standard platinum resistance thermometers. Among these, the gallium melting point is defined as exactly 29.7646 degrees Celsius. This temperature represents a highly stable reference used by national metrology institutes.
Phase Transition
Solidification and subsequent melting of high-purity gallium occurs within a specialized sealed cell to maintain the purity of the sample. To exploit the gallium melting point, the metal must be frozen around a central re-entrant well before being slowly melted from both the inside and the outside. This dual-melt method ensures a flat temperature plateau during the phase change.
Calibrating Instrument
Thermometer verification is performed by inserting the probe into the central well of the cell during the plateau. The gallium melting point provides a reference with an uncertainty of less than one tenth of a millikelvin when measured using a high-precision resistance bridge. Laboratories use this specific point to interpolate the temperature scale between the triple point of water and the melting point of indium, ensuring that the sensor responds linearly across the entire range.
This interpolation reduces the propagation of systematic errors during standard calibrations.
Interference Factor
Hydrostatic head effects and chemical impurities are the primary sources of measurement deviation in the cell. If the gallium sample contains metallic impurities at the parts per billion level, the transition temperature decreases. An increase in pressure from the column of liquid metal also introduces a slight shift that must be corrected during the calibration process.