What is a fixed point?
Fixed points rely on the constant temperature when a pure substance changes phase. For example, the freezing point of Aluminium is defined as 660.323 °C. The user takes advantage of the fact that as the pure metal passes through a phase transition (solid to liquid or liquid to solid) the temperature within the crucible remains constant due to latent heat effects. During this period (the plateau) test thermometers can be introduced into the fixed point cell.
Why use a fixed point?
Fixed point calibration has the highest accuracy / lowest uncertainty and thermometers can be calibrated to less than 0.001 °C.
Who uses fixed point cells?
National Metrology Institutes and Primary laboratories use fixed points to calibrate SPRTs and disseminate the ITS-90, outside of primary laboratories many smaller labs use smaller more affordable fixed points to both check standards and calibrate thermometers
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