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What is a Mercury Fixed Point?

Fixed points rely on the constant temperature when a pure substance changes phase. For example, the triple point of mercury is defined as -38.8433 °C. The triple point can be realised and maintained for long periods during which the test thermometers can be introduced into the fixed point cell.

What equipment is needed to operate a Mercury Fixed Point?

Mercury cells need to be cooled to around -42 °C and operated close to -38 °C, equipment types to do this includes dedicated apparatus, stirred liquid baths, and low-temperature dry blocks.

Is Mercury Not Banned?

There are a range of restrictions relating to the use and shipping of mercury and the use of mercury is being phased out, some legislation makes an exception for, “following mercury-added products: (a) products that are essential for civil protection and military uses; (b) products for research, for calibration of instrumentation, or for use as a reference standard.” As the ITS-90 specifies the use of mercury for the calibration of SPRTs many laboratories have no choice. Scientists are researching alternatives, including carbon dioxide (CO2) and sulfur hexafluoride (SF6).

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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