This website uses cookies so that we can provide you with the best user experience possible. Cookie information is stored in your browser and performs functions such as recognising you when you return to our website and helping our team to understand which sections of the website you find most interesting and useful.
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.