Isotech Ultra Pure-Metal Freezing Point Cells are designed specifically to realize the liquid-solid equilibrium temperatures of certain high-purity metal elements, for calibration of thermometers at the ITS-90 Fixed Points. When you purchase an Isotech Sealed Freeze Cell you are not just purchasing a kilo of metal inside a graphite crucible sealed within a quartz shell, you are getting the fruits of more than 35 years of experience and learning of not only how to make such an artefact without introducing contamination but an Internationally accepted embodiment of an ITS-90 fixed point. The Isotech cells have been further developed and refined from cells designed and manufactured by Henry Sostmann, with the first international inter comparison results being published in 1972. In 2007 we combined 17 years experience of producing the best Metal Clad Slim Cells, with our experience of producing the most accurate Fixed Points sealed in quartz glass; to introduce Metal Clad Optimal Cells for the Primary Laboratory. These cells can be readily shipped between labs for intercomparisons, overcoming the difficulty of transporting Quartz Cells due to the increased airport security restrictions. Isotech’s accredited laboratory has the smallest uncertainties and can issue UKAS certificates with uncertainties as low as ±0.07mK at 0.01°C to ±2mK at 961.78°C, k=2.