The most accurately defined temperatures are those defining points (fixed points) of ITS-90. The leading Primary Laboratories use large fixed point cells in deep calibration furnaces that utilise heat pipes to eliminate temperature gradients. This combination of cell and furnace gives the smallest of uncertainties. In the patented Isothermal Towers the fixed point cell and heat pipe (or heat siphon) have been combined to produce the ideal realisations for calibrating standard thermometers. Thermometers can only be calibrated accurately if they are immersed sufficiently. In Isothermal Towers a heated block (Immersion Compensator, patent applied for) sits on top the heat siphon/cell to fully compensate for the immersion characteristics of the unit under test. The Isothermal Towers performance has been fully evaluated against the most detailed and demanding requirements ever written: CCT/2000-13. All Isothermal Towers; Indium, Tin, Zinc and Aluminium meet all the requirements of CCT/2000-13 allowing laboratories to realise the smallest uncertainties, at a fraction of the cost of conventional Metrology Furnaces with Primary Standard Cells. Introducing the ISOTOwers You can purchase three Isothermal Towers; Tin, Zinc and Aluminium for a similar price as one conventional cell and heat pipe apparatus Isothermal Towers are simple to use, and very robust. Operation is risk free, as a combined apparatus there is no need to handle a fragile cell. No need for specialist training courses. Isothermal Towers remove the mystery from fixed point calibration. Easily set to provide a melt or freeze of 24 hours or more, lending themselves for automatic calibration and providing your lab with an all day long plateau.