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 utilize Heat Pipes to eliminate temperature gradients. This combination of cell and furnace gives the smallest of uncertainties. In Isothermal Towers the fixed point cell and heat pipe (or heat siphon) have been combined to produce the ideal realizations for calibrating standard thermometers. A heated block (Immersion Compensator), sits on top the heat siphon/cell to fully compensate for the immersion characteristics of the unit under test.