The freezing (or melting) temperatures of pure substances are fixed and absolute and have been given legal status in the form of the International Temperature Scale of 1990.

When calibrating sensors by directly immersing them in various Points on The Temperature Scale, there is no need to compare the sensor’s calibration to another sensor. Thus the calibration is closer to the ITS-90 Scale and has therefore more authority and smaller uncertainties.
Against this approach has been the cost of the fixed point cells, as they are called, and their apparatus combined with the complexity of use.
For this reason only a few laboratories have chosen to calibrate directly to the ITS-90 Scale using fixed point cells and apparatus.

The enhanced controller will allow for operation at 30ºC for the Gallium in ambients up to 23ºC. The Medusa 510 is a new apparatus designed to create and maintain Fixed Points simply, automatically and economically. Supplied with the very latest technology digital indicator, timer and controller.