What is a Fixed-Point Calibration?

Rather than to compare a thermometer under test to another calibrated thermometer (comparison calibration) with fixed point calibration the test thermometer is calibrated directly to an intrinsic standard or fixed-point cell.

Fixed Point Cells are used to calibrate thermometers to the highest accuracy, cells rely on the phase change of a pure substance. At higher temperatures pure metal ingots are encased in an assembly into which a thermometer can be placed. As the metal ingot melts or freezes the temperature remain constant. These fixed points are specified in the ITS-90.

Fixed point cells are not just for the Primary Laboratory, ITS-90 fixed points bring real benefit to all temperature laboratories.