What is the Fixed Point Method?
The Fixed Point Method of thermometer calibration assures the quality and accuracy of a thermometer’s measurements. Typically, only national metrology laboratories use it. Thermometer calibration by the fixed point method focuses on instruments that must measure accurately within ±.001℃.
This method uses the ITS-90 international temperature scale developed in 1990. Based on the thermodynamic or absolute temperature scale, ITS-90 is not truly a scale. It is a set of fixed points that defines an international equipment calibration standard. It helps scientists know that a temperature measured in Asia will be the same as a temperature measured in Europe. Scientists can repeat results regardless of location with properly calibrated equipment.
In creating the ITS-90 standard, a body of scientists selected seventeen fixed points. These points are based on temperatures where various elements or compounds achieve equilibrium. These points include :
Although Freezing Point and Melting Point are common terms, Triple Point and Vapor Pressure Point might not be. The Triple Point is the temperature of a substance where all the phases of matter – solid, liquid, and gas – exist in equilibrium. The Vapor Pressure Point is the temperature of a substance where the gas and liquid phases are in equilibrium.
How is the Fixed Point Method used in thermometer calibration?
To achieve fixed point calibration, the thermometer to be calibrated is placed in a specially designed flask. This flask maintains a constant temperature through both heating and cooling. The technician selects a limited number of fixed points from the ITS-90 with the goal of using as few as possible. The actual procedure varies depending on the fixed points selected. 
The points of the ITS-90 temperature scale have been identified, documented, and globally accepted. As a result, technicians with proper training can reproduce standard calibration conditions.
 B.W. Magnum. “International Temperature Scale of 1990 (ITS-90).” John R. Rumble, Editor-in-Chief. CRC Handbook of Chemistry and Physics, 98th Edition, 2017-2018. CRC Press, Boca Raton, FL (USA): 2017. 1-17.
 http://www.evitherm.org/files/795/ThermometerCalibrationMethods.pdf, last visited 01/18/2018