Comparative calibration vs. fixed point calibration

In temperature calibration technology, you can find two ways of calibration ? the comparative calibration and the fixed point calibration. The method of calibration to be utilized will be defined through the demands on the accuracy and the type of test instrument. The comparative method represents probably the most widely used method of calibration.
So how do the comparative calibration and fixed point calibration methods differ? Utilizing the exemplory case of calibrating a resistance thermometer, this blog article explains both methods and therefore tries to convey to the reader a better understanding.
The comparative calibration
As the name already implies, this kind of calibration is completed by comparing the resistance thermometer being tested with a higher-quality reference thermometer. The higher-quality reference should thus exhibit an accuracy of three or four times greater than the thermometer being tested. Both thermometers are put into a single homogenous temperature source. When placing the thermometers, care should be taken to ensure a little distance between them and that the sensitive points (meaning the measuring points) are in exactly the same height. As a temperature source, liquid baths or dry-well calibrators are generally used.
The fixed point calibration
The fixed point calibration, in contrast to the comparative calibration, is carried out against the defined triple, freezing and melting points from the International Temperature Scale of 1990 (ITS-90). The fixed points define temperatures at which the physical state of substances alter. The most crucial fixed point of the ITS-90 is the triple point of water. At a temperature of 0.010 �C the physical state of high-purity water is continually changing between steam, ice and liquid water. The quantities of the physical states always stay in proportion. Generally, only national laboratories or laboratories qualifying for really small measurement uncertainties use these fixed-point cells, as they are costly and specialised limited to this application. Highly stable PRTs (platinum resistance thermometers), which will be used as reference thermometers, are calibrated utilizing the fixed point method. The reference sensor is inserted into a closed, or also open fixed-point cell, which holds the temperature constant over an extended period.
Example: fixed points from -189 to 660 �C relative to ITS-90:
Fixed point
Temperature
Triple point of argon
-189,3442 �C
Triple point of mercury
-38,8344 �C
Triple point of water
0,01 �C
Melting point of gallium
29,7646 �C
Freezing point of indium
156,5985 �C
Freezing point of tin
231,928 �C
Freezing point of zinc
419,527 �C
Freezing point of aluminium
660,323 �C
Note
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