Thermocouples An Example of a Calibration

Consider temperature as an example. Temperature measurement is needed in a variety of laboratory applications and, in the modern laboratory, is done with a temperature sensor, such as a thermocouple. A thermocouple is a junction of two metals that produces a voltage proportional to temperature that can be measured via electrical connections to the two metals. The voltage difference between the two connections can be amplified by the difference amplifier discussed in Section 6.3.3. 

The most common reference points for temperature sensor calibration are the freezing and boiling points of water. The freezing point is a function of the purity of the water-ice system. When both pure ice and pure liquid water are present, the ice is melting and the temperature of a well-stirred mixture is, by definition, 0°C. It is the most accurate calibration point for that reason. If it is possible to use such a quantity in a calibration, then the calibration is true and without question. 

The boiling point of water, however, depends on both the purity of the water and the prevailing atmospheric pressure. Thus a boiling water bath is not as accurate a reference point as the water-ice 

This latter point begs the question Who can verify the accuracy of a reference point if its value may vary In other words, who is the ultimate source of calibration materials, such as a thermometer In the U.S., it is the National Institute of Standards and Technology (NIST). This is the same organization that we cited as the source of accurate standardization materials in Chapters 3 and 4. Experiments 16 and 17 in this chapter are exercises in the calibration of a temperature sensor and how such a calibrated sensor can be used. 

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