Resistance Thermometers and Thermocouples

Metals, as well as semiconductors have been used as resistance thermometers. As given in Fig. 4.5, the conductivity depends upon the number of charge carriers n, their charge e, and their mobility p. The mobility of the electrons is impeded by the 

The principle of resistance measurement involves a dc Wheatstone bridge, as in the sketch of Fig. 4.6. The calculations show how the lead resistances, Rq and R, can be eliminated in precision thermometry by performing two measurements (a and b) with reversed leads connected to the bridge circuit. The measured resistances are represented by Rq, the unknown resistance by Rx- 

The table in Fig. 4.7 lists the change in the emf per kelvin of temperamre difference for a number of well-known thermocouples. Copper-constantan thermocouples, which have been given the letter T by the Instrument Society of 

Based on the Seebeck effect discovered in 1821, caused by a difference in the work functions of metals. 

Electromotive Force of Thermocouples at different temperatures in microvolt per kelvin (or degree Celsius)  

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In this range (100 to 550°) the domains of pyrometry and thermometry overlap somewhat, for high-temperature resistance thermometers and thermocouples generally classed as pyrometers are often used below 550°C. These instruments will not be... 

The explanation of this phenomenon, as with many other apparatus-based developments, lies in the refinement in measurement techniques made possible by modem electronics. As a rule, heat cannot be measured directly, but must be determined instead on the basis of a temperature change in the system under investigation. Very accurate classical thermometers tend to be very slow measuring devices. Resistance thermometers and thermocouples respond much more rapidly, but they require electronic amplification, and amplifiers with the required precision have become available only in recent decades. 

The national laboratories of several countries, including the National Institute of Standards and Technology in the United States, maintain stable secondary thermometers (e.g., platinum resistance thermometers and thermocouples) that have been calibrated according to the ITS-90. These secondary thermometers are used as working standards to calibrate other laboratory and commercial temperature-measuring devices. 

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