Simple Thermocouple Circuit

Figure 4. Simple thermocouple circuit for temperature measurement (Miiza et al., 2010).

Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction the characteristics of the two metals used and the characteristics of the attached circuit. Figure 6 illustrates a simple thermocouple... 

A simple and convenient means of determining the EMF generated in complex thermocouple circuit can be derived from the relations of irreversible thermodynamics. The result of this analysis [32] is that the zero-current EMF for a single homogeneous wire of length dx is... 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

Thermoelectric potentials are quite small and should be determined at zero current so that a high precision potentiometer is required. Considerable care must be exercised to avoid parasitic e.m.f s which may be present at terminals and other connections. The e.m.f. produced per unit temperature may be increased by using multijunctions but the curve of current (on which sensitivity depends) versus number of Junctions eventually flattens to the point that further increase in thermal bulk is no longer worthwhile. Scatchard et alP have used a 128 junction thermocouple to measure depressions of the triple point of water with a reported precision of 10" K. Brown and Prue obtained a precision of about 10" K with a 24 junction couple. They used the simple circuit shown in Fig. 2.8.4 to amplify the e.m.f. by a factor of 10. 

The thermal evaluation of sohd-state devices and integrated circuits (ICs), and VLSI-based packaging takes two forms theoretical analysis and experimental charaaerization. Theoretical analysis utilizes various approaches from simple to complex closed-form analytical solutions and numerical analysis techniques, or a combination of both. Experimental characterization of the device/chip junction/surface temperature (s) of packaged/unpackaged structures takes both direct, infrared microradiometry, or Kquid crystals and thermographic phosphorous, or, to a lesser extent, thermocouples, and indirect (parametric) electrical measurements. 

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