Thermocouple circuit

Fig. 5. Basic thermocouple circuit. A and B are wires of different materials.

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

The temperature of a gas oil product flowing through a pipe is monitored using a chromel/alumel thermocouple. The measurement junction is inserted into the pipe and the reference junction is placed in the plant control room where the temperature is 20°C. The emf at the thermocouple junction is found to be 6.2 mV by means of a potentiometer connected into the thermocouple circuit adjacent to the reference junction. Find the measured temperature of the gas oil. 

Thermocouples consist of two dissimilar electrical conductors which are joined to form a measuring junction, with the free ends of the wires constituting the reference junction. When a temperature difference exists between the measuring and reference junctions, an emf is produced between the free ends of the device. This emf, which is a function of the temperature difference, can be used to determine the temperature at the measuring junction if the reference junction temperature is known. A schematic of a typical thermocouple circuit is shown in Fig. 9.12. 

Fig. 9.12. Typical thermocouple circuit. The ice water bath may be prepared in a Dewar for ex tended use of the circuit.

An elementary thermocouple circuit is shown in Fig. 16.16. The EMF generated in this circuit is a function of the materials used and the temperatures of the junctions. It is useful to describe briefly the basic thermoelectric phenomena or effects that are related to the Seebeck effect and are present in thermocouple measurements. They include two well-known irreversible phenomena—Joule heating and thermal conduction—and two reversible phenomena—the Peltier effect and the Thompson effect. 

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... 

Consider a basic thermocouple circuit with one measuring junction, as shown in Fig. 16.17. The two thermoelements A and B are joined at point c to form the measuring junction at temperature T. The thermoelements are connected to wires C at points b and d, both immersed in an ice bath (liquid water and ice in equilibrium) at T0. The two wires C are connected to the input of an EMF measuring device. The input ports, a and e, are maintained at temperature 7j. Applying Eq. 16.18 over the various legs of the circuit gives... 

Circular Foil Heat Flux Gauge. The instruments (often called Gardon heat flux gauges [114]) shown in Fig. 16.34 are also based on Fourier s law. A copper heat sink is installed in the wall of the measuring site with a thin constantan disc mounted over it. A small copper wire is attached to the center of the constantan disc. Another copper wire attached to the copper heat sink completes a thermocouple circuit. Heat flow to the constantan disc is conducted radially outward to the copper heat sink, creating a temperature difference between the center and edge of the disc. The copper and constantan (other materials could be used) act as a thermocouple pair to measure this temperature difference. 

Eab Electric potential of thermocouple circuit with materials A and B, V... 

Cutting Temperature, Fig. 3 Tool-work thermocouple circuit in turning... 

A comparison of indicated temperature rise with calc ilated temperature rise from current and voltage readings indicated a time lag in the surface thermocouple of 0.2 ms. This represented the composite lag in galvanometer response for the three readings of current, voltage, and temperature and any apparent lag due to unbalance in the thermocouple circuit which may have caused induced voltage pickup. 

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

Thermocouples may also be joined in parallel. In the parallel-connected thermocouple circuit, a mean value of the individual thermocouples is indicated, and it will be the true arithmetic mean if all thermocouple circuits are of equal resistance. A schematic diagram of a parallel-connected thermocouple circuit is shown in Fig. M-5. 

Opposed thermocouple circuits are sometimes used to obtain a direct reading of a temperature difference between two sets of thermocouples reading two levels of... 

Figure 6. Diagrammatic sketch of a suitable system for a differential thermal apparatus thin lines indicate thermocouple circuit thick lines, power supply.

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