Thermocouple construction | Thermocouple materials| Properties
Here we learn about thermocouple and construction of thermocouples, Thermocouple materials and Desirable properties thermocouples of materials
~ The thermocouple wires should be in electrical contract only at the hot junction, since contact at any other point will usually result in too low a measured emf cutting off.
~ Thermocouples are manufactured by suitable lengths of the two wires; the ends are carefully twisted together for about two turns, or sometimes butted together, and welded, brazed to form a smooth and rounded head.
~ A thermocouple consists of two pieces of dissimilar metals with their ends joined together (by twisting, soldering or welding). When heat° is applied to °the junction, a voĺtage, in the range of milli-volts (mV), is generated. A thermocouple is therefore said to be self- powered as shown in Fig. 3.5.3 is completed thermocouple circuit.
~ The voĺtage generated at each junction depends on junction temperature. If temperature T1 is higher than T2, then the voltage generated at Junction 1 will be higher than that at Junction 2. In the° above ¢ircuit, the loop current shown on the galvanometer depends on the relative magnitude of the voltages at the two junctions.
~ In 0rder to use a thermocouple t0 measure process temperature, one end of the thermocouple has to be kept in contact with the process while the other end has to be kept at a constant temperature.
~ The end that is in ¢ontact with the process is called the, hot or measurement junction. The 0ne that is kept at constant temperature is called cold or reference junction. The reĺationship between total circuit voltage (emf) and the emf at the junctions is:
Circuit emf = Measurement emf- Reference emf.
~ If ¢ircuit emf and reference emf are known, measurement emf can be calculated and the relative temperature determined.
~ The materials used commonly are copper, iron, platinum, rhodium or iridium all-alone, and alloys like constantan (60% Cu + 40% Ni), chromel (10% Cr + 90% Ni), Alumel (2AI, + SONi + balance Si and Mn).
~ The size of the wire used depends on the temperature range to be measured. For the measurement of high temperatures heavier wires are used.
1. It should have fast thermal response.
2. The material must have suitability over a wide temperature range.
3. Electromotive force (emf) that varies with temperaturein the material must be linear and reproducible.
4. It should not produce drift and should have long life.
5. Material selection is done on the basis of mechanical and chemical properties, stability
reproducibility, and cost.
A few metal combinations for making thermocouples with usable ranges are listed in Table 3.5.1.
Thermocouple Construction :
~ The thermocouple wires should be in electrical contract only at the hot junction, since contact at any other point will usually result in too low a measured emf cutting off.
~ Thermocouples are manufactured by suitable lengths of the two wires; the ends are carefully twisted together for about two turns, or sometimes butted together, and welded, brazed to form a smooth and rounded head.
~ A thermocouple consists of two pieces of dissimilar metals with their ends joined together (by twisting, soldering or welding). When heat° is applied to °the junction, a voĺtage, in the range of milli-volts (mV), is generated. A thermocouple is therefore said to be self- powered as shown in Fig. 3.5.3 is completed thermocouple circuit.
~ The voĺtage generated at each junction depends on junction temperature. If temperature T1 is higher than T2, then the voltage generated at Junction 1 will be higher than that at Junction 2. In the° above ¢ircuit, the loop current shown on the galvanometer depends on the relative magnitude of the voltages at the two junctions.
~ In 0rder to use a thermocouple t0 measure process temperature, one end of the thermocouple has to be kept in contact with the process while the other end has to be kept at a constant temperature.
~ The end that is in ¢ontact with the process is called the, hot or measurement junction. The 0ne that is kept at constant temperature is called cold or reference junction. The reĺationship between total circuit voltage (emf) and the emf at the junctions is:
Circuit emf = Measurement emf- Reference emf.
~ If ¢ircuit emf and reference emf are known, measurement emf can be calculated and the relative temperature determined.
Thermocouple Materials :
~ The materials used commonly are copper, iron, platinum, rhodium or iridium all-alone, and alloys like constantan (60% Cu + 40% Ni), chromel (10% Cr + 90% Ni), Alumel (2AI, + SONi + balance Si and Mn).
~ The size of the wire used depends on the temperature range to be measured. For the measurement of high temperatures heavier wires are used.
Desirable properties thermocouples of materials:
1. It should have fast thermal response.
2. The material must have suitability over a wide temperature range.
3. Electromotive force (emf) that varies with temperaturein the material must be linear and reproducible.
4. It should not produce drift and should have long life.
5. Material selection is done on the basis of mechanical and chemical properties, stability
reproducibility, and cost.
A few metal combinations for making thermocouples with usable ranges are listed in Table 3.5.1.
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