Thermocouple compensating lead-wire

Wiring Diagrams of Thermocouple Installations.— Figure 11 illustrates a simple thermoelectric installation for a rare-metal couple. The couple is properly protected by a porcelain or quartz tube and if necessary by an outer tube of iron, chromel, fireclay, etc. From the head of the couple compensating lead wires are carried to the bottom of a pipe driven 10 ft. under ground. From the bottom of the pipe copper lead wires are carried to the indicator. 

Figure 13 illustrates a multiple thermocouple installation connected to a single indicator. Compensating lead wires are carried from the couples to a conveniently located cold-junction box. The temperature of this box is thermostatically controlled. From the cold-junction box copper wires are carried to the terminal block and selective switch illustrated. A common return has been employed between the cold-junction box and the switchboard. In general it is preferable to use individual return wires for each couple. The switchboard illustrated is designed for six couples. By pressing one of the buttons shown any desired couple is connected directly to the indicator. 

For the connection of a thermocouple to the measurement instrument, the best way is to use leads made of the same materials (extension leads) these avoid lead junction errors. For economic reasons, however, cheaper alloys having similar e.m.f. output, at least over a limited temperature range close to room temperature, are often used. These compensating leads are often supplied by the same producers of the thermocouple wires. 

The lead wires of these thermocouples are usually made of 0.35 to 0.5 mm. diameter wire. Sometimes it is undesirable to have thermocouple wires sufficiently long to bring the connections (which also form the cold junctions) directly to constant temperature. In this case the so-called compensators are used. These may be considered lead wire lengtheners. Such compensating wire may be ordered from the companies that supply thermocouples. 

Many types of sensors and transducers have particular signal conditioning requirements. For example, thermocouples require cold-junction compensation for the thermoelectric voltages created where the thermocouple wires are connected to the data acquisition equipment. Resistive temperature devices (RTDs) require an accurate current excitation source to convert their small changes in electrical resistance into measurable changes in voltage. To avoid errors caused by the resistance in the lead wires, RTDs are often used in a 4-wire configuration. The 4-wire RTD measurement avoids lead resistance errors because two additional leads carry current to the RTD device, so that current does not flow in the sense, or... 

When compared with thermocouples, RTDs have higher accuracy, better linearity, long-term stability, do not require cold-junction compensation or extension lead wires and are less susceptible to noise. However, they have a lower maximum temperature limit and are slower in response time in applications without a thermal well (a protective well filled with conductive material in which the sensor is placed). 

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