Thermocouple cold junction compensation

Cold-junction compensation can be provided by a network which includes a constant voltage source and a temperature-sensitive bridge to provide an offset voltage which is proportional to the temperature sensitivity of the thermocouple and of opposite sign. 

The thermoelectric power, or thermopower, of the thermocouple is of the order of 2 to 50 iV/°C, depending on the metals and the temperature. In general, the thermopower decreases with decreasing temperature. Typically, in a thermocouple, the first junction is at Th, and the second, or reference junction, is held at the ice point of water (Tc = 0°C) (Fig. 10.21), or its electrical equivalent ("cold junction compensation"). 

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). 

A satisfactory environment for the 0°C reference junction is provided by a slushy mixture of ice and distilled water in a Dewar flask, with a ring stirrer and a monitoring mercury thermometer. Elaborate thermoelectric ice-water chambers are also available these are convenient for prolonged periods of use but rather expensive. As mentioned previously many commercial thermocouple systems eliminate the ice bath by placing the cold junction on an isothermal block that is at room temperature and compensating for the resulting error. This is a convenient but less accurate procedure. 

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. 

Betermination of Temperature of Buried Cold Junction.—Several methods are available for obtaining the temperature at the bottom of the junction well. The simplest is to use a thermocouple consisting of the compensating... 

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. 

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