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. 

Thermistors have the desirable characteristics of small size, narrow spans, fast response (their time constant can be under 1 second), and a very high sensitivity. They do not need a cold-junction compensation, errors due to contact or lead-wire resistance are insignificant, and they are well suited for remote temperature sensing. They are inexpensive, their stability increases with age, and they are the most sensitive differential temperature detectors available. 

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). 

It is important that the thermistor or RTD used for measuring the cold junction be physically located at the cold junction, as the temperature of the cold junction is often different from that of the room, generally because of heat leakage from the furnace. Special wire, referred to as compensating lead-wire,... 

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. 

The high cost of platinum prevents the use of compensating leads of the same metal in the case of a rare-metal couple but inexpensive lead wires of copper and an alloy of nickel-copper are now available for use with the platinum-platinum 90, rhodium 10 couple. These lead wires do not compensate individually but taken together they compensate to within 5°C. for a variation of 200 C. in the couple-lead wire junctions. Since the compensating lead wires for the rare metal couple do not compensate individually both terminals on the head of the couple should be always as nearly as possible at the same temperature. The copper wire of the compensating leads is connected to the platinum-rhodium wire of the couple and the copper-nickel wire is connected to the platinum wire of the couple, i.e., alloy wire to pure metal in each case. The cold junction is then located at the indicator end of the compensating leads. The temperature of this junction may be controlled if necessary by one of the methods described above. Copper wires are carried from this point 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. 

By means of the junction box we effect a saving of some 500 ft. in the compensating cable and need to bury only one pair of junctions, and just as satisfactory an installation is obtained. In installing a large multiple-couple equipment with a junction box, it is very important to insure that the common cold-junction couple is connected with the correct polarity as -Uustrated. Although we have used a common cold junction for all couples we have not employed the objectional common return. Individual returns are used with every couple shown in the diagram. 

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... 

All buried leads to the cold junction should be water-proof insulated, and the junction well should be made watertight. The compensating leads, particularly those for hasc-mcial couples, will generate a large voltaic electromotive force if they are wet and are not insulated with water-proof covering. 

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

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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