Thermocouples single junction

The isothermal junction block is also used in commercial thermocouple devices that utilize only a single junction. Since Tyock can have any value, it is made equal to the reference temperature Tj, as shown in Fig. 3a. Another simplification is the elimination of wire A in the junction block, so that two junctions (Cu-A and A-B) at the same temperature Tj are replaced by one (a Cu-B junction) as shown in Fig. 3A It can be shown empirically that... 

The freezing point of several samples of benzene was determined as a function of the time (Francis, Reading University). Some of the results obtained are given in table 1, where t denotes time and E the e.m.f. obtained with a copper-constantan single junction thermocouple for which 1 degree interval gave 39.0 pV. 

It is important to understand that the tables and polynomials are based on the assumption that the cold junction of the thermocouple pair is at zero degrees Celsius. In the laboratory, the cold junction is generally at room temperature or slightly above (the temperature at the screw terminals where the thermocouple wires and lead-wires join), hence a correction factor is needed. The law of successive potentials (Figure 2.6) may be stated as The sum of the EMF s from the two thermocouples is equal to the EMF of a single thermocouple spanning the entire temperature range ... 

Thermocouples, or thermal junctions, or thermoelectric thermometers have two intermetallic junctions between two different metals (or semimetals, or semiconductors) A, B in a loop (Fig. 10.21). When these two junctions are held at different temperatures (T i, and T2), then a potential difference A Vis set up this is the Seebeck98 effect. For instance, for a Cu-constantan thermocouple, with T2 = 300 K and T, 273.15 K, AV = 1.0715 mV. Its converse is the Peltier99 effect If a current at a fixed voltage is applied in a loop like in Fig. 10.21, then a temperature difference AT can be maintained (thermoelectric heaters and coolers). The Seebeck effect arises because, before the junctions are made, the two metals have different Fermi levels after the junctions are made, electrons will flow from the higher-level metal to the lower-level metal, until a single Fermi level results across the junction. 

The microcalorimeter. In the past, most immersion microcalorimetry was carried out with two of the four main categories listed at the beginning of Section 3.2.2, namely, isoperibol microcalorimeters, i.e. conventional temperature rise type, and diathermal-conduction microcalorimeters using a form of heat flowmeter. The isoperibol microcalorimeters were the only type used until the 1960s they are easily constructed and are well suited for room temperature operation. Improvements were made in the temperature stability of the surrounding isothermal shield and the sensitivity of the temperature detector. Initially the temperature detector was a single thermocouple, then a multicouple with up to 104 junctions (Laporte, 1950), and... 

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. 

Thermoelectric Circuits. A typical circuit for a single thermocouple of materials A and B is shown in Fig. 16.17. The reference temperature (at which junctions b and d are maintained) is usually the ice point, 0°C. The connecting wires C are usually copper wires. Note that, according to Eq. 16.20, the connecting (copper) wires C should not affect the EMF EAB, which, for given materials A and B, is just a function of the temperature T. 

Where t is the time constant of the system. The use of single or multiple thermocouples in calorimetry is important. If the difference between the temperatures of the sample and reference thermocouple junctions is AT, then an emf [E] is produced which depends on the thermoelectric constant e and the number of thermocouples N). Therefore... 

The emf generated is directly proportional to the number of junctions. A four-junction Type J thermopile generates 21.076 mV—or four times that of a single thermocouple. Greater precision may also be achieved by more advanced electronics with respect to potentiometers but for precise measurements of temperature differentials, a thermopile is very effective. Figure 5.10 illustrates the electrical circuit of a four-junction thermopile measuring the temperature differential between two points. 

Fig, 3, Location of wail temperature thermocouple junctions in the single-tube heat transfer... 

Thermocouples are rugged and versatile temperature sensors frequently found in industrial control systems. A thermocouple consists of a pair of dissimilar metal wires twisted or otherwise bonded at one end. The Seebeck effect is the physical phenomena which accounts for thermocouple operation, so thermocouples are known alternatively as Seebeck junctions. The potential difference (Seebeck voltage) between the fi ee ends of the wire is proportional to the difference between the temperature at the junction and the temperature at the fi ee ends. Thermocouples are available for measurement of temperature as low as —270°C and as high as 2300°C, although no single thermocouple covers this entire range. Thermocouples are identified as type B, C, D, E, G, J, K, N, R, S, or T, according to the metals used in the wire. 

In heat-flux DSC, the difference in heat flow into the sample and reference is measured while the sample temperature is changed at a constant rate. Both sample and reference are heated by a single heating unit. Heat flows into both the sample and reference material via an electrically heated constantan thermoelectric disk, as shown in Figure 31-12. Small aluminum sample and reference pans sit on raised platforms on the constantan disk." Heat is transferred through the disks and up into the material via the two pans. The dififerential heat flow to the sample and reference is monitored by Chromel-constantan area thermocouples formed by the junction between the constantan platform and Chromel disks attached to the underside of the platforms. The differential heat flow into the two pans is directly proportional to the difference in the outputs of the two thermocouple junctions. The sample temperature is estimated by the Chromel-alumel junction under the sample disk. 

---