Thermoelectric compensation

Commercially available power compensation and heat flux calorimeters are competitive in stability, accuracy, and sensitivity, fractions of a millijoule being detectable with good reproducibility. Although thermoelectric compensation is more efficient in time (larger power values are detected in shorter time intervals), the kinetics of adsorption may impose limitations on this advantage. 

Thermostats—bimetallic strip, snap-action disk, and thermostatic cable Continuous discrete conductors Thermistor sensors Rate of temperature rise Copper tubing heat-actuated devices (HADs) Thermoelectric Combined rate of rise/fixed temperature Rate compensation Oil mist ... 

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"). 

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. 

P. Cermak f has shown that equation (4) is sometimes not very well confirmed by experiment. The quantities of heat produced by the Peltier and Thomson effects are, however, very small, and the calorimetrical determinations are not nearly so accurate as the measurements of the thermoelectric e.m.f. by the compensation method, especially as the quantities of heat to be determined are differences between actual calorimetrical determinations and the Joule heats calculated from electrical data. We are therefore not yet in a position to condemn the fundamental assumptions of Thomson s theory. As the thermodynamical equations are rigorously accurate, any error in the conclusions must be sought for in the assumption of the complete reversibility of the phenomena. 

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. 

On the other hand, for slow reactions, adiabatic and isothermal calorimeters are used and in the case of very small heat effects, heat-flow micro-calorimeters are suitable. Heat effects of thermodynamic processes lower than 1J are advantageously measured by the micro-calorimeter proposed by Tian (1923) or its modifications. For temperature measurement of the calorimetric vessel and the cover, thermoelectric batteries of thermocouples are used. At exothermic processes, the electromotive force of one battery is proportional to the heat flow between the vessel and the cover. The second battery enables us to compensate the heat evolved in the calorimetric vessel using the Peltier s effect. The endothermic heat effect is compensated using Joule heat. Calvet and Prat (1955, 1958) then improved the Tian s calorimeter, introducing the differential method of measurement using two calorimetric cells, which enabled direct determination of the reaction heat. 

These authors were aware of the difficulty of establishing a comprehensive classification of calorimeters In every classification there are certain calorimeters which do not clearly belong to a particular category.The Calvet calorimeter, for instance, can be used eidier isothermally with electric compensation... or in an isoperibol manner involving the measurement of a local temperature difference. Moreover, a number of existing calorimeters remain outside our classification. One example is a calorimeter involving a compensation of the thermal effect other than by thermoelectric means or by phase transition. But such devices can be easily included in our classification by analogy. ... 

Isothermal the measuring system and the surroundings are at the same temperature the thermal resistance, between the measuring system and the surrounding thermostat is supposed to be infinitesimally small.which is not feasible in calorimetry . Consequently, isothermal operation is said to necessitate a compensation of the heat released, either by a phase transition (passive measuring system) or by thermoelectric effects (active measuring system). 

There are two main types of DSC instrumentation, heat-flux DSC and power-compensated DSC. A schematic of a commercial heat-flux DSC is presented in Figure 16.19. In a heat-flux instrument, the same furnace heats both the sample and the reference. In heat-flux DSC, the temperature is changed in a linear manner, while the differential heat flow into the sample and reference is measured. The sample and reference pans sit on the heated thermoelectric disk, made of a Cu/Ni alloy (constantan). The differential heat flow to the sample and reference is monitored by area thermocouples attached to the bottom of the sample and reference positions on the thermoelectric disk. The differential heat flow into the pans is directly proportional to the difference in the thermocouple signals. The sample temperature is measured by the alumel/chromel thermocouple under the sample position. This temperature is an estimated sample temperature because the thermocouple is not inserted into the sample itself. The accuracy of this temperature will depend on the TC of the sample and its container, the heating rate, and other factors. As shown in Figure 16.19, the sample and reference pans both have lids and the reference pan is an empty pan. A schematic of a power-compensated DSC is presented in Figure 16.20. The major difference in power-compensated DSC... 

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... 

The heat released from a sample during a process flows into the calorimeter and would cause a temperature change of the latter as a measuring effect this thermal effect is continuously suppressed by compensating the respective heat flow. The methods of compensation include the use of latent heat caused by a phase transition, thermoelectric effects, heats of chemical reactions, a change in the pressure of an ideal gas (Ter Minassian and Million, 1983), and heat exchange with a liquid (Regenass, 1977). Because the last three methods are confined to special cases, only the compensation by a physical heat of transition and by electric effects are briefly discussed here. 

Calorimeters involving compensation of the thermal effect by phase transition or thermoelectric effects. 

Scanning condition Tp = Tp(t) or Tm = TmW with Tp = constant Calorimeters involving the measurement of a temperature difference (heat flow calorimeters) or with a compensation of the thermal effect by thermoelectric effects (power compensation calorimeters). 

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