Calorimeter electric compensation

Adiabatic calorimeters have also been used for direct-reaction calorimetry. Kubaschewski and Walter (1939) designed a calorimeter to study intermetallic compoimds up to 700°C. The procedure involved dropping compressed powders of two metals into the calorimeter and maintaining an equal temperature between the main calorimetric block and a surrounding jacket of refractory alloy. Any rise in temperature due to the reaction of the metal powders in the calorimeter was compensated by electrically heating the surrounding jacket so that its temperature remained the same as the calorimeter. The heat of reaction was then directly a function of the electrical energy needed to maintain the jacket at the same temperature as the calorimeter. One of the main problems with this calorimeter was the low thermal conductivity of the refractory alloy which meant that it was very difficult to maintain true adiabatic conditions. 

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. ... 

This disadvantage does not apply to an isothermal device based on electrical compensation. Nevertheless, calorimeters of the latter type operate only in a quasi-isothermal mode, since electronic control systems depend for their response upon small deviations from an established set point, and a certain amount of time is required for changing the prevailing temperature. The use of modem circuits and components ensures that errors from this source will be negligible, however. Electrical compensation makes it possible to follow both endothermic and exothermic processes. In both cases the compensation power is readily measured and recorded or processed further with a computer. Isothermal calorimeters are used quite generally for determining heats of mixing and solution. Commercial devices are available that al.so support the precise work required for multiphase thermo-... 

It is noteworthy that any exact measurement of heat consists essentially of the measurement of electric energy or is traceable to electric energy determinations because the latter form of energy is easy to release, can be measured with great accuracy, and is directly connected to the base unit of the SI (Systeme international d unites) for the electric current, the ampere. Accordingly, all calorimeters are calibrated either directly by the use of electricity or by means of precisely known heats of reaction or transition, which in turn are measured in electrically calibrated or electrically compensated calorimeters. 

The term isothermal refers to equilibrium thermodynamics, where it means constant temperature. Strictly speaking, the temperature of isothermal calorimeters must be kept constant in every part and every moment. But in such a case, no heat transport would occur because heat can only flow when a temperature difference exists (see Chapters 4 and 5). In other words, at least the sample temperature must be more or less different from the calorimeter temperature, and therefore it is more correct to call this type of calorimeter quasi-isothermal rather than isothermal. Furthermore, the heat produced during a reaction in such a calorimeter must be compensated for immediately in one way or the other otherwise, it would change the temperature of the calorimeter. There are two possibilities to compensate for the heat produced by the sample by a phase transition or by electrical compensation. In the first case, the amount of heat produced by the sample is proportional to the amount of substance transformed in the second case, the heat flow rate from the sample is proportional to the electric compensation power. 

One peculiarity of electrical compensation should be mentioned, too every electronic temperature controller needs a minimal but nonzero temperature difference to react. Furthermore, the heat produced by the sample and that produced by the electrical heater are not at the same place, and there must be some temperature gradient to allow a heat exchange between them. Therefore, such calorimeters operate not in a strictly isothermal but in a quasi-isothermal mode. The temperature is generally kept constant but may differ locally. The deviations depend on the heat flow rate as well as on the quality of the compensation controller. Normally, these differences can be neglected in practice. 

The design of the calorimeter is shown in Figure 9.19. The calorimeter consists of a burner and an electrical compensation heater in a double-walled heat pipe filled with Freon with a boiling point of 25 °C. The Freon transports the heat, which is developed by the burner and the compensation heater, to a cooling unit that consists of water-cooled Peltier elements. A thermometer located in the vapor phase is used to control the power of the compensation heater so as to keep the temperature of the vapor constant independent of the heat of combustion of the gas. The flow of gas burned in the calorimeter is determined by means of mass... 

In power compensation calorimeters, the jacket temperature is set slightly below the desired reaction temperature. A heater in the reaction mass maintains the set temperature. A change in electrical power to the heater compensates for any change in reaction temperature. This provides a direct measure of the heat produced by the chemical reaction. 

Heat can also be pumped out from a calorimeter by other principles, e.g., by use of a cooling liquid, but such procedures are now only of historical interest in connection with laboratory calorimeters. Mainly of historical interest is also compensation of exothermic processes by use of melting of a solid, e.g., ice, surrounding the calorimeter vessel. Lavoisier used such an ice calorimeter (later often called a Bunsen calorimeter) in his pioneering biocalorimetric work (see Kleiber, 1961). For endothermic processes, compensation is easily achieved by release of electrical energy in the vessel. 

Calorimeters of any type in twin arrangements can also be used as power compensation calorimeters an exothermic process in one of the vessels can be simulated by evolution of electrical energy in the other (a thermal balance ). 

In contrast, DSC, designed in 1960 by Watson184 and O Neill,185 is a newer, more quantitative technique that does measure Ts and TR, but also measures very precisely the electrical energy used by separate heaters under either pan to make Ts = TR (this is power-compensated DSC, useable below 650° C). The power input into S minus the power input into R is plotted against Tr. High-temperature DSC (useful for TR > 1000°C) measures the heat fluxes by Tian-Calvet thermopiles rather than the electrical power, as a function of Tr. In a heat-flux DSC, both pans sit on a small slab of material with a calibrated heat resistance. The temperature of the calorimeter is raised linearly with time. A schematic DSC curve is shown in Fig. 11.80. 

A few calorimeters have a double active control in addition to the automatic cancellation of 7s - Tr by control of the thermostat temperature Ti, there is also an automatic cancellation of the temperature difference between the sample and a reference (which also forms part of the system S) by supply of heat to the cooler side. The first control is to provide the adiabatic conditions, whereas the second control is to provide a heat compensation of the phenomenon studied. The measurement of heat is not derived from a temperature increase of the system but from the electrical energy provided for the compensation. This principle was followed by Clarebrough [28], Bonjour [29] and Privalov [30]. 

Modem adiabatic calorimeters employ a technique whereby the enthalpy of vaporization is measured under conditions in which a measured amount of electrical energy is supplied to a heater immersed in the sample to compensate for the heat absorbed by the substance during the evaporation and hence the temperature is kept constant. The main differences among adiabatic calorimeters are that the vapour flows out of the calorimeter at atmospheric pressure (those of Mathews and Fehlandt [65]), into a vacuum, [67,69-71] into a gas stream [68], or into a closed recirculation system with continuous fluid flow [66]. 

ASTM E906 [99] defines the OSU calorimeter. The apparatus consists of an insulated box containing a vertical specimen, a parallel electric radiant heater, and a pilot ignition device. Air at a controlled rate flows through the box, and the inlet and outlet temperatures arc recorded. ASTM E906 also records the temperature of the box wall to compensate for the nonadiabatic characteristics of the apparatus. The box is calibrated using a preset gas flame. The vertical specimen size is 150 mm square, and the incident heat flux has a maximum value of 100 kW, m. Tests with a horizontal specimen, 110 x 150mm, which involve the use of aluminum foil to reflect heat onto the specimen, are apparently... 

The calorimeters wifli carrier gas are also used. " Evaporation of substance is accelerated by a stream of gas (for example, nitrogen) at reduced pressure. The heat loss by a calorimeter, due to evaporatioit is compensated by an electrical current to keep temperature of calorimeter constant and equal to flic temperature of the thermostating bath. 

Power compensation-type differential scanning calorimeter Instrument for measuring the differential electric power supplied between a sample and reference to maintain a minimal temperature difference between the sample and reference, in response to a temperature programme. 

In a power compensation differential scanning calorimeter (DSC), the base of the sample holder unit is in direct contact with a reservoir of coolant (Figure 2.12). The sample and reference holders are indiv idually equipped with a resistance sensor, which measures the temperature of the base of the holder, and a resistance heater. Upon detection of a temperature difference between the sample and reference, due to a phase change in the sample, electrical power is supplied so that the temperature difference falls below a threshold value, typically <0.01 K. 

In the isothermal mode of operation it is imperative that all thermal effects be somehow compensated. This is achieved either electrically or with the aid of a phase transition for some substance. Only phase-transition calorimeters can be regarded as strictly isothermal. In this case thermodynamics ensures that the temperature will remain precisely constant since it is controlled by a two-phase equilibrium of a pure substance. The most familiar example is the ice calorimeter, already in use by the end of the 18th century and developed further into a precision instrument about 100 years later by Bunsen (Fig. 16). The liquid-gas phase transition has also been used for thermal compensation purposes in this case a heat of reaction can be determined accurately by measuring the volume of a vaporized gas. 

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. 

Applications The ITC measures the heat production or adsorption when the molecules of the two components come together. When a certain amount of one component is injected into the calorimeter vessel containing the other component, the respective heat is compensated for electrically, and the power needed to maintain isothermal conditions between the two vessels is measured. The injection can be done stepwise or continuously. From the raw values, all interesting thermodynamic data (AH, AS, AG) of the reaction as well as other reaction parameters can be determined using the software of the respective instrument. 

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