Isothermal with electric compensation

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-... 

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. 

When the time interval of the isothermal situation is small compared with the previous non-isothermal phase, for the sake of simplification the measurements are to be performed from the very beginning in non-isothermal conditions, i.e. without heat compensation, hence with the electric heating power turned off,p2 = 0. When the reaction induces only a small change in the heat transfer coefficient, it can be measured without use of the intermediate thermostat (Fig. 2.31). 

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