Power compensation calorimeter

The temperature of the reactor content (Tj.) is controlled by varying the power of a compensation heater inserted directly into the reactor content. As with an electrical heater, cooling is not possible, so the compensation heater always maintains a constant temperature difference between the reactor jacket and the reactor content. Thus cooling is achieved by reducing the power of the compensation heater. The heat-flow rate from the reactor content through the wall into the cooling liquid ( Fiow) is typically not determined because the heat-flow rate of the reaction is directly visible in the power consumption of the compensation heater. The temperature of the cooling liquid (Tj) is maintained at a constant temperature by an external cryostat. The power-compensation principle was first implemented by Andersen and 

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The two basic types of reaction calorimeters commonly used for safety assessments are isothermal (including both heat flow and power compensation calorimeters) and adiabatic. 

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. 

Fig. 8.1 Standard set-up of a reaction calorimeter [4]. Left side heat-flow, heat-balance and power-compensation calorimeters. Right side Peltier calorimeters.

Figure 5. Schematic diagram of a section through a power compensation calorimeter. a, calorimetric vessel b, heat sink (e.g., a thermostatted water bath) c, air or vacuum d, thermometer e, stirrer f, thermopile g, calibration heater / is the current through the thermopile.

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 ). 

RC measurements can be classified either as devices using jacketed vessels with control of the jacket temperature (heat balance calorimeters, heat flow calorimeters and temperature oscillation calorimeters) or as devices using a constant surrounding temperature, e.g., jacketed vessels with a constant jacket temperature, (isoperibolic calorimeters and power compensation calorimeters) such instruments may also feature single or double cells. 

C) Power compensation calorimeters, where the thermal process is balanced by a cooling or heating power. 

Examples Dzhigit et al.[42], Zielenkiewicz and Chajn [60], Hansen et al.[43], Christiansen and Izatt [61]. Like the preceding one, this is also often known as a power-compensation calorimeter ... 

The major proof was that for the majority of materials the same calibration constant was found. Meanwhile, part of the AH values in Table 1 were confirmed by measurements with a Perkin-Elmer DSC-2, i.e. a power compensated calorimeter (Breuer, Eysel and Hohne ). 

In the power compensation calorimeter the temperature of the heat transfer medium is set below the desired reaction temperature, which is maintained by a heater in the reactants (Figure 3.8). Any change in heat flow is compensated by... 

DATA FROM HEAT FLOW/POWER COMPENSATION CALORIMETERS... 

Wang et al. [26] were the first to develop a power compensation calorimeter. They assumed that heat losses are constant throughout the reaction in order to link the measured and the generated heat. This can be achieved by maintaining the outside of the autoclave at a constant temperature that is slightly lower than that of the reaction. This temperature differential is maintained by the use of a smaller internal heater whose power can be externally controlled. If an exotherm occurs, then the system automatically reduces the power supplied to the internal heater to compensate and to maintain a constant temperature. The opposite happens when an endotherm occurs. 

An extensive analysis of the sawtooth modulation brought a number of interesting results. Mathematically, it could be shown that if there were no temperature gradients within the sample and if all other lags and gradients could be assessed with the Fourier heat-flow equation, Eq. (11) does allow the calculation of the precise heat capacities [33]. Temperature gradients are, however, almost impossible to avoid. Especially in the power-compensated calorimeter, the temperature sensor is much closer to the heater than the sample and cannot avoid gradients. The empirical solution to this problem was to modify Eq. (11) as follows [34] ... 

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). 

FIG. 1 The closed-loop control system of a power compensation calorimeter. 

Top-notch power compensation calorimeters can be very sensitive and are an excellent tool for academic studies. However, they can be noisy, not so easy to calibrate, and frequently present baseline stability problems. Heat flux calorimeters tend to be more robusL with better baseline... 

Stability, and the simplest models can be suitable for quality control routine operations. The most sophisticated models can match the sensitivity of the power compensation calorimeters. 

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