Heat compensation calorimeters

If there is no heat accumulation in the liquid phase (7r constant), the measured flow rate ( Pq) is equal to the heat generated by the microbial growth reac- 

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Differential scanning calorimetry (DSC) can be performed in heat compensating calorimeters (as the adiabatic calorimetry), and heat-exchanging calorimeters (Hemminger, 1989 Speyer, 1994 Brown, 1998). 

In conclusion, the heat compensation calorimeter RC-1 run in TR-mode provides an excellent on-line signal to monitor rapid changes in the metabolic activity. Especially, it is faster than the gas analyses. 

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. 

The first heat flow calorimeter based on Seebeck, Peltier, and Joule effects was built by Tian at Marseille, France, and reported in 1923 [156-158]. The set-up included two thermopiles, one to detect the temperature difference 7) — 7) and the other to compensate for that difference by using Peltier or Joule effects in the case of exothermic or endothermic phenomena, respectively. This compensation (aiming to keep 7) = T2 during an experiment) was required because, as the thermopiles had a low heat conductivity, a significant fraction of the heat transfer would otherwise not be made through the thermopile wires and hence would not be detected. 

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.

Domen, S.R. and Lamperti, P.J., A heat-loss compensated calorimeter theory, design and performance. NBS J. Research 78A, National Institute of Standard and Technology, Gaithersburg, MD 20899, USA (1974) 595-612. 

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. 

Solution calorimetry involves the measurement of heat flow when a compotmd dissolves into a solvent. There are two types of solution calorimeters, that is, isoperibol and isothermal. In the isoperibol technique, the heat change caused by the dissolution of the solute gives rise to a change in the temperature of the solution. This results in a temperature-time plot from which the heat of the solution is calculated. In contrast, in isothermal solution calorimetry (where, by definition, the temperature is maintained constant), any heat change is compensated by an equal, but opposite, energy change, which is then the heat of solution. The latest microsolution calorimeter can be used with 3-5 mg of compound. Experimentally, the sample is introduced into the equilibrated solvent system, and the heat flow is measured using a heat conduction calorimeter. 

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]. 

An original route is that proposed by Ter-Minassian and Million in 1983 [44] with their pneumatic compensation calorimeter, represented in Fig 10. The tubular sample cell 4 is in good thermal contact with four metallic bulbs. Two of them operate like bulb 1 in the figure, Le. as pneumatic thermal detectors. They are filled with gas, say around 1 bar, and their pressure is compared, by means of a differential manometer, with the constant pressure of a reference reservoir 3 immersed in the surrounding thermostat block 5. Therefore, they detect any temperature change of the sample. The two oflier bulbs operate like bulb 2, i.e. as pneumatic energy-compensating devices. They are also filled with gas, say around 1 bar, but they are connected to flie piston-cylinder 7 which enables the heat of compression (or decompression) necessary to cancel the temperature difference between the sample and thermostat (as detected with the first set of bulbs) to be produced in the bulb. More recently, Zimmermaim and Keller built a comparable pneumatic compensation calorimeter whose calorimetric performances were carefully examined [45]. 

The temperature difference family includes most adiabatic and quasi-adiabatic calorimeters (in the time dependent temperature" group) together with most heat-flowmeter calorimeters. The total probably represents between 80 and 90 % of the calorimeters used today, so that, for practical use, the above classification looks somewhat unbalanced. Moreover, the calorimeters just mentioned shift to the first family as soon as they also make use of heat compensation, hence a real overlap exists between the two main families. 

C) True Isothermal (i.e. both in space and in time) or extended isothermal (i.e. only isothermal in space) calorimeters Tq follows Ts these are proportional systems and include phase-change, power-compensation and heat-flowmeter calorimeters. 

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

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] ... 

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. 

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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