Calorimeter heat exchange sample-surroundings

Adiabatic calorimetry uses the temperature change as the measurand at nearly adiabatic conditions. When a reaction occurs in the sample chamber, or energy is supplied electrically to the sample (i.e. in heat capacity calorimetry), the temperature rise of the sample chamber is balanced by an identical temperature rise of the adiabatic shield. The heat capacity or enthalpy of a reaction can be determined directly without calibration, but corrections for heat exchange between the calorimeter and the surroundings must be applied. For a large number of isoperibol... 

A variation, which results in a more simple apparatus, is the drop calorimeter. The test piece is heated (or cooled) externally, dropped into the calorimeter and the resultant change in temperature monitored. For the simplest measurements, the calorimeter need not be surrounded by an adiabatic jacket but in that case, corrections for the heat exchange with the surroundings must be applied. A procedure using a drop calorimeter has been standardized for thermal insulation in ASTM C35l". It is possible to combine the adiabatic and drop calorimeter methods by dropping a heated sample into an adiabatic chamber and this has been used for plastics12. 

On the whole, two types of calorimeters exist one is where the sample, located in a flat holder, is placed npon the sensitive part of the calorimeter, shaped as a horizontal plate. Thus, the sample may exchange a vertical heat flux between this temperature-sensitive plate. The other consists of a cylindrical sample surrounded by a temperature-sensitive cylinder, and the heat flux exchanged is radial. When this... 

Passive diathermal calorimeters are those in which good heat-exchange between the system S and the surrounding thermostat T is achieved by good thermal conduction. The sample temperature passively follows, here, the thermostat temperature and, except in transient situations, there is no heat stored in the system S, These calorimeters can also be called, quite correctly, thermal conduction calorimeters . 

These are still diathermal calorimeters, where everything happens as if there was excellent heat exchange between the system S and the surrounding thermostat T. This means that the system temperature Ts closely follows that of the thermostat, Tj. The good heat exchange is, in reality, simulated . For that purpose, the thermal transfer between sample and thermostat is complemented or replaced by a physical phenomenon taking place in close contact with die sample and able to generate or remove heat in situ. 

To sum up, to yield repeatable results and to be capable of calibration, a calorimeter must be constructed in such a way that the entire heat exchange of the sample with the surroundings takes place in a defined manner through the measuring system only. In any uncertainty analysis (see Section 6.5), heat transport phenomena and their consequences must be taken into account. 

Calorimeters with Heat Exchange between the Sample and Surroundings... 

For measurement of the heat exchanged between the sample in the calorimeter vessel and the surroundings, different methods exist, which have already been explained in Part One of this book. We will not repeat the fundamentals in this chapter unless the details are of essential importance for the respective calorimeter type. 

It follows from these considerations that in calorimeters with large thermal resistance between sample vessel and surroundings, the measured temperature signal is proportional to the heat exchanged with the calorimeter substance. In... 

Calorimeters with Heat Exchange between the Sample and Surroundings 1175 Here is a summary of the difficulties encountered in flow calorimeters ... 

To allow the sample to follow the temperature change of the furnace as closely as possible, there must be a quick but controlled heat transfer between the furnace and the sample. Most scanning calorimeters can therefore be considered as heat flow calorimeters (see Section 7.9.2), and the comments concerning the controlled heat exchange between the surroundings (furnace) and the sample are valid for the scanning calorimeters, too. Some essential issues shall, however, be repeated here ... 

Calorimeters ivith Heat Exchange betiveen the Sample and Surroundings 183 Table 7.2 Specifications of some commercially available heat flow DSCs. 

In adiabatic calorimeters, under ideal circumstances, no heat exchange whatsoever occurs between the sample under investigation and the surroundings. All heat produced by or in the sample stays there and changes its temperature no heat leakage occurs. This can be achieved in three ways ... 

The immense development of computer techniques and electronics made it possible to construct adiabatic calorimeters in which the surroundings (e.g., the heat shield) follow the temperature of the sample container perfectly. The identical temperatures of the sample container and the surroundings reduce the uncontrolled heat exchange to a negligible amount and raise the accuracy of the measurements accordingly. Of course, even the best electronics cannot eliminate... 

The construction principles of such chip calorimeters are similar to those of conventional calorimeters The heater corresponds to the furnace, and the center of the membrane corresponds to the calorimeter system, including the sample container. The thin membrane serves as the thermal path between the heater and the sample with very low thermal resistance and very low effective heat capacity. The thermopile measures the temperature difference between the sample site and the chip frame (surroundings). Because of the much larger lateral dimension of the membrane of at least two orders of magnitude, the heat exchange between the sample and the frame can be neglected. The chip calorimeter can therefore be considered a quasi-adiabatic calorimeter when vacuum is applied. 

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