Calorimeter classification

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

B1.27.4.1 CLASSIFICATION BY PRINCIPLE OF OPERATION ISOTHERMAL CALORIMETERS (MORE PRECISELY, QUASI-ISOTHERMAL)... 

The number of small scale test methods, used for classification purposes, should be limited and based on ISO tests, presumably the Cone Calorimeter /10/ (see Fig. 8) and possibly the ISO Surface Spread of Flame test /11/. 

Furthermore, it has been shown that the time period until ignition occurs, in the Cone calorimeter, is proportional to the inverse of the flame spread rate [16]. The Cone calorimeter can also be used to provide the mass loss rate information required for the simplified classification into categories of toxic hazard [1] quick toxic hazard assessment. Thus, the NBS Cone calorimeter is a very useful tool to overcome some of the disadvantages associated with measuring a single property at a time. 

The classical calorimetric methods addressed in chapters 7-9, 11, and 12 were designed to study thermally activated processes involving long-lived species. As discussed in chapter 10, some of those calorimeters were modified to allow the thermochemical study of radiation-activated reactions. However, these photocalorimeters are not suitable when reactants or products are shortlived molecules, such as most free radicals. To study the thermochemistry of those species, the technique of photoacoustic calorimetry was developed (see chapter 13). It may be labeled as a nonclassical calorimetric technique because it relies on concepts that do not fit into the classification schemes just outlined. 

There are also other criteria for the classification of calorimeters. They can also be divided according to the aim for which they are used, i.e. 

Radiation thermometers can be sensitive to radiation in all wavelengths (total-radiation thermometers) or only to radiation in a band of wavelengths (spectral-radiation thermometers). Thermocouple and thermopile junctions or a calorimeter are the usual detectors in a total-radiation thermometer. For spectral systems, the classification is normally based on the effective wavelength or wavelength band used—as determined, for example, by a filter, which allows only near-monochromatic radiation to reach the detector, or by the use of a detector sensitive only to radiation in a specific wavelength band. Radiation thermometers utilize the visible portion of the radiation spectrum, infrared thermometers or scanners measure infrared radiation, and spectroscopic thermometers operate with radiation that is normally of shorter wavelength than the other two methods. 

The above classification is now applied to the real world of calorimeters. 

Evans introduced his classification in the following way We need only briefly consider classification. Calorimeters are either isothermal or adiabatic, thou some calorimeters do not belong strictly to either category . He finally distinguishes ... 

Comment this is similar to the Calvet and Prat classification (1956), still with an unjustified separation between isothermal and heat conduction calorimeters. The adiabatic calorimeters are more correctly defined (i.e. after the heat exchange, not after the thermal conductance) but, still, in an idealized way, because the author restricts the term to calorimeters where the heat exchange is eliminated . 

Skinner considered Two extreme types either the adiabatic or the conduction calorimeter. Between the two extremes lies the well-known and commonly used isothermally jacketed calorimeter . He ended with the following classification ... 

The aim of this classification was to unquestionably classify any calorimeter within only one main class. The relationship existing between the sample temperature and that of the surrounding thermostat seems to be an appropriate criterion . For that purpose, a clear distinction is made between two parts of the calorimeter, namely (i) the sample together with the container with which it is in good thermal contact and (ii) the surrounding thermostat. To make this distinction clear, these two distinct parts are shown on the schematic representation of 11 types of calorimeters. This finally leads to the following classification ... 

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. ... 

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. 

Hemminger and Hohne finally proposed (p 132) to simultaneously use the three classifications above to classify any calorimeter. 

The aim of this classification was to be simple, easy to apply (both for actud and hypothetical calorimeters) and to give rise to the minimum overlaps. It starts from the general equation ... 

This classification still separates from each other the Adiabatic and the Ordinary (or Isoperibo)l calorimeters, but, under the heading of Isothermal or Extended Isothermal , introduces, as a whole, the family of calorimeters which are called, in sections 4,4. and 4,5 above, "Diathermal , a term certainly more appropriate than "Isothermal . 

These authors do not classify calorimeters, but rather calorimetric methods in thermochemistry , which is a more general term able to include things other than the principles of construction or detection. Also, their classification is... 

Being in the scope of a review of modern trends, the above classification did not need to explicitly list the phase-change calorimeters. 

This nomenclature is close to that proposed by Hemminger and Hohne in 1984. It makes use of the same three primary criteria the principle of measurement, the mode of operation and the construction principle. Each criterion leads to its own classification, as shown hereafter. The main difference from the 1984 classification is that, instead of only proposing two major methods of calorimetry (compensation of the thermal effects and measurement of the temperature differences, respectively) there are now three. This is obtained by splitting the second one into calorimeters that measure a heat-accumulation (including the adiabatic and the isoperibol calorimeters) and calorimeters that measure a heat-flow. 

Most unfortunately, when applied to existing calorimeters, this classification is unbalanced, since one group (N 7) is void, whereas another one (N" 5) embraces more than 60% of the real calorimeters, Le. all isoperibol and most conduction calorimeters. 

Because of the profusion of calorimeter designs there is no agreed system of classification. Hemminger and Hohne have suggested a method based on three criteria ... 

Classification of calorimeters Methods of determination of heat effects... 

Various classifications of calorimeters have been presented [75-83]. The classification given here [84] is based on the assumption that the calorimeter is a dynamic object in which heat is generated. Calorimeters are graded by applying the criteria of the temperature conditions under which the measurement was made. As an initial basis for further considerations, the Fourier - Kirchhoff equation (Eq. (1.29)) has been used in the following form ... 

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