Calorimeter cooling methods

A number of other thermodynamic properties of adamantane and diamantane in different phases are reported by Kabo et al. [5]. They include (1) standard molar thermodynamic functions for adamantane in the ideal gas state as calculated by statistical thermodynamics methods and (2) temperature dependence of the heat capacities of adamantane in the condensed state between 340 and 600 K as measured by a scanning calorimeter and reported here in Fig. 8. According to this figure, liquid adamantane converts to a solid plastic with simple cubic crystal structure upon freezing. After further cooling it moves into another solid state, an fee crystalline phase. 

The efficiency of this method has been demonstrated for several types of heat-flow calorimeters. The rather long time constant of a Calvet-type calorimeter (200 sec), for instance, is decreased to 10 sec, when exact Peltier cooling is used (61). Similarly, the time constant of calorimeters... 

Heat contents can be measured accurately by a number of techniques based on a drop method. This involves heating the sample to a high temperature and dropping it directly into a calorimeter held at a lower temperature. The calorimeter then measures the heat evolved while the sample cools to the temperature of the calorimeter. The temperature at which the sample is initially heated is varied and a plot of Ht — 298.15 vs temperature is drawn (Fig. 4.1). Heat capacities can then be calculated using Eq. (3.9). A popular calorimeter for this is the diphenyl ether calorimeter (Hultgren et al. 1958, Davies and Pritchard 1972) but its temperature range is limited below about 1050 K. 

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. 

Consider the flow-calorimeter method for measurement of heats of combustion of fuel gases. The fuel is mixed with air at room temperature and ignited. Combustion takes place in a chamber surrounded by a cooling jacket through which water flows. In addition there is a long water-jacketed section in which the products of combustion are cooled to the temperature of the reactants. Whatever the details of this steady-flow process, the overall energy balance [Eq. (2.10)] reduces to... 

This method measures the total heat evolved by the vapour condensing to liquid and the liquid cooling from the condensing point to the temperature of the calorimeter, arid the latter part must be subtracted. 

The graphical method mentioned on p. 16 is inadequate for very accurate determinations. There are essentially three methods which have been devised with a view to avoiding this error. Rumford first determines roughly the rise in temperature Lt produced by the reaction, and cools the calorimeter... 

The slow heat release appliances are operated in an insulated and air cooled calorimeter room (Fig. 3), where the heat output can be measured directly. In parallel, the efficiency is determined indirectly with measurement of flue gas temperature and concentrations of carbon dioxide (CO3) and carbon monoxide (CO) in the flue gas, in accordance with the CEN/prEN flue loss method. 

Adiabatic calorimeter. With the adiabatic calorimeter, exchange of heat between the calorimetric vessel and the cover is suppressed. This happens so that the temperatures of the vessel and the cover are maintained at almost the same temperature. The condition (Tc-Tfi) = 0 can be attained at constant cover temperature by heating or cooling the calorimetric vessel using an internal heater or heat sink placed inside the calorimetric vessel. This compensation method is suitable for endothermic processes. For the adiabatic method, the characteristic feature is not only the equality of temperatures of the calorimetric vessel and of the cover, but also their changing value - the measurement proceeds at dynamic conditions, where the temperature of the calorimetric cover follows the temperature of the calorimetric vessel. 

In the high-temperature region, the main method of measurement is the drop calorimetry, where the sample is heated to the chosen temperature outside the calorimeter in a furnace and the heat capacity is calculated from the temperature dependence of the enthalpy changes measured after dropping the sample into the calorimeter. The application of this technique affects, however, the behavior of the sample heated in the furnace (decomposition, reaction with the crucible, etc. should be avoided) as well as at the cooling from the furnace temperature to that of the calorimeter. Sometimes the sample does not complete its phase transition at cooling (e.g. at the temperature of fusion, a part of the sample crystallizes while the other part becomes glassy). In such a case, the drop calorimeter must be supplemented by a solution calorimeter in order to get the enthalpy differences of all the samples to a defined reference state. 

The solution of the sample in a 2 1 mixture of concentrated hydrofluoric and nitric acids at Tref = 298 K was chosen as the reference state. The relative enthalpy, 7/rei(7m), was measured by indirect method of double calorimetry. This procedure enables us to determine Hiei(Tm) as the sum of enthalpy increase measured during the cooling of the system in a drop calorimeter (Acooi and during its dissolution in a solution calorimeter (Asoi//). Equation (4.34) can thus be written in the form... 

Standard Method for Measuring Heat Flux Using A Water-Cooled Calorimeter, ASTM Std. E-422, Annual Standard of ASTM, Pt. 41,1981. 

In this method, a cooled solid is used as a calorimeter. It is commonly internally cooled with water or glycol. The flow rate and the temperature rise of the coolant are measured after steady-state conditions have been reached. The heat flux to the solid is calculated from... 

American Society for Testing and Materials Standard E 422 - 83, Standard Method for Measuring Heat Flux Using a Water-Cooled Calorimeter, Philadelphia, PA, 1983. 

The heat capacities of Th, U, Be, Na and a Pb-Bi alloy were determined from 323 to 973 K by two methods, the precise details of which are not clear. For thorium (99.81% purity), two measurements were made at 323 and 373 K by comparing the cooling rate with an unspecified standard, and ten measnrements from 523 to 973 K in an electron radiation calorimeter. The uncertainties in the valnes were estimated by the author to be 1.5-2%. 

E 422 (1994) Test method for measuring heat flux using a water-cooled calorimeter E 472 (1991) Reporting thermoanalytical data E 473 (1999) Terminology relating to thermal analysis E 476 (1993) Standard test method for thermal instability of confined condensed phase systems (confinement test)... 

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