Heat flow calibration

If the y-axis scale is to be calibrated directly then the approach used for Cp measurement (see Section 1.2.3) should be employed. Values obtained for a Cp standard are then entered against standard values. 

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Here k represents a proportionality factor, which can be found by calibration (heat flow calibration, see following discussion). 

Careful heat-flow calibrations have to be performed. Chemical calibrations present many disadvantages they rely on prior results, with no general agreement and no control of rate, and are generally available only at a single temperature. On the contrary, electrical calibrations (Joule effect) provide many advantages and they are easy to perform at any temperature [103],... 

ASTM E 968-83, Standard Practice for Heat Flow Calibration of Differential Scanning Calorimeters, 1983. 

Differential Scanning Calorimetry. DSC scans were made at 20°C min"1 on a Mettler TA300O system equipped with a DSC-30 low temperature module. Temperature calibration was done with a multiple Indium-lead-nickel standard. An indium standard was used for heat flow calibration. Thin shavings (ca. 0.5 mm thick) were cut with a razor blade from the cross-sectional edge of a plaque. These sections contained both surface and center portions. 

Calibration is a potentially complex subject as there can be cell asymmetries in terms of heat capacity, thermal resistances, and other effects such as convection and emissivity. However, here we shall confine ourselves to a simple procedure that will be effective in most cases. We can consider that calibration of an MTDSC consists of three steps (1) heat flow calibration (by calculation of a cell constant), (2) baseline calibration, and (3) heat capacity calibration (10). 

The standard DSC heat-flow calibration procedure with indium is not longer accurate enough, due to the change from a closed to an open cell system (the standard platinum cell lids are removed). Demineralised water is used, therefore, as calibration substance for the vaporisation experiments. The heat of vaporisation of demineralised water was measured six times which each of the sample cups. The average heat of vaporisation measured was compared with the known heat of vaporisation of water (43.9 kJ/mol., [20]) to calculate a correction factor for each sample cup ... 

Heat flow calibration of the apparatus is also required. This is obtained by running baseline... 

E 968 (1999) Standard practice for heat flow calibration of differential scanning calorimeters E 1131 (1998) Test method for compositional analysis by thermo gravimetry... 

Heat flow calibration of the apparatus is also required. This is obtained by running baseline and experimental traces for a material whose specific heat capacity is well known. Sapphire is the calibration material of choice since it is easily available and its specific heat capacity is accurately known. 

After baseline calibration is performed, heat flow calibration is done by melting a known quantity of a material with a well-known heat of fusion. Indium is the most often used standard. Indium is placed in the sample pan and scanned against an empty reference pan. The area of the melting peak is related to the known enthalpy of fusion by a calibration factor known as the celt constant. This procedure also calibrates the temperature axis from the known melting temperature of indium. Temperature calibration should also be performed over a wider temperature range, by measuring the melting points of several well-known standards. 

Heat flow calibration of differential scanning calorimeters ASTM E %8... 

Temperature and heat flow calibrations were performed upon heating at 10 K/min with indium (Tm = 156.6 °C) and a liquid crystal standard (+)-4-n-hexyloxyphenyl-4 -(2 -methylbutyl)-biphenyl-4-carboxylate [20] (CE-3 from T.M. Leslie, University of Alabama smectic to cholesteric transition at 78.8 °C). The temperature and heat flow are considered to be within 0.10 K and 0.20 J/g, respectively. The calibrations were checked at regular intervals during the DSC studies by performing check runs using CE-3 and indium. 

Experiments were performed in tlie SIMULAR calorimeter using the power compensation method of calorimetry (note that it can also be used in the heat flow mode). In this case, the jacket temperature was held at conditions, which always maintain a temperature difference ( 20°C) below the reactor solution. A calibration heater was used to... 

No theory can possibly take into account the arrangement of a real heat-flow calorimeter in all its details. Theoretical models of heat-flow calorimeters, which are necessarily simplified versions of the actual instruments, will therefore be used in the following calculations. It must be remarked that because of the limitations of the theory, no absolute measurements can be made with a heat-flow calorimeter, nor with any calorimeter. It is possible, however, to compare successive measurements with precision. A calorimetric study necessarily involves the calibration of the calorimeter and, upon this operation, depends the accuracy of the whole series of measurements. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

The RC1 Reaction Calorimeter is marketed by Mettler-Toledo. The heat-flow calorimetric principle used by the RC1 relies on continuous measurement of the temperature difference between the reactor contents and the heat transfer fluid in the reactor jacket. The heat transfer coefficient is obtained through calibration, using known energy input to the reactor contents. The heat trans-... 

A heat flow calorimeter and the drop calorimetric method were used by Connor, Skinner, and Virmani to investigate the thermal decomposition of Cr(CO)6 at 514 K (the calibration was made with iodine as described above) [164], The only peak observed corresponded to an endothermic process ... 

A marginal but very important application of the drop calorimetric method is that it also allows enthalpies of vaporization or sublimation [162,169] to be determined with very small samples. The procedure is similar to that described for the calibration with iodine—which indeed is a sublimation experiment. Other methods to determine vaporization or sublimation enthalpies using heat flow calorimeters have been described [170-172], Although they may provide more accurate data, the drop method is often preferred due to the simplicity of the experimental procedure and to the inexpensive additional hardware required. The drop method can also be used to measure heat capacities of solids or liquids above ambient temperature [1,173],... 

The heat flux and energy calibrations are usually performed using electrically generated heat or reference substances with well-established heat capacities (in the case of k ) or enthalpies of phase transition (in the case of kg). Because kd, and kg are complex and generally unknown functions of various parameters, such as the heating rate, the calibration experiment should be as similar as possible to the main experiment. Very detailed recommendations for a correct calibration of differential scanning calorimeters in terms of heat flow and energy have been published in the literature [254,258-260,269]. 

If, as illustrated in figure 12.6, the isothermal starting lines of the various curves do not coincide, then A< >o, A< cai, and Aheat transfer change between runs, for example, due to a variation in the purge gas flow or the fact that it is virtually impossible to relocate the crucible containing the sample exactly in the position used for the calibrant run (normally the reference crucible remains in place throughout a series of runs). Note that a similar correction should have been used in the computation of heat flow or area quantities if, in the example of figure 12.4, the isothermal baselines of the main experiment and the zero line were not coincident. 

E. Gmelin, S. M. Sarge. Temperature, Heat and Heat Flow Rate Calibration of Differential Scanning Calorimeters. Thermochim. Acta 2000, 347, 9-13. 

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