Calvet type DSC

However another calibration technique is available when the Calvet type DSC is used. The principle is to apply a known amount of power in a dedicated calibration vessel. To reach this target, a resistance is embedded in the crucible. A known current I is delivered and the corresponding tension U measured, providing the power P = UI that is applied. The corresponding Joule effect provides a DSC exothermic deviation in microvolt (Fig. 2.8). Such an electrical calibration is very interesting as it can apply at any temperature, even at constant temperature. This will be more detailed in paragraph 5 for the calorimetric techniques. 

Two other types of crucibles, that can be only used with a Calvet type DSC are also described as they find specific applicafions for cafalytic investigations. 

The quartz tube is introduced in the Calvet type DSC (set in the vertical position). A fritted glass substrate is located in the middle of the tube to receive the powdered sample in order to be surrounded by the calorimetric detector. Tight connections are adjusted at both ends of the tubes for the gas inlet and outlet. 

After introducing the sample, the cmcible (made of incoloy) is tightly closed with a screwable stopper, then introduced in the tube of the Calvet type DSC (Fig. 2.14). A pressure of gas is applied through an external high pressure panel. 

The Sensys TG-DSC is based on the Calvet type DSC (Fig. 2.36) used in the vertical position [30]. On top of the DSC, is adjusted a symmetrical balance corresponding to the principle described on Fig. 2.29. The crucibles containing the sample and the inert material are hung on each side of the balance and introduced in the calorimetric zone of the DSC without touching the walls. In such a situation, the crucibles are fully surrounded by the fluxmeters, providing an accurate DSC determination. In the same time, the symmetrical balance allows a compensation of the buoyancy effect resulting on a very high sensitive TG determination. 

Classical DTA has been developed into heat-flux DSC by the application of multiple sensors (e.g., a Calvet-type arrangement) or with a controlled heat... 

A microcaloiimeter of Calvet type or a Differential Scanning Calorimeter (DSC) apparatus can be used to measure the heat flow produced or consumed by the reaction at any time. This method is very practical, particularly for gas-sohd reactions, provided that the experimenter controls the initiation of the reaction, but this method is more difficult for decompositions or reactions between sohds. 

The Calvet calorimeters have their roots in the work of Tian [26] and the later modifications by Calvet [7]. Presently this calorimeter type is commercially available from Setaram and the models C80 and BT215 are particularly well adapted for safety studies. It is a differential calorimeter that may be operated isothermally or in the scanning mode as a DSC in the temperature range from room temperature to 300°C for the C80 and -196 to 275°C for the BT215. They show a high... 

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

Figure 1 Different types of differential scanning calorimeters, (a) Three-dimensional cylindrical calorimeter (Tian-Calvet). (b) Three-dimensional calorimeter with power compensation, (c) Two-dimensional plate-like calorimeter, (d) Scheme of a twin-chip sensor (Mettler Toledo Flash 1 DSC ) for fast scanning calorimetry.

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