Power compensated system

DSC (Differential Scanning Calorimetry) has also been used in cement science investigations to some extent. It is based on a power compensated system. In this technique the reference and the sample imder investigation are maintained at a constant temperature throughout the heating schedule. The heat energy required to maintain the isothermal condition is recorded as a function of time or temperature. There are some similarities between DTA and DSC ineluding the appearance of thermal curves. DSC can be used to measure the heat capacities of materials. DSC measures directly the heat effects involved in a reaction. 

Two different conventions exist for the display of the heat flow curve one shows en-dotherms in the downward direction, the other upward. The operator has a choice with most software packages. Traditionally, with heat flux systems (Section 1.7.2) endotherms are shown as going down, since endothermic transitions result in a negative temperature differential, whilst with power compensation systems (Section 1.7.1) they are shown as going up since with this principle endothermic transitions result in an increase in power supplied to the sample. In this chapter data are shown with endotherms up. 

This variant on DSC involves recording the heat flow cure of a sample as it is heated very rapidly (e.g. at 500°C/min), as opposed to a standard 20°C/min heating ramp. This technique can only be used with power compensation systems, as an analysis chamber having a low thermal mass is essential and the ability to record data very quickly is another prerequisite. Although the technique of HyperDSC is relatively new, there are a number of publications available which provide further information on its use and application [103,104]. 

Fig. 9 Schematic diagrams illustrating the sample cell configurations for (a) power-compensation and (b) heat-flux modes of DSC detection. Each cell system is contained in the furnace assembly, and the differential heat flow between sample and reference is monitored as the experimental observable and ultimately is plotted as a function of the system temperature.

The equipment for power-compensated DSC involves two parallel temperature measurement systems. Sample (ca. 50 mg) and reference in... 

The main problem, the height-dependent differential pressure between the catho-lyte gap and oxygen across the ODC, could be solved with the pressure compensation system based on oxygen supply via gas pockets [1] (see Fig. 4.2). The first successful piloting was done in October 1995 on the basis of Bayer engineering and construction. In a four-gas-pocket element with each electrode segment of 180 x 180 mm2 individually supplied with power, the proper function of pressure compensation could be proven for a total height of 90 cm. 

One method proposed for estimating the cost of fuel cell power plants is to calculate distributive (bulk) costs as a function of the equipment cost using established factors based on conventional generating technologies. When applied in such a way as to compensate for the differences associated with a fuel cell plant, this approach can yield reasonable results. NETL has elected, based on the international prominence of the Association for the Advancement of Cost Engineering (AACE), to utilize this approach in estimating the costs for fuel cell/turbine power plant systems currently under study. 

Fig. 1 Schematic diagrams of the (A) differential thermal analysis (DTA) (B) power-compensated DSC and (C) heat-flux DSC cells. (From Ref adapted from DuPont Instruments Systems Brochure.)...

Manufacturers use two methods of measurements. In the first method called heat flux DSC, the instrument measures this temperature difference (DTA). Through calibration, this temperature difference is transformed into a heat flow, dq/dt. Therefore, there is a thermal factor that may vary with temperature. In the second method, called power compensation DSC, two individual heaters are used in order to monitor the individual heating rates of the two individual ovens. A system controls the temperature difference between sample and reference. If any temperature difference is detected, the individual heatings are corrected in such a way that the temperature is kept the same in both pans. That is, when an endothermic or exothermic process occurs, the instrument delivers the compensation energy in order to maintain equal temperature in both pans. 

DSC works on a similar principle but has several advantages over DTA, not least of which being the ability to easily measure the energy associated with the transition as well as the temperature at which it occurs. There are two main forms of DSC. power compensation and heat fiux. Power compensation DSC involves the use of two furnaces (rather than the one used for DTA), one placed under the. sample and the other under the reference (Fig. 1B). The system operates on the basis of keeping the sample and reference at the same temperature. This therefore means that energy must be supplied to the reference in order to make it follow the predetermined temperature programme and... 

Fig. 3 Schematic diagram of a power-compensated DSC system (A) and a typical DSC curve (B).

DSC is a precise method of measuring the endothermic and exothermic behaviors of sample materials. Unlike the earlier version of the thermal analyzer, the differential thermometric analyzer (DTA) measures the temperature difference between two cells heating in the same furnace. The power-compensated DSC uses two independent furnaces, one for the sample and one for the reference. When an exothermic or endothermic change occurs in the sample materials, energy is applied to or removed from one or both furnaces to compensate for energy changes in the sample. This means that the system directly measures energy flows to or from a sample at all times. 

Differenhal scanning calorimetry (DSC) conshtutes one of the most widely used techniques for the study of polymers, parhcularly those systems that crystallize. Although the term DSC is used in conjunchon with many different instruments, fundamentally, these can be divided into two categories heat flow instruments based upon differenhal thermal analysis (DTA) and those which are true power compensated instruments. 

In contrast to the heat flux approach, in these instruments it is the sample temperature that is programmed, and this should, ideally, conform exactly to the selected temperature profile. In principle, therefore, power compensation instruments provide for a better-defined experiment. In practice, however, the control system requires a finite temperature difference in order to operate and has a finite response time, so perfect control is never achieved. 

Two types of systems are commonly used power compensation and heat flux DSCs. In the power compensation apparatus temperatures of the sample and the reference are controlled independently by using separate but identical furnaces. The power input to the two furnaces is adjusted to equalize the temperatures. The energy required for the temperature equalization is a measure of the enthalpy or heat capacity in the sample relative to the reference. In heat flux DSC, the sample and the reference are interconnected by a metal disk that acts as a low-resistance heat-flow path. The entire assembly is placed inside a furnace. The changes in the enthalpy or heat capacity of the sample cause a difference in its temperature compared to the reference. The resulting heat flow is small because of the thermal contact between the sample and the reference. Calibration experiments are conducted to correlate enthalpy changes with the temperature differences. In both cases, the enthalpy changes are expressed in the units of energy per unit mass. 

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