Temperature sensors, sample container

Figure 8.1 Scheme of a Dewar vessel isoperibol reaction-solution calorimeter. A ampule containing the sample B ampule breaking system C calorimeter head D temperature sensor E stirrer F electrical resistance G Dewar vessel H plunger of the ampule breaking system I, J inlets K plug connecting the calibration resistance to the calibration circuit. 

The basic components of a DTA apparatus are a temperature-controlled furnace containing sample and reference cells and a pair of matched temperature sensors connected to recording apparatus, as indicated in Figure 1.1. The temperature sensors (usually thermocouples) are in contact with the sample and reference or their containers, and the output is amplified and recorded. DTA data may be plotted as a function of sample temperature, reference temperature (as is usually the case), or time. In both DTA and DSC, the measurement relies on the occurrence of a temperature difference between a sample and reference (AT) as a result of the thermal event in question. 

Figure 1. Refrigeration system to keep samples at-13 "C. A) external view with the temperature sensor B) samples container and Peltier system.

One system, which combines calorimetric measurements with monitoring of temperature and pressure, encloses the sample in an inert cell, to which pressure and temperature sensors are attached. The cell is then enclosed in a bomb-like container and heated at a slow rate, between 0.5 and 2°C min by the integral furnace. Any rapid rise in pressure or in temperature during the heating indicates that the sample is undergoing a degradation, which may pose a risk. ... 

In the classic and Boersma DTA systems, both sample and reference are heated by a single heat source. Temperatures are measured by sensors embedded in the sample and reference materials (classic) or attached to the pans that contain the materials (Boersma). A plot is made, usually by means of a recorder, of the temperature difference AT = Tj - Tq between sample and reference as ordinate against time as abscissa. The magnitude of AT at a given time is proportional to (a) the enthalpy change, (b) the heat capacities, and (c) the total thermal resistance to heat flow, R. High sensitivity requires a large value of R, but unfortunately the value of R depends on the nature of the sample, the way it is packed into the sample pan, and the extent of thermal contact between sample pan and holder also, R varies with temperature. Attachment of the temperature sensors to the pans in the Boersma method is made in an attempt to reduce the effect of variations in the thermal resistance caused by the sample itself. 

In the absence of carbon monoxide in the sample cell, the two sensor chambers are heated equally by IR radiation from the two sources. If the sample contains carbon monoxide, however, the right-hand beam is attenuated somewhat and the corresponding sensor chamber becomes cooler with respect to its reference counterpart. As a result, the diaphragm moves to the right and the capacitance of the capacitor changes. This change in capacitance is sensed by the amplifier system. The amplifier output drives a servomotor that moves the beam attenuator into the reference beam until the two compartments are again at the same temperature. The instrument thus operates as a null balance device. 

The techniques of DTA and DSC are not identical. Let us consider the essential difference between them. In DTA, the heat changes within the material are monitored by measuring the difference in temperature (AT) between a sample and an inert reference. In a classical DTA equipment both the sample (S) and the reference (R) are heated by the same furnace (Fig. la). The temperature sensors are inserted directly into the sample and reference, while in a modification of classical DTA, called Boersma DTA (Fig. lb) they are in contact with the container but not with the materials under test. The temperature difference between the sample and the inert reference is recorded as a function of temperature (T) or time (t). In DSC, the sample and the reference materials are provided with their own separate furnaces as well as with their own separate temperature (T) sensors. In DSC, the sample and reference are maintained at identical temperatures by controlling the rate at which heat is transferred to them (Fig. Ic). 

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