Thermopile design

In the following section, textile auxiliary wall conception with weaving process and thermopile design with two different methods will be described. 

Figure 12. Schematic design of thermocouple or thermopile signal read-out.

As a second transducer thermopile sensors are well suited for performing differential measurements because of their underlying physical principle. With conventional design there are some difficulties related to size, sensitivity and mechanical stability of thermocouples. 

Most calorimeters described above rely on a measurement of temperature (heat-Flow Calorimeters). The Tian182-Calvet183 calorimeters (some with a twin calorimeter design) use a thermopile (instead of a thermocouple) to measure heat flow directly (Fig. 11.78). 

In contrast, DSC, designed in 1960 by Watson184 and O Neill,185 is a newer, more quantitative technique that does measure Ts and TR, but also measures very precisely the electrical energy used by separate heaters under either pan to make Ts = TR (this is power-compensated DSC, useable below 650° C). The power input into S minus the power input into R is plotted against Tr. High-temperature DSC (useful for TR > 1000°C) measures the heat fluxes by Tian-Calvet thermopiles rather than the electrical power, as a function of Tr. In a heat-flux DSC, both pans sit on a small slab of material with a calibrated heat resistance. The temperature of the calorimeter is raised linearly with time. A schematic DSC curve is shown in Fig. 11.80. 

A batch microcalorimetric experiment, very similar to the one just described, is possible with a diathermal heat flowmeter type of microcalorimeter, which is less versatile than the Tian-Calvet microcalorimeter (especially in its temperature range and ultimate sensitivity), but of a simpler design. In the Montcal microcalorimeter (Partyka et al., 1989), the thermopile with up to 1000 thermocouples is replaced by a few thermistors. 

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. 

The reference junction, which is usually housed in Ihe same chamber as the active junclion, is designed to have a relatively large heal capacity and is carefully shielded from the incident radiation. Because the analyte signal is chopped, only the difference in temperature between the two junctions is important therefore, the reference junction does not need to be maintained at constant temperature. To enhance sensitivity, several thermocouples may be connected in series to fab ricale what is called a thermopile. 

The parallel circular plate (PCP) and compensated ionization chambers and the fission chamber are of ORNL design and manufacture. The air wall and water monitor chambers are made by General Electric Company, and the. boron thermopiles are made by Nuclear Instrument and Chemical Corporation. 

Figure 16.23 (continued) A 2D plate detector versus 3D Calvet detector designs, (e) A 3D heat flow detector using Peltier elements for the Setaram pDSC7. (f) Detail of the Peltier thermopile. (Courtesy of Setaram Instrumentation, SA, Caluire, France, www. setaram.com. Used with permission.)... 

The recent development of thin film techniques has enabled cheap thermopiles to be designed which can be fabricated as complex arrays with good reliability. By using antimony and bismuth for the sensitive elements devices can be produced which have some of the advantages of the metal wire thermopiles but with a higher sensitivity. Table 3.2 summarizes the characteristics of the different types of modern thermopiles. Figure 3.3 which shows the performance of a number of thermal detectors operating at room temperature includes two thermopiles. The performance obtained with the spectroscopic thermopile... 

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