Thermocouples amplification

This instrument utilises a silver block chamber with an external heater. The chamber contains a constantan disc with raised platforms for the sample and reference containers. The temperature difference between sample and reference is monitored by area thermocouples formed by the constantan disc and chromel wafers under the platforms. Amplification and electronic compensation of the differential temperature signal provides a linear calorimetric response over a wide temperature range. The theory of this instrument is discussed by Lee and Levy 6). Other available examples of... 

The home-made heat-flow calorimeter used consisted of a high vacuum line for adsorption measurements applying the volumetric method. This equipment comprised of a Pyrex glass, vacuum system including a sample holder, a dead volume, a dose volume, a U-tube manometer, and a thermostat (Figure 6.3). In the sample holder, the adsorbent (thermostated with 0.1% of temperature fluctuation) is in contact with a chromel-alumel thermocouple included in an amplifier circuit (amplification factor 10), and connected with an x-y plotter [3,31,34,49], The calibration of the calorimeter, that is, the determination of the constant, k, was performed using the data reported in the literature for the adsorption of NH3 at 300 K in a Na-X zeolite [51]. 

Generally the y-axis is left as an amplification of differential thermocouple voltage, showing exothermic and endothermic trends, where no effort is made to convert the abscissa values into temperatures. 

Since some thermocouple signals were less than 3 meticulous care was taken to shield the leads and maintain electrical terminals at uniform temperatures. Noise level for the amplified thermocouple signals was far below 1 /xv and the amplifier drift during a t5rpical one-day operation was usually less than 1 IJLV. Calibration signals of 1/xvat the amplifier inputs were very discernable on the recorders when high amplification was used. 

The explanation of this phenomenon, as with many other apparatus-based developments, lies in the refinement in measurement techniques made possible by modem electronics. As a rule, heat cannot be measured directly, but must be determined instead on the basis of a temperature change in the system under investigation. Very accurate classical thermometers tend to be very slow measuring devices. Resistance thermometers and thermocouples respond much more rapidly, but they require electronic amplification, and amplifiers with the required precision have become available only in recent decades. 

Give the thermocouple design parameters for a DTA working between 1000 and 2000 K. Assume your recorder can resolve, after amplification, 0.001 mV in temperature difference and 0.05 mV in temperature. 

Thermocouples, also called thermopiles, in which several couples are combined electrically, played an important role as infrared detectors in the last decades of the nineteenth and the first half of the twentieth century. Thermocouples provide low impedance energy converters, well-suited to drive sensitive galvanometers. Before the use of electronic amplification this was of great value. Even today many laboratory spectrometers still use thermopiles, but, in nearly all cases, with electronic amplifiers and recorders. An excellent discussion of various constmction techniques and test results of thermoelectric detectors is given by Smith et al., (1957). 

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