Thermocouple series connection

This instrument was designed to yield information intermediate between the ARC and the DSC. A sample of 0.2-0.5 g is loaded into a tube-like container and placed into the device (larger sample sizes may be used at slower scan rates). A thermocouple is connected to the outside of the tube and the cell is fitted with a pressure transducer. A similar, empty cell in the same oven with thermocouple serves as a thermal reference. The oven is heated at a slow, linear rate (0.5 to 1 °C/min), and the pressure and differential thermal data are collected. The data are presented in a fashion similar to DSC - Heat Rate (mW) vs. Temperature (°C). The thermal data are enthalpically calibrated by means of a series of standards (cahbration at high heat rates may be non-linear). Detection of thermal events approaches the sensitivity of the ARC. 

When several thermocouples are connected to each other in series, then a multijunction thermocouple, or thermopile (e.g., Tian -Calvet thermopile), is obtained, with a much higher potential difference about 500 thermocouples may be combined to form a single thermopile. 

A common improvement to the simple system described above is to use multiple thermocouple pairs connected in series. This increases the signal roughly in propor-... 

The sensitivity of a thermocouple is related to the voltage generated. The voltage is greater for a larger temperature differential it is 5.269 mV for a Type J thermocouple measuring a temperature of 100 and approximately double that at 200 °C. To increase the sensitivity, thermocouples may be connected in series and this series connection is referred to as a thermopile. 

Thermoelectric Effect. In a circuit consisting of two different conductors, usually metals, and the junctions between them, preferential heating of one junction will generate a voltage which is a measure of the temperature difference. This thermoelectric effect is employed in laboratories in the form of thermocouples as a precise measurement of temperature. When the temperature difference arises from the absorption of radiation at one junction, the device is known as a radiation thermocouple. In order to increase the signal voltage, radiation thermocouples are connected in series to form a radiation thermopile. 

Low power is less than a watt, The first miniature thermoelectric generator fabricated using integrated circuit technology is shown in Figure 22. Altemate n- and p-type active thermoelements are ion implanted into an undoped substrate. Several hundred thermocouples are connected electrically in series and occupy an area approximately 25mm square. 

Thermal detectors measure the temperature difference generated within the detector by the heating from the laser radiation. Modem commercial devices consist of a metal disk, connected to a heat sink at its edge, and an array of thermocouples located across the disk such detectors are known as thermopiles. The thermocouples are connected in series to produce a voltage proportional to the temperature difference from the disk centre to the edge. 

As can be seen from their principle of operation (Figure 2.2), thermocouples are particularly suitable for the measurement of temperature differences, there being no need for a thermostat for the reference junction. Series connection into thermopiles, comprising up to 1000 thermocouples, permits the rapid measurement of temperature differences of 10 K, which is not achieved by other measuring techniques. Thus, thermocouples constitute ideal measuring instm-ments for the determination and continuous monitoring or control of minor differences of temperature. They are consequently used in many calorimeters. At temperatures above 1300 K, thermocouples surpass resistance thermometers for absolute temperature measurements because the precision of the latter instruments deteriorates rapidly in this range. The measurement uncertainty is on the order of 1% of the absolute temperature. 

Thermocouples may be joined in series. The series connection, in which the output is the arithmetic sum of the emfs of the individual thermocouples, may be used to obtain greater measurement sensitivity and accuracy. A series-connected thermocouple assembly is generally referred to as a thermopile and is used primarily in measuring small temperature differences. A schematic diagram of a series-connected thermocouple is shown in Fig. M-4. 

The ARC is controlled by its own hardwired control module. The temperature is monitored by a set of seven thermocouples connected in series which measure the difference between the temperature of the sample and that of its surroundings. The temperature is maintained by heaters which receive their inputs from the control module. A pressure transducer is attached to the sample container, giving both an analog readout on a pressure gauge and a digital readout on the control module panel. It should be noted that pressure is monitored but it is not part of the control loop. 

As all the thermocouples are identical, xe /X is constant. Therefore, the thermoelectric force of the pile formed by the n thermocouples connected in series is given by... 

Notice that two thermocouples can be differentially connected, for instance in a differential thermal analysis apparatus (see Fig. 2.39), in order to be able to measure at the same time the specimen temperature and the temperature difference in comparison to a reference sample. Several thermocouples, moreover, may be connected in series to form a thermopile, which is a device with an increased sensitivity relative to a simple couple. 

Several thermocouples may be connected in series in order to increase the sensitivity of the instrument. This arrangement is called a thermopile and, for n identical thermocouples, gives an output which is n times greater than that from a single thermocouple. Thermopiles are frequently used to detect and measure heat radiation (Section 6.4.2). In this case the surface of the detector is blackened to maximise the absorption of the incoming radiation. 

Total Radiation Pyrometers In total radiation pyrometers, the thermal radiation is detected over a large range of wavelengths from the object at high temperature. The detector is normally a thermopile, which is built by connecting several thermocouples in series to increase the temperature measurement range. The pyrometer is calibrated for black bodies, so the indicated temperature Tp should be converted for non-black body temperature. 

Procedure. The furnace should be turned on the day before measurements are to be made so that it will achieve a steady temperature. Set the power supply to a predetermined voltage appropriate for the desired reaction temperature. The heating current should be measured with a series ammeter and recorded periodically. At the start of the experiment, place the reference thermocouple junction in a Dewar flask filled with ice and distilled water (see Chapter XVII) and connect the thermoconple to a potentiometer or digital voltmeter (see Chapter XVI). Measure the thermocouple emf and check to see if the furnace is at the proper... 

The pressure of the system was controlled to within 0.5 mm Hg for each series by a Cartesian diver manostat connected to a positive air leak, a vacuum pump, and a surge volume of 12 liters. An absolute mercury-in-glass manometer with a 0.1 mm sliding vernier was used to measure the pressure. Temperatures were monitored via two copper-constantan thermocouples one was located just above the liquid surface and the other just below it. Maximum differences of 0.4°C were detected, but an average of the two readings was reported to 0.2°C. Glacial acetic acid and acetone, both meeting ACS specifications, were used. 

Differential thermal analysis. The phenomena of adsorption and desorption of water and the loss of physically bound water have been investigated by differential thermal analysis (112,151a). In this method, a small amount of the sample to be examined and a sample of inert material (e.g., calcined alumina) are placed in separate wells in a nickel block. A thermocouple junction is placed in each of the samples and the thermocouple leads are connected in series but with opposing e.m.f. s. The nickel... 

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