Thermocouple Operation

Thermocouples operate on the principle of thermoelectricity (Seebeck effect). When the junction of two dissimilar electronic conductors - typically two metals, two semimetals, or semiconductors - is held at temperature T (the so-called cold junction ), a measurable voltage develops between it and the hot (Tz) junction. This thermoelectric voltage (V) of the two conductors can be measured according to (3.9). 

In the thermal circuit, thermocouples operate in parallel, whereas in the electrical circuit they are in series, which is expressed by the sums... 

Thermocouples are rugged and versatile temperature sensors frequently found in industrial control systems. A thermocouple consists of a pair of dissimilar metal wires twisted or otherwise bonded at one end. The Seebeck effect is the physical phenomena which accounts for thermocouple operation, so thermocouples are known alternatively as Seebeck junctions. The potential difference (Seebeck voltage) between the fi ee ends of the wire is proportional to the difference between the temperature at the junction and the temperature at the fi ee ends. Thermocouples are available for measurement of temperature as low as —270°C and as high as 2300°C, although no single thermocouple covers this entire range. Thermocouples are identified as type B, C, D, E, G, J, K, N, R, S, or T, according to the metals used in the wire. 

Thermocouples/Thennopiles. Thermocouples operate on the basis of the Seebeck effect. If wires fabricated from two different metals or semiconductors are soldered together to form a circuit, any temperature difference that exists between the joined points leads to a measurable potential difference, the magnitude of which depends on the extent of the temperature difference and the materials involved. The use of several thermocouples connected in series thermopiles) increases the sensitivity of the sensor, but if a single thermocouple is damaged the entire sensor is affected. It is more difficult to miniaturize thermoelectric sensors than resi.stance devices, and the long-term stability of thermocouples is often not good, A typical response time is less than one second. 

A thermocouple operates on the principle that an electromotive force (EMF) is generated in a closed circuit of ... 

Type B thermocouples (Table 11.56) offer distinct advantages of improved stability, increased mechanical strength, and higher possible operating temperatures. They have the unique advantage that the reference junction potential is almost immaterial, as long as it is between 0°C and 40°C. Type B is virtually useless below 50°C because it exhibits a double-value ambiguity from 0°C to 42°C. 

Control Devices. Control devices have advanced from manual control to sophisticated computet-assisted operation. Radiation pyrometers in conjunction with thermocouples monitor furnace temperatures at several locations (see Temperature measurement). Batch tilting is usually automatically controlled. Combustion air and fuel are metered and controlled for optimum efficiency. For regeneration-type units, furnace reversal also operates on a timed program. Data acquisition and digital display of operating parameters are part of a supervisory control system. The grouping of display information at the control center is typical of modem furnaces. 

Verifying temperature is the second most important aspect of any compressor operation. As with pressure, the basic form of measurement is a simple temperature gauge. The construction of the gauges is quite varied, ranging from a bimetallic device to the filled systems. When transmis sion is involved, the sensor becomes quite simple, taking the form v)l a thermocouple or a resistance temperature detector (RTD). The monitor does the translation from the native signal to a temperature readout ()r signal proportional to temperature. 

Each pilot shall be spark operated complete with a windshield, pilot tip and pilot tube. It shall also have an igniter tube, thermocouple and remote venmri, if applicable. 

In the ARC (Figure 12-9), the sample of approximately 5 g or 4 ml is placed in a one-inch diameter metal sphere (bomb) and situated in a heated oven under adiabatic conditions. Tliese conditions are achieved by heating the chamber surrounding the bomb to the same temperature as the bomb. The thermocouple attached to the sample bomb is used to measure the sample temperature. A heat-wait-search mode of operation is used to detect an exotherm. If the temperature of the bomb increases due to an exotherm, the temperature of the surrounding chamber increases accordingly. The rate of temperature increase (selfheat rate) and bomb pressure are also tracked. Adiabatic conditions of the sample and the bomb are both maintained for self-heat rates up to 10°C/min. If the self-heat rate exceeds a predetermined value ( 0.02°C/min), an exotherm is registered. Figure 12-10 shows the temperature versus time curve of a reaction sample in the ARC test. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

The reactor is an 8-mm i.d. quartz tube located in a tube furnace. The quartz tube is packed with 20 by 30 mesh catalyst particles. The catalyst bed is positioned in the tube using quartz wool above and below the bed, with quartz chips filling the remainder of the reactor. The furnace temperature is controlled by a thermocouple inserted into the reactor tube and positioned about 3 mm above the catalyst bed. This allows operation at constant feed temperature into the reactor. 

The sample is assembled into the permeation cell after first having had its thickness accurately measured, with the sample supported by a stainless steel porous sinter (not visible in the photographs). The test cell is heated to test temperature by suitably controlled band-heaters, test temperature being measured by a thermocouple located close to the sample. Test gases are boosted to test pressure using a gas intensifier operated by pressurized air. 

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