Thermocouple Construction

Some other applications of technetium are the following [283] use of some of its alloys as superconductors with high critical temperature, use of 99Tc in high-temperature thermocouples, construction of basic anticorrosive covers for nuclear reactors, etc. 

Turbine inlet temperature. Thermocouple is constructed of platinum-platinum rhodium with the junction enclosed with ceramic insulation. Typically, 9-12 units are required at this stage. 

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. 

Muffle furnaces. An electrically heated furnace of muffle form should be available in every well-equipped laboratory. The maximum temperature should be about 1200 °C. If possible, a thermocouple and indicating pyrometer should be provided otherwise the ammeter in the circuit should be calibrated, and a chart constructed showing ammeter and corresponding temperature readings. Gas-heated muffle furnaces are marketed these may give temperatures up to about 1200 °C. 

Calvet and Guillaud (S3) noted in 1965 that in order to increase the sensitivity of a heat-flow microcalorimeter, thermoelectric elements with a high factor of merit must be used. (The factor of merit / is defined by the relation / = e2/pc, where e is the thermoelectric power of the element, p its electrical resistivity, and c its thermal conductivity.) They remarked that the factor of merit of thermoelements constructed with semiconductors (doped bismuth tellurides usually) is approximately 19 times greater than the factor of merit of chromel-to-constantan thermocouples. They described a Calvet-type microcalorimeter in which 195 semiconducting thermoelements were used instead of the usual thermoelectric pile. 

A TMA analyser will need to measure accurately both the temperature of the sample, and very small movements of a probe in contact with the surface of the sample. A typical analyser, as illustrated in Figure 11.20(a) and (b), uses a quartz probe containing a thermocouple for temperature measurement, and is coupled to the core of a linear variable differential transformer (LVDT). Small movements at the sample surface are transmitted to the core of the LVDT and converted into an electrical signal. In this way samples ranging from a few microns to centimetre thicknesses may be studied with sensitivity to movements of a few microns. For studying different mechanical properties the detailed construction of the probe will vary as is illustrated in Figure 11.20(c). 

As far as the equipment is concerned, the miniaturization of many components has lead to construction of smaller apparatus without sacrificing the accuracy, efficiency or versatility. Of importance for this development was also the availability of better and new construction materials, e.g. for measuring heads, thermocouples, furnaces etc. 

Platinum metal and its alloys have numerous applications. As a precious metal it is used extensively in jewelry. Other important applications include construction of laboratory crucibles and high temperature electric furnaces in instruments as thermocouple elements as wire for electrical contacts as electrodes in dentistry in cigarette lighters and for coating missile and jet engine parts. 

Two alloys containing tungsten are commercially available. The first, containing about 3 wt % rhenium, is used for heating filaments. The rhenium contributes improved resistance to thermal and mechanical shock. The second alloy contains about 25 wt % rhenium. This latter alloy is sold as sheet, rod, and heavy wire and may be fabricated for various uses. An important use of these rhenium alloys is in the construction of thermocouples. Various combinations, 3 wt % Re—97 wt % W, or 25 wt % Re—75 wt % W, are useful for measurement of temperatures to 2500°C (see Temperaturemeasurement). 

Twin differential microcalorimeters have been described by Berghausen el al. (S), by Hackerman (8), and by Whalen and Johnson (9). Hacker-man employs thermistors, whereas the other two are based on thermocouples and in addition are run adiabatically. These calorimeters appear to have about 10 times the sensitivity of simpler designs, but for many purposes the large additional diflSculties in design, construction, and operation do not seem to be warranted. Berghausen and coworkers, however, have shown that they can estimate slow heat evolutions, after the first few minutes, due to surface reactions. 

The temperature of the furnace is measured by a thermocouple which can be moved to various positions in the thermocouple well. The catalyst temperature should be maintained at 325-345°, although it may be as low as 320° at the ends of the catalyst zone, depending unon the construction of the furnace. 

The construction of DTA apparatus is simple and consists of a furnace, differential thermocouple, temperature thermocouple, specimen holders, temperature programmer and recorder. The schematic of a typical DTA apparatus is shown in Figure 3.5. 

The column of the pilot plant is 7.5 feet in diameter and is 18 feet above the slurry distributor. It is made in sections, so that different slurry distributors may be tested and the height may be 6, 12, or 18 feet. The bottom section is constructed of Monel and the two upper sections are coated to reduce ice adhesion. The column is instrumented with thermocouples and pressure taps at 2-foot intervals. 

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