Thermocouples metal-shielded

Some electric tube furnaces incorporate a p3n ometer, which is not required in any of the experiments described in this manual, though Experiments 13 and 14 can be run by using an electric tube furnace at controlled temperature instead of an oil bath if so desired. A pyrometer may easily be improvised with iron and constantan wires twisted together and spot-welded to provide the junctions. The hot junction is placed against the outside of the combustion tube in the center of the heated portion and bound in position by asbestos tape, which also serves as insulation. The cold junction is kept in ice and water, and the electromotive force is measured with a millivoltmeter. With the cold junction at 0°C and the hot junction at 200°C, the electromotive force of the iron-constantan couple is given in tables as 10.77 millivolts. For accurate work the couple used should be calibrated and to assure uniform temperature distribution and electrical shielding, the combustion-tube and thermocouple wires should be encased in a tubular metal shield that fits inside the furnace and is groimded. 

Fiber-optic thermometers can be applied up to 300°C, but are too fragile for real industrial applications. In turn, optical pyrometers and thermocouples can be used, but pyrometers measure only surface temperatures which in fact can be lower than the interior temperatures in reaction mixtures. Application of thermocouples which in case of microwaves are metallic probes, screened against microwaves, can result in arcing between the thermocouple shield and the cavity walls leading to failures in thermocouple performance. 

Figure 5. Assembly drawing (a) and general view before assembly (b) of M-H container 1 -case, 2 - heat exchanger, 3 - filter with gas input/output fitting, 4 - point for metal-hydride loading, 5 - thermocouple shield, 6 - position for electric heater, - welded seams.

Several DTA instruments have been described by Barrall et al. (94, 95). A DTA calorimeter cell in which the AT-sensing thermocouples are attached to the sample container is shown in Figure 6.35. In this apparatus, copper-constantan thermocouples are soldered to a 4-mm-OD copper cup fitted with a copper lid. The thermocouples and sample cups are supported on ceramic insulator tubes which are attached to a metal base. All the cups were heated by thermal radiation received from the blackened copper radiation shield this prevented radiation hot spots due to furnace windings. The entire DTA cell was enclosed by a glass bell jar which provided a controlled atmosphere from reduced pressures to about 2 atm. 

At the desired measurement location, gases are extracted from the flame through a sampling tube in the suction pyrometer. A thermocouple is positioned just inside this tube, typically made of a ceramic or high temperature metal, which acts as a radiation shield. In some designs, multiple shields are used (see Figure 5.6). Examples of suction pyrometers are shown in Figure 5.7. 

A ideal Knudsen cell holds the source in an isothermal box with a relatively small exit orfice. This can be implemented by placing the crucible and filament inside a radiation shield without making contact between the three elements. The filament heats the crucible radiatively (there is no gas or direct mechanical connection to conduct heat), and this multiple reflection and conduction inside the crucible homogenizes the temperature along the crucible length. This configuration also increases the temperature that can be achieved for a given power input, since less heat is lost to the environment via radiation. The maximum temperature which can be achieved by a K-cell is limited by the metal elements used to fabricate the cell and the thermocouples, and is typically 1200-1400°C. 

Figure 1.6.1 Electrolysis cell schematic. 1 Alumina shields 2 metallic cap 3 gas purge outlet 4 thermocouple 5 gas purge inlet 6 alumina feeding inlet 7 alumina tube 8 crucible 9 electrolyte 10 anode 11 cathodes 12 stainless steel shell...

Much more common are the oven sources known as effusion cells. A typical example of such a source is shown in Figure 11.3. These consist of a crucible made of an inert material (usually boron nitride) containing the evaporant and surrounded by a refractory metal heater coil. This assembly is enclosed in a heat shield to reduce the power needed to operate the source and reduce the heat load on surrounding fixtures. The heat shield also increases the thermal mass of the oven so that it operates in a more stable manner over long periods. The temperature of the crucible is measured by a thermocouple contained within the heat shield and in conact with the base of the crucible. Because the environment within the heat shield is, to a good approximation, a black body, the thermocouple reads the outside temperature of the crucible quite reliably, usually to within 0.5°C. 

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