Thermocouple wires insulation

Thermocouple wire insulated by compressed magnesimn oxide powder and housed in various alloy tubes is available from the Thermo Electric Company. Another commonly used wire supplied by the Claud S. Gordon Company is insulated as follows each strand is coated with phenol formaldehyde varnish and Fiberglas-impregnated with a silicone alkyd copolymer, and the entire wire is Fiberglas-impregnated with a silicone alkyd copolymer. 

Another source of problems are the thermal losses induced by instrumenting a typical high-temperature system. Thermocouple wiring and metallic capillary tubing to pressure and flow transducers can easily double the thermal losses of the system. Therefore, nonmetallic and thermally insulating sensor leads are required at reasonable costs for reliable operation at 1200°C. 

Thermocouple wire is available commercially either as bare wire or as wire protected with a variety of insulators. The best choice of insulation for use up to 280°C is Teflon. Its resistance to abrasion, chemical reaction, solvents, and humidity is excellent, and its flexibility is good. Braided fiberglass is very good for use up to 480°C, but it has poor abrasion resistance and quite high porosity. Special braided ceramic fibers are available for use... 

Iron-Constantan thermocouples, which had been calibrated against an N.B.S.-standardized platinum resistance thermometer, measured both the sample temperature and the difference in temperature between sample and reference tubes. The thermocouple wires were embedded in magnesia and electrically insulated from their Inconel sheaths. The signal of the thermocouple in the sample tube could be determined either by a recording potentiometer or by a manual potentiometer and null meter. 

Thermocouples were fabricated from commercial thermocouple wire by stripping the insulation approximately 1 cm from the end. The two wires were then twisted together, doubled over, and soldered. After fabrication, the thermocouple was tested for electrical continuity and for temperature measurement fidelity. Thermocouples made from the same spool of wire were found to provide closely identical signals giving a relative temperature measurement error of less than 0.05 K. Thermocouples were made from 0.25 mm diameter wire to allow insertion of the thermocouple in the capillaries. Because of the fine wire, all operations described in this note require considerable care to avoid breaking the wire. 

Fluoropolymers are used to insulate wire for critical aerospace and industrial applications where chemical and thermal resistance is essential. They are also materials of construction for connectors for high-frequency cables and for thermocouple wiring that must resist high temperatures. 

Commercial off-the-shelf thermocouple wires are available in different forms bare wires, insulated wires, and sheathed wires. Bare wires come on spools in a wide range of gauge sizes. They have to be individually matched, fabricated, and insulated before use. 

Insulated wires come as single-insulated thermoelements or double-insulated duplex wires. Duplex wires can be obtained with a stainless steel overbraid for wear and abrasion protection. Table 16.13 lists characteristics of insulations used with thermocouple wires. 

The furnace consisted of an alundum core, which was wound with platinum-20% rhodium resistance wire. A booster coil of Nichrome wire was also wound on the two ends of the core this was used only for the very high-temperature work. The differential temperatures were detected with platinum versus platinum-10% rhodium thermocouples, inserted in the indentations of the platinum sample and reference cups. To shield the thermocouple wires, platinum foil was wound around the ceramic insulating tubes used to bring them into the furnace hot zone. 

Figure 11.24. Apparatus used by Chiu (102) for parallel TG—DTG-DTA and ETA measurements. A. balance housing B, balance beam sheath C, beam stop D, quartz beam E, sample container F. thermocouple block G, sample measuring thermocouple H. ceramic tubing I, platinum jacket J. reference quartz tube K. sample quartz tube L, outer platinum electrode M, center platinum electrode N, cold beam member O. P. platinum lead wires Q, sample thermocouple junction R, reference thermocouple junction S. spacer T, ceramic insulation U, V. sample thermocouple wires W. platinum grounding wire.

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. 

Ultrapure silica or quartz fibers are used in fabrics, yams, rovings and threads. Fabrics are used to reinforce radomes, antenna windows for missiles, high temperature circuit boards, and rocket nose cones. Braided yarns provide high temperature electrical insulation, e.g., for coaxial cables, thermocouple wires, and space separators. Rovings are used to reinforce polymer matrix composites for ablative and electrical uses, as well as high performance sporting goods, e.g., tennis racquets and skis, especially when hybridized with carbon fibers. Threads are used to stitch cable tray insulation for nuclear power plants. 

Temperature Measurement shift. Measurement not representative of process. Indicator reading varies second to second. Ambient temperature change. Fast changing process temperature. Electrical power wires near thermocouple extension wires. Increase immersion length. Insulate surface. Use quick response or low thermal time constant device. Use shielded, twisted pair thermocouple extension wire, and/or install in conduit. 

Electric tube furnaces of appropriate dimensions are available from various manufacturers. A model RO 4/25 by Heraeus GmbH, Hanau, FRG is suitable. However, a very satisfactory furnace can be built by any well equipped laboratory workshop at little cost and effort. The material required consists of thin walled ceramic tubing, 3.5 cm i.d., nichrome resistance wire, heat resistant insulation, and ordinary hardware material. A technical drawing will be provided by the submitters upon request. The temperature of the furnace can be adjusted by an electronic temperature controller using a thermocouple sensor. A 1.5 kW-Variac transformer and any high temperature thermometer would do as well for the budget-minded chemist. 

A thermocouple consists of two wires of dissimilar metals or alloys insulated from each other by placing them in a two-channel porcelain tube. The wires are welded together at one end (the hot or measuring junction of the thermocouple)... 

In order to protect the thermocouple against chemical or mechanical damage, it is normally enclosed in a sheath of mineral packing or within a thermowell (Fig. 6.24). Any material which contains the junction should be a good conductor of heat on the one hand, but an electrical insulator on the other. A potentiometric converter is frequently employed to convert the thermocouple signal to the standard 4-20 mA current range prior to further processing and control room presentation. The extension wires which connect the thermocouple element to the control room should have similar thermoelectric properties to those of the thermocouple junction wires. 

The pressurized volume in this type of apparatus is relatively large, at least a few milliliters, and easily holds an electrically heated furnace. The pistons carry the heating current. Thermocouple or other sensing wires can be led out through the gaskets. Maximum steady temperatures can be as high as 3000°C, depending on the thermal insulation used. Temperatures over 4000° C can be reached in brief (millisecond) pulses. An apparatus of this type... 

Although two reactors are shown in Figure 1, they were not used simultaneously. The reactor shown in the center was the fixed bed reactor which is of primary interest in this contribution. It consisted of a 12.7 mm diameter X 250 mm long steel tube packed with 40/50 mesh catalyst (0.3 mm average particle diameter). The reactor was heated by a nichrome wire coil and was well insulated. The coil spacing was adjusted and was packed in insulation with the intent of making the reactor crudely adiabatic. A variac controlled heater on the reactor inlet and a thermocouple sensor kept the feed to the reactor at the nominal reaction (or feed inlet) temperature of 400°C. The tube of the fixed-bed, reactor was fitted with 12 thermocouples to record the axial temperature profile in the bed (Figure 1). 

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