Thermocouple wires sheathed

Thermocouples. Their protection. In the various, often hostile, environments suitable sheathing materials must be used to protect the thermocouple wires. 

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. 

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. 

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.

The thermocouple wires are enclosed by a sheath which is itself housed within the well tube. The tip of well tube is inserted horizontally into the center of the pipe. This tip (some 15 cm in length) is thinner in diameter than the base of the sensor. (Fig.3)... 

X-ray photog-aphs were taken to estimate the extent of adherent sodium compounds around the temperature sensor and to assess structural condition. These revealed that the tip of the well tube was missing and the sheath containing the thermocouple wires was bent downstream. Other anomalies were not observed by the photographs. At that time, it was clear that the sodium leaked into the temperature sensor through the broken well tube. (Fig.4)... 

The noble metal thermocouples, Types B, R, and S, are all platinum or platinum-rhodium thermocouples and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures can readily change the platinum wire calibration, hence platinum wires should only be used inside a nonmetallic sheath such as high-purity alumina. 

The measurement ranges for the base-metal thermocouples are 0 to +750 °C (type J), -200 to +1200 °C (type K), and -200 to +350 °C (type T). The noble-metal thermocouples can be used at higher temperatures up to 1700 °C. The dynamic response of sheathed thermocouples is not very fast however, a probe made from bare, thin wires can have very fast dynamic properties. One of the best features of thermocouples is the simplicity of making new probes by soldering or welding the ends of two wires together. 

At the m.p. of aluminium (600°C) an aluminium-sheathed palladium thermocouple formed an alloy with a flash and an exotherm to 2800°C [1]. The use of thin layers of palladium or platinum on aluminium foil or wire as igniters derives from the intense heat of alloy formation, which is sufficient to melt the intermetallic compounds [2],... 

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. 

A thermocouple is constructed of two dissimilar wires joined at one end and encased in a metal sheath. 

The same type of conical leadthrough may be used for swaged thermocouples (Fig. 3.2) the outside sheath of which is either silver soldered to the cone or simply glued with epoxy resin. If the outer sheath is cut and only the centre wire is used, porous MgO or AI2O3 powder insulator will be in contact with the pressurized fluid, and some kind of impervious filler, such as uncured epoxy resin, has to be forced under pressure into the insulator. 

Fig. 4.42 A schematic drawing of a high-pressure dilatometry apparatus (1) high-pressure pipe, (2) a sheathed Alumel-Chromel thermocouple, (3) a sheathed heater, (4) a support, (5) a liquid medium (silicon oil), (6) the sample, (7) a bellows cell, (8) steel wire, (9) high-pressure vessel, and (10) a differential transformer. (Reprinted with permission from Polymer J. 1992, 24, 778.)...

Note This table gives the recommended upper temperature limits for the various thermocouples and wire sizes. These limits apply to protected thermocouples that is, thermocouples in conventional closed-end protecting tubes. They do not apply to sheathed thermocouples having compacted mineral oxide insulation. Properly designed and applied sheathed thermocouples may be used at temperatures above those shown in the tables. Other literature sources should be consulted. 

Bare wires or insulated wires are normally fabricated into butt-welded or beaded (welded or soldered) junctions. Sheathed wires can be fabricated into these exposed types as well as protected (grounded or ungrounded) types. Figure 16.22 shows different thermocouple junctions. 

The reactor was fitted with an iron-consteuitan thermocouple housed in a steiinless steel sheath. A small cunount of oil was maintained in the latter to ensure good thermal contact. The thermocouple was wired to a temperature compensated recorder and... 

We wish to estimate the dynamics of a cold thermocouple probe suddenly placed in a hot flowing fluid stream for the purpose of temperature measurement. The probe consists of two dissimilar metal wires joined by soldering at the tip, and the wires are then encased in a metal sheath and the tip is finally coated with a bead of plastic to protect it from corrosion. Take the mass of the soldered tip plus plastic bead to be m, with specific heat Cp. Denote the transfer coefficient as h. 

Two types of thermocouples were used to monitor the temperature during testing Type R and Type K. The vast majority of the testing was done with a Type R thermocouple while the long-term creep testing was done with a Type K thermocouple with the addition of a Ni sheath. A type R thermocouple is a bi-metallic joint between a Platinum -13% Rhodium alloy and Platinum [8]. A type K. thermocouple is a bi-metallic joint between a Nickel-Chromium alloy and a Nickel-Aluminum alloy [8], The thermocouples were held in place by wires. 

A Kanthal wire-wound furnace enabled the temperature to be controlled to 5 °C at 700, 725, 750, 775 and 800 °C. The cell assembly consisted of a round-bottomed silica container with a ground fiat flange at the upper open end, tightly connected to a flanged Pyrex manifold, through which a thermocouple sheath and various... 

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