Thermocouple Hardware

Material Sources

Thermoelectric alloys and thermoelements are manufactured by a very few specialized sources. These manufacture bare thermoelement bar stock, wire, and foil, and selectively pair thermoelements for thermocouples. Fewer sources fabricate the MIMS thermocouple materials that are incorporated by others into finished products.

These producers of basic forms and very many other sources fabricate these starting forms into finished specialized thermocouple cables and a multitude of products. Few vendors of thermocouples make the thermoelements. Some vendors do not manufacture thermometry products they offer.

Quality Assurance

Pioneer thermocouple users worked with commercial-grade metals and alloys that were not controlled for thermoelectric sensitivity or homogeneity. Each thermocouple had to be individually calibrated and tested for uniformity. Now, thermoelectric quality assurance is first applied by the thermoelement manufacturer. However, properties might later be modified during additional fabrication.

Additional thermoelectric testing could be performed by the provider of the final thermocouple product. Infrequently, through the production chain, materials might be modified or misidentified in type or color code. The prudent user confirms the materials.

Forms of Thermoelements

Bar Stock: Some thermoelectric products, such as links, connectors, foil, and so on, require bulk feedstock available in a form not usually seen by consumers. The processed materials may be re-characterized as later fabricated. Circuit components that are short and of high thermal conductivity tend to remain nearly isothermal in application; therefore, they introduce little Seebeck emf.

Foil and Film: The Seebeck coefficient of metals is independent of material dimension down to about 3500 Å, or even much less, thickness. Commercial thermoelement foils have thickness down to 0.013 mm (0.0005 in.), leaf to 0.01 in. Vapor-deposited alloy films, properly conditioned, can have Seebeck characteristics like thicker material. That allows fabrication of intricate thermoelectric circuits.

Very thin thermoelements are more vulnerable to mechanical damage and degradation than are the wire sizes most used in thermometry. Rolled thin foil, ribbon, or vapor-deposited film thermoelements are used in thermocouples specialized for transient or much localized thermometry.

Wire: Thermoelements are most familiar to users as wire or cylindrical rod in diameters from 0.013 to 3.3 mm (0.0005 – 0.128 in.) or more. Individual or thermocouple-paired bare thermoelement wire is available to consumers. Such bare materials are not indelibly identified by color code. Identification is by label. Labels are easily separated from the material.

It is important to maintain identification of all thermoelectric materials by type, color code, source, and batch throughout their use. Individual thermoelement wire is also available with color-coded insulation. Insulating tubing in all the international coding colors is available for use on bare thermoelements. Casually applied by a user, the color code might be incorrect or misinterpreted.

Cable: A very wide variety of measurement and extension-grade thermoelements and thermocouple pairs are available cabled as insulated in thermally and chemically resistant plastics and other materials. Individual pairs in solid or stranded form may be covered with protective insulation and wire braid or shielding foil.

For severe duty field and process industry use, they are assembled into cables of many separately color-coded pairs that have mechanically and chemically resistant jackets. Some have rugged steel reinforcement. Special connectors can connect extension cables of 24 or more thermocouple pairs.

Mineral-Insulated, Metal-Sheathed Thermocouple Material: Many fabricated thermocouple assemblies have thermoelements tightly drawn within a thin-walled tubular metal sheath. Such bulk material is available drawn to overall diameters of 0.075–6.35 mm (0.003–0.25 in.).

Materials are controlled to conform to ASTM E585. The process results in highly compressed mineral insulation that allows bending to a radius as small as a few sheath diameters without harm.

Many measuring thermocouples of special design use MIMS thermocouple construction. Needle-sized hypodermic probes must be rigid; lengthy small-diameter material is very flexible. High isolation resistance between thermoelements is essential. The crushed insulant is very hygroscopic. The resistance is difficult to maintain in very small-diameter MIMS material.

Immediate sealing is particularly important for MIMS material of small diameter. Ends opened for fabrication must be quickly sealed to avoid absorption of moisture that decimates resistive isolation of thermoelements. Special epoxy sealants are used for the lower temperature end. Sealants for the exposed measuring junctions of MIMS thermocouples require fused glass or other high-temperature material. Such seals may breach in service.

MIMS material is available in all standardized thermocouple types. Different sheath and insulation materials are adapted to a variety of application environments and to temperatures up to 2315°C. MIMS thermocouple material as seen by most users is as commercially fabricated into the variety of special measuring thermocouple assemblies. Like bare wire, the bulk MIMS material is not color-coded. It is identified by batch or product label.

The sheath of a MIMS thermocouple can contain several thermoelements of the same or different thermocouple types. Additionally, for concurrent longitudinal temperature profile measurement, several individual small-diameter MIMS thermocouples can be enclosed and swaged tightly within an overall metal sheath with measuring junctions longitudinally spaced at intervals.

The thin-walled MIMS thermocouple sheaths appear impervious to external contaminants but unrecognized pinholes and fine cracks can allow penetration of moisture or other contaminants. Some surrounding materials at very high temperature can actually diffuse through intact sheaths.

Moreover, constituents of the sheath alloy or of its thermoelements can migrate through intact mineral insulation to locally affect the Seebeck properties of the thermoelements. An externally similar less costly metal-sheathed form simply encloses insulated thermoelements loosely within a tubular sheath without drawing. This less expensive rigid form cannot be bent. It lacks most benefits of MIMS construction.

Insulation

Formerly, most measuring thermocouples were of bare wire insulated using simple ceramic beads. Beads of nesting fish-spine and many other forms still provide flexible high temperature insulation for bare thermoelements. The insulators and exposed thermoelement are vulnerable to contamination.

Specialized insulators now include lengthy rigid cylindrical forms with one to six holes for thermoelements. Lengths may be up to 5 ft. Some are usable to temperatures up to 1600 °C. Typical materials are alumina or mullite.

Diverse applications require a variety of insulating materials of different forms and costs adapted to particular temperature ranges. Individual thermoelements and thermocouple pairs usually are insulated by flexible insulation.

Typical materials are Kapton, Teflon, and glass or ceramic braid. Flexible insulation temperature upper limits range from 230 °C to 1200 °C. Extension pairs are normally exposed to temperatures near room temperature. Insulations of connectors or extensions may be rated to 220 °C or even to 650 °C.

However, if used to such high temperatures, a significant part of the emf will be from the uncalibrated extension rather than from the measuring thermocouple.

Special thermally conducting and electrically isolating silicone pastes and epoxies are used for coupling or temporarily bonding thermocouple sensors to the test subject. Typical temperature limits are between −40 °C and 200 °C. Special adhesives for thermal coupling, electrical isolation, and bonding are for use to 260 °C.

Connectors

Quick-connect plug and jack connectors using thermoelement materials as contacts are of full, miniature, and sub-miniature sizes with either two or three pins. For MIMS dual element thermocouples, four-pin styles are used. The largest pin marks negative polarity. Pins can be of cylindrical, flat, or banana-plug form.

Other styles include specialty connectors of circuit board or “Sub-D” form and multi-pin connectors for 25 or more pairs. For thermometry in electrically noisy environments, connectors for shielded cable add shield continuity straps. Some have built-in ferrite core noise filters.

Most connector shells are of plastic with temperature limits up to 220 °C or of ceramic rated to 650 °C. Connector shells are color coded like the associated thermocouples. For large installations of multi-pair cable, special barrier terminals or panel-mounted connectors, maintained isothermal, can be used.

Some thermocouple measurements must be made within high-pressure or vacuum chambers. The circuit must cross an impervious wall. The temperature difference across thermoelectric feedthroughs is likely to be much greater than for connectors used near ambient temperature. Therefore, they are more subject to error than are quick-connects.

An alternative penetration method uses a compression-style feedthrough that allows several uninterrupted insulated thermoelements to pass sealed through a wall while maintaining pressure or vacuum integrity. That avoids adding incidental thermoelements.

Thermocouple Junction Styles

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Specialized Thermocouple Assemblies

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• Thermocouple Working Principle
• Absolute Seebeck Effect
• Basic Thermocouple Circuits
• Extensions of Thermocouple
• Functional Model of Thermoelectric Circuits
• T/X Sketch of Dual-Reference Junction Circuit
• Applications of Functional Model
• Characteristics of Thermocouples
• Thermocouple Hardware
• Thermocouple Junction Styles
• Active Tests of Thermocouple
• Calibration of Thermocouples
• Thermocouple Thermometry Practice
• Distinctive Thermocouple Noise Problems