Thermocouple wire is used to make thermocouple elements which are then integrated into thermocouple probes. Thermocouples provide a cost effective means to measure temperature compared to other, more complicated temperature sensing technologies. They provide relatively accurate results for many industrial applications, especially after the thermocouple element has been calibrated against published standards.

An advantage of thermocouples is their ability to be used at high temperatures (up to 2600°C depending on the wire material). By grounding the device, thermocouple probes are also able to directly contact the object being measured. However, thermocouples have a number of potential sources of error. These include the influence of aging, corrosion, material defects, calibration uncertainty, and the non-linear correlation between temperature and voltage. In addition, electronic compensation is needed for the second temperature measurement taken at the cold junction of the probe.

Thermocouples work based on the Seeback effect, which is the production of an electromotive force (emf) or Seebeck voltage when a conductor is heated. This effect was derived from the observation that an electrical current is present in a series circuit of two dissimilar metals, provided the junctions of the two metals are at different temperatures. This effect is pictured in figure 1. The thermoelectric effect increases as t₂ - t₁ increases. Different conductors or alloys will generate different voltages.

Figure 1 - Seebeck effect with copper and iron conductors. Image Credit: Encyclopedia of Science

Types of Thermocouple Wire

Thermocouple wires are classified into different types based on alloy composition of each wire in the pair. Code letters for wire types are designated by the American National Standards Institute (ANSI). Table 1 provides an overview of the different types of wires.

Table 1 - Overview of thermocouple wire types. Table Credit: Pelican Wire Company, Inc.

Selection Properties

In addition to selecting by type, industrial buyers may want to consider the type of insulation, physical size, number of elements, and features of thermocouple wires.

Insulation Type

Insulation selection is important for protecting thermocouple wires from various types of corrosion. Tables 2 and 3 describe different types of insulation and the properties and resistances they exhibit.

Table 2 - Overview of insulation types and their various physical properties. Table Credit: Omega Engineering

Table 3 - Resistance properties of thermocouple wire insulation and additional characteristics. Table Credit: Omega Engineering

Physical Size

Thermocouple wires may be sized differently depending on the application. Dimensions include diameter, gauge thickness, and length.

• Diameter of the wire can have an impact on the response time of the thermocouple element.
• Gauge thickness refers to the numerical gauge or AWG thickness of the wire product.
• Length is the distance the wire extends.

Number of Elements

The number of elements should also be specified when considering a thermocouple cable. This refers to the number of thermocouple wire pairs or elements in the thermocouple cable. A thermocouple wire with 12 pairs or elements can measure 12 points within a furnace, which provides greater temperature measurement capability compared to a single pair or two pair thermocouple wire. The cost of multi-pair thermocouple wire is much higher than single pair because the alloy material cost is multiplied by the number of pairs.

Features

Thermocouple wires can include a number of different features which may important to consider.

Extension wiring is used to connect thermocouple elements or probes to a measuring instrument where the instrument is located away from the thermal process being measured. The connections to the extension wires, being of like metals, do not generate a voltage. Extension grade thermocouple wires are ideally made of the same metals as a higher-grade thermocouple element or probes in order to avoid generation of unwanted voltagesfrom additional dissimilar material junctions between in the system. Copper extension wires are used with noble or platinum thermocouples because the cost of platinum extension wires would be prohibitive. The extension wire selected should have a very similar thermal coefficient of EMF to the thermocouple over ambient or operating temperatures.

Metal sheathing can be incorporated on a thermocouple cable or wire, encasing the insulation and thermocouple pairs. It is composed of stainless steel and nickel alloys such as Inconel^TM and Hastelloy^TM and is used to provide heat, oxidation, and corrosion protection.

Electromagnetic shielding can be incorporated on a thermocouple cable or wire to protect against electromagnetic interference, which could affect the results of the voltage reading.

Special limits of error (SLE) are used to distinguish highly purified thermocouple wires. The level of impurities in the wire material controls the thermocouple wire errors. Standards allow errors as much of 2.2°C. Thermocouples manufacturedwith tighter metallurgical controls and to SLE standards can have error less than 0.5°C. Calibration of the finished wirecanalso reduceerrors and improve temperature measurement accuracy.

Vacuum compatible thermocouple wires are suitable for vacuum measuring applications. Specifically, the wire and wire insulation have low outgassing characteristics to help maintain a vacuum during measurement.

Standards

Some of the thermocouple types are standardized with calibration tables, color codes, and assigned letter-designations. The ASTM (American Society for Testing and Materials) Standard E230 provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits, and the complete voltage versus temperature tables for cold junctions maintained at 32^o F and 0^o C.

References

Pelican Wire Company, Inc. - Types of Thermocouple Wire

Encyclopedia of Science - The Seebeck Effect, Figure 1

Omega Process Measurement and Control - Wire Insulation Identification, Table 2, 3 (pdf)

Image credit:

Omega Engineering, Inc.