What is a Thermocouple? What is it Used For?

Thermocouples are connected to wires that are used to measure the temperature of applications like diesel engines, exhausts, and other industrial processes where temperature monitoring is critical. The thermocouple wire is connected to a sensor used for determining temperature.

It consists of diverse metals joined at the ends in a single connecting point, which generates electric energy when the temperature of one end differs from the other. When this junction experiences a change in temperature a voltage is created, this voltage can then be measured and referenced back to the temperature.

Thermocouple wire is used to link the thermocouple to control instrumentation. It functions even in exceptionally high service temperatures. There are multiple types and grades of thermocouple wire. Each type of thermocouple wire has a specific combination of metal alloys. This combination is what defines the type of thermocouple. It may also be used to build the sensing point.

What are the Different Types of Thermocouple Wire?

This high-temperature wire is produced using a wide range of materials and is available with PVC, FEP, TFE, PFA, Fiberglass, and Ceramaflex insulation. However, if you see it referenced by "type" on a spec sheet, this is referring to the kind of metal alloy used for the wire's conductor.

The different kinds of thermocouple wire include:

• Type K- Probably the most common general-purpose thermocouple type, Type K has the widest operating temperature range. It is constructed with Chromel and Alumel legs. It is the most universal type of thermocouple because of its low-cost, accurate, dependable, and wide temperature range characteristics. The type K is administered in nuclear applications because of its relative radiation stability and has a temperature range of -454 to 2,300F (-270 to 1260C)
• Type J- Made with Iron and Constantan (Copper-Nickel) wires, Type J thermocouples are similar in budget and dependability to Type K, but have a more limited temperature range and a higher sensitivity. Type J thermocouple wire is composed of a negative constantan wire and a positive iron wire and has a wide temperature range, a great voltage output, and is low cost.
• Type N- This type is nickel-based, composed of a Nicrosil and a Nisil leg. Designed to be more stable and resistant to high-temperature oxidation, Type N is often considered an “improved” Type K and is gaining popularity.
• Type T- Stable at extremely low temperatures, Type T thermocouples are often used in cryogenics and ultra-low freezers. This type of thermocouple is made with positive copper and negative constantan wires. This type is also administered in laboratory environments and has exceptional temperature repeatability between –380F to 392F (–200C to 200C).
• Type E- Also suited to low-temp applications, Type E has the highest sensitivity of common thermocouples. Thermocouple Wire Type E is constructed with a Chromel leg opposite a Constantan leg. Type E thermocouple works well for cryogenic and chemical applications and has a temperature range of -454 to 1600F (-270 to 870C).
• Types S, R, and B- The higher costs and lower sensitivities of these noble metal thermocouples restrict them to high-temperature measurements for which they are well-suited. They are, however, among the most stable of thermocouples. This makes them ideal for calibration standards because they are reproducible. Type S, for example, is used as the standard calibration for the melting point of gold. Both legs of all three types are composed of varying ratios of Platinum to Rhodium.

The cable is extension grade when an "X" follows the letter indicating which alloy is used in the cable.

What is the Difference Between a Thermocouple Grade Wire and an Extension Grade Wire?

Thermocouple grade wire is a wire that is used to make the sensing point of the instrument. Extension grade wire is only used to extend a thermocouple signal from a probe back to the instrument reading the signal.

What is Thermocouple Wire Insulated With?

There are many different types of insulation available, all with their own benefits:

• Fiberglass Insultaed Thermocouple Wire - moisture resistant and can withstand moderate abrasion
• PFA Insulated Thermocouple Wire - good resistance to moisture, oil, and chemicals
• TFE Insulated Thermocouple Wire - low friction coefficient, in addition to being resistant to moisture, oil, and certain chemicals
• FEP Insulated Thermocouple Wire - resistant against abrasion, chemicals, moisture, nicking, and flame.
• PVC Insulated Thermocouple Wire- weather, oil, liquid, and chemical resistant, and flame retardant.
• CERAMAFLEX Insulated Thermocouple Wire - provides the highest amount of thermal protection without sacrificing flexibility and abrasion resistance

How are Insulated Wires Identified?

The insulation is usually color-coded for easy identification. There are some common guidelines for the colors. The negative lead insulated wire is red and the positive lead is the same color as the thermocouple and the overall color of insulated extension grade wire. The outer jacket of thermocouple grade wire is typically brown. For high-temperature wire, it is common to have a color-coded tracer thread in the white material. For information on usable temperature ranges for insulation, see the "Wire Insulation Identification" table.

Why is Vab(T) Generated?

Each metal at a given temperature (T) is composed of atoms that have an electronic distribution of different energy (calculated by using Quantum Mechanics). This distribution is like a fingerprint: each element is different. Due to the difference in distribution, when A and B come into contact there are electrons that migrate from one metal to the other. This means that one loses electrons (and becomes electrically positive) and the other wins electrons (and becomes electrically negative), which generates an electric field that points to the negative metal, the metal that receives the electrons.

The process does not continue indefinitely, because each electron will have an electric force that will oppose migration. Then, the process ends when the electric field generated by the charge difference reaches equilibrium. Consequently, putting the metals in contact generates an electric field that determines the voltage Vab(T) between this pair of metals A and B at the temperature "T".

Can Thermocouple Wire be Used at All Temperatures?

For operations involving high temperatures, this type of wire functions well. Although the service temperature of the wire varies depending on the specific wire type, the CERAMAFLEX insulated wire works to an extended high temperature of 1204 C.

*limited use in these conditions

**approximate values for bare conductors