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HomeTips & GuidesThermo Wire – All You Need to Know

Thermo Wire – All You Need to Know

About Thermo Wire: While you may not know, gadgets that monitor your vehicle components’ temperature play a key role in their optimal functionality. 

For instance, monitoring the brake disc temperature ensures optimal braking, while that of the engine parts ensures optimized combustion and power.

 Although there may be several devices for monitoring your car temperature, one of the most reliable gadgets is the thermo wire. 

Here, we discuss what a thermocouple wire is, how it works, its types, plus how to choose the right one.

What Is Thermocouple Wire?

Before discussing a thermocouple wire, it’s only fair first to understand what a thermocouple is. 

A thermocouple is typically a sensor type that you can use to monitor the temperature of your car components.

 A thermocouple is found mainly in the engine, catalytic converter, exhaust, brake, and climate control systems. 

Apart from your car, you can also use this sensor in ovens, furnaces, and other systems requiring precise temperature control.

Now, the thermocouple sensor uses a special wire called the thermocouple wire. It features two dissimilar metals joined together to create a junction that measures temperature, a process we will discuss below. 

Your thermocouple wire then features fiberglass, PVC, or Teflon insulation to protect it from harsh environments and mechanical damage. 

Most systems prefer it due to its real-time accuracy, which in turn ensures the performance and safety of your car systems.

Thermocouple device calculating temperature

Thermocouple device calculating temperature

How Does a Thermocouple Wire Work

As we established earlier, your thermocouple wire consists of two dissimilar metals or alloys joined together to create a junction at one end. 

The metals you choose mostly depend on the temperature accuracy and range you require.

 Some of the most common metals I know to make thermocouple wires include copper/constantan, iron/constantan, and others. 

To measure the temperature with this device, you expose this measuring junction to heat away from the reference junction. 

When you do so, it creates a temperature variation along the wire length, causing Seebeck effects. 

The Seebeck principle is the creation of a voltage between the measuring and reference junctions of a thermocouple wire.

 The voltage you produce here is proportional to the temperature difference between the two junctions and depends metals used. 

However, this voltage is so small that you can only measure it accurately using a temperature controller or thermocouple thermometer.

 Once it measures this voltage and the characteristics of the metal combination in the wire, it determines the temperature accurately. 

It then sends this data to your car computer (ECU), prompting the affected components to act accordingly.

A temperature controler

A temperature controler

Thermocouple Wire Parts

As you may expect, the thermocouple wire features several parts that work in tandem to ensure accurate temperature readings. Some of the main parts include:

Metal wire: We already established that this wire features two wires that join to form a temperature-measuring junction.

Measuring junction: It’s where your two metals combine and the part you expose to the temperature you intend to measure. 

Reference junction: Typically, to know the temperature of your wire, you must have another end to act as a control or reference point. This is where the reference junction comes in, which you can call the cold connection. It is the other side of the thermocouple wire built inside your device.

Insulation: As we said earlier, the thermocouple wire features high-quality insulation for environmental protection, hence longevity and functionality. While the insulation material depends on the application, some common ones include ceramic, Teflon, Fiberglass, mineral insulation, and PVC.

Extension wire: Now, you usually place these devices in areas you can’t touch, e.g., very hot or cold metals. For this reason, the temperature reader or display is at a distance from the junction where you can access it. Note that these cables may use insulation and shielding to prevent EMI, and signal loss to enhance accuracy.

Connectors: These wires may feature different connector types to facilitate easy installation to the control systems.

Thermocouple wire probe

Thermocouple wire probe

Thermocouple Wire Types

When we say type here, we mean the combination of metals used to make the thermocouple wire. With that said, below are the most common types of thermocouple wires and their characteristics: 

Type K

One of the most popular universal thermocouple wires on the market, it features Chromel and Alumel construction and has an operating temperature of between -270 and 1260 degrees Celcius. 

With this huge temperature range, it finds its application in various industries, such as nuclear applications. In addition, it’s radiation stable, accurate, affordable, and dependable.

Type J

It features iron position iron metal plus negative constantan negative metal construction (an alloy of Copper and Nickel). It is expected to have high sensitivity, excellent voltage output, and a wide temperature range (-210-760 degrees Celcius).

 Like type K, it’s also low-budget with a wide application, but its temperature range is smaller.

Type N

Type N thermocouple wire is the newest and improved version of type K which is gaining popularity for its excellent characteristics.

 Its construction is mostly nickel metal, with Nicrosil (Nickel/Chromium/Silicon) being positive and Nisil (Nickel/Silicon) being negative.

 Expect stability and resistance to high-temperature oxidation of -270 to 1300 degrees Celsius. With these characteristics, you can find it in various applications including car engines and exhaust systems.

Type T

The above thermocouple wire type consists of negative constantan plus positive copper metal. It is one of the most accurate thermocouples, especially in extremely low temperatures between -250 and 350 degrees Celcius. This low-temperature measuring capability makes it useful in freezers and cryogenics

Type E

Another popular thermocouple to expect is Type E, which is also good for low temperatures of between -270-870 degrees Celcius). It features Chromel and Costantan metals, making it the most sensitive device on the list.

How to Choose the Right Thermocouple Wire

Before choosing any thermocouple wire, you must consider what you will pay. As you may expect, devices with special features and high accuracy always cost more than normal ones. 

After the budget, know the temperature range of your application, as different thermocouples have different working temperatures.

 For instance, when your application is in high temperatures, go for type K. In contrast, in low temperatures, require type T. 

From there, check your application environments for dirt, corrosive chemicals, moisture, and electromagnetic interference. 

If your usage is in such environments, I recommend a device with proper insulation and shielding that can withstand such environments. Then check the accuracy level you require, but note that thermocouples with high accuracy, such as types S and R, are expensive.

You might also want to check mechanical durability, such as flexibility and the ability to endure bending and vibrations. Lastly, choose a proper gauge wire device depending on your application and ensure compatibility with your control system.

Many thermocouples

Many thermocouples

How to Identify Insulated Thermocouple Wires

Typically, international electrical bodies require color coding in any insulated thermocouple for universality. However, you should note that the negative lead of the insulated thermocouple is always red.

On the other hand, the color of the positive lead depends on the type of wire in the thermocouple. For instance, type K is yellow, type J is white, T is blue, E is purple, and N is orange.

Frequently Asked Questions

What is the Difference Between Thermocouple Probes and Thermocouple Wires?

A thermocouple is a complete gadget that senses temperature. On the other hand, a thermocouple wire connects sensors to your temperature-measuring device to facilitate temperature data transmission.

What is the difference between Thermocouple Grade Wire and Extension Grade Wire?

Thermocouple-grade wires refer to the wires that make the thermocouple sensor itself and measure the temperature directly. On the other hand, extension-grade wires connect the thermocouple sensor to the temperature-measuring device.


Using extensive research, I trust you can now choose the right thermocouple wire for your application.

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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