--- title: How Does a Thermocouple Work? description: Thermocouples are remarkably versatile, reliable and cost-effective. But how does a thermocouple work? 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Subscribe now and get the latest blog posts delivered straight to your inbox. # How Does a Thermocouple Work? Print/Save as PDF ![John Carissimi, Technical Sales Leader's headshot](https://blog.ashcroft.com/hs-fs/hubfs/John%20Carissimi%20hs.jpg?width=50&height=50&name=John%20Carissimi%20hs.jpg) **By:** [John Carissimi](https://blog.ashcroft.com/author/john-carissimi) Technical Sales Leader **Published on:** February 6th, 2023 **Last updated on:** October 7th, 2026 **Topics:** [temperature sensor](https://blog.ashcroft.com/topic/temperature-sensor) [thermocouple](https://blog.ashcroft.com/topic/thermocouple) **Share:** [mailto:?subject=How+Does+a+Thermocouple+Work%3F&body=https://blog.ashcroft.com/how-thermocouples-work](mailto:?subject=How+Does+a+Thermocouple+Work%3F&body=https://blog.ashcroft.com/how-thermocouples-work) ![](https://blog.ashcroft.com/hs-fs/hubfs/Repost-HowThermocouplesWork_BLOG.jpg?width=1600&height=534&name=Repost-HowThermocouplesWork_BLOG.jpg) A [thermocouple](https://www.ashcroft.com/products/temperature/thermocouples) works by using two dissimilar metals to produce a measurable voltage that changes with temperature. This simple operating principle makes thermocouples versatile, reliable and cost-effective for temperature measurement across many applications. Temperature can be measured using several methods, including glass [thermometers](https://www.ashcroft.com/products/temperature/thermometers), resistance temperature detectors ([RTDs](https://www.ashcroft.com/products/temperature/rtds)), bimetal instruments, [gas-actuated thermometers](https://www.ashcroft.com/products/temperature/thermometers/?pageNumber=1&postTypes=product&postsPerPage=6&search=gas-actuated&sortType=menu_order&sortOrder=ASC) and infrared devices. Each offers advantages for specific applications. Thermocouples are another extremely common temperature measurement device. This article explains what a thermocouple is, how it works, the different thermocouple types and the materials used to construct them. ## What is a thermocouple? A thermocouple is a temperature sensor made from two wires of dissimilar metals joined together at one end, typically by welding or twisting them together. This connection forms the measuring junction that comes in contact with the temperature source. For example, a Type J thermocouple consists of one iron wire and one copper-nickel (constantan) wire. Depending on the installation, the joined wires may make direct contact with the measurement surface. In other applications, the wires are embedded in the sheath of a rigid or flexible probe. **The sheath protects the sensing elements and can make installation easier, particularly when the thermocouple is inserted into a thermowell.** ## How does a thermocouple work? A thermocouple measures temperature by producing a voltage when its two dissimilar metals experience a temperature difference. When two dissimilar wires are joined together and exposed to different temperatures, they form a thermoelectric circuit that produces a measurable voltage. As the temperature changes, the resulting voltage also changes. This phenomenon is known as the **Seebeck effect**, and the resulting voltage is called the **Seebeck voltage**. The voltage depends on the temperature difference between the junctions and the composition of the two metals. By measuring this voltage and using the known voltage-temperature relationship for the thermocouple type, an instrument can determine the temperature at the measuring junction. *Figure 1: Thermoelectric circuit*  *![](https://blog.ashcroft.com/hs-fs/hubfs/thermoelectric%20circuit.png?width=392&height=152&name=thermoelectric%20circuit.png)*  ## How does a thermocouple circuit measure temperature? A thermocouple circuit measures temperature by connecting the thermocouple wires to an instrument that measures the voltage generated by the temperature difference. To turn the basic thermoelectric circuit into a practical temperature measurement system, the circuit is connected to a voltage-measuring device. *Figure 2: Thermocouple circuit*  *![](https://blog.ashcroft.com/hs-fs/hubfs/thermocouple%20circuit.png?width=477&height=173&name=thermocouple%20circuit.png)*  Connecting each thermocouple wire to the terminals of a voltage meter introduces additional junctions into the circuit. Junctions between like materials do not create an additional thermoelectric voltage. However, when thermocouple wire is connected to a dissimilar metal, another electromotive force (EMF) can be generated at the junction. This additional voltage must be accounted for so the instrument can accurately determine temperature from the thermocouple's Seebeck voltage. ## What are the most common thermocouple types? The most common thermocouple types are J, K, T, E, R, S, B and N, with each type using a specific combination of metals. Different metal combinations produce different voltage-temperature characteristics, allowing thermocouples to be selected for different temperature ranges and operating environments. *Figure 3: Types of thermocouples* | Type | Materials | | --- | --- | | J | Iron & Copper-Nickel (Constantan) | | K | Nickel-Chromium & Nickel-Aluminum | | T | Copper & Copper-Nickel (Constantan) | | E | Nickel-Chromium & Copper-Nickel (Constantan) | | R | Platinum-13% Rhodium & Platinum | | S | Platinum-10% Rhodium & Platinum | | B | Platinum-30% Rhodium & Platinum-6% Rhodium | | N | Nickel-Chromium-Silicon & Nickel-Silicon-Magnesium | For example, consider a Type T thermocouple made from copper and copper-nickel (constantan). Because most voltage meters use copper terminals and internal wiring, the secondary junction between the copper side of the thermocouple and the meter (J₃) does not generate an opposing EMF. However, the junction between copper and copper-nickel (J₂) does produce EMF. This voltage must be accounted for when calculating temperature from the measured Seebeck voltage. One of the values needed to determine this EMF is the temperature at the secondary junction (J₂). ## What is a thermocouple reference junction? A thermocouple reference junction provides a known temperature that allows the instrument to accurately determine the temperature at the primary measuring junction. The thermocouple's purpose is to measure temperature at the primary junction (J₁). If the secondary junction (J₂) is exposed to a different and unknown temperature, its voltage contribution can affect the measurement. Historically, one way to establish a known reference temperature was to place J₂ in an ice bath at 0 °C. With J₂ held at a known 0 °C reference, the temperature at J₁ can be calculated from the measured thermoelectric voltage. This is known as the **reference junction**. *Figure 4: Reference junction of thermocouple circuit*  *![](https://blog.ashcroft.com/hs-fs/hubfs/tc%20reference%20junction.png?width=536&height=284&name=tc%20reference%20junction.png)*  For thermocouple types that do not contain copper, such as Type J, connecting the thermocouple to copper instrument wiring creates additional dissimilar-metal junctions. An isothermal block can be used to keep these junctions at the same temperature. **Because the junctions experience the same temperature, their opposing voltage effects can be accounted for without introducing additional measurement error.** *Figure 5: Isothermal block of thermocouple circuit*  *![](https://blog.ashcroft.com/hs-fs/hubfs/isothermal%20block.png?width=589&height=321&name=isothermal%20block.png)*  ## What is cold junction compensation? Cold junction compensation (CJC) electronically accounts for the temperature at the thermocouple's reference junction so an ice bath is not required. Although an ice bath provides a known 0 °C reference, using one is impractical for most industrial and portable applications. Modern temperature instruments, including transmitters and meters, typically use an internal circuit to provide this reference. This process is known as **cold junction compensation**. The CJC circuitry measures the temperature where the thermocouple connects to the instrument and applies the appropriate correction relative to the 0 °C reference. This allows the instrument to calculate and display the temperature at the primary measuring junction (J₁). ## What shapes and sizes are thermocouples available in? Thermocouples can be incorporated into many probe styles and configurations to meet the requirements of different temperature measurement applications. Although the sensing principle requires only two wires, thermocouples can be designed for everything from permanent installations in harsh industrial environments to portable equipment used for spot measurements. Examples of Ashcroft thermocouple configurations include:  ![](https://blog.ashcroft.com/hs-fs/hubfs/s50.jpg?width=183&height=177&name=s50.jpg)  [S50](https://www.ashcroft.com/products/temperature/s50-industrial-thermocouple/)Industrial probe for installation in a thermowell  ![](https://blog.ashcroft.com/hs-fs/hubfs/Ashcroft_Rueger_S70_RTD_small.jpg?width=161&height=249&name=Ashcroft_Rueger_S70_RTD_small.jpg)  [S70](https://www.ashcroft.com/products/temperature/s70-tube-skin-thermocouple/) Industrial probe with thermocouple pad welding to the contact surface  ![](https://blog.ashcroft.com/hs-fs/hubfs/multipoint.jpg?width=68&height=179&name=multipoint.jpg)  Heavy industrial "multi-point" probe with several thermocouple probes  ![](https://blog.ashcroft.com/hs-fs/hubfs/ATE2%20w-Thermocouple.jpg?width=353&height=235&name=ATE2%20w-Thermocouple.jpg)  [ATE-2](https://www.ashcroft.com/products/test-instruments/calibrators/ate-2-handheld-digital-calibrator/) portable calibrator with thermocouple interface and probe ## Ready to learn more? Thermocouples provide a simple and versatile method for measuring temperature by converting temperature differences into a measurable electrical voltage. Understanding the thermocouple circuit, reference junction and cold junction compensation can help you better understand how these sensors provide reliable temperature measurements in industrial applications. Review the related resources below for more information about thermocouples or [contact us](https://www.ashcroft.com/contact/) for help selecting the right thermocouple or other temperature instrument for your application. In the meantime, feel free to [download our guide](https://www.ashcroft.com/learn/product-documentation?pageNumber=1&search=RTD&sortType=date&sortOrder=DESC&content_type=31)to learn more. [![New call-to-action](https://no-cache.hubspot.com/cta/default/4021582/c81b75cc-efe3-4427-a0d5-bf410e9b3cd4.png)](https://cta-redirect.hubspot.com/cta/redirect/4021582/c81b75cc-efe3-4427-a0d5-bf410e9b3cd4) [John Carissimi, Technical Sales Leader](https://blog.ashcroft.com/author/john-carissimi) For four decades, John Carissimi has served Ashcroft Inc. in a wide variety of disciplines that span from advertising to applications engineering. John specializes in pressure system applications and precision calibration instrumentation. As the Technical Sales Leader, John also manages the customer support team, addressing technical questions, website and phone leads, and military quotes. 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