The Most Common Applications for Thermocouples

We all measure temperature in some way just about every day. The temp gauge on our car, or the temperature of our dryer, or how hot the water is coming out of the tap. Most of these measurements are made with a product used heavily in the industry because of its durability. This device is called a thermocouple.

In a process facility like a chemical plant, refinery, or power plant, thermocouples play the vital role of sending a signal back to the control room so operators can monitor and control temperature. Thermocouples have been around for a long time, and are well-understood devices which can be configured in infinite variations to accommodate almost every application. Let’s examine these devices a little closer.

What is a Thermocouple?

A thermocouple is a device used to measure temperature. Thermocouple construction consists of 2 dissimilar metals joined together to form what is termed as a junction. The thermocouple junction, due to its construction, outputs a predictable voltage over a given temperature range. Thermocouples use the thermoelectric effect, or Seebeck effect, to convert a voltage measurement into a temperature measurement. The Seebeck effect works because different metals have different electron orbitals. This means the electrons change energy levels in different ways. This creates a difference in potential energy over a distance which produces a voltage across the junction.

We can change the dissimilar metals to change the temperature range. Thermocouples can be used to measure temperatures well over 1200 degrees F. This allows thermocouples to be used in process environments in a wide array of industries. uses two pieces of different conductive materials in contact with each other. There are many types of thermocouples that find use in a wide variety of situations and environments.

Types of Thermocouples

There are lots of types of thermocouples, utilizing different metals and alloys. They all have different uses for various temperature environments.

Nickel-Alloy Thermocouples

• Type E uses wires made of chromel and constantan and even works in cryogenic environments.
• Type J uses iron and constantan and has a very high sensitivity.
• Type K uses chromel and alumel and is the most widely used type of thermocouple. They are inexpensive and can be used in most ordinary circumstances.
• Type M uses one wire with 82% nickel and 18% molybdenum and one wire with 99.2% nickel and 0.8% cobalt and is useful in vacuum furnaces.
• Type N uses nicrosil and nisil and is a very stable type of thermocouple, resisting oxidation very well.
• Type T uses copper and constantan and has a much higher thermal conductivity because of the use of copper.

Platinum/Rhodium-Alloy Thermocouples

• Type B uses one wire with 70% platinum and 30% rhodium and one wire with 94% platinum and 6% rhodium.
• Type R uses one wire with 87% platinum and 13% rhodium and one wire that is completely platinum.
• Type S uses one wire with 90% platinum and 10% rhodium and one wire that is completely platinum.
• Types B, R, and S are more expensive than most other thermocouples because of the use of platinum.
They are very stable and work well in high-temperature environments.

Tungsten/Rhenium-Alloy Thermocouples

• Type C uses one wire with 95% tungsten and 5% rhenium and one wire with 74% tungsten and 26% rhenium.
• Type D uses one wire with 97% tungsten and 3% rhenium and one wire with 75% tungsten and 25% rhenium.
• Type G uses one wire with 74% tungsten and 26% rhenium and one wire that is completely made of tungsten.
• Although types C, D, and G become brittle in high oxidation environments, they handle very high temperatures well.

Other Thermocouples

• A thermocouple using one chromel wire and one gold/iron-alloy wire has a high sensitivity at low temperatures.
• Type P uses one wire with 55% palladium, 31% platinum, and 14% and one wire with 65% gold and 35% palladium. They work similarly to type K thermocouples but are more resistant to corrosion.
• Thermocouples that use a platinum/molybdenum alloy work well in nuclear reactors.  • Iridium/rhodium alloy thermocouples are sometimes used in high-temperature environments.
• Thermocouples that use a gold/platinum alloy or a platinum/palladium alloy have low melting points; they also have very high accuracy.
• HTIR, or high energy irradiation resistant, thermocouples are also useful in nuclear reactors. They are also useful in high-temperature situations where radiation might be an issue.

Applications

Thermocouples have many uses in the home as well as in industry. Factories that produce steel and iron products use them to keep track of the temperature of products throughout their manufacturing processes.

Appliances that use gas often have a pilot light. A thermocouple can monitor the pilot light to make sure it doesn’t go out and release gas.
Thermocouples can also measure radiation intensity, typically in the visible or infrared portion of the spectrum, which is useful in measuring the power of lasers, for example. Research and development departments also use thermocouples to test electrical and mechanical
prototypes. Chemical plants use thermocouples to monitor hundreds of different temperatures throughout the chemical manufacturing process as the end products quality often depends on precise temperature control.

Replacement and Repairs

Depending on which metals are used, thermocouples can be subject to degradation by very high or low temperatures, high voltages, or even chemical processes like oxidation. It may be necessary to repair your device or replace it with a new thermocouple. Over time, the metal in thermocouples exposed to high temperatures can change thermoelectric coefficients. These are used to calculate the measured temperature given a voltage, so changes here would result in inaccurate temperature readings. If the entire thermocouple is exposed to excessive temperatures for its range, the wires will degrade homogeneously, that is, the same way throughout the entire length of the wire. Recalibration is necessary to ensure the continued accuracy of the device.

Next Steps

Thermocouples use the Seebeck effect to measure temperature. They come in a wide variety of types and have lots of applications.
Now that you know how thermocouples work, their types, and their applications, feel free to contact us for more information. Whether you need engineering services, fabrication services, or commissioning and spare parts, we are happy to help you find the components and resources you need.

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