What is the Difference Between Thermocouple Types and How Do I Know Which One to Choose?

We’ve talked about the types of thermocouples we offer in previous articles, but maybe you’re not sure of which style of thermocouple would work best for a new project. No problem at all, we’re happy to help! If you need help to choose an electrical temperature sensor, we’ve got you covered.

Thermocouples: Function, Types, Selection and Application

The types of thermocouples available on the market vary according to their end use. Here’s a quick rundown of the typical categories you’ll be choosing from:

• Beaded Wire: Two thermocouple wires are connected via a weld. They’re the simplest form, provide quick measurements, and are available in an array of sizes. 
• Thermocouple Probes: The wire is protected by a sheath that determines its properties. They may be highly compatible, provide quick readings, or reduce electrical interference.
• Surface Probes: A thin, flat junction is used to measure temperature on flat surfaces. They may also rotate, which is helpful when measuring on moving surfaces.
• Wireless Thermocouples: A computerized measurement device that can be configured to operate in various conditions. They are exceptionally versatile and accurate and are compatible with a variety of electronic devices.

Every type of thermocouple also has its pros and cons for specific applications. While beaded wire models are very useful in many instances, they’re susceptible to corrosion and oxidation. Thermocouple wire probes are equally as useful but may be rendered inaccurate due to electrical interference.

Beyond these standard categories, thermocouples can also be distinguished by the following application requirements:

• abrasion and vibration resistance
• temperature range
• chemical resistance
• specific installation considerations
• compatibility with existing equipment
• desired response time

Another thing to keep in mind that is specific to application and site is the need for accessories. These might include cable, fittings, floats, a thermowell, or if a sheath is necessary for the probe. For a rundown of the temperatures that each lettered type of thermocouple can withstand, check our previous blog post. And for a quick tip, it’s good to know that for many tasks, a type T, type E, type K, or type J thermocouple wire is a common solution.

Grounding In Temperature Sensors

In addition to all the above considerations, thermocouple wires can also be grounded, ungrounded, or exposed. What are the key differences in these temperature sensors? In a grounded junction thermocouple, the thermocouple wires are attached to the interior of the probe wall. This allows for accurate heat transfer through the probe wall. To measure the temperature of static or flowing corrosive gas or liquid, a grounded thermocouple is ideal and offers a fast response time.

Ungrounded junction thermocouples are detached within the probe wall, resulting in a slower response time. The upside is that because the probe is isolated, electrical interference is not as much of an issue, so even lower level temperature signals can be picked up accurately. Both grounded and ungrounded probes can be used in corrosive and high-pressure environments.

An exposed junction thermocouple wire is not protected by a sheath. Instead, it protrudes from the tip of the temperature sensor and so is directly exposed to the environment. This type of unit can only be used in non-corrosive and non-pressurized applications, but the tradeoff is a lightning fast response time. To protect the temperature sensor and to ensure accuracy of measurements, sheath insulation is sealed at a point where it can prevent moisture or gas from penetrating and causing errors.

How To Choose Between the Different Types of Thermocouples

Once you know how, where, and why the thermocouple will be used, you can then use that information to inform your selection. The next consideration is to know whether the temperature in the environment will be extremely hot or cold. You’ll also want to assess whether the thermocouple will need to withstand potentially corrosive, abrasive, oxidizing, vibrating, or otherwise hostile environments and still accurately measure and transmit data. Finally, once you’ve found thermocouples that can stand up to the anticipated application, you need to determine how important its response time is. In some situations, it could be vital to have an up-to-the-second response time for certain critical processes. Spending some time upfront to determine all the necessary criteria for your project will ensure that you are able to choose the most fitting thermocouple for your specific desired use.

Contact RAM Sensors for Expert Advice on Temperature Sensors

For over 40 years, RAM Sensors, Inc. has been an industry leader in manufacturing thermocouple wires and temperature sensors. For help to determine which of our many products would work best for your desired application, get in touch! Contact us for expert advice, recommendations, and to learn more about our wide array of temperature sensing products.