What is a thermocouple? A temperature measurement manufacturer explains its principle, types, and how to choose one in an easy-to-understand way.

Thermocouples measure temperature using a combination of different metals.

A thermocouple is a temperature sensor made by combining different types of metals. When two different metals are connected and a temperature difference occurs between contact on the measuring side and the reference side, a voltage is generated. This voltage is used to determine the temperature.

Thermocouples come in several types, including K, N, E, J, T, R, S, and B, and their thermoelectric characteristic differ depending on the combination of metals used.

What are the Seebeck effect and thermoelectric power?

The phenomenon in which a voltage is generated when a temperature difference is applied to two different metals is called the Seebeck effect. The voltage generated at this time is called thermoelectric power.

Thermocouples use the relationship between thermoelectric power and temperature to determine the temperature of the object being measured. Because the thermoelectric power is very small, measuring instruments and appropriate wiring are required that are compatible with the type of thermocouple.

A mechanism for determining temperature from thermoelectric power.

The thermoelectric voltage generated by a thermocouple is not simply determined by the temperature of the sensing contact alone. The measuring instrument must take the temperature of the reference junction into account and make a correction. This correction is called cold contact compensation.

What is the difference between a thermocouple and resistance thermometer?

Thermocouples and resistance thermometer are both widely used sensors for contact-type temperature measurement, but they differ in their principles of temperature detection. Thermocouples utilize thermoelectric power, while resistance thermometer utilize the change resistance of a metal.

Thermocouples are suitable for measuring high temperatures or wide temperature ranges, while resistance thermometer may be preferred when relatively high accuracy and stability are important. However, in practice, the choice depends on the object being measured, the operating environment, response speed, and installation method.

If you want to organize your selection criteria for temperature sensors as a whole, the guide on choosing temperature measuring equipment can also be helpful.

Features of K.N.E.J.T. thermocouples

K, N, E, J, and T are representative thermocouples used in industrial temperature measurement and other applications. While K thermocouples are particularly widely used, it's important to choose the type that best suits the measurement temperature and operating environment rather than simply selecting "K" because it's the most common.

Each type of sensor has different thermoelectric characteristic, suitable temperature ranges, and environmental requirements. In actual selection, you should check the applicable standards and specifications of the sensor you will be using, and choose the type that best suits your measurement conditions.

Characteristics of R-S-B thermocouples

R, S, and B thermocouples are candidates for temperature measurement in high-temperature ranges. They are used in high-temperature furnaces and other applications, but the decision is not simply made based on the temperature ("high temperature means R, S, or B"), but rather by considering the measurement temperature, usage time, ambient conditions, required stability, and protective structure.

The type of thermocouple is not determined solely by its temperature range.

When choosing a thermocouple, it's important to first check the temperature range. However, that alone isn't enough. The appropriate configuration varies depending on factors such as the oxidizing/reducing atmosphere, corrosive gases, continuous use time, required measurement accuracy, and response speed.

Compare the characteristics of each type.

For specific measurable ranges and tolerances, please refer to the applicable standards such as JIS and specifications of the product you will actually be using.

What is a sheathed thermocouple?

Sheathed thermocouples are temperature sensors in which thermocouple wires are enclosed within a metal sheath and surrounded by a high-density packing of inorganic materials such as magnesium oxide (MgO) insulation structure. Because they can be easily manufactured with small diameters and readily ensure insulation, airtightness, and vibration resistance, they are used in a wide range of industrial applications.

There are multiple options for the sheath outer diameter, length, sheath material, thermocouple type, and temperature sensing contact structure. The combination should be considered based on the required responsiveness, durability, and mounting conditions.

What is a solid-pack thermocouple?

Solid-pack thermocouples are relatively robust industrial temperature sensors that incorporate thermocouple wires and insulation material within a protective tube. Because structure and dimensions differ from sheathed thermocouples, their selection should consider not only responsiveness but also mechanical strength, operating environment, and mounting conditions.

General-purpose type, coated type, etc., structure suited to the application.

Thermocouples come in various types, including sheathed thermocouples, general-purpose thermocouples, and insulated thermocouples. Insulated thermocouples have a covering made of vinyl or glass wool on the thermocouple wires and are used without being enclosed in a protective structure.

In other words, "type of thermocouple" and "sensor structure" must be considered as separate selection criteria.

Differences between ground, ground, and exposed types

The most suitable structure will vary depending on whether you prioritize responsiveness, insulation, durability, or the operating environment.

① Decide what to measure and where.

The first thing to consider is not just "what temperature to measure," but what temperature to measure and where. The appropriate sensor shape and mounting method will vary depending on whether you are measuring a solid surface or interior, an atmosphere, a gas, or a liquid.

② Narrow down the type of thermocouple based on the measured temperature.

This document outlines the minimum temperature, normal operating temperature, maximum temperature, temporary peak temperature, and the duration of exposure to high temperatures. Especially in high-temperature applications, it's crucial to distinguish between "temperatures reached instantaneously" and "temperatures maintained over extended periods."

③ Choose the material based on the usage environment.

We check for oxidizing and reducing atmospheres, corrosive gases, vibration, moisture, and pressure. For high-temperature applications, the material of the sheath and protective tube is just as important as the thermocouple wire itself.

Sheaths and protective tubes are made from a variety of materials, including stainless steel and heat-resistant alloys. Since each material has different heat resistance, oxidation resistance, and corrosion resistance, the choice should not be based solely on the name, but rather on conditions such as temperature, atmosphere, and usage time.

④ Select structure and outer diameter based on the required response speed.

In process with rapid temperature changes, sensor responsiveness is crucial. Generally, reducing the size of the temperature-sensing element reduces the heat capacity, making it easier to achieve a faster response.

On the other hand, reducing the diameter also changes the mechanical conditions. We need to consider the balance not only with responsiveness, but also with vibration, strength, mounting method, and operating temperature.

⑤ Confirm the required accuracy.

Being able to "measure" and being able to "measure with the required accuracy" are not the same thing. We need to clarify the acceptable error in process and consider not only the tolerance of the thermocouple, but also the effects of the measuring instrument, wiring, and mounting.

⑥ Determine the dimensions and installation method.

We will check the outer diameter, sensor length, insertion length, flange, nipple, compression fitting, and mounting method. In addition to being able to be attached to the equipment, it is important that the temperature sensing element can be properly positioned at the location where measurement is desired.

Mounting dimensions are important not only for whether the device will fit the equipment, but also for its measurement performance, specifically whether the temperature sensor can be properly positioned at the desired location.

⑦ Check the wiring, terminal, and measuring instruments to be connected.

We will check the wiring distance from the thermocouple to the measuring instrument, compensating and extension wires, terminal boxes, cables, shield, noise environment, and whether explosion proof is required. We will select the measurement system including not only the thermocouple itself, but also the connected display, recorder, controller, and converter.

Measurements are taken over a long period of time in a high-temperature, oxidizing atmosphere.

For high-temperature applications, in addition to the type of thermocouple, the material of the sheath and protective tube, as well as the operating environment, must be checked. Even at the same measurement temperature, the suitable material will vary depending on the atmosphere (oxidizing or reducing) and the continuous operating time. When using at high temperatures for extended periods, not only heat resistance but also oxidation resistance and material changes over time must be considered.

Measuring narrow spaces and small objects

For small components or confined spaces, the size of the sensor itself becomes a measurement constraint. Small-diameter sheathed thermocouples are a viable option for insertion into narrow spaces or for measuring the temperature of small objects.

Measuring rapid temperature changes

If high-speed response is required, check the sheath diameter and structure of the temperature sensing contact. Generally, reducing the sheath diameter and thus the heat capacity of the temperature sensing element makes it easier to speed up the response to temperature changes. However, it is necessary to balance this with mechanical strength and durability.

Furthermore, "ultra-fine sheathed thermocouples" are available for measuring the temperature of extremely small parts. When measuring small objects, the influence of the sensor itself on the target temperature must also be considered.

Measurements are taken in harsh environments such as those involving vibration and corrosion.

In environments with vibration or corrosive conditions, we consider not only the thermocouple wires themselves but also the sheath, protective tube, fixing method, and terminal section. Especially for equipment used over long periods, durability and maintainability are also selection criteria.

Measurements taken during furnace and heat treatment process.

When designing furnaces and heat treatment equipment, it's necessary to consider not only high temperatures, but also the atmosphere, temperature distribution, degradation due to prolonged use, and even the recording and calibration of measurement values.

Instead of just choosing a sensor, thinking of it as a series of temperature management steps—measuring → recording → control → verifying accuracy—makes it easier to organize the necessary configuration.

What is cold contact compensation?

In thermocouples, a thermoelectric voltage is generated in response to the temperature difference between the sensing contact and the reference side. Therefore, the measuring instrument needs to compensate for the temperature of the reference side. This is called cold contact compensation.

Why are compensatory and extension wires necessary?

When installing thermocouples and measuring instruments separately, it is necessary to extend the wiring. In this case, it is important to select compensation and extension wires that are compatible with the type of thermocouple being used.

Compensation wires and extension wires are available for different types of thermocouples, so do not mix different types and connect them according to the standards and specifications of the equipment being used.

What happens if you get the polarity wrong?

Thermocouples have polarity. Connecting the positive and negative sides incorrectly will prevent the thermocouple from processing the temperature change as a signal in the correct direction, resulting in unnatural display. Check the polarity of the sensor, wires, terminal, and measuring instrument before connecting them.

Prevent errors caused by wiring noise

Because thermocouple signals are weak, they can be affected by surrounding electrical noise. Consider countermeasures tailored to the equipment environment, such as separating the wiring from noise sources like power lines and using shield as needed.

Influence of mounting position and insertion depth on measurement values

Even if the sensor itself is functioning correctly, it may not accurately capture the temperature of the object being measured if it is not mounted in the appropriate location. In particular, in areas such as piping and furnaces, measurement errors can occur due to heat escaping from the measurement point into the surrounding structure.

It is important to consider not only the selection of thermocouples, but also the insertion depth, fixing method, and the influence of heat from the surroundings.

characteristic of a thermocouple change when used at high temperatures.

Over time, the condition of the wires in a thermocouple can change, altering its thermoelectric power characteristic. It is especially important to monitor changes in measured values when using thermocouples at high temperatures for extended periods.

Effects of oxidizing, reducing, and corrosive atmospheres

Thermocouple degradation is affected not only by temperature but also by the surrounding environment. The rate at which thermocouple wires, sheaths, and protective tubes deteriorate varies depending on conditions such as oxidizing, reducing, carburizing, nitriding, and corrosiveness.

Even when used at the same temperature, the suitable materials and structure will vary depending on conditions such as oxidation/reduction properties, carburizing/nitriding properties, and the type of combustion furnace.

Pay attention to the deterioration of the protective tube and sheath.

During sensor inspection, we check not only the thermocouple wires but also the sheath and protective tube for corrosion, deformation, and wear. Deterioration of the outer protective structure can affect the internal thermocouple.

The replacement timing will be determined based on working conditions and measurement results.

It is impossible to uniformly determine the lifespan of a thermocouple in a single number of years, because factors such as operating temperature, operating time, ambient conditions, thermal cycles, and vibration vary.

In practice, the inspection, calibration, and replacement cycles are determined by combining factors such as usage history, visual inspection, occurrence of abnormal values, past calibration results, and the importance of process.

What is the difference between inspection and calibration?

The inspection involves checking the sensor's appearance, for disconnection, and for proper mounting to ensure it is functioning correctly.

Calibration is the process of comparing a measurement to a reference temperature and checking the difference. It's important to distinguish between "it's working" and "it's showing the correct value."

What do you check when calibrating a thermocouple?

The measurement system is compared to a reference point at the required temperature to confirm the difference. The specific calibration method and range will vary depending on the type of thermocouple, application, and required accuracy.

How do you determine the calibration cycle?

Calibration cycles are not uniform. They are determined based on factors such as operating temperature, operating environment, process importance, past calibration results, internal standards, and standard requirements.

Consider the entire measurement system, not just the sensor.

Actual temperature measurement involves multiple elements, including thermocouples, wiring, converters, display, recorders, and controllers. Therefore, it is important to verify the reliability of the entire measurement system as needed.