Differences Between Thermocouples: Type S, K, N, J, E, T

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Differences Between Thermocouples

Thermocouples have the advantages of simple structure, wide measurement range, and easy processing of output signals. Therefore, thermocouples have become our preferred sensor for temperature measurement.

The graduation numbers of thermocouples mainly include S, R, B, N, K, E, J, T, etc. Among them, S, R, and B belong to precious metal thermocouples, and N, K, E, J, and T belong to cheap metal thermocouples. So what’s the difference? Understanding the Differences between Thermocouples can help us choose the most suitable thermocouple.

RFQ

Thermocouple working principle

A thermocouple is a passive sensor that uses the thermoelectric effect as its measurement principle.

The so-called thermoelectric effect is when the two ends of conductors with different components (called thermocouple wires or hot electrodes) are connected to form a loop. When the temperatures of the two junctions are different, an electromotive force will be generated in the circuit. This will also involve the calculation of the effect of cold junction temperature.

Therefore, different types of thermocouples can be obtained according to the different materials of the two wires that make up the thermocouple. They have different advantages, disadvantages and scope of application.

Let’s talk about it in detail below.

spring thermocouple

Types of thermocouples

What are the different types of thermocouples? There are 8 main types of thermocouple graduation numbers: S, R, B, N, K, E, J, and T.

Among them, S, R, and B belong to precious metal thermocouples, and N, K, E, J, and T belong to cheap metal thermocouples.

  • The S grade is characterized by strong oxidation resistance and should be used continuously in oxidizing and inert atmospheres. The long-term use temperature is 1400°C and the short-term use temperature is 1600°C. Among all thermocouples, the S graduation number has the highest accuracy level and is usually used as a standard thermocouple;
  • Compared with the S-grading type, the heat removal electromotive force of the R-grading type is about 15% larger, and other properties are almost identical;
  • The thermal electromotive force of the B graduation number is extremely small at room temperature, so compensation wires are generally not needed during measurement. Its long-term use temperature is 1600℃ and short-term use temperature is 1800℃. Can be used in oxidizing or neutral atmospheres, and can also be used under vacuum conditions for short periods of time;
  • The characteristics of the N graduation number are strong high-temperature oxidation resistance at 1300°C, good long-term stability of thermoelectromotive force and short-term thermal cycle reproducibility, and good nuclear radiation resistance and low temperature resistance. It can partially replace the S graduation number. thermocouple;
  • The K grade is characterized by strong oxidation resistance and is suitable for continuous use in oxidizing and inert atmospheres. The long-term use temperature is 1000°C and the short-term use temperature is 1200°C. The most widely used of all thermocouples;
  • The characteristic of the E graduation number is that it has the largest thermal electromotive force among commonly used thermocouples, that is, the highest sensitivity. It should be used continuously in an oxidizing and inert atmosphere, with a service temperature of 0-800°C;
  • The characteristic of the J graduation number is that it can be used in both oxidizing atmospheres (the upper limit of the operating temperature is 750°C) and reducing atmospheres (the upper limit of the operating temperature is 950°C), and is resistant to H2 and CO gas corrosion. It is mostly used in oil refining and chemical industry;
  • The T graduation number is characterized by the highest accuracy level among all low-cost metal thermocouples, and is usually used to measure temperatures below 300°C.
Types of thermocouples

Differences Between Thermocouples

The main differences between various thermocouples are based on the different metal materials used, which results in different measurement ranges and characteristics.

The materials of a K-type thermocouple are: the positive electrode is a nickel-chromium alloy (containing 10% chromium), and the negative electrode is a nickel-silicon alloy (containing 2.5% silicon).

K-type thermocouple is a base metal thermocouple with strong oxidation resistance. It can measure the medium temperature from 0 to 1300°C. It is suitable for continuous use in oxidizing and inert gases. The short-term use temperature is 1200°C and the long-term use temperature is 1000°C. The relationship between thermoelectric potential and temperature is approximately linear, and it is currently the most commonly used thermocouple.

The nominal chemical composition of the positive electrode (KP) is: Ni:Cr=90:10.
The nominal chemical composition of the negative electrode (KN) is: Ni:Si=97:3.

However, K-type thermocouples are not suitable for use with bare wire in vacuum, sulfur-containing, carbon-containing atmospheres and alternating redox atmospheres. When the oxygen partial pressure is low, the chromium in the nickel-chromium electrode will be preferentially oxidized, causing the thermoelectric potential to occur. Great changes, but metal gas has little impact on it. Therefore, metal protective tubes are often used.

Type S thermocouple is a precious metal thermocouple.

The material of a s type thermocouple is:

  • The nominal chemical composition of the positive electrode (SP) is a platinum-rhodium alloy, which contains 10% rhodium and 90% platinum.
  • The negative electrode (SN) is pure platinum.

The long-term use temperature is 1300℃, and the short-term use temperature is 1600℃.

Nickel-chromium-copper-nickel type E thermocouple is also called nickel-chromium-constantan thermocouple.

The e type thermocouple material：

  • The positive electrode (EP) is: nickel-chromium 10 alloy, with the same chemical composition as KP.
  • The negative electrode (EN) is copper-nickel alloy.
  • The operating temperature of this thermocouple is -200~900℃.

The thermoelectromotive force of type E thermocouple has the highest sensitivity and is suitable for measuring small temperature changes. It is suitable for use in environments with high humidity.
E thermocouple has good stability and better oxidation resistance than T type and J type thermocouple, and can be used in oxidizing and inert atmospheres.

E-type thermocouples cannot be used directly for sulfur at high temperatures. In reducing atmospheres, the uniformity of thermoelectric potential is poor.

The materials for the N-type thermocouple are: the positive leg is a nickel-chromium-silicon alloy (containing 14% chromium and 1.5% silicon), and the negative leg is a nickel-silicon alloy (containing 4.5% silicon).

The nominal chemical composition of the positive electrode (NP) is: Ni:Cr:Si=84.4:14.2:1.4.
The nominal chemical composition of the negative electrode (NN) is: Ni:Si:Mg=95.5:4.4:0.1.

It has strong anti-oxidation ability in temperature regulation below 1300℃, good long-term stability and short-term thermal cycle reproducibility, and good nuclear radiation resistance and low temperature resistance.

In the range of 400 to 1300°C, the linearity of the thermoelectric characteristics of the N-type thermocouple is better than that of the K-type thermocouple.

N-type thermocouples cannot be used directly in sulfur, reducing or reducing-oxidizing alternating atmospheres at high temperatures, nor can they be used directly in vacuum. It is also not recommended in weakly oxidizing atmospheres.

Iron-copper-nickel J-type thermocouple is also called iron-constantan thermocouple.

J type thermocouple material: The nominal chemical composition of the positive electrode (JP) is pure iron.
The negative electrode (JN) is copper-nickel alloy.

The J-type thermocouple is characterized by being cheap and suitable for vacuum oxidation in reducing or inert atmospheres. The temperature range is from -200 to 800°C, but the commonly used temperature is only below 500°C. Because beyond this temperature, the oxidation of the iron hot electrode will The speed is accelerated. If thick wire diameter wire is used, it can be used in high temperatures and has a longer life;

J-type thermocouples are resistant to hydrogen (H2) and carbon monoxide (CO) gas corrosion, but cannot be used in high-temperature (such as 500°C) sulfur (S)-containing atmospheres.

Type R thermocouple is a precious metal thermocouple. The long-term use temperature is 1300℃, and the short-term use temperature is 1600℃.

The materials for the R-type thermocouple are:

  • The nominal chemical composition of the positive electrode (RP) is platinum-rhodium alloy, containing 13% rhodium and 87% platinum.
  • The negative electrode (RN) is pure platinum.

Compared with the S type, its potential rate is about 15% larger. Other properties are almost the same.

A copper-copper-nickel T-type thermocouple is also called copper-constantan thermocouple.

T type thermocouple materials are: The positive electrode (TP) is pure copper. The negative electrode (TN) is copper-nickel alloy.

Among base metal thermocouples, it has the highest accuracy and good uniformity of the hot electrode;
Its operating temperature is -200~350℃. Because copper thermoelectrode is easy to oxidize and the oxide film is easy to fall off, when used in an oxidizing atmosphere, it generally cannot exceed 300℃;
In the range of -200~300℃, their sensitivity is relatively high;

Another feature of copper-constantan thermocouples is that they are cheap and are the cheapest among several commonly used stereotyped products.

Type B thermocouple is a precious metal thermocouple. The long-term use temperature is 1600℃, and the short-term use temperature is 1800℃.

B type thermocouple materials are:

  • The nominal chemical composition of the positive electrode (BP) is platinum-rhodium alloy, which contains 30% rhodium and 70% platinum.
  • The negative electrode (BN) is a platinum-rhodium alloy containing 6% rhodium.

Type B thermocouples are suitable for use in oxidizing and inert atmospheres, and can also be used in vacuum for short periods of time.

Type B thermocouples are not suitable for reducing atmospheres or atmospheres containing metal or non-metal vapors.

An obvious advantage of type B thermocouple is that there is no need to use compensation wires for compensation, because the thermoelectric potential is less than 3μV in the range of 0~50℃.

These different types of thermocouples can be used directly, or as part of a sheathed assembly, or with a Thermocouple Sheath.

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FAQ

Thermocouples produce direct current (DC).

The working principle of a thermocouple depends on the Seebeck effect. The Seebeck effect occurs between two different metals; when heat is applied to either metal, electrons begin to flow from the hot metal to the cold metal, thus generating a direct current in the circuit. The Seebeck effect is also called the thermoelectric effect.

We can differentiate between Type J and Type K thermocouples by observing and testing their magnetic properties.

  1. Using a magnet test (the fastest and most accurate method)

Type J thermocouple: Its positive leg is made of iron. One of the wires (the positive leg) is strongly magnetic and will be firmly attracted to a magnet.

Type K thermocouple: Its negative leg is made of Alumel (nickel-aluminum alloy). One of the wires (the negative leg) is weakly magnetic. Compared to the iron wire of the Type J thermocouple, the magnetism of the Type K is usually much weaker.

  1. We can also differentiate them by observing the color of the connector:

According to international or national standards, the wire and connector colors of K-type and J-type thermocouples usually differ.

StandardsThermocouple TypePositive terminal (+)Negative terminal (-)Overall sheath/plug
ANSI (American)Type KYellowRedYellow
Type JWhiteRedBlack
IEC (International)Type KGreenWhiteGreen
Type JBlackWhiteBlack

In the field of industrial temperature measurement, K-type thermocouples have become the most widely used temperature sensors due to their unique performance advantages. The core reason is that they achieve an optimal balance between measurement range, stability, response speed, and cost through the precise matching of material science principles and engineering requirements. This balance allows them to meet the demands of extreme conditions such as high-temperature smelting and chemical reactions, while also being suitable for medium and low-temperature applications such as food processing and environmental monitoring, making them the preferred choice for industrial temperature measurement.

The response time of a thermocouple refers to the time required for the thermocouple’s output change to reach a certain specified percentage when the temperature undergoes a step change, usually denoted by τ. Generally, the response time of a thermocouple is typically between a few hundred milliseconds and a few seconds, and the specific value varies depending on the model and manufacturer.

The response time is affected by the heat transfer rate between the thermocouple and the surrounding medium; the higher the heat transfer rate, the shorter the response time.

The above is an introduction to Differences Between Thermocouples. All precious metal thermocouples have the advantages of high accuracy, best stability, wide temperature measurement range, long service life, and good oxidation resistance at high temperatures. The disadvantages are that the thermoelectric potential rate is small, the sensitivity is low, the mechanical strength decreases at high temperatures, it is very sensitive to pollution, and it is expensive.

We at Sino-Inst supply various types of industrial thermocouples, including K-type, T-type, etc. When selecting and purchasing, the thermocouple model should be selected reasonably based on the needs of the working conditions. If you have relevant technical questions, please feel free to contact us.

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