In the field of industrial instrumentation, various types of thermocouples are commonly utilized for temperature measurement. These thermocouples play a crucial role in accurately gauging temperature in diverse applications. Below are the commonly used types of thermocouples:
K-Type Thermocouple (Nickel-Chromium/Nickel-Silicon [Nickel-Aluminum] Thermocouple)
This type of thermocouple is renowned for its reliability and versatility, making it widely employed in diverse industrial settings.
It’s a noble metal thermocouple with strong oxidation resistance, capable of measuring temperatures in the range of 0 to 1300°C. It is suitable for continuous use in oxidizing and inert gases, with a short-term usage temperature of up to 1000°C. Its thermoelectric potential has an approximately linear relationship with temperature, making it the most widely used thermocouple. However, it is not suitable for bare wire use in vacuum, sulfur-containing, carbon-containing atmospheres, and atmospheres with alternating oxidation-reduction conditions. In low oxygen pressure, the chromium in the nickel-chromium electrode will preferentially oxidize, causing a significant change in thermoelectric potential. Therefore, it is often enclosed in a metal protective tube to mitigate these effects.
- Lower high-temperature stability of thermoelectric potential compared to N-type thermocouples and precious metal thermocouples. Besides, it often damaged due to oxidation at temperatures exceeding 1000°C.
- Poor short-term cyclic stability in the range of 250 to 500°C, showing variations in thermoelectric potential values at the same temperature point during heating and cooling cycles, with differences up to 2 to 3°C.
- Magnetic transformation of the negative electrode occurs in the range of 150 to 200°C, leading to deviation from calibration values within the range of room temperature to 230°C, especially when used in a magnetic field, causing time-independent thermoelectric potential interference.
- Long-term exposure to high-flux radiation environments can lead to degradation in stability due to the transformation of elements like manganese (Mn) and cobalt (Co) in the negative electrode, causing significant changes in thermoelectric potential.
S-Type Thermocouple (Platinum-Rhodium 10%/Platinum Thermocouple)
Known for its high accuracy and stability, the S-Type thermocouple is often chosen for applications where precision temperature measurement is paramount.
The S-type thermocouple consists of a positive electrode made of a platinum-rhodium alloy containing 10% rhodium and a negative electrode made of pure platinum.
- Stable thermoelectric performance, strong oxidation resistance.
- Suitable for continuous use in oxidizing atmospheres, with a long-term usage temperature of up to 1300°C. Beyond 1400°C, even in the air, the pure platinum wire will recrystallize, resulting in coarse grains and fractures.
- High accuracy, with the highest accuracy grade among all thermocouples, commonly used as a standard or for measuring higher temperatures.
- Wide range of applications, with good uniformity and interchangeability.
- The main disadvantages include small differential thermoelectric potential, resulting in lower sensitivity, higher cost, and low mechanical strength. It makes it unsuitable for use in reducing atmospheres or conditions with metal vapors.
E-Type Thermocouple (Nickel-Chromium/Copper-Nickel [Constantan] Thermocouple)
Renowned for its suitability in a wide range of temperatures, the E-Type thermocouple is particularly effective in applications where rapid temperature changes occur.
The E-type thermocouple is a relatively new product, with a positive electrode made of a nickel-chromium alloy and a negative electrode made of a copper-nickel alloy (constantan). Its most notable feature is having the highest thermoelectric potential among commonly used thermocouples, indicating the highest sensitivity. While its application range is not as extensive as the K-type thermocouple, it is often chosen for conditions requiring high sensitivity, low thermal conductivity, and high permissible resistance. Limitations in use are similar to the K-type, but it is less sensitive to corrosion in atmospheres with higher humidity.
N-Type Thermocouple (Nickel-Chromium-Silicon/Nickel-Chromium Thermocouple)
With its resistance to high temperatures and corrosion, the Type N thermocouple is a reliable choice for demanding industrial environments.
The N-type thermocouple is characterized by strong oxidation resistance up to 1300°C, good long-term stability, and excellent short-term cyclic reproducibility. It performs well in radiation resistance and low-temperature environments. Additionally, in the range of 400 to 1300°C, the N-type thermocouple exhibits better linearity in thermoelectric characteristics compared to the K-type thermocouple. However, it has significant non-linear errors at low temperatures (-200 to 400°C) and is challenging to process due to its hard material.
J-Type Thermocouple (Iron/Constantan Thermocouple)
Widely used for its affordability and durability, the Type J thermocouple is effective in measuring temperatures in various industrial processes.
The J-type thermocouple has a positive electrode made of pure iron and a negative electrode made of constantan (copper-nickel alloy). It is characterized by its affordability and suitability for use in vacuum oxidation, reducing, or inert atmospheres within a temperature range of -200 to 800°C. However, its practical temperature range is usually below 500°C, as the oxidation rate of the iron thermoelectric electrode increases significantly beyond this temperature. With the use of fine or coarse wire, it can have a longer lifespan at high temperatures. It is resistant to corrosion by hydrogen (H2) and carbon monoxide (CO) gases. However, it should not be used in sulfur-containing atmospheres at high temperatures (e.g., 500°C).
T-Type Thermocouple (Copper/Constantan Thermocouple)
Recognized for its suitability in low-temperature applications, the T-Type thermocouple is commonly employed where precision is essential at lower temperature ranges.
The T-type thermocouple has a positive electrode made of pure copper and a negative electrode made of constantan (copper-nickel alloy). Its main features include:
In the category of noble metal thermocouples, it has the highest accuracy and good electrode uniformity. Its operating temperature is -200 to 350°C. Due to the oxidation susceptibility of the copper electrode, it’s generally not recommended to exceed 300°C in oxidizing atmospheres. In the range of -200 to 300°C, it exhibits high sensitivity. The copper-constantan thermocouple is also characterized by its affordability, being the cheapest among commonly used standardized products.
R-Type Thermocouple (Platinum-Rhodium 13-Platinum Thermocouple)
The Type R thermocouple, also known as the platinum-rhodium 13-platinum thermocouple, is a crucial component in industrial temperature measurement instruments. This thermocouple is renowned for its high accuracy and reliability in capturing temperature data, especially in high-temperature environments.
Crafted with precision, the R-Type thermocouple consists of a platinum-rhodium 13 (PtRh13) positive leg and a pure platinum (Pt) negative leg. The utilization of these specific materials enhances the thermocouple’s performance and ensures durability under challenging conditions.
One of the notable features of the R-Type thermocouple is its ability to measure temperature in the range of 0 to 1600 degrees Celsius, making it suitable for a wide range of industrial applications. Its robust construction and high-temperature capabilities make it ideal for processes where accuracy and stability are paramount.
Understanding the characteristics and advantages of each type of thermocouple is crucial for selecting the most suitable one for specific industrial requirements. The versatility of these thermocouples ensures their widespread use in temperature measurement across various industrial processes.