What is the difference between thermocouple types?

Thermocouples come in various types, each designed with different metal combinations and suited for different temperature ranges and environments. The most common thermocouple types are designated by letters such as K, J, T, E, N, S, R, and B. Here’s a breakdown of the most common thermocouple types and their characteristics:

Type K (Nickel-Chromium / Nickel-Alumel)

Characteristics:

  • Temperature Range: -200°C to 1260°C (-328°F to 2300°F)
  • Accuracy: ±1.5°C or ±0.4%
  • Features: Most common and widely used thermocouple type. Suitable for a wide range of temperatures, good for oxidizing environments.

Type J (Iron / Constantan)

Characteristics:

  • Temperature Range: -210°C to 760°C (-346°F to 1400°F)
  • Accuracy: ±2.2°C or ±0.75%
  • Features: Suitable for lower temperature ranges. Not suitable for oxidizing environments at high temperatures as the iron can rust.

Type T (Copper / Constantan)

Characteristics:

  • Temperature Range: -200°C to 370°C (-328°F to 700°F)
  • Accuracy: ±1.0°C or ±0.75%
  • Features: Excellent for low-temperature measurements, especially in cryogenics. Suitable for oxidizing and reducing environments.

Type E (Nickel-Chromium / Constantan)

Characteristics:

  • Temperature Range: -200°C to 900°C (-328°F to 1652°F)
  • Accuracy: ±1.7°C or ±0.5%
  • Features: High output (mV) which makes it suitable for cryogenic measurements. Suitable for oxidizing or inert atmospheres.

Type N (Nickel-Chromium-Silicon / Nickel-Silicon)

Characteristics:

  • Temperature Range: -200°C to 1300°C (-328°F to 2372°F)
  • Accuracy: ±2.2°C or ±0.75%
  • Features: Improved stability and resistance to oxidation compared to Type K. Suitable for high-temperature measurements.

Type S (Platinum Rhodium – 10% / Platinum)

Characteristics:

  • Temperature Range: 0°C to 1450°C (32°F to 2642°F)
  • Accuracy: ±1.5°C or ±0.25%
  • Features: Very stable, used in high-temperature applications, typically in laboratories. Suitable for oxidizing environments.

Type R (Platinum Rhodium – 13% / Platinum)

Characteristics:

  • Temperature Range: 0°C to 1450°C (32°F to 2642°F)
  • Accuracy: ±1.5°C or ±0.25%
  • Features: Similar to Type S but with a higher output. Suitable for high-temperature applications and oxidizing environments.

Type B (Platinum Rhodium – 30% / Platinum Rhodium – 6%)

Characteristics:

  • Temperature Range: 0°C to 1700°C (32°F to 3092°F)
  • Accuracy: ±0.5%
  • Features: Suitable for very high temperatures. High stability and accuracy. Limited use at lower temperatures due to low output.

Summary

  • Type K: Versatile, high-temperature range, general-purpose.
  • Type J: Suitable for lower temperatures, but prone to rust.
  • Type T: Excellent for low-temperature applications.
  • Type E: High output, good for cryogenic and moderate temperatures.
  • Type N: Improved stability over Type K, high temperature.
  • Type S, R, B: High stability, high-temperature applications, often used in labs and industry.

The choice of thermocouple type depends on the specific requirements of temperature range, environment (oxidizing, reducing, etc.), accuracy, and stability needed for the application.