Application
Comparison of Thermocouples
Learn more about the characteristics of the most common thermocouple types including temperature and tolerance ranges, materials, performance, and applications.
While all thermocouples operate upon the same basic scientific principles, differences in their materials, construction, and ultimately their operating temperature ranges, create different types of thermocouples. These different thermocouple types are identified with a letter, such as a “Type K Thermocouple”. The following will focus on the difference between thermocouple types.
Thermocouple Calibration Types
Thermocouples are analog temperature sensors, which means that their output data must be interpreted and converted. In the case of thermocouples, the voltage output is calibrated to account for electromotive force (EMF) and the temperature curve of each thermocouple construction. For example, some thermocouples produce significant voltage at low temperatures, while others do not begin to generate voltage until much higher temperatures are reached.
Calibration types aim to make it easy for instruments or temperature controllers to correctly match voltage to its corresponding temperature value by creating the straightest possible line for the voltage curve within the prescribed temperature range. Thermocouples are also calibrated according to the application atmosphere, so it is possible to avoid chemical reactions between the operating atmosphere and the thermocouple type. By choosing the correct thermocouple calibration type, one can better guarantee the life of the sensor, its accuracy requirements, and avoid metallurgy degradation.
Thermocouple Types
Type K Thermocouple
Widest temperature range
This is the most common thermocouple type and provides the widest operating temperature range. Type K thermocouples generally will work in most applications because they are nickel based and have a good corrosion resistance.
- Positive wire is non-magnetic (yellow), negative wire is magnetic (red)
- Traditional base metal choice for high temperature work
- Appropriate for use in oxidizing or inert atmospheres at temperatures up to 1260°C (2300°F)
- Performs best in clean oxidizing atmospheres
- Vulnerable to sulfur attack (regrain from exposure to sulfur-containing atmospheres)
- Not recommended for use under partially oxidizing conditions in vacuum, or when subjected to alternating cycles of oxidization and reduction
Type J Thermocouple
General purpose applications (no moisture)
This is the second most common thermocouple. It is a good choice for general purpose applications if moisture is not present.
- Positive (iron) wire is magnetic (white), negative is non-magnetic (red)
- Appropriate for use in vacuum, air, reducing, or oxidizing atmospheres to 760°C (1400°F) in the heavier gauge sizes
- The expected service life of the finer sized wires is limited due to the rapid oxidation of the iron wire at temperatures above 540°C (1000°F)
- Avoid use in sulfurous atmospheres above 540°C (1000°F)
- Limited subzero use due to rusting and embrittlement of the iron conductor
Type E Thermocouple
Highest output EMF
- Both wires are non-magnetic. Positive wire is purple, negative wire is red.
- The highest output EMF of any standard type
- Recommended for use to 900°C (1600°F) in oxidizing or inert atmospheres
- Performs best in clean oxidizing atmospheres
- Appropriate for low temperature to about -230°C (-380°F)
- Not recommended for use in the following conditions, except in short periods:
- Under partially oxidizing conditions
- When subjected to alternating cycles of oxidation and reduction
- In vacuum
Type T Thermocouple
Appropriate for use to -200°C
- Both wires are non-magnetic. Positive wire is blue, negative wire is red
- When used in air, it is moisture resistant, very stable and useful to 370°C (700°F)
- Higher temperature use is possible when used in a vacuum, or in reducing or inert atmospheres
- Appropriate for use down to -200°C (370°F). Special selection may be required of the materials.
Design Specifications
Temperature Ranges
| Type | Application Range | Trade Names | Color Code |
|---|---|---|---|
| K | 95-1260°C (200-2300°F) | Chromel / Alumel | Red(-) / Yellow(+) |
| J | 95-760°C (200-1400°F) | Iron / Constantan | Red(-) / White(+) |
| E | 95-900°C (200-1650°F) | Chromel / Constantan | Red(-) / Purple(+) |
| T | 0-350°C (32-660°F) | Copper / Constantan | Red(-) / Blue(+) |
Tolerance Ranges
Initial Calibration Tolerances
| Type | Temperature Range | Standard Limits | Special Limits |
|---|---|---|---|
| K | -200°C to 0°C* 0°C to 1250°C |
±2.2°C or ±2%* ±2.2°C or ±0.75% |
N/A ±1.1°C or ±0.4% |
| J | 0°C to 750°C |
±2.2°C or ±0.75% | ± 1.1°C or ± 0.4% |
| E | -200°C to 0°C* 0°C to 900°C |
±1.7°C or ±1%* ±1.7°C or ±0.5% |
±1°C or ±0.5%* ±1°C or ±0.4% |
| T | -200°C to 0°C* 0°C to 350°C |
±1°C or ±1.5%* ±1°C or ±0.75% |
±0.5°C or ±0.8%* ±0.5°C or ±0.4% |
*Thermocouple wire is usually supplied to meet tolerances for temperatures above 0°C. Special selection and testing may be required for these same materials in order for them to fall within the sub-zero tolerances given.
Wire Size Temperature Limits
The table below gives the recommended upper temperature limits for the various thermocouples and wire sizes. These limits apply to protected thermocouples, that is, thermocouples in conventional closed-end protecting tubes (sheaths).
| Gauge | K | J | E | T |
| 20 | 980°C (1800°F) | 480°C (900°F) | 540°C (1000°F) | 260°C (500°F) |
| 24 | 870°C (1600°F) | 370°C (700°F) | 430°C (800°F) | 200°C (400°F) |
| 28 | 870°C (1600°F) | 370°C (700°F) | 430°C (800°F) | 200°C (400°F) |
| 30 | 760°C (1400°F) | 320°C (600°F) | 370°C (700°F) | 150°C (300°F) |
