Thermocouples are temperature sensing devices that are made of two dissimilar metal wires joined together at one end. The junction where the two wires are connected is called the hot or measuring junction. When heat is applied to this junction, a small voltage is generated that can be correlated back to the temperature. This phenomenon is known as the thermoelectric effect or Seebeck effect.
Thermocouples come in different combinations of metals, which are identified by a letter designation such as J, K, T, E, and more. Each type has its own temperature range, accuracy, and color coding used for the wires. Understanding the color codes for the different thermocouple types is important for proper installation and use.
Common Thermocouple Types and Color Codes
The most common thermocouple types used in industrial and commercial applications are J, K, T, E, N, S, R, and B. Here are the different thermocouple types with their associated color codes:
Type J
Made of iron and constantan. Used in reducing atmospheres up to 750°C.
– Black wire – Iron (+)
– White wire – Constantan (-)
Type K
Made of chromel and alumel. General purpose up to 1150°C. Most common type.
– Yellow wire – Chromel (+)
– Red wire – Alumel (-)
Type T
Made of copper and constantan. Accurate and stable up to 350°C.
– Blue wire – Copper (+)
– White wire – Constantan (-)
Type E
Made of chromel and constantan. Useful up to 900°C. Low output requires amplification.
– Purple wire – Chromel (+)
– Red wire – Constantan (-)
Type N
Made of nicrosil and nisil. Stable and oxidation resistant up to 1250°C.
– Orange wire – Nicrosil (+)
– White wire – Nisil (-)
Type S
Made of platinum/10% rhodium and platinum. Most stable and accurate up to 1480°C.
– Black wire – Platinum/Rhodium (+)
– Red wire – Platinum (-)
Type R
Made of platinum/13% rhodium and platinum. Used up to 1480°C.
– Black wire – Platinum/Rhodium (+)
– Red wire – Platinum (-)
Type B
Made of platinum/30% rhodium and platinum/6% rhodium. Highest temperature range up to 1700°C.
– Grey wire – Platinum/30% Rhodium (+)
– Red wire – Platinum/6% Rhodium (-)
Thermocouple Extension and Compensating Cables
In many applications, thermocouples have long lead wires to connect back to the temperature measuring or controlling device. It’s important that any extension cables use the same alloys as the thermocouple probes to maintain accuracy. Compensating cables and wire will follow the same color coding as the thermocouple type.
For example, a type K thermocouple extension cable would have yellow (+) and red (-) wires. A type J extension cable would have black (+) and white (-) wires. Using the wrong wire alloys for extensions can introduce errors due to dissimilar metal junctions.
Polarity Matters
One key consideration when working with thermocouples is maintaining proper polarity. Reversing the positive and negative leads results in the measured temperature being opposite of the true value. Having standard color codes makes it easier to keep polarity correct during installation and use.
Tips for Working with Thermocouple Wiring
Here are some useful tips when working with thermocouple wiring and color codes:
– Always double check that extension wires match the thermocouple type. Mismatched alloys will cause inaccurate readings.
– Don’t splice thermocouple wires of different types together. This creates a third alloy that does not conform to standard tables.
– Use ceramic wire connectors rated for high temperatures when splicing thermocouple wires.
– Clearly label both ends of extension or compensating cables to prevent polarity confusion.
– For multipoint installations, label each thermocouple channel at both ends according to the temperature device inputs.
– Check for continuity of each wire run and proper polarity before powering up.
Standardized Color Codes
The color codes used for common thermocouple types are standardized according to guidelines published by organizations such as ASTM, IEC, and JIS. This helps ensure consistency between manufacturers and compatibility across industries. While there are some slight national variations, the main codes are aligned globally.
Having standardized color codes aids troubleshooting in the field. A technician can quickly identify the thermocouple type and match it with any extension wiring. Color codes also facilitate safe installations and prevent polarity reversals.
Special Situations and Exceptions
While standard color codes are the norm, there can be exceptions:
– If using thermocouples at very high temperatures, the wires may use fiberglass insulation which can alter the apparent wire color. Proper labels should still indicate polarity.
– Some manufacturers may use non-standard codes, especially for specialty alloys. Always check the wiring diagram for these exceptions.
– In multipoint installations with many thermocouples, other colors may be used to help identify specific temperature channels. Polarity should still follow conventions.
– Hook up wires that connect the thermocouple to terminal blocks may use other colors, but polarity should match at both ends.
The standard color codes give a good starting point, but additional verification of polarity may be needed in these special situations. Use a combination of codes, diagrams, labels, and testing to validate correct connections.
Proper Installation Matters
Taking the time to properly install thermocouples using the correct color codes and polarity will ensure accurate and reliable temperature measurements. Here are some final installation tips:
– Use twisted pair or shielded cables for long thermocouple lead wire runs to minimize noise.
– Anchor wires to prevent strain on sensor junction connections which can cause failures.
– Isolate thermocouple wiring from power cables to avoid electrical noise interference.
– Calibrate the overall installation periodically to account for any drift in sensors, connections, or meters.
– Maintain spare thermocouples, extension wires, and connectors on hand for repairs and replacements.
Following standard wiring practices and color codes for each thermocouple type will make installation, troubleshooting, and maintenance much easier. Consult thermocouple reference tables for details on the letter designations, temperature ranges, material composition, and color codes when using these important temperature sensors.
Summary Table of Common Thermocouple Types and Color Codes
| Thermocouple Type | Positive Wire Color | Negative Wire Color |
| J | Black (Iron) | White (Constantan) |
| K | Yellow (Chromel) | Red (Alumel) |
| T | Blue (Copper) | White (Constantan) |
| E | Purple (Chromel) | Red (Constantan) |
| N | Orange (Nicrosil) | White (Nisil) |
| S | Black (Platinum/Rhodium) | Red (Platinum) |
| R | Black (Platinum/Rhodium) | Red (Platinum) |
| B | Grey (Platinum/30%Rhodium) | Red (Platinum/6%Rhodium) |
Conclusion
Understanding thermocouple color codes and wiring conventions is critical for proper temperature measurement and control. The codes provide a standard means to identify thermocouple types and maintain correct polarity through the system. Following the established installation guidelines, labeling wires, and testing for continuity ensures a reliable setup. With the right techniques and vigilance, thermocouples can provide accurate temperature data for many industrial applications.