A complete technical guide to thermocouple types — materials, temperature ranges, colour codes, and how to select the right thermocouple for your industrial application.
The thermoelectric effect — also known as the Seebeck effect — describes what happens when two dissimilar metals are joined. At the junction, a voltage is generated that depends on the temperature and the specific metals used. The millivolt output from this junction is converted to a temperature reading by the measuring instrument using the thermocouple's calibration tables.
Each thermocouple type has a unique voltage-temperature relationship. This is why instruments must be configured for the exact thermocouple type connected — using the wrong setting produces incorrect readings.
| Type | Materials | Range | IEC Colour | Best For |
|---|---|---|---|---|
| J | Iron / Constantan | 0 to 750°C | Black | General industrial, plastics, food |
| K | Chromel / Alumel | 0 to 1250°C | Green | Most common — heat treatment, HVAC, furnaces |
| R | Pt13%Rh / Platinum | 0 to 1700°C | Orange | High-temp furnaces, kilns, glass, ceramics |
| S | Pt10%Rh / Platinum | 0 to 1600°C | Orange | High-temperature, steel industry |
| T | Copper / Constantan | −200 to 350°C | Brown | Food, refrigeration, cryogenic |
| E | Chromel / Constantan | 0 to 870°C | Violet | Highest output, vacuum applications |
| N | Nicrosil / Nisil | 0 to 1300°C | Pink | High-temperature stability, improved K-type replacement |
The J-type thermocouple uses iron (positive) and constantan (negative) wires. It is one of the oldest thermocouple types and remains widely used for general industrial applications at temperatures up to 750°C. J-type produces a relatively high millivolt output, making it easy to measure accurately. However, above 750°C the iron wire oxidises rapidly, limiting its upper range. J-type is common in plastic moulding machines, food processing ovens, and general manufacturing processes where temperatures stay below 600°C.
K-type (Chromel-Alumel) is the most popular thermocouple worldwide. Its wide temperature range (0 to 1250°C), good accuracy, relatively low cost, and wide availability make it the default choice for most industrial applications. K-type is used in heat treatment furnaces, kilns, HVAC systems, automotive testing, gas turbines, food processing, and general manufacturing. When someone refers to "a thermocouple" without specifying a type in an industrial context, K-type is usually implied.
K-type has good resistance to oxidation in air up to 1250°C but should not be used in sulphurous or reducing atmospheres, which cause rapid degradation.
R-type thermocouples use platinum-rhodium (13% rhodium) as the positive wire and pure platinum as the negative wire. This noble metal construction allows R-type to measure temperatures up to 1700°C with good stability and accuracy — making it the standard choice for high-temperature applications that exceed the capability of base metal thermocouples.
R-type is used in industrial furnaces for heat treatment of metals, glass manufacturing tanks, ceramic kilns, precious metal refineries, and laboratory high-temperature research. The high cost of platinum-rhodium makes R-type significantly more expensive than J or K type, but for applications above 1250°C it is often the only viable option.
T-type (Copper-Constantan) is designed for low-temperature measurement from −200°C to 350°C. It offers excellent accuracy and stability in this range and is moisture-resistant, making it suitable for food processing, cold storage monitoring, refrigeration systems, and cryogenic research. T-type thermocouples are often used in food safety applications where temperatures near 0°C must be measured precisely.
Thermocouple cables are colour-coded to identify the type. Two international standards are in common use. The IEC standard (used in India, Europe, and most of Asia) and the older ANSI/ASTM standard (used in the USA). Always confirm which standard applies to your thermocouples before connecting them to instruments.
| Type | IEC (India/Europe) | ANSI (USA) |
|---|---|---|
| J | Black overall, Black(+) White(−) | Black overall, White(+) Red(−) |
| K | Green overall, Green(+) White(−) | Yellow overall, Yellow(+) Red(−) |
| R | Orange overall, Orange(+) White(−) | Black overall, Black(+) Red(−) |
| T | Brown overall, Brown(+) White(−) | Blue overall, Blue(+) Red(−) |
Thermocouple signals cannot be extended using ordinary copper wire — the connection introduces a new thermocouple junction that creates measurement errors. Two types of cables are available for extending thermocouple connections. Extension cables are made from the same alloys as the thermocouple itself — most accurate but more expensive. Compensating cables are made from cheaper alloys that approximate the thermocouple's voltage-temperature characteristics over the range of ambient temperatures — suitable for most industrial installations.
K-type (Chromel-Alumel) is the most common thermocouple in Indian industry. It covers 0–1250°C, is versatile, widely available, and used in heat treatment, furnaces, HVAC, and food processing. J-type is also common for lower temperature applications.
R-type thermocouple (platinum-rhodium/platinum) for up to 1700°C. K-type for up to 1250°C. For most heat treatment furnaces in India operating up to 1000°C, K-type is standard. For higher temperature furnaces and kilns, R-type is required.
No. Ordinary copper wire creates junction errors. Use proper thermocouple extension cables (same alloy as the sensor) or compensating cables (alloy approximating thermocouple characteristics). Using wrong cables gives seriously inaccurate readings.
India uses the IEC colour code — K-type overall cable is green, positive wire is green, negative wire is white. Do not use US ANSI colour codes (yellow) which are different.
The CT716 accepts J, K, and R-type thermocouples and PT100 RTD sensors simultaneously. Each channel can be independently configured for a different sensor type — ideal for mixed-sensor installations in furnaces and industrial processes.
Tell us your temperature range and application — we will recommend the right thermocouple type and compatible data logger or controller.