J type and K type are the two most common thermocouples in Indian industry. Both are inexpensive and reliable, but they are not interchangeable. Using the wrong type – or the wrong extension cable – gives wrong temperature readings.
How a thermocouple works
A thermocouple is two different metal wires joined at one end. When that junction is heated, a small voltage is produced. The temperature controller reads this voltage and converts it into temperature. Each thermocouple type uses a different pair of metals, so each produces a different voltage for the same temperature.
J type thermocouple
- Materials: iron (+) and constantan (−)
- Typical range: up to about 750 °C (lower in continuous use)
- Strengths: low cost, good sensitivity, very common on plastic processing machines
- Limitations: the iron wire can rust in moist or oxidising conditions; not for high temperatures
Typical uses: injection moulding and extrusion barrels, hot runners, moulds, dies and packaging machines.
K type thermocouple
- Materials: chromel (+) and alumel (−)
- Typical range: up to about 1200 °C (depending on sheath and wire size)
- Strengths: wide temperature range, good oxidation resistance, general-purpose
- Limitations: slightly less sensitive than J type at low temperatures
Typical uses: furnaces, heat treatment, ovens, kilns, exhaust gas and general process measurement.
Quick comparison
| J type | K type | |
|---|---|---|
| Wires | Iron / Constantan | Chromel / Alumel |
| Approx. max temperature | ~750 °C | ~1200 °C |
| Best for | Plastic machinery, moulds | Furnaces, ovens, high temperature |
| Cost | Low | Low |
| Moisture resistance | Lower (iron can rust) | Better |
The most common mistake
The controller, the thermocouple and the extension/compensating cable must all be the same type. A K type sensor connected to a controller set for J type – or joined with J type cable – will show a wrong temperature, and your heaters may run much hotter or colder than you think.
When to choose an RTD (PT100) instead
An RTD such as a PT100 gives better accuracy and stability at lower temperatures (roughly up to 400–500 °C). It is a good choice when precise control matters – for example in food, pharma and liquid heating – but it is less rugged and more expensive than a thermocouple.
Thermocouples and RTDs from Thermosys
Thermosys Technologies supplies J and K type thermocouples and PT100 RTD sensors in Pune, in the probe length, sheath and termination your machine needs. Cartridge heaters can also be supplied with a built-in thermocouple.