What is the difference between type J and type K thermocouples?
The primary difference between type J and type K thermocouples is the materials they are made of. The positive terminal of a type J thermocouple is iron, and the negative terminal is a nickel-thallium alloy. Its constant value is approximately 94% nickel and 6% silicon.
The positive terminal of a type K thermocouple is a nickel-aluminum alloy, and the negative terminal is a nickel-aluminum-silicon (or nickel-chromium-silicon) alloy.
In addition, type J and type K thermocouples have different temperature ranges. Type J thermocouples are suitable for measuring temperatures from -210°C to 1200°C, while type K thermocouples are suitable for measuring temperatures from -270°C to 1372°C.
How do I know which thermocouple to buy?
Based on Sino-Inst’s many years of experience supplying thermocouples, we recommend the following standard selection process:
1. Determine the calibration (e.g., K, S);
2. Select the protective tube material (stainless steel, ceramic, etc.);
3. Set the accuracy class (Class I/Class II);
4. Match the mounting method (thread, flange, etc.);
5. Confirm the actual insertion depth and wire length.
Selecting the right thermocouple requires comprehensive consideration of both economic and technical considerations. Only by precisely matching the operating conditions can performance and life be maximized. For special applications (such as ultra-high pressure and severe corrosion), we recommend in-depth consultation with us for customized solutions.
What are common problems with thermocouples?
| Fault phenomenon | Causes of failure | Treatment method |
| The thermocouple thermoelectric potential value is low | Thermocouple short-circuit | Identify the cause of the short circuit. If it’s due to moisture, dry it. If it’s due to insulation damage, replace the insulator. |
| Dust accumulation on the thermocouple terminals, causing a short circuit | Clean up any accumulated dust. | |
| Short circuit between compensating wires | Identify the short circuit point and reinforce the insulation or replace the compensating wire. | |
| Deterioration of the thermocouple terminals or damage to the hot end | If length permits, cut off the deteriorated section and re-weld it or replace it with a new thermocouple. | |
| Reversed polarity of the compensating wire and thermocouple | Reconnect the wires correctly. | |
| Incorrect compensating wire and thermocouple installation | Replace the thermocouple compensating wire. | |
| Improper thermocouple installation position or insertion depth | Adjust the cold-junction compensator. | |
| Incorrect thermocouple and display instrument | Reset the display input signal type or replace the display. | |
| The thermocouple thermoelectric potential is too high | Incorrect thermocouple and display instrument | Reset the display input signal type or replace the display. |
| Incorrect compensating wire and thermocouple | Replace the thermocouple compensating wire. | |
| DC interference signal input | Eliminate DC interference. | |
| The thermocouple thermoelectric potential output is unstable | Poor contact between the thermocouple terminals and the thermocouple | Tighten the terminal screws. |
| Damage on the thermocouple measurement line, causing intermittent short circuit or grounding | Identify the fault point and repair the insulation. | |
| Thermocouple is about to break | Repair or replace the thermocouple. | |
| Deterioration of the thermocouple | Replace the thermocouple. | |
| Incorrect thermocouple installation position | Change the installation location. | |
| Dust accumulation on the protective tube surface | Clear any accumulated dust. |
