There are a few important variables for selecting the proper (exotic) thermocouple. Most common criteria are temperature range, reaction time, abrasion / vibration resistance, chemical resistance, calibrations, installation and compatibility.
There are numerous styles of sensors to choose from, we have written down three considerations to help you determine the best thermocouple for your installation;
Temperature sensors can give unreliable measurement data if they are used for temperature measurements that are outside the specified working range. If a temperature measurement has to be done within a process where temperatures higher than 600-700°C can be expected, it is recommended to use a thermocouple.
When you know the temperature range that you need, you can check out MI Cable or Bare wire products to see which TC types are suitable for your temperature requirements. The type K thermocouple is the most used thermocouple. It is inexpensive and can be used for a wide variety of purposes. This is because of the wide temperature range and its sensitivity of approximately 41 µV/°C. The type N Thermocouple has been developed as a substitute for the type K thermocouple in the range of 0°C to 1250°C. Where a type K thermocouple shows cumulative instability above 900°C, the Type N exhibits thermal stability.
| Thermocouple Symbol | Thermocouple Chemical composition | Recommended max. working temperature |
| Type E | Cu-CuNi | 850°C |
| Type J | Fe-CuNi | 750°C |
| Type K | NiCr-Ni | 1200°C |
| Type N | NiCrSil-NiSil | 1200°C |
| Type T | Cu-Ni | 350°C |
| Type S | Pt10%Rh-Pt | 1450°C |
| Type R | Pt13%Rh-Pt | 1450°C |
| Type B | Pt30%Rh-Pt6%Rh | 1700°C |
| Type C | W5%Re-W26%Re | 2200°C |
| Type D | W3%Re-W25%Re | 2200°C |
Above 600°C a thermocouple is more accurate than a resistance thermometers (RTD sensors) and recommended to use. TC sensors have three types of thermocouple junctions: exposed, grounded or ungrounded which influence responds time.
A grounded thermocouple has the junction welded to the sheath wall of the sensor tip. The advantage of using a grounded thermocouple is that heat is easier transferred causing improved response times to temperature changes. The disadvantage is that the ground can cause a electrical ground loop that may damage your equipment. An ungrounded thermocouple has insulation, like magnesium oxide, between the junction and the sheath wall. The insulation material reduces the change of a ground loop because it is non-conductive. Therefore, the response time is also lower.
An exposed thermocouple has the junction outside the sheath. This provides fast response times but is also vulnerable for external hazards like oxidation. These thermocouple styles are useful hen you want a very fast response and have a low risk of damaging the measuring tip.
Are there vibrations in the process (machines, pipes, etc.) that need to be taken into account? Then thermocouples are more resistant to vibration than resistance sensors because of the thick wiring. However, thin film resistance sensors can also withstand vibrations well and can therefore also be considered.
For exposed thermocouples the use in noncorrosive applications is limited. For applications in corrosive or high-pressure environments you can use both a grounded or ungrounded thermocouple. Where an ungrounded thermocouple is best if there is a need to have the thermocouple electronically isolated from and shielded by the sheath.
For thermocouples electronically isolated and shielded by the sheath it is best to use an ungrounded thermocouple and for faster response times in a corrosive environment, a grounded thermocouple is best
Also consider specifications of the existing equipment. For example, existing holes may determine the thermocouple diameter.
Kamet can and will help you to find the right thermocouple for your process or application. Before we make an offer we always like to have a call or meeting to discuss the required specifications. You want to scedule such a call? Please contact us.
