Thermocouples are available in various types of calibrations ranges, it is important to carefully select the right thermocouple for our application such as oxidizing, and reducing atmospheres taken into consideration.
In the table below are listed the main types of thermocouples:
|Thermocouple type||Positive Element, Negative Element||Typical Temperature Range||Standard Error Line|
|Type T||Copper Constantan||-200°C~370°C||+/-1°C|
|Type J||Iron Constantan||0°C~760°C||+/-2.2°C|
|Type E||Chromel Constantan||0°C~870°C||+/-1.7°C|
|Type K||Chromel Alumel||0°C~1260°C||+/-2.2°C|
|Type N||Nicrosil Nisil||0°C~1260°C||+/-2.2°C|
|Type S||90% Platinum / 10% Rhodium Pt 100%||0°C~1480°C||+/-1.5°C|
|Type R||Platinum / 13% Rhodium Pt 100%||0°C~1480°C||+/-1.5°C|
|Type B||Platinum / 30% Rhodium 94% Platinum / 06% Rhodium||0°C~1700°C||+/- 0.5%|
Thermocouple Type T
It can be used in inert, oxidizing, or reducing environments. It has good precision due to a high homogeneity with which copper can be processed.
At temperatures above 300°C, the oxidation of copper becomes very intense, thus reducing its useful life and causing deviations in its original response curve.
Thermocouple Type J
The material can be suitable in reducing, neutral or oxidizing environments. However, it is not recommended in high humidity atmospheres and also at low temperatures, as the J-type thermocouple can become brittle.
Thermocouple Type B
It has very similar characteristics to the R and S models. They are more stable, however, due to their reduced sensitivity, they are generally used only to measure temperatures above 300°C, up to 1800°C.
It may also be suitable for use under a vacuum for short periods. However, it should not be used in vapor-containing or reducing atmospheres, both metallic and non-metallic. This equipment should not be inserted directly into a primary metal protection tube. It requires the use of high alumina ceramic insulators and protection tubes.
Thermocouple Type E
It can be used in oxidizing, inert, or vacuum conditions. In contrast, it is not suitable for alternating oxidizing and reducing atmospheres.
Compared to other commonly used thermocouples, it has a higher thermoelectric power, which is very advantageous for those who want to detect small variations in temperature.
Thermocouple Type K
It stands out for being of generic use. It has a low cost, and due to its popularity, it is available in the most diverse probes. This type of thermocouple is suitable for the temperature range from -200°C to 1200°C.
Thermocouple Type N
Its great stability and resistance to oxidation at high temperatures make it more suitable for measurements at high temperatures, without resorting to thermocouples that incorporate platinum in their constitution (types B, R, and S). This was designed to be a K-type ‘evolution’.
Thermocouple Type S
It can be used in inert or oxidizing environments. It has a high stability index when exposed to high temperatures over time, which is higher than that of non-platinum thermocouples.
However, the thermocouples in this apparatus must not be exposed to reducing atmospheres or metallic fumes. What is indicated is that they are never inserted directly into metal protection tubes, but first into a ceramic protection tube with the following specifications: made of high purity alumina (Al2O3) (99.7%), commercially called type 799 (old 710).
However, it should be noted that ceramic tubes with 67% Lumina content are available on the market, called type 610, but their use for platinum thermocouples is not recommended.
Thermocouple Type R
They have the same characteristics as the Type S thermocouples, being suitable for measuring temperatures up to 1600 °C, but due to their higher cost, their use in general industry is not as common.