Essential for industrial instrumentation, the thermocouples They convert heat into electrical signals via the Seebeck effect to monitor extreme processes. Among the extensive line of sensors available, the type B thermocouple It stands out as the elite solution: its noble composition of platinum and rhodium guarantees precision at temperatures where others would fail. But when should you use it and how does it work?
What is a thermocouple and how does it work in practice?
A thermocouple is a robust and simple temperature sensor, consisting of two wires of different metals or metal alloys, joined at one end, called the junction. measuring junction (or hot junction).
The operation is based on Seebeck effect, discovered by Thomas Johann Seebeck in 1821. When a temperature gradient exists between the measuring junction and the other end (reference or cold junction), an electromotive force (EMF) is generated, expressed in millivolts.
Contrary to popular belief, voltage is not generated solely at the soldering point, but rather along the conductors that traverse the thermal gradient. This "thermoelectric signature" is what allows the instrument to translate voltage into degrees Celsius or Fahrenheit with high repeatability.
What does a type B thermocouple look like?
The type B thermocouple represents the pinnacle of thermoelectric engineering for extreme heat. Its durability, compliance with stringent standards, and stability make it a safe investment for industries that cannot afford inaccurate measurements.
The type B thermocouple belongs to the group of thermocouples of noble metals (Types S, R, and BIts chemical composition is what dictates its superior performance:
- Positive Leg: Platinum alloy with 30% rhodium.
- Negative Leg: Platinum alloy with 6% rhodium.
This sensor is designed to operate in ranges that vary from +100 °C to +1.800 °CAlthough it can detect lower temperatures, its response curve below 500 °C is almost zero, making it unsuitable for low-temperature processes.
Its great advantage lies in its chemical stability. Because it is composed primarily of platinum, it resists oxidation and corrosion in oxidizing atmospheres exceptionally well, maintaining precision even after long periods in industrial furnaces.
How does a type B thermocouple compare to types R and S?
When designing a high-temperature system, the question arises: should we choose Type B, R, or S? They all use Platinum and Rhodium, but the proportions alter the sensor's behavior.
| Feature | Type B Thermocouple | Type R Thermocouple | Type S thermocouple |
| Composition | PtRh30% / PtRh6% | PtRh13% / Pt | PtRh10% / Pt |
| Maximum Temperature | Up to 1.800 °C | Up to 1.600 °C | Up to 1.600 °C |
| Sensitivity | Low (< 600 °C) | High | High |
| Cost | Highest | High | High |
| Mechanical Resistance | Superior in extreme heat. | High | High |
Type B is the "heavyweight champion." While Types R and S are more sensitive and accurate in intermediate ranges, Type B is the only one that maintains structural integrity and reliability continuously above 1.600 °C.
What are the advantages and disadvantages of type B?
The decision to implement a type B thermocouple should consider the balance between performance and investment.
Advantages:
- Exceptional Thermal Resistance: It is one of the few sensors capable of operating at temperatures up to 1.800 °C without immediate degradation.
- Chemical Inertia: Excellent performance in oxidizing and inert atmospheres.
- Regulatory Compliance: It strictly adheres to the standards. AMS 2750 E e CQI-9, essential for the aerospace and automotive industries.
- Minimum Cold Junction Error: Due to its response curve, temperature variation at the reference junction (between 0 °C and 50 °C) has a negligible impact on the final measurement, often eliminating the need for expensive compensation cables for short distances.
Disadvantages:
- Acquisition Cost: Rhodium is an extremely valuable metal. The high content of this metal in type B makes it considerably more expensive than type K or J.
- Zero Sensitivity at Low Temperatures: It should not be used to measure anything below 600 °C, where the percentage error is extremely high.
- Vulnerability to Contamination: Although it resists oxidation, it is sensitive to contamination by metallic vapors or silica reduction in reducing atmospheres, which necessitates the use of high-purity ceramic protective tubes (such as alumina).
Where are type B thermocouples used in industry?
Steel and Metallurgical IndustryIn the heart of steel mills, the Type B monitors pig iron and liquid steel. It is essential to ensure that the metal reaches the exact melting point for casting.
Glass manufacturingMolten glass is highly corrosive and requires constant temperatures above 1.500 °C. The type B thermocouple, coated with platinum or special ceramic protection, is the gold standard for this application.
Aerospace and Defense IndustryIn rocket engine and aircraft turbine tests, where combustion temperatures are extreme, type B provides critical data for the safety and efficiency of the components.
Research and DevelopmentMaterials science laboratories use type B in calcination and sintering furnaces to develop new ceramic compounds and metal alloys.
Do you need a custom design? At Alutal, we manufacture custom sensors for the most challenging applications on the market, ensuring your operation never stops due to a lack of thermal precision.
FAQ
For many industrial applications, type B cables can use ordinary copper cables if the reference junction is between 0°C and 50°C, as the voltage generated in this range is negligible. However, for laboratory precision, specific compensation cables are recommended.
In critical processes (such as aerospace heat treatment), calibration must follow AMS 2750 standards. In general high-temperature use, semi-annual verification is recommended, as rhodium can migrate between the thermocouple legs at extreme temperatures, causing drift. Alutal has the only CGCRE-accredited laboratory for the calibration of noble metal thermocouples up to 1600 ºC.
Yes. Type B is used in vacuum furnaces, provided there are no metal vapors that could contaminate the platinum. The use of high-purity (99,7%) alumina insulators is mandatory in these cases.
