The differences between type B and type S thermocouples go far beyond the temperature range, and understanding what truly changes between these sensors can be crucial for the accuracy of an industrial process. Although both are used in high-performance measurements, there are technical details that completely separate their applications. This difference explains why one excels in extreme environments while the other is chosen for fine control.
What is a type B thermocouple?
the range of thermocouples Type B is ideal for applications in extremely high temperatures. This thermocouple remains stable and accurate up to 1700°C – 1800°C. The specialty of the type B thermocouple lies in its higher temperature limit among all thermocouple types. These thermocouples produce the same output at 0°C and 42°C, limiting their use to temperatures below approximately 50°C. The electromotive force (EMF) function exhibits a minimum around 21°C, facilitating cold junction compensation.
Type B thermocouples (PtRh30-PtRh6) are composed of platinum and rhodium, making them extremely resistant to corrosion and high temperatures. These temperature sensors are capable of measuring high temperatures up to 1700°C – 1800°C, making them ideal for applications in extreme environments, such as measuring the temperature of molten glass and also in steel mills.
It requires rigorous care:
- It should not be used in reducing atmospheres, nor in environments with metallic or non-metallic vapors, which can contaminate the platinum and compromise accuracy.
- It should never be inserted directly into metal protective tubing.
Benefits
- Measurements across a wide temperature range: Continuous: 500 – 1800°C.
- Recommended for use in higher temperatures (above 500°C).
Disadvantages
- It provides the lowest electromotive force (emf) output per degree Celsius of temperature change.
- More expensive than other types, as it uses platinum alloys.
- Slower response for measurements below 500°C.
- It cannot be used for measurements at temperatures below zero.
- Lower efficiency than types R and S below 600°C (1112°F)
What is a type S thermocouple?
Among sensors designed for high-temperature measurements, the Type S thermocouple is frequently used. The equipment is constructed with noble metals – Platinum with 10% Rhodium (Pt10%Rh) at the positive pole and pure Platinum at the negative pole – a combination that makes it especially resistant to heat and the harsh conditions typical of advanced industrial processes.
The Type S has a standardized temperature range of -50°C to +1760°C, although continuous use is recommended within more controlled intervals.
In industrial practice, the sensor is suitable for continuous measurements from 100°C to 1480°C, and can reach up to 1760°C for short periods, provided it is adequately protected.
This model is recommended for clean, oxidizing, neutral (air) atmospheres and non-aggressive gases, such as noble gases. It also tolerates vacuum, provided it is for short periods, without risk of deterioration.
At the same time, it requires rigorous care:
- It should not be used in reducing atmospheres, nor in environments with metallic or non-metallic vapors, which can contaminate the platinum and compromise accuracy.
- It should never be inserted directly into metal protective tubing.
- It requires high-alumina ceramic insulators and protective tubes, ensuring structural and chemical stability under extreme conditions.
Benefits
The design features of the Type S make it an excellent sensor when the focus is on advanced thermal performance:
- High precision, ideal for critical applications.
- Excellent reproducibility, maintaining consistent results even after long thermal cycles.
- High thermal stability, used in continuous processes.
- Reliable operation in medium and high temperature ranges, where other thermocouples begin to lose efficiency.
Disadvantages
Despite its superior performance, the Type S has significant limitations:
- Low sensitivity, around 10 μV/°C, requiring high-quality reading instruments.
- High cost, a direct consequence of the use of precious metals such as platinum and rhodium.
- Chemical fragility, especially in unsuitable atmospheres, demands robust protection and the correct selection of ceramic materials.
Comparison of Type B and Type S thermocouples
The main difference between Type S and Type B thermocouples lies in their temperature limits and sensitivity: while Type B withstands higher temperatures, reaching up to 1800°C, Type S offers greater sensitivity and stability, especially in intermediate ranges, making it more accurate in many industrial applications. In short, Type B is more resistant to extreme heat; Type S is more sensitive and stable for continuous measurements.
The Type S thermocouple (PtRh10–Pt) uses a platinum alloy with 10% rhodium and operates reliably up to 1480°C. Its sensitivity is similar to that of type R and superior to that of type B, ensuring good response to thermal variations in industrial and laboratory processes. Stability is its strong point: even after long cycles in ceramic, metallurgical, or calibration furnaces, it maintains a predictable thermal curve, which is why it is widely used as a reference in high-temperature measurements.
The Type B thermocouple (PtRh30–PtRh6) was developed for even harsher environments. With 30% rhodium in one conductor and 6% in the other, it is highly resistant to corrosion and operates up to 1700°C, surpassing both types S and R in this respect. This robustness makes it ideal for heavy metallurgy furnaces, continuous processes, and situations where sensor durability is more critical than accuracy in lower ranges. On the other hand, its performance is less sensitive below 50°C, which limits its use in finer measurements outside of extreme heat conditions.
Glass and Ceramics Industry
- Type S: used to control the temperature of furnaces and casting stages, promoting uniformity and quality of the final product.
- Type B: preferred in furnaces that operate consistently above 1.200°C, delivering greater durability in harsh thermal environments.
Steel and Metallurgy
- Type S: Used to monitor melting, annealing, and heat treatments that require precision even over long cycles.
- Type B: suitable for measurement in steel mills, high-temperature furnaces, and processes that exceed the critical range supported by other sensors.
Calibration and Research Laboratories
- Type S: often chosen as a reference sensor, thanks to its stability and predictable behavior at high temperatures.
- Type B: also used in high-temperature standards, especially in tests requiring extreme thermal resistance for extended periods.
Chemical and Petrochemical Industry
- Type S: accurately monitors reactors, boilers, and units that operate with intense heat and require rapid responses.
- Type B: suitable for severe and continuous processes, where maintenance must be reduced and the sensor needs to withstand temperatures close to the operational limit of the materials.
How to choose between the different types?
Choosing the right type of thermocouple is a matter of matching the thermocouple to your measurement needs. Here are some aspects to consider:
temperature rangeDifferent types of thermocouples have different temperature ranges. For example, type T, with its copper terminal, has a maximum temperature of 370°C or 700°F. Type K, on the other hand, can be used up to 1260°C or 2300°F.
Driver's dimensionsThe diameter of the thermocouple wires also needs to be taken into account when long-duration measurements are required. For example, type T thermocouples are rated for 370°C/700°F; however, if your thermocouple has 14 AWG gauge wires (0,064” diameter), the rating will be 370°C/700°F. If your thermocouple has 30 AWG gauge wires, the rating drops to 150°C/300°F. More information can be found here (see the table at the bottom of page H-7).
PrecisionType T thermocouples have the highest accuracy among all base metal thermocouples, with ±1°C or ±0,75%, whichever is greater. Next come type E thermocouples (±1,7°C or 0,5%) and types J, K, and N (±2,2°C or 0,75%) for standard error limits (according to ANSI/ASTM E230).
Other important considerations are the sheath materials (if it is an immersion probe type), the insulating material (if it is a wire or surface sensor), and the sensor geometry.
Alutal Thermocouples
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