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Thermoelectric temperature sensors, such as thermocouples, are widely used in industrial applications due to their durability and accuracy. However, they must be designed to withstand harsh environments, including high temperatures, oxidation, and corrosion. Metal thermocouples made from common materials are not suitable for use in oxidizing or corrosive atmospheres, while precious metal thermocouples should avoid reducing environments. To protect the sensor, a stainless steel protective sleeve is often used, along with a ceramic insulating tube to isolate the conductive parts. This setup ensures electrical insulation and mechanical protection, allowing the thermocouple to function reliably over time.
Another design approach involves enclosing the thermocouple wires in a ceramic powder and then pulling them into a stainless steel tube, creating an armored thermocouple. This structure provides enhanced mechanical strength and durability, making it ideal for challenging conditions. As shown in Figure 2.3.5(b), this type of thermocouple is commonly used in industrial settings where physical stress is a concern.
Despite their advantages, thermocouples can experience issues such as leakage current at high temperatures, especially when measuring electrically sensitive systems. This can introduce common-mode interference on the signal line, which may disrupt the operation of connected instruments or computers. Therefore, careful consideration must be given to the environment and the need for shielding or isolation.
The thermal inertia caused by the protective sleeve and ceramic tube can also affect the response time of the thermocouple. This increased time constant means that the sensor may not react immediately to rapid temperature changes. In scientific experiments where fast response is critical, exposed thermocouples are preferred, particularly those with smaller diameters that offer quicker thermal response.
Thermocouples generate a small electromotive force (EMF) based on the temperature difference between two dissimilar metals. This EMF arises from two main effects: contact potential and temperature gradient potential. The contact potential occurs at the junction of two different materials, while the temperature gradient potential develops along a single conductor when there is a temperature difference across its length. Together, these effects produce the total EMF measured in the closed circuit.
To measure this EMF accurately, the thermocouple is connected to a measuring instrument. According to the third-conductor rule, the presence of a third material in the circuit does not affect the total EMF as long as both ends of that material are at the same temperature. This allows for the cold junction to be moved away from the hot junction, typically to a stable reference point, ensuring more accurate readings.
In cases where the thermocouple uses expensive materials, compensation wires are used to extend the cold junction to a more controlled area. These wires have similar thermoelectric properties to the original materials but are more cost-effective and easier to handle. Proper polarity connections are essential to ensure correct operation and minimize measurement errors.
Overall, thermocouples remain a vital tool in temperature measurement, offering reliable performance in a wide range of applications. Understanding their construction, limitations, and proper usage is essential for achieving accurate and consistent results.
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