How to judge the quality of thermocouples? The correct selection method of thermocouples

What is a thermocouple? A thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures temperature and converts the temperature signal into a thermoelectromotive force (EMF). This signal is then converted to the temperature of the measured medium via an electrical instrument (secondary instrument). The appearance of various thermocouples often varies greatly depending on the application, but their basic structure is generally the same. They typically consist of a thermocouple, an insulating protective tube, and a junction box. They are typically used in conjunction with display instruments, recording instruments, and electronic regulators. This article has compiled some relevant information about thermocouples and hopes it will be of great reference value to readers.

How to determine the quality of a thermocouple?

1. Visually inspect the protective tube for corrosion, penetration, and leakage.

2. Use a multimeter to measure continuity. The resistance of an assembled thermocouple is generally no greater than 2 ohms, and for a wire-type thermocouple, no greater than 50 ohms. A resistance greater than 1K generally indicates a defective thermocouple.

3. Use a multimeter to measure the resistance; resistance exceeding 100K is considered bad.

4. Use a multimeter to measure resistance using the ohmmeter method. Adjust the resistance, connect the two terminals, and burn them with a lighter. If the multimeter needle noticeably increases or decreases, it's probably working. If it doesn't move, it's broken.

5. Use a multimeter to measure the voltage across the terminals in the millivolt range. If there's no voltage, it's broken.

6. Remove the thermocouple from the instrument's thermocouple input terminals and short-circuit them with any wire. Turn on the power. If the digital display on the top of the meter shows a value approximately equal to room temperature, the thermocouple's internal wiring is open and you should replace it with a thermocouple of the same model. If the same condition persists, the instrument's input terminals were damaged during transportation and the instrument should be replaced.

7. Remove the thermocouple from the faulty instrument and replace it with a thermocouple connected to a similar, functioning, adjacent meter. After powering on, if the digital display on the original faulty instrument shows the heating element temperature, the thermocouple's wiring is open and you should replace it with a thermocouple of the same model.

8. Remove the faulty thermocouple from the instrument and use a multimeter to measure ohms (R)*1. Use the two probes of the multimeter to measure both ends of the thermocouple. If the multimeter displays a high resistance value, it indicates an open circuit in the thermocouple's internal connection. Replace the thermocouple with a similar type. Otherwise, if there is a certain resistance, there is a problem with the instrument's input terminal and the instrument should be replaced.

9. Connect the thermocouple correctly according to the instrument wiring diagram. If the digital display on the instrument shows a negative value after powering on the instrument, the thermocouple connections between the "+" and "-" terminals are incorrect. Replace them again.

How to Correctly Select a Thermocouple

In traditional industrial applications, thermocouple elements are generally terminated at the connector; however, the reference junction is rarely located there. Instead, use appropriate thermocouple extension cable to connect to a controlled environment with relatively stable temperature. Thermocouple selection should be based on a comprehensive consideration of the operating temperature range, required accuracy, operating atmosphere, the performance of the measurement object, response time, and cost-effectiveness.

1. Measurement Accuracy and Temperature Range

For operating temperatures between 1300°C and 1800°C with relatively high accuracy requirements, a B-type thermocouple is generally selected. For lower accuracy requirements and acceptable atmospheres, a tungsten-rhenium thermocouple can be used. For temperatures above 1800°C, a tungsten-rhenium thermocouple is generally used. For operating temperatures between 1000°C and 1300°C with relatively high accuracy requirements, an S-type or N-type thermocouple can be used. Below 1000°C, a K-type or N-type thermocouple is generally used, while below 400°C, an E-type thermocouple is generally used. For temperatures below 250°C and negative temperatures, a T-type thermocouple is generally used. At low temperatures, a T-type thermocouple offers stability and high accuracy.

2. Atmosphere Selection

S-type, B-type, and K-type thermocouples are suitable for use in strongly oxidizing and weakly reducing atmospheres. J-type and T-type thermocouples are suitable for weakly oxidizing and reducing atmospheres. If a relatively airtight protective tube is used, the atmosphere requirements are less stringent.

3. Durability and Thermal Response Selection

Thermocouples with larger wire diameters offer better durability but slower response. This slow response also affects thermocouples with larger heat capacities, resulting in poor temperature control when measuring large temperature gradients. For those requiring both fast response and a certain degree of durability, armored thermocouples are more suitable.

4. Thermocouple Selection Considers the Nature and State of the Measured Object

Temperature measurement of moving objects, vibrating objects, and high-pressure vessels requires high mechanical strength. Chemically contaminated atmospheres require a protective tube. Electrical interference requires high insulation.

In summary, I believe that the above information has provided you with a basic understanding of thermocouples. I also encourage you to review these lessons as you learn, so that you can continuously improve your professional skills.