Thermocouple and thermal resistance selection and installation - Database & Sql Blog Articles

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Thermocouples are widely used temperature measurement devices that offer reliable performance in a variety of environments. Choosing the right thermocouple or RTD (Resistance Temperature Detector) is essential for accurate readings, safety, and long-term stability. Below are some key considerations to help you make an informed decision.

1. For objects with normal operating temperatures below 300°C, thermal resistors (RTDs) are typically preferred due to their high accuracy and linearity in this range.

2. When the measured object operates above 300°C, thermocouples are more suitable because they can withstand higher temperatures and provide faster response times.

Proper installation is crucial for ensuring accurate and consistent temperature measurements. Here's a step-by-step guide to installing thermocouples and thermal resistors:

1. Measure the dimensions of the thermocouple or RTD flange or threaded fitting, and prepare the corresponding base or flange accordingly.

2. Drill holes in the pipe or equipment based on the size of the flange or threaded base.

3. Weld the flange or threaded base into the drilled hole to ensure a secure connection to the pipe or vessel.

4. Insert and tighten the thermocouple or RTD into the welded base. Make sure it is properly secured but not over-tightened to avoid damage.

5. Connect the junction box of the thermocouple or RTD according to the wiring diagram. Ensure that the junction box is isolated from the pipe wall to prevent heat transfer and maintain internal temperature below 100°C. Also, seal the outlet hole to prevent moisture or dust ingress, which could cause short circuits.

6. Choose an installation location that allows for easy access during maintenance and inspection.

When installing thermocouples and thermal resistors, it’s important to consider accuracy, safety, and ease of maintenance. The following guidelines will help ensure optimal performance and longevity:

1. To reduce measurement errors caused by heat loss, ensure that thermocouples and RTDs with protective sheaths are inserted deeply enough. For example:

(a) If the insertion depth exceeds 1 meter, install it vertically or use a support frame and protective sleeve.

(b) In flue gas applications, even if the flue diameter is 1000mm, the thermocouple or sensor can be inserted up to 500mm.

(c) For fluid temperature measurement in pipes, the sensing element should be placed in the center of the flow. If the pipe diameter is 100mm, the insertion depth should be between 50–100mm, depending on the mounting base or flange size.

(d) For high-temperature and high-pressure fluids like main steam, use shallow-insertion or sleeve-type thermocouples to minimize resistance and avoid damage. The minimum insertion depth should be at least 75mm for shallow types, and 100mm for standard types.

2. Avoid placing thermocouples or RTDs near valves, elbows, or other dead zones where the flow may be stagnant. This ensures better heat exchange between the sensor and the medium, leading to more accurate readings.

By following these guidelines, you can ensure that your temperature measurement system performs reliably and efficiently, meeting the needs of your application while maintaining safety and accuracy.