The first step in accurate temperature measurement: How to determine the optimal insertion depth of a resistance temperature detector (RTD)?

Process safety and efficiency begin with correct installation details.

In industrial temperature measurement, resistance temperature detectors (RTDs) are among the most commonly used temperature detectors in the medium- and low-temperature range. Their measurement accuracy and stability are crucial to production processes. The choice of insertion depth during installation is often a key factor determining the success or failure of temperature measurement.

Why is insertion depth so important?

Insertion depth directly affects the heat exchange efficiency between the RTD's measuring end and the measured medium.

If the insertion is too shallow, the measuring element cannot fully contact the medium, making it susceptible to interference from ambient temperature or pipe wall temperature, leading to distorted measurements.

If the insertion is too deep, it may increase unnecessary mechanical stress, affecting equipment safety, especially dangerous for high-pressure, high-speed fluids.

How to determine the optimal insertion depth?

Determining the optimal insertion depth for an RTD is not fixed; it requires comprehensive consideration of multiple factors, including medium characteristics, pipe or equipment dimensions, and the installation environment. Here are some general core principles:

1. Pipe Installation: Aim for the Center Point

For measuring the temperature of fluids within pipes, it is generally required that the measuring end of the RTD reach the center of the pipe. This is because the fluid temperature at the center of the pipe best represents the average temperature of the fluid and is least affected by the pipe wall temperature.

General rule: The insertion depth of the RTD should reach half the pipe diameter. For example, for a pipe with a diameter of 200 mm, an insertion depth of 100 mm is recommended.

Large pipe rule: When the outer diameter of the pipe is greater than 500 mm, the insertion depth is usually 300 mm.

2. Special fluids: Safety and accuracy are equally important.

For high-temperature, high-pressure, and high-speed fluids (such as main steam), to protect the RTD and reduce its resistance to the fluid, a shallow insertion method with a protective tube or a heat-shrouded RTD can be used.

The insertion depth of the protective sleeve of a shallow-insertion RTD into the main steam pipe should be no less than 75 mm.

The standard insertion depth of a heat-shrouded RTD is generally 100 mm.

3. Large Equipment and Flue Gas: Follow Empirical Values

When measuring the temperature of large containers (such as distillation towers and storage tanks) or flue gas, the situation is slightly different:

Flue Gas Measurement: Although the flue gas diameter may be large (e.g., 4 meters), an insertion depth of about 1 meter for the RTD is sufficient.

Tower Equipment: Generally, empirical values ​​can be used as a reference; the insertion depth is typically between 300mm and 500mm. Specific details should be determined in consultation with the equipment's technical personnel.

4. Exceeding Experience: Ensure Sufficient Heat Exchange

In all cases, the core principle is to ensure sufficient heat exchange between the measuring end of the RTD and the measured medium.

The measuring point location should be carefully selected, avoiding installation near valves, bends, and dead angles in pipes and equipment.

When the insertion depth of the measuring element exceeds 1m, it should be installed as vertically as possible, or a support frame and protective sleeve should be added to improve stability and protect the sensor.

Installation Precautions Summary

Besides depth, the following points are equally important:

Leading Method: In industrial measurements, a three-wire connection is preferred to eliminate the influence of connecting wire resistance. This is crucial for ensuring measurement accuracy.

Installation Direction: RTDs should be installed as vertically as possible on horizontal or vertical pipes to facilitate stress distribution and reduce wear.

Protective Sleeve: Protective sleeves are often required for easy inspection and replacement, but attention should be paid to potential heat transfer and dissipation losses due to the sleeve.

Conclusion: Leave professional tasks to professionals.

Determining the optimal insertion depth of a RTD is a technical task that combines theoretical knowledge and field experience. There is no single, absolute answer; rather, it requires a deep understanding and accurate judgment of the operating conditions.