The insertion depth of a temperature sensor used in the field can affect measurement accuracy. In most applications, the insertion depth is calculated based on the centerline of the pipeline. This method is suitable for most applications. However, for large-diameter pipes, towers, furnaces, and other temperature measurement applications, this method may not always be appropriate. It may result in unnecessary material costs or make installation and fixation difficult.

When fluid flows through a pipeline, different flow velocities can result in two main flow patterns: laminar flow and turbulent flow.

The fluid velocity is generally highest at the center of the pipe and lowest near the pipe wall. When the flow velocity is low, the fluid flows in layers with little mixing between them. This is known as laminar flow.

As the flow velocity gradually increases, the flow lines begin to oscillate in a wave-like pattern. The frequency and amplitude of these oscillations increase with the flow velocity. This is known as transitional flow.

When the flow velocity becomes sufficiently high, the flow lines are no longer clearly distinguishable. Numerous small eddies form in the flow field, and the laminar structure breaks down. Adjacent fluid layers not only slide against each other but also mix together. The fluid then moves irregularly, with velocity components perpendicular to the pipe axis. This type of flow is called turbulent flow.

Under turbulent flow conditions, the temperature sensor only needs to penetrate through the relatively thin laminar boundary layer near the pipe wall to measure the temperature of the medium with relatively little variation.

Under laminar flow conditions, changes in the upstream temperature are detected first at the center of the pipe and then gradually near the pipe wall. The temperature change detected at approximately one-quarter of the pipe diameter is only slightly delayed compared with the center.

For this reason, the insertion depth of a temperature sensor does not always need to reach the center of the pipeline. This is especially important for large-diameter pipes. Otherwise, the temperature sensor would need to be unnecessarily long, increasing costs and the risk of damage.

To ensure accurate measurement, the sensing element of the temperature sensor should be fully immersed in the measured medium.

The typical required immersion lengths are as follows:

1. Gas measurement:

Thermocouple: insertion depth should be greater than 95 mm.

RTD and bimetallic temperature sensors: insertion depth should be greater than 115 mm.

2.Liquid measurement:

Thermocouples, RTDs, and bimetallic temperature sensors: insertion depth should be greater than 46 mm.

3.Calculation of insertion depth:
Temperature sensor insertion depth = connection length + pipe wall thickness + immersion length

4.Small-diameter pipelines:
If the process pipe diameter is too small (less than 80 mm), an enlarged pipe section should be installed when mounting the temperature sensor.

6.Temperature measurement in towers:
For general tower equipment, when the temperature sensor is installed horizontally, an insertion depth of approximately 300–400 mm is generally sufficient.

Temperature measurement in boiler furnaces:
Temperature sensors are usually installed horizontally. After excluding the thickness of the refractory material, a thermocouple only needs to extend approximately 150 mm into the furnace.

Proper selection of the temperature sensor insertion depth can help ensure accurate temperature measurement while avoiding unnecessary costs and potential damage to the sensor.