Why does the M38 threaded radar read 1105 mm when the actual level is 510 mm inside a narrow pipe?

Answer: The radar was mounted off-center (or tilted) in the 108 mm pipe, so part of the beam hit the pipe wall and the sensor locked onto the wall echo — about 1105–1106 mm — instead of the true 500–510 mm level. Adjusting the installation position so the antenna sits centered in the pipe and perpendicular to the liquid surface fixes the reading.

Symptom

The M38 thread radar level transmitter (38 mm body, model JJY-YLD128D-W-32) was mounted on a pipe with a diameter of 108 mm. When the actual level was around 500 mm, the output jumped around and read about 1106 mm. With the true distance at about 510 mm, the sensor reported 1105 mm. Aimed at open ground the same unit read normally, which confirmed that the beam was hitting the pipe wall.

Field installation photos — the M38 transmitter mounted on the pipe:

Echo spectrum at the moment of the fault — the sensor reported a false distance of 1.1056 m:

Root cause

In a narrow 108 mm pipe, an off-center or tilted antenna lets part of the beam cone reach the pipe wall. The wall returns a strong echo from a longer path, so the radar locks onto the wall instead of the liquid surface. The output therefore shows a much longer and unstable distance, and it jumps whenever the beam footprint touches the wall.

Solution

1. Install the antenna in the center of the pipe so the whole beam stays inside the pipe diameter.
2. Keep the antenna axis perpendicular (90°) to the liquid surface — do not tilt the transmitter.
3. Avoid positions where the beam grazes the pipe wall, weld seams or internal fittings; shorten the mounting nozzle if one is used.

After the installation position was corrected, the readings became stable and matched the actual level.

If you meet a similar application, contact our support team with your installation photo and the echo spectrum from the upper-computer software — we will help you select and tune the right radar level transmitter.

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