Radar Level Transmitter Output Variation

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Fault Log · Field Report

Radar Level Transmitter — Output Variation Traced to a Cable a Multimeter Couldn't Catch

Plant: Cement  |  System: Radar level transmitter  |  Downtime: None

Symptom

Radar level transmitters are normally rock solid — that's the whole reason plants trust them for level measurement. So when this one started showing variation in the level reading with no real process reason for it, it stood out immediately. Radar isn't supposed to drift around like this.

Radar Level Measurement: Time-of-Flight Principle
Radar
Transmitter
The transmitter sends a microwave pulse down toward the liquid surface, then measures the time it takes for the echo to return. Since the pulse travels at the speed of light, that time-of-flight — combined with the known tank height — gives a precise, non-contact level reading regardless of vapor, dust, or turbulence at the surface.

Investigation

Worked through the usual suspects for a radar transmitter one at a time:

  • Antenna condition — cleaned the antenna thoroughly in case buildup or coating was distorting the echo. Variation continued.
  • Range and parameters — checked the configured range and parameter settings for anything abnormal. All found correct.
  • Damping time — increased the damping time slightly to see if it was a fast, noise-like variation that damping could smooth out. No improvement — this ruled out.
  • Supply voltage — checked the supply voltage since unstable supply voltage can absolutely cause output signal variation on a transmitter. Voltage measured stable on the multimeter.

With the transmitter itself, its settings, and the supply all checked clean, the remaining suspect was the cable. A thorough check of the cable condition found it to be in bad shape — degraded enough to be introducing variation into the output signal, even though the multimeter reading on supply voltage hadn't shown anything wrong.

Root Cause

Degraded cable condition was introducing variation into the transmitter's output signal. The key detail here: a multimeter check on the supply voltage came back stable, but that doesn't rule out a cable problem — a multimeter is a static, snapshot-in-time check and can miss the kind of intermittent or high-frequency variation that a cable fault introduces, especially variation that a PLC input card, sampling continuously, will pick up.

Fix

Replaced the cable. Output stabilized and the level reading returned to normal, steady radar performance.

Lesson

Don't fully clear a cable just because a multimeter reads stable voltage. A multimeter gives you a single point-in-time reading — it can easily miss the kind of variation a degraded cable introduces that a PLC card, sampling continuously, will detect. When every other cause has been ruled out and a "stable" multimeter reading is the only thing standing between you and condemning the cable, don't let that reading stop you from checking the cable's physical condition directly.

Ever chased a signal variation that your test meter said didn't exist? Drop a comment — always good to compare notes on the gap between what a meter shows and what the PLC actually sees.

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