Unstable Tank Radar Readings: Diagnosis and Acceptance Tests

An unstable tank level may be a real process condition, a radar echo problem or a downstream signal problem. Agitation, inlet flow, foam and boiling can move the surface; condensation, coating, nozzle reflections and internals can change the selected echo; wiring, scaling and filtering can alter the displayed value. Buying a replacement before locating the layer wastes time and can reproduce the same symptom. A structured investigation records the condition, verifies an independent level, compares raw distance and echo diagnostics, traces the signal and repeats the failed state after correction.

Classify the instability

Record amplitude, frequency, duration and relationship to filling, agitation, temperature, pressure, cleaning or level. Determine whether the local distance, local level, loop current, PLC tag and HMI all move together. Time-aligned trends and operator observations distinguish genuine process motion from instrument or software behaviour.

Verify the real process state

Use a safe independent gauge, known transfer, mass balance or stable operating point. Inspect whether foam, vapour, boiling, inlet jets or a moving internal surface could explain the changes. Record product and conditions with the reference; an uncertain manual reading should not be used as an exact calibration point.

Inspect geometry and antenna

Measure reference plane, nozzle and fixed structures and compare their distances with the echo curve. Check antenna condensation, deposits, damage, alignment and direct spray. A recurring jump to one distance often indicates a fixed target, while gradual bias after cleaning may indicate a changed reference or coating.

Review target selection and configuration

Save the original curve, selected echo, confidence, false-echo map, response rate and blocking distance. Confirm the product group and firmware against the accepted baseline. Avoid increasing damping or suppression until the physical target is identified; filtering may create a calm but delayed or incorrect value.

Trace electrical and software processing

Compare transmitter output with calibrated current or digital register, PLC raw value, scaling, filters and HMI. Check loop voltage, grounding, shielding, register byte order and stale-data logic. If local distance is stable while the PLC changes, troubleshooting should move downstream rather than retuning the radar.

Correct and prove stability

Change one justified cause, repeat the operating state and compare before-and-after trends and curves. Test high-level response, lost echo and communication faults after any filter change. Define an acceptable stability band and observation period, record residual limitations and schedule follow-up under untested process conditions.

Define the acceptance observation window

Choose an observation period that includes the conditions associated with the instability, such as a complete batch, transfer, heating cycle, agitator sequence or rainfall event. Define permitted noise, maximum deviation, lost-echo count and alarm performance before the test. Record process state and compare raw distance, signal quality and downstream trend throughout the window. If a condition cannot be observed, keep it as an explicit limitation with a planned test. Stable readings during an unrelated calm period should not be used to close a fault that only occurs during demanding operation.

Fault closure criteria

Close the instability only when the agreed observation window shows acceptable raw distance, echo confidence, output and alarm behavior with no unexplained configuration changes. If the symptom is reduced but not eliminated, quantify the remaining frequency and consequence and obtain an explicit risk decision. Attach trend and echo evidence rather than a verbal report. Define a recurrence trigger that reopens the investigation. Clear closure criteria prevent intermittent faults from being declared solved merely because they were absent during a short technician visit.

Engineering checklist

Record where and when the value changes.
Establish an independent process reference.
Inspect geometry, antenna and fixed targets.
Preserve curves before changing parameters.
Trace output through PLC and HMI.
Repeat the original condition after correction.

Frequently asked questions

Should the radar be replaced first?

No. Confirm whether the cause is process, installation, electronics or PLC before selecting corrective action.

Can damping solve fluctuation?

It can reduce displayed noise but may delay alarms and hide an incorrect selected echo.

What proves the issue is resolved?

Stable, correct readings under the original operating condition with documented fault and alarm behaviour.

Need a project-specific review? Send process data, drawings, photographs and acceptance criteria through our contact page.