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Visible foam can disappear while a radar display remains high, fixed or unavailable. A radar reading after foam collapse deserves its own investigation: the original disturbance may have ended, but residue, altered tracking or a held controller value may remain. Do not immediately move the zero point to match the now-visible liquid.
Compare the local reading with the value used by the plant
Record local distance and available validity information alongside the PLC or historian value. If the instrument has resumed following liquid while the plant value remains fixed, investigate hold logic, communication age and filtering. Changing antenna settings will not release a value held elsewhere.
If the local measurement is also incorrect, retain its selected distance and the process timeline. Check whether the display is actually updating. A plausible number can be an old observation rather than a new measurement of the settled surface.
| Post-event pattern | Next comparison |
|---|---|
| Local value recovered | Trace the first downstream point that still differs or reports old data. |
| Local distance still fixed | Compare the selected target with geometry and available echo information. |
| Recovery after cleaning | Document the deposit and repeat a comparable observation before attributing the cause. |
Investigate a fixed radar reading after foam collapse
When access is safe and authorised, check for deposits or moisture around the antenna and nozzle. Foam reaching the upper assembly can leave a different condition from the clean installation. Do not open a pressurised vessel or dismantle the sensor to obtain a quick visual check.
The METRAVON MV-RD12 has a PTFE lens, but material identification alone does not define a permitted cleaning method. Use the instructions for the actual assembly. Record what was found before approved cleaning so a later improvement can be interpreted.
Review settings changed during the disturbance
Operators may have increased damping, held a value or changed another setting during the disturbance. Compare the present configuration with the accepted baseline. Resolve any temporary control changes through the site's procedure rather than assuming they cleared when the foam disappeared.
Verify the required liquid reading at a known condition after correction. Then consider whether the original foaming stage is still an unqualified operating condition. Successful recovery does not prove reliable measurement during foam formation, and a corrected local reading does not automatically restore every alarm or control function.
Q1. Should the transmitter be reset before collecting evidence?
Not solely to make the display normal. Preserve the event and settings where safe, then use a supported recovery procedure if needed. An early reset can remove evidence of a recurring fault.
Q2. Does foam disappearing prove the antenna is clean?
No. The visible surface and the condition at the antenna are separate observations. Check for remaining deposits through the approved access method.
Summary
Trace why the fault persists after the visible disturbance ends. Send the MV-RD12 local and PLC records with the foam and recovery timeline before applying offsets or replacing hardware.
