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Foam level measurement must specify which surface matters. The top of the foam, the liquid beneath it and the space remaining below a vent are different quantities. A stable radar reading is useful only when its target matches the decision the operator or controller needs to make.
Foam Level Measurement: Define the Target Before the Trial
| Target | Used for | Required proof |
|---|---|---|
| Foam top | Occupancy or carryover assessment. | The detected surface follows the foam through its relevant changes. |
| Liquid | Liquid-level regulation or inventory conversion. | The measurement follows the liquid rather than switching to the foam. |
| Headspace | Remaining clearance to a defined fitting or limit. | The relevant highest surface and physical reference are identified. |
| Foam alarm | A defined operator action or separately assessed protective duty. | Detection and response are verified for the specified foam condition. |
Describe how foam changes with recipe, agitation, aeration, cleaning and time. Wetness, structure, thickness and persistence affect the observed signal. Frequency alone does not establish whether a particular radar will follow the foam or the underlying liquid.
What a Liquid Radar Specification Does Establish
The METRAVON MV-RD12 documents a 32 mm PTFE lens, an 8-degree beam and a liquid measuring interval of 0.1-30 m. Those values help assess geometry and operating distance. They do not establish simultaneous foam-top and liquid-level outputs or guaranteed penetration of the foam in a particular process.
Locate the antenna away from direct spray and the foam-generating inlet where the vessel permits. Check nozzle reflections and possible lens deposits. Changing to another antenna may improve the installation, but it should not be presented as a guaranteed cure for an undefined foam condition.
Design a Trial That Can Reveal a Target Change
- Select an independent reference for the quantity being assessed. A liquid reference alone does not establish foam-top height; a visual foam-top observation alone does not establish liquid volume.
- Record the liquid and foam conditions before formation, during the relevant foaming stage and after collapse, using the approved observation method.
- Save raw distance, echo information and device status with the process state. Compare the movement of the indicated target with the chosen reference.
- Check what reaches the PLC or data logger when the target becomes unavailable or changes. Include the response delay introduced by filtering.
Define acceptance conditions before seeing the results. If the instrument follows liquid in one state and foam in another, that is a limitation to resolve, not a reason to average the two into a plausible level. A water-only test cannot qualify the foaming stage.
Keep Inventory and Carryover Decisions Separate
A tank-volume table converts liquid height to volume only when its input is actually liquid height. Feeding it the foam-top height can overstate the liquid quantity. Conversely, a radar that continues seeing liquid may not warn that foam is approaching a vent.
If the two decisions require different evidence, specify separate measurements or an approved operating arrangement. Do not claim that a second echo visible during a trial proves two reliable output channels. Obtain the supported functions of the exact electronics and verify them across the required process states.
The controller should distinguish current valid data, invalid measurement and a held previous value. If a process-state restriction is used, define who releases it and what proves that normal measurement has returned. Avoid undocumented operator workarounds that quietly turn a liquid-level display into an estimated quantity.
Questions About Foam Trials
Q1. Can longer damping make the instrument see through foam?
No. Filtering changes the reported response; it does not establish which surface produced the signal. It can conceal a target switch and delay an alarm. Identify the echo behaviour before deciding the acceptable response time.
Q2. Why is a settled sample insufficient?
The foam may collapse or change structure before it is examined. Observe the relevant operating stage, including recipe and temperature conditions. Qualification should address the difficult intended state, not only the condition remaining after the process stops.
Q3. Can an accepted trial cover later recipe changes?
Only within the demonstrated operating range. Review changes that alter foaming, deposits or cleaning exposure, and retain the earlier trial conditions. A model number alone does not establish that a new recipe remains within that evidence.
Summary
Choose the wanted surface, a suitable independent reference and a trial that includes foam formation and collapse. Request a target-specific measurement review with the vessel and foam observations. Accept the result for the decision it actually supports, without turning one stable reading into several unsupported measurements.
