After the interface is commissioned, each immersed level probe operates one isolated relay contact, and the PLC receives four stable 24 VDC level states referenced to the fifth probe. Probe branding does not determine success; electrical compatibility, wiring, contamination tolerance, and fail-state behavior do.
What should the PLC input screen show?
Start online at the PLC input table. With the tank below all four level probes, every level input should show the configured dry state. Raising the conductive liquid should change inputs individually and in physical level order. An input that changes on the interface relay but not in the PLC points to the 24 VDC contact circuit, not the probe.
| Observed state | Likely test boundary | Next check |
|---|---|---|
| No relay or PLC change | Probe circuit | Reference connection, level-probe conductor, liquid conductivity, and interface power |
| Relay changes; PLC does not | Dry-contact/PLC circuit | 24 VDC source, common, input address, and contact wiring |
| PLC input changes; screen indication does not | Program or HMI binding | Input mapping, controller tag, communication path, and display tag |
| Input flickers near one level | Process or sensitivity | Wetting, foam, deposits, grounding, and available switching hysteresis or delay |
Proof before continuing: force or temporarily operate each relay output using an approved test method and confirm that its assigned physical PLC input changes. This establishes the complete 24 VDC side independently of the vessel probes.
How should the five probes map to four level channels?
Use one probe as the common reference electrode and connect each of the other four probes to a separate conductive-level channel. When conductive liquid touches both the reference and a level probe, the liquid completes the sensing path. The interface detects that path and changes its isolated relay output.
| Connection | Location | Effect |
|---|---|---|
| Reference probe | Common terminal of each sensing channel | Provides the shared return path through the liquid |
| Level probe 1 | First sensing channel | Reports the lowest monitored level |
| Level probes 2 and 3 | Separate sensing channels | Report intermediate levels |
| Level probe 4 | Fourth sensing channel | Reports the highest monitored level |
| Steel tank | Protective bonding system | Must not silently replace the designated reference unless the design specifically permits it |
Two arrangements can function: a dedicated fifth reference probe, or the conductive tank as the return electrode. Use the dedicated probe because that is the stated installation architecture and it keeps sensing independent of uncertain tank bonding, coatings, joints, and corrosion paths.
Proof before continuing: label all five conductors, disconnect them from the old board, and verify that the reference conductor is electrically isolated from the four level conductors when the probes are dry.
Will the KFD2-ER-2.W.LB work with unknown probes?
The probes appear to be passive electrodes rather than powered sensors. Their manufacturer is therefore less important than the electrical circuit formed by the electrode alloy, insulation, process liquid, wiring, and sensing module. A suitable conductive-level interface applies a low-voltage AC sensing signal; AC excitation limits electrode polarization and electrochemical buildup compared with continuous DC excitation.
Compatibility still requires checks against the current KFD2-ER-2.W.LB documentation. Read the module datasheet and installation instructions rather than treating the product code as proof of channel count or suitability.
| Required check | Where to obtain it | Commissioning effect |
|---|---|---|
| Accepted electrode resistance or conductance range | Interface datasheet | Determines whether sewage and cable leakage create a detectable state |
| Sensing waveform and electrode limits | Interface datasheet | Confirms suitability for passive conductive probes |
| Available relay channels | Terminal diagram | Determines whether two modules provide four independent outputs |
| Lead-break or line-fault requirements | Wiring diagram | May require components or wiring at the probe end |
| Output contact rating | Output specification | Must cover the PLC input voltage and wetting-current requirement |
| Supply and isolation requirements | Installation instructions | Defines module power and panel segregation |
Proof before continuing: measure the resistance between the reference and each wetted level probe under actual process conditions, then confirm every measured state falls inside the module's documented detection range.
How do I wire the relay contacts to 24 VDC inputs?
- Power down and isolate the old control board before transferring conductors.
- Connect the reference electrode to the common sensing connection required for each assigned channel.
- Connect each level electrode to its own sensing input. Do not parallel level probes if four independent states are required.
- Wire one isolated relay contact per channel into the PLC's 24 VDC input circuit. Match the input module's sourcing or sinking arrangement using its wiring diagram.
- Choose normally open or normally closed logic from the required safe state. A normally closed run-state path can reveal loss of module power or broken output wiring, but only if the interface and PLC logic distinguish that failure from a real level state.
- Map each physical input to a clearly named controller level tag, preserving bottom-to-top order.
Keep probe conductors away from noisy power wiring and avoid unintended connections to shield, protective earth, or tank metal. Long, wet, or damaged cables can create leakage paths that resemble an immersed probe.
Proof before continuing: simulate each relay contact at the panel and verify the correct physical input, controller tag, and displayed indication change together.
Why can a correctly wired channel still chatter or stick?
Sewage service changes the sensing resistance. Foam can bridge an electrode before the bulk liquid reaches it; residue can keep a leakage path after the level falls; grease can insulate a wetted probe; and a compromised probe insulator can conduct to the steel tank. These faults occur upstream of the relay contact, so changing PLC logic alone does not correct them.
| Symptom | Diagnostic | Correction |
|---|---|---|
| False wet after draining | Measure probe-to-reference resistance dry and inspect deposits | Clean the probe and insulator; repair contaminated terminations |
| No wet indication | Measure through actual liquid and check reference continuity | Restore the reference path or correct sensitivity within documented limits |
| Rapid switching | Observe liquid turbulence and relay state together | Use documented hysteresis or delay, or add qualified PLC filtering |
| Several levels change together | Disconnect level wires one at a time | Remove cross-connections, moisture tracks, or shared cable faults |
Apply delay only after proving the electrical signal. Filtering a leakage fault hides the diagnostic evidence and may delay a genuine high-level response.
Proof before continuing: wet and dry every electrode repeatedly, confirming one clean relay transition per physical level without adjacent channels changing.
How do I verify the complete level sequence?
- Begin below the lowest level and record the four relay, PLC, controller-tag, and screen states.
- Raise the liquid slowly past each probe. Confirm transitions occur in bottom-to-top order.
- Hold the liquid near each electrode and check for chatter, delayed pickup, or simultaneous transitions.
- Lower the liquid through the same levels and confirm each channel releases predictably despite residual wetting.
- Disconnect each level lead in turn and then the reference lead. Confirm the resulting indication and alarm behavior match the documented failure strategy.
- Remove power from each interface module and verify that the PLC and operator display show the intended fail state.
Record the final terminal assignments, PLC addresses, controller tags, normal dry states, alarm states, and measured wet/dry resistance values. The final acceptance check is a complete fill-and-drain cycle in which every physical level, relay LED, PLC input, controller tag, and screen indication agrees.
FAQ
How do I identify an unknown conductive level probe?
Isolate it and check whether it is a passive electrode insulated from the tank. Identification by brand is unnecessary when its material, insulation condition, process compatibility, and measured wet/dry resistance meet the sensing interface requirements.
How do I use one reference probe with four level probes?
Connect the reference electrode to the common sensing path for four independent channels, then connect each level electrode to one channel. Confirm the selected module terminal diagram permits the shared-reference arrangement before wiring.
How do I prove the conductive-probe retrofit is finished?
Run one complete fill-and-drain cycle and compare all four physical levels with their relay outputs, 24 VDC PLC inputs, controller tags, and operator indications. Finish by testing reference loss, each level-wire disconnection, and interface power loss against the documented fail-state design.