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Two silo level instruments can give apparently conflicting indications without either being defective. A radar sees a surface region, while a point switch detects material at its own sensing location. In a sloping or uneven pile, those locations may be covered at different times.
Locate Both Silo Level Instruments on the Same Drawing
Mark the radar's target region and the switch's active sensing position, not just their roof connections. Add the fill inlet, outlet and expected pile shape. Compare these locations at the time of the disagreement. A radar level expressed as a height is not proof that material has reached the same height everywhere in the silo.
The MV-RD21 radar provides a continuous distance measurement, whereas MV-SP601 is an RF admittance point-detection family. Their outputs answer different questions. Combining them can be useful, but it does not create an automatically validated voting or safety system.
| Observed state | Possible explanation | Evidence to collect |
|---|---|---|
| Radar high | The pile peak is under the radar but not at the probe. | Filling position, surface geometry and local switch state. |
| Switch covered | Material has reached the probe while another region remains lower. | Probe elevation, active region and radar target position. |
| Switch stays on | Residue or retained material remains around the probe. | As-found condition after discharge and return-to-clear test. |
| Timing differs | Filtering, polling or separate event times distort the comparison. | Time-aligned raw values, state changes and configured delays. |
Test the Physical States Before Changing the Logic
Check that the switch changes state with representative material present and absent, including normal residue. Then verify how the controller interprets that state. An electrical input simulation proves the receiving logic but not whether the actual probe is covered.
For the radar, retain raw distance and diagnostic status rather than comparing only a rounded percentage. Align the records in time. If one indication is delayed, identify the delay before moving an alarm threshold to make the two displays agree.
Trace the switch state into the PLC
Compare the local indication with the selected output state, the PLC input and the alarm message during the same material transition. If the local switch changes correctly but the controller indication does not, investigate the output circuit and input interpretation before moving the probe. For a contact-output version, confirm the selected terminals and contact action from its wiring diagram rather than assuming that a closed contact always means material present.
Check loss of instrument power as a separate condition under the approved test plan. Material absent, instrument unpowered and a broken circuit must not be treated as equivalent without the control design explicitly defining their meaning. Record the observed response and any unresolved distinction; neither a local LED nor a PLC label alone proves the complete alarm path.
Questions About Combined Indications
Q1. Should a disagreement automatically stop filling?
The appropriate action depends on the process risk assessment and the defined roles of the instruments. Establish it during control design. Do not invent a generic rule from the presence of one continuous sensor and one switch.
Q2. Can the switch calibrate the radar at one known height?
It can provide a useful local event only when the active sensing elevation, material state and relationship to the radar target are established. On an uneven surface, switch operation does not give an exact reference for the radar's separate target region.
Summary
Investigate location and timing before calling different indications a fault. Submit the joint layout, time-aligned trends and intended control actions for a silo instrument-duty review so each device is selected and tested for its own role.
