Troubleshooting a switch that remains covered after draining

Check the actual local condition

Operators may call a vessel empty while a nozzle, coating or product bridge remains at the sensor. Inspect through an approved isolation and access procedure. Compare the location with the installation drawing and identify any retention pocket.

If residue is present, reproduce that state during an application test. A clean sensor in air is not an equivalent empty condition.

Separate sensor output from controller interpretation

Check the local indication and the PLC input against the documented truth table. An inverted logic setting, incorrect PNP/NPN interface or latched alarm can look like a sensing failure. Follow qualified electrical test procedures and the exact wiring instructions.

Correct the cause

If the issue is installation geometry, changing thresholds may create a new failure to detect product. If configuration is changed, document the previous and new values and verify all required states. Escalate damaged or unstable equipment to the supplier rather than opening a sealed sensor for unauthorized repair.

What to confirm for your application

• Local residue or trapped product
• Sensor indication versus PLC state
• Correct wiring and output logic
• Configuration history
• Retest both wet and empty conditions