Build a wet/dry and power-loss truth table

Start at the machine, not the indicator

Write the required action in process terms: allow transfer, stop filling, warn the operator or hold the machine in a defined state. Then map that action to the PLC input and the sensor output. A green indicator does not necessarily mean that the plant should run. Its meaning depends on the sensor configuration and the program that consumes the signal. Keep the sensor state, input voltage, PLC bit and final control action as separate columns.

Record the normal operating states

Include the clean dry and fully covered tip, plus any condition that the process routinely produces: drained film, foam, product change or cleaning solution. State whether each condition should count as material present. If a medium-response window is being used, record the approved window and the evidence behind it. The same physical wet state can require a different machine action during production and during a cleaning sequence.

Add faults deliberately

Loss of sensor power, a disconnected signal conductor and loss of PLC input power are separate faults. A conventional transistor output does not automatically distinguish every fault from a legitimate dry state. Decide which faults must be diagnosable and whether additional diagnostics or independent protection are required. Have the responsible engineer define and supervise fault simulation; do not introduce faults into an operating plant without a controlled procedure.

Verify the final response

Observe the output state at the sensor and at the PLC, then confirm the requested final action through the approved test arrangement. Include startup and restart. A signal can be correct while the PLC uses the wrong tag, an inversion is applied twice, or an alarm is latched unexpectedly. Record actual values and timing instead of marking every row simply “OK”. Retain the software revision with the test result.

What to keep in the project record

• Operating mode and required final action
• Wet/dry output configuration and input channel
• Observed response to each approved fault simulation
• Startup, reset and restart conditions

Explore the Point Level Sensor advantages guide or compare the available sensor families for this duty.