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A high-level alarm and a lost level measurement are different events. One describes the process; the other means the control system cannot rely on its observation. The logic for radar level alarms must distinguish these states when a receiver holds the last value, substitutes zero or labels an input fault as a real high level.
Define value and validity as separate inputs
For an analogue loop, establish the transmitter's configured response to a fault and how the receiving channel recognises it. Do not assume a particular out-of-range current is available or enabled without the selected instrument's documentation. For a digital connection, identify supported status information, communication timeout and how data age is calculated.
The application logic should state what happens when the value is invalid. That decision belongs to the process and control design. Holding the previous value can support continuity of a display, but it must not make an old reading appear to be a current measurement.
| Condition | Question the control design must answer |
|---|---|
| Valid high level | Which process alarm and operating response apply? |
| Invalid input | How is unavailable measurement distinguished from a real level condition? |
| Stale value | When does data age make the displayed value unsuitable for control? |
| Signal recovery | What checks or operator actions are required before normal control resumes? |
Test radar level alarms and measurement faults separately
Prepare an authorised test with the control engineer and operations. Verify normal, alarm, unavailable and recovered states using the approved simulation or test method. Keep the physical process protected during the test. A wiring interruption is not a casual diagnostic step when the channel can stop a pump, open a valve or initiate a shutdown.
For METRAVON MV-RD12, choose the 4-20 mA/HART or separately powered RS485 version to suit the receiver. HART availability does not mean the PLC automatically consumes digital device status; the input hardware and host configuration must support that use.
Show operators which condition has priority
Consider a level that was high just before communication failed. The screen may need to retain the last known high value while prominently showing that it is no longer current. Specify alarm presentation and latching behaviour deliberately rather than allowing one colour or message to conceal the other condition.
Include the return from fault in acceptance testing. A recovered communication link does not prove the process is back within its normal range. Check the new value, its validity and the operating mode before returning control authority.
Q1. Should every bad input be converted to a high-level value?
No universal substitution is appropriate. The required response depends on the process hazards and control design. Preserve the distinction between a measured high level and an unavailable measurement even where both require equipment to stop.
Q2. Is an alarm banner enough evidence of correct integration?
It verifies presentation only. Test the underlying tag quality, control response, event record and recovery behaviour that the project actually requires.
Summary
Make measurement failure visible without disguising it as a process reading. Provide the MV-RD12 interface and receiver specification to establish what instrument information the alarm design can use.
