Radar Output Direction: Why a 12 mA Check Can Miss a Reversed Level Signal

A reversed linear signal can pass a midpoint test. At 12 mA, both normal and reversed mappings of a 4-20 mA span represent 50 percent. Radar output direction therefore needs checking away from the midpoint and against the physical quantity assigned to the output.

Distance falls when liquid level rises

For a fixed overhead mounting, antenna-to-surface distance decreases as liquid rises. Level measured upward from a tank datum increases. A transmitter sending distance to a PLC configured for level can produce a smooth but inverted trend. Applying another inversion in the display may hide the problem while leaving alarms or calculations incorrect.

Document the output variable, engineering endpoints and where any distance-to-level calculation takes place. Inspect the PLC input and the variable used by control logic, not only the label shown beside the operator's number.

Use asymmetric points to verify radar output direction

The table illustrates two hypothetical linear mappings over a 0-10 m level span. It is not a recommended MV-RD12 setup. The midpoint agrees in both cases; the other points expose the reversal.

Choose suitable off-centre values in the approved loop test, then verify a safely observed physical change. Injected current proves the receiving scale from the injection point onward; it does not establish the radar's selected variable or prove that its target is the real surface.

For the METRAVON MV-RD12 two-wire 4-20 mA/HART configuration, confirm the documented variable and output assignment. If the selected electronics use RS485, check the meaning and units of the mapped quantity instead of applying analogue assumptions to a register value.

Correct the mapping at its source

When a reversal is found, identify which device or calculation differs from the approved design. Correct that point through the change procedure and review all dependent tags. Renaming a display or introducing a compensating formula in one screen leaves other users of the value exposed.

Repeat low and upper checks after the correction, including the required alarm responses under authorised test conditions. Confirm that temporary simulations are removed and retain the final endpoint record with the instrument tag.

Q1. Is an increasing percentage proof of correct level?

Only when the percentage is tied to the intended variable and endpoints. A percentage of distance can increase while the tank is emptying.

Q2. Can a technician safely force test currents during production?

Only within an approved test arrangement that addresses the connected equipment. A simulated level can trigger real control actions, so coordinate with operations before altering the signal.

Summary

Test direction where the two possible mappings differ. Send the MV-RD12 output assignment and PLC endpoint settings when local measurement and plant indication move in opposite directions.