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During 4-20 mA loop testing, a calibrator set to source current does a different job from one set to simulate a two-wire transmitter. Selecting the mode from habit can create an incorrect circuit or an inconclusive test. First identify where the loop receives power and what has been disconnected.
Identify the power source on the loop drawing
An input arrangement may provide loop power, or a separate supply may power the transmitter through the receiving circuit. Equipment manufacturers use active and passive terminology differently, so verify the terminal diagram and electrical function rather than relying on the label alone.
For the selected two-wire METRAVON MV-RD12, power and current signal share the loop. The separately powered RS485 construction is not tested by substituting a current source. Confirm the instrument electronics and receiving input before preparing the procedure.
| Test function | Electrical role |
|---|---|
| Current source | The calibrator drives a selected current into a compatible receiving circuit within its capability. |
| Transmitter simulation | The calibrator regulates current using the loop's external power, acting as a two-wire transmitter substitute. |
| Current measurement | The meter observes current; the exact connection and any loop-power function must follow its instructions. |
Define what the 4-20 mA loop testing point includes
A test at the input cabinet can validate the receiver while leaving the field cable and remote supply untested. A correctly planned field-side simulation includes more of the electrical path but still bypasses the radar's measurement. State the injection location and scope in the record so a successful test is not overinterpreted.
Have qualified personnel use the approved circuit and calibrator instructions. Do not connect competing supplies or select a test mode by trial and error. Where barriers, isolators or hazardous-area requirements apply, verify the permitted test equipment and connection arrangement before work.
Interpret a failed test before adjusting the input card
If the commanded current cannot be established, inspect the test setup, available supply, continuity and the calibrator's operating limits. A failed simulation does not automatically mean the PLC input requires calibration. If current is correct but the engineering value is wrong, investigate the receiving scale and assigned quantity.
Coordinate the test with operations because substituted values may trigger real alarms or equipment actions. After testing, restore the field wiring and verify the live measurement. Record which temporary control arrangements were removed through the authorised process.
Q1. Can current-source mode replace every two-wire transmitter?
No. The correct mode depends on the existing power arrangement and approved test circuit. Transmitter simulation uses external loop power; source mode has a different electrical role.
Q2. Does a successful simulation prove the radar sees the liquid?
No. It tests the electrical path included in the setup. Verify the physical measurement separately with the actual transmitter connected.
Summary
Choose the calibrator mode from the circuit, then state what the test proves. Share the MV-RD12 version and complete loop drawing before investigating a field-to-PLC mismatch.
