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A 4-20 mA radar loop can work at low level and then stop increasing near the upper range. One possible cause is insufficient voltage left at the transmitter as current rises through the loop resistance. Other causes include a configured output limit, receiver clipping or a measurement problem, so diagnose the circuit before replacing the radar.
Compare local measurement, loop current and displayed level
If the local distance continues changing but measured current stops rising, investigate output configuration and the electrical loop. If current reaches the expected value but the PLC indication does not, focus on the input channel and scaling. If the local measurement also freezes, a voltage check remains useful, but the measurement conditions must be investigated too.
Live measurements and signal injection should be performed by qualified personnel using the site's permitted method. Opening a current loop or simulating its signal can affect connected control and protective functions. Plan the test with operations rather than treating it as an isolated meter reading.
Budget voltage for the 4-20 mA radar loop
For a simplified DC loop, the transmitter voltage equals the available supply minus the drops across cable, receiver and other series components. Resistive drop is I x R. Active isolators and barriers may require their own specified voltage allowance instead of being treated as a fixed resistor.
In an illustrative circuit, a 24 V supply and 500 ohms of total series resistance leave 14 V at 20 mA: 24 - (0.020 x 500) = 14. At 4 mA they leave 22 V. These assumptions do not specify the MV-RD12 minimum terminal voltage, permitted load or an approved barrier arrangement. Compare the actual worst-case remainder with the ordered instrument's electrical requirements.
| Input to the budget | Use the installed value |
|---|---|
| Supply | Minimum available voltage under load, including any relevant tolerance. |
| Cable | Round-trip resistance for the installed length and operating conditions. |
| Receiver | Input burden and any additional series resistance actually fitted. |
| Other devices | Specified drops for isolators, barriers, indicators or protection components. |
Specify the METRAVON MV-RD12 two-wire 4-20 mA/HART construction for this analysis. The separately powered RS485 configuration is a different circuit. Obtain terminal and load information for the ordered version before changing supply voltage or removing any protective component.
Q1. Does a successful 4 mA check prove the loop has enough supply?
No. A resistive voltage drop increases with current. Test the required operating range and any configured fault output using the approved procedure and electrical limits.
Q2. Can increasing the supply always solve clipping?
No. The supply must remain within every connected device's limits, and clipping may originate elsewhere. Establish the voltage budget and fault location before modifying the circuit.
Summary
Use measurements to distinguish electrical headroom from scaling or echo faults. Provide the MV-RD12 version, loop drawing and measured voltage/current pairs for a focused electrical review.
