4–20 mA Radar Level Loop Testing for PLC Acceptance

A 4–20 mA loop can carry the wrong level perfectly. PLC acceptance must therefore verify electrical integrity and measurement meaning: which radar variable drives current, whether 4 mA represents empty or full, the reference plane and span, input-card scaling, fault currents and alarm response. Testing only an injected current proves part of the control system but not the radar measurement; observing only the transmitter proves neither wiring nor PLC conversion. A complete procedure combines physical or simulated radar points with independent electrical checks and fault tests.

Document loop design

Record transmitter power requirement, supply voltage, cable length, conductor resistance, barriers, isolators, surge devices, input type and grounding. Calculate available terminal voltage at 20 mA and worst-case conditions. Identify whether the transmitter sources current or uses a two-wire loop and ensure drawings match the installed channel.

Define scaling and direction

State the 4 mA and 20 mA physical values, engineering units, antenna reference and whether level increases or decreases with distance. Keep distance and level distinct. Verify PLC raw counts, scaling blocks, clamping and decimal handling, and control the tank table separately if current represents volume.

Measure electrical performance

Check polarity, continuity, insulation where appropriate, shield termination and voltage at the transmitter. Compare a calibrated loop meter with transmitter output and PLC current at several points. Excessive resistance, ground loops or barrier mismatch can create saturation or noise that configuration cannot fix.

Test the radar-to-current conversion

Use safe known levels, target simulation or transmitter output simulation according to procedure. Compare raw distance, local level, commanded or measured current, PLC value and HMI display at low, middle and high range. Document which tests include the antenna and which verify electronics only.

Verify fault and alarm states

Generate lost echo, under-range, over-range, open circuit and power loss where safe. Confirm configured fault current, PLC quality, diagnostic alarm and process response. Avoid clamping values so tightly that a NAMUR high or low fault becomes an apparently valid process measurement.

Close with traceable evidence

Record test equipment identity, calibration status, measured current, terminal voltage, PLC values, alarm timing and final settings. Remove current sources, forces and bypasses, restore wiring and verify live operation. Repeat affected tests after range, card, barrier, cable or transmitter replacement.

Account for measurement uncertainty

State the accuracy of the current source or meter, analog input card, resistor and independent level reference used in the test. Define acceptable combined error rather than requiring every displayed value to match an unrealistic number. At each point, record expected and measured current, PLC counts and engineering value and check hysteresis where relevant. A small stable difference may be consistent with the measurement chain, while a nonlinearity or direction error indicates configuration or hardware. This evidence supports a defensible PLC acceptance without confusing resolution with accuracy.

Acceptance limits

Agree numerical limits before testing: current tolerance at each point, PLC engineering-unit tolerance, fault-current recognition time and permitted recovery behavior. Include values near alarm and trip setpoints rather than relying only on zero, midpoint and span. Where a safety or independent overfill function exists, test it under its separate procedure and do not infer success from the control loop. Predetermined limits make the result auditable and prevent acceptance criteria from being relaxed after an unexpected error is observed.

Engineering checklist

• Calculate loop voltage and resistance.
• Define 4 mA, 20 mA and reference direction.
• Measure current at several representative points.
• Separate physical measurement from simulation.
• Test fault currents and PLC quality.
• Remove overrides and store signed results.

Frequently asked questions

Does current injection calibrate the radar?

No. It verifies downstream electronics and scaling, not antenna measurement or reference geometry.

Why test fault current?

The PLC must distinguish a failed instrument from a real low or high level.

What causes 20 mA saturation?

Insufficient loop voltage, excess resistance, barriers, wiring faults or configuration limits can all contribute.

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