4–20 mA Radar Level Loop Check: Field-to-PLC Acceptance

A 4–20 mA loop check should prove the complete measurement path from radar output to the value and action used by the PLC or DCS. Merely forcing 4, 12 and 20 mA at the input card does not verify transmitter range, field wiring or failure behavior, while checking only the local display does not verify the loop. A defensible test combines physical inspection, electrical measurements, known level or transmitter simulation, system scaling, alarm checks and fault states using predetermined tolerances and a signed record.

Confirm the signal definition

Document whether 4 mA represents empty or full, the exact engineering range and units, antenna reference, fault-current values, damping and receiving channel. Identify where distance converts to level or volume. The transmitter, input card and PLC should not each apply unexplained offsets. Match the I/O list, loop drawing and configured values before testing.

Inspect the physical loop

Verify power source, polarity, two-wire or four-wire arrangement, barriers, isolators, fuses, terminals, cable, shield and grounding. Check cable entries and hazardous compliance. Identify test points and safe isolation. Confirm that HART communication, if used, has adequate loop resistance without compromising available transmitter voltage.

Calculate and measure loop voltage

Use worst-case supply and load to confirm voltage at the radar remains above its required minimum through barriers, cable and input resistance. Measure terminal voltage and current during representative states. A loop that works at 12 mA may fail near 20 mA or during cold startup if the voltage margin is inadequate.

Use meaningful test points

Test low, middle and upper range plus values near alarm setpoints. Establish physical levels where practical or use documented transmitter simulation, then distinguish that from external current injection. Record expected current, measured current, input counts and engineering value. Define combined tolerance from source, meter, output and analog input accuracy.

Verify scaling and direction

Compare local distance and level with PLC or DCS units, direction, decimal resolution and limits. Check that 4–20 mA scaling is applied once. Verify historian, trends and remote displays. If the tank uses a volume table, test it separately and keep raw level available. A linear loop test cannot validate an untested nonlinear conversion.

Test fault behavior

Test configured under-range, over-range, sensor fault, lost echo, open circuit and power loss using approved methods. Confirm the input module and logic distinguish these from real process extremes. Check alarm priority, delay and operator message. A held last value should carry bad or stale quality and must not remain a healthy control input.

Verify alarms and restoration

Approach alarm points in both directions to test pickup, reset, hysteresis and delay. Follow critical permissives or final actions under the approved procedure. Remove forces and bypasses under independent review, restore normal configuration and verify the live value. Cycle power where required and confirm predictable startup and recovery.

Account for total loop uncertainty

Combine the uncertainty of the field reference, transmitter output, test meter, barrier and analog input when setting acceptance limits. Record resolution separately from accuracy and avoid requiring identical displayed digits at every layer. A small stable offset may be consistent with the test chain, whereas wrong direction, nonlinear error or changing difference indicates a configuration or hardware problem. Predetermined limits make the loop result defensible and stop criteria from being relaxed only after an unexpected reading appears.

Engineering checklist

Match drawings, range, units and channel.
Inspect barriers, shield and grounding.
Measure loop voltage at representative current.
Test field, transmitter and PLC layers.
Verify fault current and stale behavior.
Sign off restoration of every bypass.

Frequently asked questions

Do three injected currents prove the radar?

No. They prove part of the input and logic path, not field measurement or transmitter configuration.

Why test near alarm setpoints?

It verifies scaling, hysteresis and action where small errors matter operationally.

What should an open circuit display?

A defined invalid or fault condition, not a normal zero or believable held value.

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