Check 4–20 mA scaling before changing hardware.
Use this field calculator to convert measured loop current into engineering units, calculate the expected current for a process value and estimate the resistance budget available to a two-wire transmitter loop.
Measured current → process value
Formula: PV = LRV + ((mA − 4) / 16) × (URV − LRV).
Process value → expected current
Formula: mA = 4 + 16 × ((PV − LRV) / (URV − LRV)).
Loop resistance & transmitter voltage budget
Maximum external resistance = (supply voltage − minimum transmitter voltage − reserved margin) / design current (A). Estimated transmitter terminal voltage = supply voltage − design current (A) × total series burden. Enter the combined burden of AI input resistors, cables, barriers, isolators and other series devices; do not add a barrier twice if its specified voltage drop has already been included in an equivalent resistance.
The default 20 mA checks only nominal full scale. If the installed transmitter is configured to deliver a higher overrange or high-alarm current, enter the documented maximum relevant current to check voltage compliance. For nonlinear or actively powered interfaces, a simple resistor equivalent may not describe the device accurately; use its voltage-drop specification and the vendor's wiring requirements.
Field fault-isolation sequence
1. Confirm the configured range
Record the transmitter LRV and URV. Do not assume the PLC scaling matches the transmitter nameplate or an old drawing.
2. Measure loop current
Measure current using the approved site method. Compare the measured mA with both the process condition and the analog-input raw value.
3. Check loop voltage
Confirm the transmitter still has enough terminal voltage at the actual loop current. Excess resistance can cause clipping near the top of span.
4. Check the AI channel
Verify channel type, input resistor or shunt arrangement, isolation, grounding and software scaling before replacing the transmitter.
5. Separate signal and process faults
If loop current matches the transmitter output but the engineering value is wrong, investigate scaling and input configuration before process instrumentation.
6. Record the final state
Capture measured current, terminal voltage, configured range, card channel and any barriers or isolators so the fault can be reproduced later.
Quick reference
| Loop current | Percent of span | Example 0–100 range |
|---|---|---|
| 4 mA | 0% | 0 |
| 8 mA | 25% | 25 |
| 12 mA | 50% | 50 |
| 16 mA | 75% | 75 |
| 20 mA | 100% | 100 |
Reusable field sheet
Download 4–20 mA loop check CSVMaintainer
Maintained by PLCProTech Ltd. as an open industrial automation engineering reference.