Standard 4–20 mA Milestone Values
In a standard linear two-wire transmitter, every milliamp corresponds to exactly 6.25% of the total measurement span. Use this quick reference matrix when testing with a signal generator or calibrator.
| Signal Current (mA) | Percentage of Span | Operating Condition | Diagnostic Meaning (NAMUR NE43) |
|---|---|---|---|
| 0.00 mA | — | Dead Circuit / Fault | Broken loop wire, blown supply fuse, or unpowered transmitter |
| 3.60 mA | -2.5% | Sensor Downscale Fault | Primary element open circuit (thermocouple burn-out, RTD broken) |
| 3.80 mA | -1.25% | Under-range | Process slightly below physical lower range value (LRV) |
| 4.00 mA | 0.00% | Live Zero (LRV) | Process at 0% calibration point (e.g. 0.0 bar, 0.0 °C, 0.0 m³/h) |
| 8.00 mA | 25.00% | Quarter Scale | Process at 25% calibration point |
| 12.00 mA | 50.00% | Mid Scale | Process at 50% calibration point (center of measurement span) |
| 16.00 mA | 75.00% | Three-Quarter Scale | Process at 75% calibration point |
| 20.00 mA | 100.00% | Full Scale (URV) | Process at 100% calibration point (e.g. 10.0 bar, 100.0 °C) |
| 20.50 mA | 103.12% | Over-range | Process slightly above physical upper range value (URV) |
| 21.00 mA | 106.25% | Sensor Upscale Fault | Transmitter hardware failure or shorted sensing element |
The Mathematical Scaling Formula
Converting a milliamp measurement to an engineering value is a linear two-point interpolation problem. The mathematical formula implemented by this calculator and standard PLC scaling instructions is:
Value = Low_EU + [(I_measured − I_min) / (I_max − I_min)] × (High_EU − Low_EU) Where:
- I_measured: The measured current in mA from your digital multimeter or process clamp.
- I_min: The lower baseline current (typically 4.00 mA).
- I_max: The upper baseline current (typically 20.00 mA).
- Low_EU: The process value represented at 4 mA (e.g. -20 °C or 0 PSI).
- High_EU: The process value represented at 20 mA (e.g. +150 °C or 100 PSI).
0–20 mA vs. 4–20 mA Channel Card Mismatch
A frequent error during plant commissioning occurs when a technician wires a 4–20 mA transmitter to an analog input card configured for 0–20 mA mode. At 4.00 mA (0% process), the PLC reads 20% of scale instead of zero. When the transmitter reaches 20.00 mA, the PLC reads 100%, but the entire lower 20% of the input resolution is permanently distorted. Always confirm that both the hardware dip switches (or software channel properties) and the scaling logic are set to 4–20 mA live-zero mode.
Related Field Guides & Troubleshooting Notes
Why current loops dominate long distances and high-noise plant environments.
A calm, meter-first sequence for separating power, polarity, and loop breaks.
How Siemens 0-27648 and Rockwell 0-32767 convert raw analog registers.