Instrumentation
4-20 mA Signal Reference
Current, percent of span, and voltage across a 250 ohm input for a 4-20 mA loop, including the NAMUR fault thresholds.
Source of these values
Arithmetic relationships of the 4-20 mA standard, with fault thresholds from NAMUR NE 43.
| Current | Percent of span | Volts across 250 ohm | Meaning |
|---|---|---|---|
| 0 mA | — | 0.00 V | Open loop: broken conductor, dead supply, or failed transmitter |
| 3.6 mA | -2.5% | 0.90 V | NAMUR NE 43 low fault threshold |
| 3.8 mA | -1.25% | 0.95 V | Below range, often a slightly under-ranged calibration |
| 4 mA | 0% | 1.00 V | Bottom of the calibrated range |
| 8 mA | 25% | 2.00 V | |
| 12 mA | 50% | 3.00 V | Midpoint. The check that catches span errors. |
| 16 mA | 75% | 4.00 V | |
| 20 mA | 100% | 5.00 V | Top of the calibrated range |
| 20.5 mA | 103% | 5.13 V | Above range but still measuring on many transmitters |
| 21 mA | 106% | 5.25 V | NAMUR NE 43 high fault threshold |
| 22 mA | 112.5% | 5.50 V | Typical maximum a transmitter will drive |
In short
In a 4-20 mA loop, 4 mA is 0% of span and 20 mA is 100%. Across a 250 ohm input this becomes 1 to 5 volts. Under NAMUR NE 43, a reading below 3.6 mA or above 21 mA indicates a device fault rather than a process value.
Notes and limits
- The 4 mA live zero is what makes a broken conductor detectable. At 0 mA the loop is open; a 0-20 mA scheme cannot tell that apart from a genuine zero reading.
- Configure the fault direction deliberately. On a wet well level, failing low can make the controller think the well is empty and stop pumping while it fills.
- HART communication generally requires a minimum loop resistance of about 230 ohms, which the standard 250 ohm input satisfies.
Direct contact
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