What the calculator is for
A 4–20 mA transmitter maps its calibrated range linearly onto the loop current: 4 mA at the lower range value (LRV), 20 mA at the upper range value (URV). The live zero at 4 mA lets a receiver tell a true 0 % from a broken wire. Use this calculator when commissioning, loop checking, setting trip points in a DCS or SIS, or reading a current with a multimeter.
- % of span = (I − 4) / 16 × 100
- PV = LRV + % / 100 × (URV − LRV)
- I = 4 + 16 × (PV − LRV) / (URV − LRV)
Worked example
A temperature transmitter is ranged −50 to 150 °C and you measure 14.2 mA. The signal is (14.2 − 4) / 16 = 63.75 % of span, so the temperature is −50 + 0.6375 × 200 = 77.5 °C.
The same loop showing 3.5 mA is not reading −56 °C: 3.5 mA is at or below 3.6 mA, which NAMUR NE43 reserves for a transmitter failure signal. The control system should treat the value as bad, and a safety function should go to its defined fault action.
Reference values
| % of span | mA | Square-root flow (% of max) |
|---|---|---|
| 0 % | 4.00 | 0 % |
| 10 % | 5.60 | 31.62 % |
| 25 % | 8.00 | 50 % |
| 50 % | 12.00 | 70.71 % |
| 75 % | 16.00 | 86.6 % |
| 90 % | 18.40 | 94.87 % |
| 100 % | 20.00 | 100 % |
| Current | Meaning |
|---|---|
| ≤ 3.6 mA | Failure signal (downscale) |
| 3.6 to 3.8 mA | Outside the limits: not a valid measurement |
| 3.8 to 4 mA | Measurement, under range (saturation) |
| 4 to 20 mA | Measurement in range |
| 20 to 20.5 mA | Measurement, over range (saturation) |
| 20.5 to 21 mA | Outside the limits: not a valid measurement |
| ≥ 21 mA | Failure signal (upscale) |
Common mistakes
- Forgetting the live zero. 12 mA is 50 %, not 60 %: the span is 16 mA, not 20 mA.
- Square root twice. Extract the root in the transmitter or in the DCS, never both. A DP flow at 25 % signal is 50 % flow.
- Trip points in the saturation band. A low trip set below 3.8 mA can never be reached by a healthy NE43 transmitter, which saturates at 3.8 mA.
- Fault direction. Choose upscale (≥ 21 mA) or downscale (≤ 3.6 mA) burnout so a transmitter fault drives the SIF towards its safe state, and check the logic solver detects it.
Basis
4–20 mA analogue signals are defined in IEC 60381-1. Signal levels for failure information follow NAMUR recommendation NE 43; the calculator applies its status only to the standard 4–20 mA range. Change the 0 % and 100 % signal under Advanced for other ranges.