4–20 mA scaling calculator
Enter the transmitter range to get the process value for a signal, or the signal for a value. Works with square-root extraction for differential-pressure flow too.
Result
- Signal percent
- 50%
- NAMUR NE 43
- Within measuring range
PV = LRV + (URV − LRV) × (S − 4) ÷ 16
PV = 0 + 10 × (0.5) = 5
| % | Signal [mA] | Value [bar] |
|---|---|---|
| 0 | 4 | 0 |
| 10 | 5.6 | 1 |
| 25 | 8 | 2.5 |
| 50 | 12 | 5 |
| 75 | 16 | 7.5 |
| 90 | 18.4 | 9 |
| 100 | 20 | 10 |
How scaling works
A transmitter maps the process value to the signal linearly: the lower range value (LRV) gives 4 mA and the upper range value (URV) gives 20 mA. So the value follows from a proportion:
PV = LRV + (URV − LRV) × (I − 4) ÷ 16
For a 0–10 bar range, 12 mA is 50%, which is 5 bar.
Square-root extraction for flow
With an orifice plate or venturi, the transmitter measures differential pressure, which rises with the square of flow. If the transmitter doesn't extract the root itself, the controller must: Q = Qmax × √((I − 4) ÷ 16). Half signal (12 mA) is then about 70.7% flow.
Signal states per NAMUR NE 43
| Current | Meaning |
|---|---|
| 3.8–20.5 mA | valid measurement (a small margin outside 4–20 mA) |
| below 3.6 mA | fault, such as a broken loop |
| above 21 mA | fault, such as a short or transmitter failure |
Common questions
What is 12 mA on a 0–10 bar range?
12 mA is 50% of 4–20 mA, so 5 bar. Each 1 mA is 1/16 of the span, here 0.625 bar.
Why does the signal start at 4 mA rather than 0?
A live zero tells 0% apart from a broken wire, and it powers two-wire transmitters, which need a few milliamps to run.
When should I turn on square-root extraction?
When a differential pressure transmitter measures flow across an orifice or venturi and doesn’t extract the root itself.