Use PV = LRV + (I − 4) ÷ 16 × (URV − LRV) to turn current into a process value, and I = 4 + 16 × (PV − LRV) ÷ (URV − LRV) to go the other way. 12 mA is mid-scale; each 1.6 mA is 10 % of span.
4–20 mA scaling calculator
| Current | % of span | Process value |
|---|
Fault check follows the common NAMUR NE 43 convention: below 3.6 mA or above 21 mA = sensor or wiring fault; 3.8–20.5 mA = valid measurement with saturation.
The 4–20 mA scaling formula
Current → value: PV = LRV + (I − 4 mA) ÷ 16 mA × (URV − LRV)
Value → current: I = 4 mA + 16 mA × (PV − LRV) ÷ (URV − LRV)
Percent of span: % = (I − 4) ÷ 16 × 100
LRV and URV are the lower and upper range values set in the transmitter. The 16 mA span is fixed, so the same formula works for pressure, level, flow, temperature or position — only the range changes.
Worked examples
| Transmitter range | Reading | Calculation | Result |
|---|---|---|---|
| Pressure 0–10 bar | 8 mA | 0 + (8 − 4) ÷ 16 × 10 | 2.5 bar |
| Pressure 0–400 bar | 15.2 mA | (15.2 − 4) ÷ 16 × 400 | 280 bar |
| Level 0–5 m | 17.6 mA | (17.6 − 4) ÷ 16 × 5 | 4.25 m |
| Position 25–1000 mm | 12 mA | 25 + 0.5 × 975 | 512.5 mm |
| Compound −1 to +5 bar | 4 mA → 20 mA | LRV = −1, URV = 5 | −1 bar → 5 bar |
Why the signal starts at 4 mA, not 0 mA
The 4 mA "live zero" powers two-wire transmitters through the loop itself and lets the receiver distinguish a genuine 0 % reading (4 mA) from a broken wire or dead transmitter (0 mA). Current is also immune to the voltage drop of long cable runs, which is why 4–20 mA remains the standard analogue signal for process instruments (IEC 60381-1).
Reading fault currents
| Loop current | Usual meaning | First check |
|---|---|---|
| 0 mA | Open loop: broken wire, no supply, blown fuse | Measure supply voltage at the transmitter terminals |
| < 3.6 mA | Transmitter signals an internal fault (NE 43 low alarm) | Transmitter diagnostics, sensor element |
| 3.8–20.5 mA | Valid measurement (including slight over/under-range) | — |
| > 21 mA | High alarm or short circuit across the loop | Wiring for shorts, transmitter alarm setting |
Wiring two-wire and three-wire transmitters is covered step by step in our 4–20 mA wiring guide; for choosing a transmitter see the selection guide and the 4–20 mA pressure transmitter page.
Frequently Asked Questions
12 mA is always 50 % of the configured range, because 12 mA is halfway between 4 mA and 20 mA. On a 0–10 bar transmitter, 12 mA means 5 bar.
Use PV = LRV + (I − 4) ÷ 16 × (URV − LRV). For a 0–16 bar transmitter reading 10 mA: (10 − 4) ÷ 16 × 16 = 6 bar.
0 mA almost always means an open loop (broken wire or no supply). Readings below about 3.6 mA are commonly used by transmitters to signal an internal fault (NAMUR NE 43 convention).
0.16 mA, because the span is 16 mA. 10 % of span is 1.6 mA.
Yes. Set the lower range value below zero, e.g. −1 to +5 bar for a compound range; 4 mA then means −1 bar.