4-20 mA scaling
mA = 4 + 16 × (value − min) / (max − min)
The 4-20 mA current loop is the standard of industrial instrumentation: this tool converts both ways between the physical quantity (pressure, level, temperature, in your unit) and the current, for any range, reversed scale included. Currents outside the live span are flagged instead of silently extrapolated: below 4 mA is a probable loop fault, not a low range; above 20 mA, saturation or an alarm. The note explains the live zero, the reason for the 4.
12 mA
- Calculation
- mA = 4 + 16 × (5 − 0) / (10 − 0) = 12
- Position in the range
- 50 %
The sensor outputs 12 mA, that is 50% of the range.
Why 4 mA and not 0: the "live zero". A cut wire or a dead sensor gives 0 mA, distinct from the low range at 4 mA, and the loop thus carries its own diagnosis. It is also what lets the loop power the sensor itself: the 4 mA at rest keep it alive.
The conversion is affine, sixteen milliamps for the whole range: every percent of range is 0.16 mA. Reading precision therefore depends on the chosen range: a 0-100 bar sensor read at ±0.01 mA gives ±0.06 bar.
Scientific dossier
What the tool computes, what it assumes, where it stops being valid, and where its data comes from.
Method & formulasmA = 4 + 16 × (value − min) / (max − min)
mA = 4 + 16 × (value − min) / (max − min)
value = min + (max − min) × (mA − 4) / 16
0% ↔ 4 mA, 50% ↔ 12 mA, 100% ↔ 20 mA
The conversion is affine: sixteen milliamps carry the whole range, every percent of range is 0.16 mA. The mid-range 12 mA is the quickest checkpoint with a multimeter.
- Span
- · the sensor’s physical range, from low (4 mA) to high (20 mA).
- Live zero
- · the low range at 4 mA rather than 0: a zero current signals a fault, not a reading.
- Reversed scale
- · 4 mA at the physical maximum, common in level measurement and safety logic.
- NAMUR alarms
- · the usual fault conventions: ≤ 3.6 mA low, ≥ 21 mA high (NE 43).
Validity domainThe tool assumes a LINEAR transmitter between its calibration bounds, which is the usual setting.
The tool assumes a LINEAR transmitter between its calibration bounds, which is the usual setting. Some transmitters apply a square root (flow measurement by differential pressure) or a characterisation: the affine conversion does not apply there as is. Physical precision depends on the chosen range, sixteen milliamps carrying the whole span: calibrating wide means reading coarse.
Reading the resultA current outside [4; 20] is not a reading: below 4 mA the loop is probably cut or the sensor in low alarm; above 20 it saturates or signals a high alarm.
A current outside [4; 20] is not a reading: below 4 mA the loop is probably cut or the sensor in low alarm; above 20 it saturates or signals a high alarm. The tool then shows the extrapolated value, because it helps diagnosis, but marks it as such. The range percentage is the reading common to both directions: it is what gets compared between transmitter and PLC.