4–20 mA Scaling & Calculation
Scaling converts the 4–20 mA signal into meaningful engineering units such as bar, °C, %, ppm, or flow rate. Every PLC, DCS, and analyzer system depends on correct scaling.
Basic Concept
- 4 mA = Lower Range Value (LRV)
- 20 mA = Upper Range Value (URV)
- Span = URV − LRV
- Total mA span = 16 mA
mA → Engineering Units (Forward Calculation)
Process Value = (Current − 4) ÷ 16 × Span + LRV
This formula works for pressure, temperature, flow, level, and analyzers.
Worked Example 1 (Simple Range)
Pressure transmitter range: 0–100 bar
- LRV = 0
- URV = 100
- Span = 100
If measured current = 12 mA:
(12 − 4) ÷ 16 × 100 = 8 ÷ 16 × 100 = 50 bar
Worked Example 2 (Negative Range)
DP transmitter range: -50 to +50 mbar
- LRV = -50
- URV = +50
- Span = 100
If current = 12 mA:
(12 − 4) ÷ 16 × 100 + (-50) = 50 − 50 = 0 mbar
Negative ranges are common in DP level and flow applications.
Engineering Units → mA (Reverse Calculation)
mA = ((Process Value − LRV) ÷ Span) × 16 + 4
Example
Flow range: 0–200 m³/h Actual flow = 100 m³/h
((100 − 0) ÷ 200) × 16 + 4 = 0.5 × 16 + 4 = 12 mA
Quick Percentage Method
If signal is at 50% of span:
- 50% of 16 mA = 8 mA
- 8 + 4 = 12 mA
Useful for quick field estimations without full calculation.
Common Field Errors
- Wrong LRV / URV in PLC
- Units mismatch (bar vs kPa)
- 4–20 reversed (20–4 configuration)
- Scaling applied twice (transmitter + DCS)
- Incorrect square root extraction for DP flow
Golden Field Rule
Always confirm transmitter range from nameplate or configuration before adjusting PLC/DCS scaling. Most “wrong readings” are configuration errors — not hardware faults.