How 4–20 mA scaling works
A 4–20 mA signal has a live zero: 4 mA represents the lower range value (LRV), 20 mA the upper range value (URV), and the 16 mA between them is the span. Because 0 mA is not a valid reading, a broken wire (0 mA) can be told apart from a process sitting at the bottom of its range (4 mA).
mA = 4 + (% of span ÷ 100) × 16
process value = LRV + (% of span ÷ 100) × (URV − LRV)
Worked example: pressure transmitter ranged 0–200 kPa
The transmitter reads 12.8 mA. What is the pressure?
- % of span = (12.8 − 4) ÷ 16 × 100 = 55%
- Pressure = 0 + 0.55 × 200 = 110 kPa
Worked example: temperature transmitter ranged −50 to 150 °C
What current should you see at 20 °C?
- % of span = (20 − (−50)) ÷ (150 − (−50)) × 100 = 70 ÷ 200 × 100 = 35%
- mA = 4 + 0.35 × 16 = 9.6 mA
Ranges that don't start at zero are where most mistakes happen: always subtract the LRV before dividing by the span, and keep the sign on negative values.
NAMUR NE 43 signal limits
Most modern transmitters follow NAMUR NE 43, which keeps measurement values separate from failure signals:
| Current | Meaning |
|---|---|
| 3.8 – 20.5 mA | Valid measurement (3.8–4 and 20–20.5 mA are saturation: under or over range) |
| ≤ 3.6 mA | Low failure signal (downscale alarm) |
| ≥ 21.0 mA | High failure signal (upscale alarm) |
Whether a transmitter fails upscale or downscale is usually selectable (a jumper or a configuration setting). Choose the direction that drives the control system to the safe action for that loop.
Field checks
- Reading a loop without breaking it: many transmitters have test terminals, or use a clamp-on mA meter.
- A reading stuck at exactly 4 mA or 20 mA (not following the process) often means the transmitter is in a fixed-output or test mode.
- If the DCS value is wrong but the loop current is right, check the DCS scaling (LRV/URV) against the transmitter's configured range.
Checking whether the loop has enough voltage? Use the 4–20 mA loop power calculator.