Why 4–20 mA?
- Current, not voltage: the current is the same everywhere in a series loop, so cable resistance doesn't change the signal.
- Live zero: 4 mA means 0 %, so 0 mA clearly means a broken wire or dead instrument.
- Loop powered: a 2-wire transmitter runs on the 3.5–4 mA that is always flowing, so two wires carry both power and signal.
Scaling
mA = 4 + 16 · (PV − LRV) / (URV − LRV)
PV = LRV + (mA − 4) / 16 · (URV − LRV)
Example: a 0–250 °C transmitter at 12 mA reads 125 °C (50 %). Scale any signal both ways with the Analog Signal Scaler, which also generates calibration test points.
NAMUR NE 43: failure currents
| Current | Meaning |
|---|---|
| ≤ 3.6 mA | Failure: low alarm (downscale burnout) |
| 3.6 – 3.8 mA | Undefined: treat as fault |
| 3.8 – 4.0 mA | Under-range: measurement below LRV, still valid |
| 4 – 20 mA | Normal measurement |
| 20.0 – 20.5 mA | Over-range: measurement above URV, still valid |
| 20.5 – 21.0 mA | Undefined: treat as fault |
| ≥ 21 mA | Failure: high alarm (upscale burnout) |
Set the transmitter's failure direction to suit the process: a level transmitter protecting against overfill should normally fail high, so that a fault trips safe. Configure the DCS/PLC input to recognise the NE 43 bands as bad quality.
Loop power and load
The supply must cover the transmitter's minimum voltage plus every voltage drop around the loop, at the highest current the loop will carry (alarm current, e.g. 22 mA):
V_supply − V_tx,min ≥ I_max · (R_input + R_barrier + R_cable)
R_cable = 2 · L · 0.0172 / A (copper, Ω, L in m, A in mm²)
Example: 24 V supply, 10.5 V lift-off and 22 mA allows up to (24 − 10.5) / 0.022 ≈ 614 Ω of total loop load. Check yours with the Loop Load Check. A loop that works at 12 mA but drops out near 20 mA is almost always a load or supply problem.
HART
HART superimposes a small digital signal (1200 Hz / 2200 Hz FSK, ±0.5 mA) on the analogue current. Because it averages to zero, the 4–20 mA reading is unaffected. It gives access to configuration, diagnostics and secondary variables.
- HART needs 230–600 Ω in the loop. A 250 Ω resistor is the usual choice; some DCS cards have it built in.
- Connect the communicator across the resistor or across the transmitter terminals, not in series.
- Use twisted, screened pairs; HART is sensitive to noise and to excessive cable capacitance on long runs.
Fieldbus and wireless
- FOUNDATION Fieldbus H1 and PROFIBUS PA: fully digital, 31.25 kbit/s, with many devices on one segment powered by a fieldbus power conditioner. Segment design (voltage drop, spur lengths, device count) is critical.
- WirelessHART / ISA100: battery-powered mesh networks for monitoring points where cabling is expensive.
- Ethernet-APL: two-wire Ethernet to the field, intrinsically safe. This is where new projects are heading.
Calibration, ranging and trimming
- Re-ranging: changing LRV/URV in configuration. No reference standard is needed, and it doesn't verify accuracy.
- Sensor trim: correcting the transmitter's digital reading against a known input from a reference standard (e.g. a dead-weight tester or pressure calibrator).
- Output (D/A) trim: making the 4.000 / 20.000 mA output match a reference milliammeter.
- Calibration means applying known inputs (typically 0, 25, 50, 75, 100 %, rising and falling), recording the as-found and as-left values, and adjusting if the error exceeds tolerance. The reference should be at least 4× more accurate than the device under test.