Loop Diagnostics + Calculator

// NAMUR NE43 decoder · Loop resistance calculator · mA converter · If X see Y

// NAMUR NE43 — CLICK ANY ZONE TO SEE WHAT IT MEANS

FAIL

WARN

VALID MEASUREMENT — 4.0 to 20.0 mA

OVER

FAIL

Click any zone on the bar above to see what that reading means and what to check.

mA Range	NAMUR NE43 Zone	What It Means	First Check
<3.6 mA	High failure — downscale burnout	Device fault — sensor or electronics failure. Transmitter configured for downscale burnout.	Check sensor — likely open or severely damaged. Check burnout configuration.
3.6–3.9 mA	Underrange / sensor failure warning	Sensor failed low, or process genuinely below calibrated zero. Distinct from a valid 4.0 mA live zero.	Verify actual process. Check sensor connection. HART diagnostic read.
3.9–4.0 mA	Approaching live zero	Process near minimum range, or slight zero drift. Watch for 3.8 mA HART alarm.	Verify process condition. Check zero calibration.
4.0–20.0 mA	Valid measurement range	Normal operation. Live zero (4mA) = minimum, 20mA = maximum of calibrated span.	Calculate % span: (mA−4)÷16×100
20.0–20.5 mA	Overrange — process high	Process exceeded calibrated range. May be genuine or indicates re-ranging needed.	Verify actual process against independent measurement.
20.5–21.5 mA	High overrange / saturation	Process well beyond range OR loading issue (loop resistance too high). Transmitter running out of headroom.	Calculate loop resistance. Verify supply voltage AT transmitter terminals.
>21.5 mA	High failure — upscale burnout	Device fault with upscale burnout configuration. Critical failure indication.	Check sensor — likely short or overpressure. Transmitter may need replacement.
0 mA	Open circuit — complete loop failure	No current flowing at all. Loop is broken somewhere.	Power first: fuse → supply → wiring → transmitter power terminals.

// THE FORMULA: R_max = (V_supply − V_min_transmitter) ÷ 0.020

Loop Resistance Calculator

Supply voltage

VDC

Min transmitter voltage
(12V = HART standard)

VDC

Wire resistance
(18AWG ≈ 6.5Ω/1000ft)

IS barrier resistance
(0 if no barrier)

PLC input burden
(250Ω for 1-5V input)

// RESULT

600 Ω

Maximum allowable loop resistance for this supply voltage.

HART Minimum Check

HART communication requires 250–1100Ω loop resistance and minimum 12VDC at transmitter terminals.

If HART won't communicate, first check:

Loop resistance <250Ω? → Add 250Ω resistor

Voltage at TX <12V? → Reduce resistance or increase supply

AC noise >0.2Vpp? → Ground loop or EMI — fix shielding

Wrong device address? → Check HART address config

Wire Resistance Quick Ref

AWG	Ω per 1000ft	Notes
14 AWG	2.5	Long runs
16 AWG	4.0	Common field wire
18 AWG	6.5	Most common instrument
20 AWG	10.2	Watch on long runs
22 AWG	16.1	Short runs only

* Total loop = 2× cable length (out and return)

// DRAG THE SLIDER — SEE WHAT THAT mA VALUE MEANS

0 mA4 mA12 mA20 mA22 mA

12.0

mA READING

50.0%

% OF SPAN

VALID

NAMUR ZONE

Mid-range — process at 50% of calibrated span

MEANING

Normal operation. Calculate process value: (mA − 4) ÷ 16 × span + LRV

// QUICK CONVERSION FORMULAS

mA → PERCENT

% = (mA − 4) ÷ 16 × 100

8mA → (8−4)÷16×100 = 25%
12mA → 50% · 16mA → 75%

PERCENT → mA

mA = (% ÷ 100 × 16) + 4

25% → 8mA · 50% → 12mA
75% → 16mA · 100% → 20mA

mA → PROCESS VALUE

PV = ((mA−4)÷16) × span + LRV

0-100 PSI, 12mA → 50 PSI
0-10ft, 16mA → 7.5ft

mA → VOLTAGE (250Ω LOAD)

V = mA × 0.250

4mA → 1.0V · 12mA → 3.0V
20mA → 5.0V (standard 1-5V range)

// PATTERN RECOGNITION — IF YOU SEE THIS, START LOOKING HERE

If You See...	First Consider...
0 mA	Power first. Check fuse → check polarity → measure voltage AT transmitter terminals (not just supply) → look for broken wire. 35% of dead transmitters are power issues.
Fixed 4 mA, process active	Verify not in test mode. HART communicator check → apply known input with calibrator → compare to independent measurement. May be genuine zero — confirm process state.
3.6–3.9 mA	Sensor failure low or underrange. Distinct from valid 4mA live zero. Check sensor connection, read HART diagnostics. Not a power issue.
Fixed 20 mA, normal process	Loading or overrange. Calculate loop resistance — R_max = (Vsupply−12)÷0.020. Measure voltage at transmitter at full output. If voltage <12V, loop resistance is the issue.
>21.5 mA	Upscale burnout — device fault. Sensor shorted or overpressure damaged. Check sensor resistance. Transmitter may need replacement.
Noisy, fluctuating at 50/60 Hz	Ground loop or power line coupling. Measure shield voltage to ground — should be near 0V. Verify single-point shield grounding. Only ground at one end.
Noisy at VFD frequencies (kHz)	EMI from VFD. Check cable routing — minimum 12 inches separation from VFD cables. Add ferrite suppressors. Improve shield grounding.
Non-linear — error increases at high end	Loading confirmed. Measure voltage at transmitter terminals at full 20mA output. Should be >12VDC. If not — reduce loop resistance.
HART communicates but analog wrong	Output circuit fault. Transmitter may be in digital/multidrop mode, analog output failed, or forced output active. HART communicator to check output mode.
Signal good, process value wrong	Calibration or configuration. LRV/URV settings, square root extraction mode, engineering unit scaling. Not a hardware fault.
Multiple transmitters fail simultaneously	Common supply issue. One fuse, one supply rail, one breaker. In a parallel supply arrangement, individual failures are independent — simultaneous failure points to shared supply element.
Drift after pressure spikes	Zero shift from overpressure. Check if sensor was exposed to pressure beyond URL. Zero trim may restore it — if not, sensor damaged.