Why is 4-20mA preferred over 0-20mA?

The live zero at 4 mA lets you distinguish a valid zero measurement from a dead loop. It also leaves room for underrange and fault signaling below the calibrated low end.

What do the NAMUR NE43 limits mean in practice?

A common interpretation is below 3.6 mA means low fault, 3.6 to 3.8 mA means underrange, 3.8 to 20.5 mA is usable measurement range, 20.5 to 21.0 mA means overrange, and above 21.0 mA means high fault.

Why does 250 ohm create a 1-5V signal?

Ohm’s law gives V = I x R. At 4 mA across 250 ohm you get 1.0 V, and at 20 mA you get 5.0 V. That makes 250 ohm a standard shunt for converting current loops into voltage inputs.

Should my PLC clamp scaled values or extrapolate them?

Clamp if you want the displayed engineering value limited to the declared range. Extrapolate if you need to preserve out-of-range information for diagnostics, but pair that with explicit fault and overrange handling.

Can this tool handle reverse-acting ranges?

Yes. If your engineering maximum is numerically lower than the minimum, the tool treats the signal as reverse acting and keeps the slope negative.

Home / Tools / 4-20mA Scaling Calculator

Signal Utility

4-20mA Scaling Calculator

4-20mA Scaling Calculator converts current loops into engineering units and back, interprets NAMUR NE43 fault bands, generates calibration checkpoints, and shows shunt-resistor voltage.

Range Setup And Solve Target

Quick Presets

Preset: Tank Level

Engineering Min

Engineering Max

Unit Label

Receiver Resistor

Use 250 ohm for a standard 1-5V conversion reference.

Output Handling

Extrapolate to see the raw linear result. Clamp to emulate PLC logic that limits displayed values back into the calibrated range.

Loop Current Input (mA)

Enter the actual loop current from a transmitter, calibrator, or input card.

Live Scaling Result

Awaiting valid input

Scaled process value

Enter a current in mA

Use this to turn an actual loop current into an engineering value.

Span position

Signal percent is calculated from the 4-20mA span.

Voltage Across Burden

250 ohm receiver resistor

Engineering per 1 mA

6.25 %

Signed value reflects whether the signal is direct acting or reverse acting.

1% Span Current

0.16 mA

Fixed for a 4-20mA loop because the signal span is always 16.00 mA.

Interpretation

Set the calibrated process range, then solve from either actual loop current or desired engineering value.

NAMUR NE43 Status Rail

Low fault

< 3.6

Under

3.6 to 3.8

Normal

3.8 to 20.5

Over

20.5 to 21.0

High fault

> 21.0

Scaling Equations

ENG = 0 + ((mA - 4) * 100 / 16)

mA = 4 + ((ENG - 0) * 16 / 100)

V_receiver = (mA / 1000) * R_burden

Calibration Checkpoints

% Span	Ideal Current	Engineering Value	Voltage @ Burden	Typical Check
0%	4 mA	0 %	1 V	Zero trim / low-end verification
25%	8 mA	25 %	2 V	Quarter-point calibration checkpoint
50%	12 mA	50 %	3 V	Mid-scale linearity check
75%	16 mA	75 %	4 V	Quarter-point calibration checkpoint
100%	20 mA	100 %	5 V	Span trim / high-end verification

Fault Band Decoder

Low fault

< 3.6 mA

Open loop, transmitter fault output, or supply issue.

Underrange

3.6 to 3.8 mA

Below calibrated low end but not yet a hard fault.

Normal

3.8 to 20.5 mA

Usable measurement band for a 4-20mA loop.

Overrange

20.5 to 21.0 mA

Above calibrated high end but below the hard-fault band.

High fault

> 21.0 mA

Forced high fault, device diagnostic, or configuration issue.

Assumptions & Usage Notes

This tool assumes a linear transmitter and a linear receiver scaling model. It does not replace final verification of device accuracy, compliance voltage, loop impedance, or controller-specific filtering and fault-handling behavior.

Equation And Standards Reference

These are the core relationships behind loop scaling, fault-band interpretation, and shunt-resistor conversion.

Topic	Expression	Meaning
Current to Engineering	ENG = EUlow + ((mA - 4) x (EUhigh - EUlow) / 16)	Maps a measured current into the calibrated process range.
Engineering to Current	mA = 4 + ((ENG - EUlow) x 16 / (EUhigh - EUlow))	Calculates the loop current needed to represent a target process value.
Signal Slope	1% span = 0.16 mA	Useful for quick calibration math and sanity checks in the field.
Receiver Voltage	V = (mA / 1000) x Rburden	Shows why 250 ohm converts 4-20mA into a 1-5V signal for legacy analog inputs.

Field Workflow Matrix

Use the tool differently depending on whether you are commissioning, troubleshooting, interfacing, or writing PLC logic.

Scenario	Objective	Recommendation	Critical Checks
Commissioning a transmitter	Verify zero, quarter, half, three-quarter, and span points	Inject 4, 8, 12, 16, and 20 mA, then compare measured loop current, receiver voltage, and PLC displayed value.	Burden resistor tolerance, AI scaling constants, clamp policy, transmitter trim state
Diagnosing a strange analog value	Determine whether the process is truly under-range or the loop is faulting	Read actual loop current first, then compare it against NE43 boundaries before blaming the PLC scaling logic.	3.6 mA low fault, 3.8 mA usable low limit, 20.5 mA usable high limit, 21.0 mA high fault
Converting 4-20mA to 1-5V	Validate a shunt-resistor interface into a voltage-style analog input	Confirm the burden resistor value and ensure the transmitter still has enough compliance voltage left.	250 ohm nominal burden, loop supply margin, card input impedance, cable resistance
Writing PLC scaling logic	Build a block that behaves well during overrange and fault states	Use the linear equation plus an explicit clamp strategy, and treat NE43 fault bands separately from normal scaling.	Raw current source, clamp vs extrapolate, alarming thresholds, reverse-acting spans

Generic PLC Scaling Snippet

Use the linear calculation, then decide deliberately whether your application should clamp, alarm, or preserve the raw out-of-range information.

raw_eng := eu_low + ((input_ma - 4.0) * (eu_high - eu_low) / 16.0);
display_eng := LIMIT(MIN(eu_low, eu_high), raw_eng, MAX(eu_low, eu_high));

low_fault := input_ma < 3.6;
underrange := input_ma >= 3.6 AND input_ma < 3.8;
overrange := input_ma > 20.5 AND input_ma <= 21.0;
high_fault := input_ma > 21.0;

Separate the measurement calculation from fault interpretation. That keeps your trend value useful while still preserving a clean alarm model for NE43 or transmitter diagnostics.

Worked Example

Suppose a 0 to 10 bar transmitter is reading 13.6 mA. The signal is 9.6 mA above live zero, or 60% of span.

The engineering result is therefore 6.0 bar. If the receiver uses a 250 ohm shunt, the equivalent input voltage is 3.4 V.

That one check lets you validate the transmitter, the shunt, and the PLC display against the same physical signal.

Why This Tool Matters

A simple formula is not enough in real automation work. You also need to know whether a given current is normal, under-range, over-range, or a real fault signal.

You need calibration checkpoints that match field calibrators, not just a single answer. And when a loop is shunted into a voltage input, you need to understand the receiver voltage immediately without switching tools.

That is why this page combines scaling math, NE43 interpretation, checkpoint generation, and burden-voltage conversion in one workflow.

Frequently Asked Questions

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