Floor 03 · Workshop

4-20 mA & loop calculator

Scale current to engineering units and check the loop voltage budget.

Workshop

Scaling

Current mA: 12.00

Lower range LRV

Upper range URV

Unit

5.000 bar

50.0% of spanWithin measuring range (NAMUR NE43)

Loop voltage budget

Supply V

Load resistor Ω (HART 250)

Total cable resistance Ω

Barrier drop V

Transmitter lift-off V

Voltage at transmitter at 22 mA: 17.62 V
Max allowed loop resistance: 614 Ω

Loop is adequate

Linear scaling: PV = LRV + (URV − LRV) × (mA − 4) / 16. NAMUR NE43: measurement between 3.8 and 20.5 mA; failure at ≤3.6 mA or ≥21 mA.

The loop budget is checked at 22 mA (worst case): voltage at the transmitter = supply − barrier drop − 0.022 × (load + cable). It must stay above the transmitter lift-off voltage.