4–20 mA loop scaling

The 4–20 mA current loop is the default analog signal in process plants: 4 mA represents 0% of span, 20 mA represents 100%, and anything near 0 mA means a broken wire (the "live zero"). Scaling converts that current into the engineering units the operator sees — °C, bar, liters per minute.

How scaling works

Percent of span is linear in current: percent = (mA − 4) / 16 × 100. Engineering value is linear in percent: value = LRV + percent × (URV − LRV) / 100, where LRV/URV are the transmitter's lower and upper range values. The reverse path — from a desired engineering value back to mA — is what you need when simulating a transmitter with a loop calibrator during a loop check.

Loop check table

Before commissioning, drive the loop to 4, 8, 12, 16, and 20 mA and confirm the PLC or DCS reads 0, 25, 50, 75, and 100% within tolerance. The table below the calculator gives the expected engineering value at each point for your range. If 4 mA reads slightly off but 20 mA is exact (or vice versa), suspect a zero/span trim issue at the transmitter rather than a scaling error in the controller.

See also

• 4–20 mA current loop and HART protocol — Shopfloor glossary
• PID loop tuning: a field guide — the loops this signal feeds