Why does a 4–20 mA signal start at 4?

4 mA means the lower measurement limit

For a 0–10 bar transmitter, 4 mA represents 0 bar and 20 mA represents 10 bar. Zero current is a different condition and warrants investigation of the loop or its power supply. This separation is called live zero.

What does 12 mA mean?

Process value = minimum + ((mA − 4) / 16) × (maximum − minimum). For 0–10 bar, 12 mA gives 0 + (8 / 16) × 10 = 5 bar. The same sensor gives 2.5 bar at 8 mA and 7.5 bar at 16 mA.

12 mA is not always 5 bar. On a −20…80 °C sensor it means −20 + 0.5 × 100 = 30 °C. Current expresses the fraction of the range; the sensor supplies the limits and unit.

Compare three points

Write down the lower, middle and upper points: 4 mA → 0 bar; 12 mA → 5 bar; 20 mA → 10 bar. Match both the physical input type and the software scale to the transmitter.

If the current is correct but the displayed process value is wrong, inspect scaling and channel settings. If both differ from expectations, check the transmitter range and process condition first.

Do not treat an out-of-range value as a measurement

Zero current may indicate an open loop or missing power. Values below 4 mA or above 20 mA may represent overrange or a diagnostic state depending on the equipment. Check the manual. Applying a linear formula to an out-of-range current does not make the resulting number a valid measurement.

Record enough information to diagnose the fault

Record the transmitter model and configured range, observed mA, displayed process value and channel type. For example: “0–10 bar; 12.0 mA; display 50 bar; 4–20 mA input.” This is more useful than simply reporting an incorrect sensor value.