A 4-20 mA transmitter represents its measured quantity as a current proportional to the position within the configured range. PV is the process variable, the standard instrumentation term for that measured quantity, whatever it happens to be: a concentration in ppm, a pressure in bar, a temperature in degrees. PV_min and PV_max are the values the transmitter is configured to report at 4 mA and at 20 mA, which is the range set in the transmitter rather than the physical span of the sensor.

I is the loop current in milliamps. The 16 is the span of the signal, 20 minus 4. PV_max below PV_min is allowed and gives a reverse-acting output, where rising current means a falling measured value.

Why the zero is live

The signal starts at 4 mA rather than 0 so that a broken wire, a dead transmitter or a lost supply reads as 0 mA, which is outside the measuring range and therefore distinguishable from a genuine reading of zero. That property is the reason the standard survives, and it is why any current below about 3.6 mA should be treated as a fault rather than scaled into a value.

Status bands

Transmitters are allowed to drive slightly outside 4 to 20 mA so that a reading just past the end of the range is still readable, and to use currents beyond that to signal a failure. The bands below follow NAMUR NE 43, the common convention for how a transmitter signals its own health. Check the transmitter manual, since the exact failure current is configurable on most devices and some default to driving high on failure while others drive low.

What this does not cover

The scaling is linear. A transmitter configured with square root extraction, common on differential pressure flow measurement, does not follow this relationship, and neither does one with a custom characterisation curve. Loop resistance, supply voltage and burden are separate questions: they determine whether the loop works at all, not what the current means once it flows.