In this guide
Most pressure, level, flow and temperature transmitters in a plant send their reading as a 4 to 20 mA current. It is the quiet workhorse of process control and it fails in predictable ways.
Why current, and why 4 mA
Current stays the same along a wire regardless of resistance, so cable length matters less than it would for a voltage. The signal starts at 4 mA, not zero, so the system can tell a real zero reading from a broken wire. A loop reading 0 mA is a fault.
How the value maps
The value is Low + (mA − 4) ÷ 16 × (High − Low). On a 0 to 10 bar transmitter, 12 mA is 5 bar. The scaling calculator does this in either direction and shows the Siemens normalised value.
Types of transmitter
| Type | How it is powered | Common use |
|---|---|---|
| Two-wire (loop-powered) | From the loop itself, on the same two wires as the signal | Most process transmitters |
| Three-wire | Separate supply and signal sharing a common | Some sensors |
| Four-wire (active) | Own supply, separate signal output | Larger instruments |
Fault bands
NAMUR NE 43 defines 3.8 to 20.5 mA as the valid measuring range. Below 3.6 mA or above 21 mA a transmitter is signalling a fault. A good PLC program treats those values as a fault and not as a real reading.
Finding a bad loop
- Measure the current with a meter in series, or measure the voltage across a known resistor in the loop. 250 Ω gives 1 to 5 V
- 0 mA points to a broken wire, a blown fuse, a missing loop supply or a dead transmitter
- Stuck at about 4 mA may be a genuine zero, a transmitter fault, or a blocked impulse line
- Unable to reach 20 mA suggests too little supply voltage for the loop resistance, or a transmitter limited by its own range
- Unstable or noisy suggests a loose terminal, wet cable, missing screen or an earth loop
- PLC reads nothing though the loop is healthy often means the input channel is configured for voltage, not current
Common mistakes
- Wiring a two-wire transmitter as if it were four-wire, or the reverse
- Forgetting that the loop needs a supply voltage that exceeds the transmitter's minimum plus the voltage dropped across everything in series
- Earthing the screen at both ends and creating a loop
What to send us
- The transmitter's model and range
- The PLC or indicator input it connects to
- What you measured, if anything