How to read a 4–20 mA signal
The most common analog signal in a plant is a current loop. Once you know five numbers, you can read it.
The five numbers
| Current | Means |
|---|---|
| 4 mA | 0 % of range (live zero) |
| 8 mA | 25 % |
| 12 mA | 50 % |
| 16 mA | 75 % |
| 20 mA | 100 % |
Formula: % = (mA − 4) ÷ 16 × 100. Example: a 0–100 psi transmitter reading 14.4 mA → (14.4 − 4) ÷ 16 = 65 % → 65 psi.
Why it starts at 4, not 0
The "live zero" lets you tell an empty tank from a dead wire. A healthy loop never sits at 0 mA.
- 0 mA — no current flows: broken wire, blown loop fuse, or no loop power.
- 3.6 mA or less — below the normal range: the transmitter is signalling a fault (many follow the NAMUR NE 43 convention).
- 21 mA or more — above range: transmitter fault or overrange.
Measuring without breaking the loop
Breaking a running loop to put your meter in series makes the PLC see 0 mA. That can trip alarms, shut down a process or start a pump. Better options:
- Transmitter test terminals: many transmitters have a test diode across two terminals. Put your meter (mA range) across them and read loop current with the loop still closed.
- mA clamp meter: clamp around one loop wire. No disconnection at all.
- If you really must open the loop, tell operations first and put the loop in manual or bypass through the approved procedure, never by jumpering safety functions.
Compare field, PLC and HMI
A loop check compares the same value at 0/25/50/75/100 % in three places: the transmitter, the PLC raw counts and the HMI display. If the loop current is steady but the PLC value jumps, the problem is inside the PLC (channel config or scaling), not the transmitter.
Practise this in the Technician and Programmer ranks of Service Call: PLC Trainer.