Why 4–⁠20 mA starts at 4

Walk through almost any plant and you will find the same signal on hundreds of wires: a current between 4 and 20 mA. Pressure transmitters, level sensors, flow meters and valve positioners all speak it.

The range looks odd at first. Why not 0 to 20? The answer is one of my favourite small ideas in engineering.

On a 4–⁠20 mA loop, 4 mA means 0 % and 20 mA means 100 %. So the bottom of the range is not zero current. It’s a live zero.

Now imagine the cable gets cut. The current drops to 0 mA. If 0 mA meant 0 %, the control system would read “tank empty” or “no pressure” and carry on as if nothing happened. With a live zero, 0 mA is outside the range. The system knows it’s a fault, not a reading, and it can raise an alarm.

Field note

Many transmitters follow NAMUR NE 43. From 3.8 to 20.5 mA the signal is a reading. At 3.6 mA or below, or at 21 mA or above, the transmitter is telling you it has found a fault.

The live zero has a second job. Many transmitters are two-wire, or loop-powered, devices: the same two wires carry the signal and the power for the transmitter’s own electronics.

A device can’t run on nothing. The current never drops much below 4 mA. Even a fault signal stays at about 3.6 mA. So there is always a little current left to keep the transmitter alive, even at the bottom of the range. The transmitter controls how much current flows round the loop and quietly uses a small part of it to power itself.

A simple loop has three parts:

• a 24 V DC power supply
• the transmitter out in the field
• an analogue input in the control panel that measures the current, often across a 250 Ω resistor, which turns 4–⁠20 mA into 1–⁠5 V

Current is the same at every point of a series loop. A long cable adds resistance, which would change a voltage signal, but the current stays the same as long as the supply has enough voltage to push it. That makes 4–⁠20 mA a good choice for long cable runs in electrically noisy places.

It also means there is a budget to check: at 20 mA, the supply must cover the transmitter’s minimum voltage plus the drop across the cable and the input. The datasheet gives you the first number.

Turning the current into a value is a straight line:

value = (mA − 4) / 16 × span + bottom of range

For a 0–⁠10 bar transmitter, 12 mA gives (12 − 4) / 16 × 10 = 5 bar. Right in the middle.

It’s an old standard, and still a good one. Next time a meter shows 4 mA, remember that it isn’t nothing. It’s a healthy loop saying zero.