Telemetry, data and integration

What is a 4-20mA sensor loop?

• Current, not voltage. In a series loop the current is the same everywhere, so cable length and resistance do not change the reading.
• 4mA is live zero. Zero measurement is not zero signal. That distinction is what makes the next point possible.
• 0mA means a fault. Break detection is built into the range: cut cable, dead transmitter or lost supply.
• Two wires can do both jobs. A loop-powered transmitter takes its supply from the same pair that carries its signal.

Why current rather than voltage

A voltage signal degrades over distance. Cable has resistance, resistance drops voltage, and the value arriving at the panel is therefore not the value that left the transmitter. Correcting for it means knowing the exact cable run, and the correction stops being right the moment anything is rewired.

Current does not have that problem. In a series loop the same current flows at every point in the circuit, so a transmitter several hundred metres away delivers the same reading as one on the next wall. The loop is also considerably more tolerant of the electrical noise found around pumps, drives and motor starters. That combination is why 4-20mA is still the default in plant rooms and treatment works decades after it might have been expected to disappear.

Live zero, and why it matters

The choice of 4mA rather than 0mA as the bottom of the range is the clever part of the standard. If zero measurement produced zero current, an empty tank and a severed cable would look identical, and every reading of nothing would be ambiguous.

By placing the bottom of the range at 4mA, a healthy loop always draws current. Zero current is then unambiguous and means the circuit is broken, the transmitter has failed, or its supply has gone. Some transmitters go further and drive the current slightly outside the 4 to 20 band to signal a specific diagnostic condition, so a reading of 3.6mA or 21mA is worth looking up rather than dismissing as noise.

This is a genuine safety property rather than a convenience. A monitoring system built on 4-20mA can distinguish between an instrument telling it nothing is happening and an instrument that has stopped telling it anything at all.

Loop powered versus four wire

A two-wire, loop-powered transmitter draws its own operating power from the loop and signals by varying how much current it lets through. One pair of wires carries both supply and measurement, which keeps installations simple and cable counts low. The constraint is power: the transmitter has to run on less than 4mA, which limits what it can do.

A four-wire transmitter has a separate supply and outputs its 4-20mA signal independently. This is used where the instrument needs more power than a loop can provide, such as an analyser with a pump, a heater or a display.

Either way there is a voltage budget to respect. The supply has to cover the transmitter’s minimum operating voltage plus the drop across every device in series plus the sense resistor at the receiving end. Loops usually fail when something is added to an existing circuit without redoing that arithmetic, and the symptom is an instrument that reads correctly at low values and saturates at high ones.

Getting an existing loop into a monitoring platform

A 4-20mA instrument on its own is not connected to anything. It reports to whatever reads the loop, normally a PLC, a local controller, a panel indicator or a chart recorder. On a great many UK sites that is exactly where the signal stops. The instrument works, the measurement is good, the panel displays it, and nobody more than ten metres away can see it.

There are two ways to change that without replacing the instrument. The signal can be read where it already terminates, by a device at the panel that converts the current to a digital value and transmits it. Alternatively a device can be placed in series with the existing loop so that it measures the same current the original receiver does, leaving the panel working exactly as before and adding a remote reading alongside it.

The second approach is attractive on a live plant because nothing about the existing control arrangement changes. The caution is the voltage budget again: anything inserted in series consumes part of it, so the sum has to be checked against the supply before the loop is opened, not after.

What to check before specifying anything

Establish the range mapping first. 4-20mA is only a scale, and it means nothing until you know what 4 and 20 correspond to. The same signal can be zero to ten metres of level or four to twenty bar of pressure, and that mapping lives in the transmitter configuration, not in the wiring.

Confirm the loop voltage budget and what is already in series.

Check whether the location is a hazardous area, because in a classified zone the loop may require an intrinsically safe barrier and any device added to it must be certified as part of that arrangement. The zone classification governs this, not the instrument on its own.

Finally, ask whether the measurement is already available digitally. Many modern instruments carry Modbus over RS485 alongside their analogue output, and reading the digital value is usually simpler, carries diagnostics the analogue signal cannot, and avoids touching the loop at all. It is worth ten minutes with the instrument manual before committing to an analogue tap. This matters most on water quality instruments, where a single analyser may expose several parameters digitally but only one over 4-20mA.