Mag flow meters sit at the heart of almost everything we install, across irrigation, municipal, and industrial sites alike. What differs enormously between those sites is how the meter actually talks to the rest of the system. A simple irrigation take might never touch anything beyond a basic pulse output. A complex treatment plant or process line, on the other hand, needs the output to marry cleanly into an existing SCADA or control system, and that means picking the right communication protocol from a surprisingly long list of options.
This is a reference guide to every output type you'll find across the major mag meter brands, what each one actually is, how it works, and when you'd reach for it.
Pulse and frequency output
A pulse output represents volume, not flow rate, directly. Each pulse corresponds to a fixed, pre-configured volume passing through the meter, sometimes called the pulse value or K-factor, for example one pulse per litre, or one pulse per 10 litres. A running pulse count is a running volume total, since every pulse simply adds another fixed unit, which is why it's such a natural fit for simple totalising and remote counters.
A frequency output works on the same underlying principle but is scaled continuously, the pulse rate (frequency) rises and falls in proportion to the instantaneous flow rate, rather than each pulse representing a fixed lump of volume. To calculate a flow rate from a pulse output, you count pulses over a known time window and multiply by the volume per pulse, then divide by the time elapsed. Count 120 pulses in one minute at 1 litre per pulse, and you've got 120 litres per minute for that window.
Pulse width matters more than people expect. As flow increases, pulses arrive more frequently, and if the pulse width (how long each pulse stays "on") is fixed, there's an upper frequency where pulses start to run into each other, capping the maximum flow rate the output can reliably represent. Most modern meters let you configure pulse width to suit the receiving equipment's input speed. Pulses are typically square-wave, delivered as either an open collector/open drain switched output or an active voltage pulse, depending on how the receiving device needs to see it.
Limitations: a pulse output on its own only gives you accumulated volume, not a live flow rate, unless something downstream is doing the time-based calculation. It also doesn't indicate flow direction without a separate output, and long cable runs can distort pulse shape enough to cause missed or false counts if wiring isn't done properly.
Current output (4-20mA)
The 4-20mA current loop is the industry's long-standing analog standard, and unlike pulse, it represents instantaneous flow rate directly and continuously. The loop current is scaled linearly between a defined lower range value (typically 4mA) and upper range value (20mA), so at any given moment, the current level tells you exactly what the flow rate is, without needing to count anything over time.
It can be wired as a 2-wire loop-powered device, where the same two wires carry both power and signal, or as a 4-wire active output with separate power. For most applications you'll see a straightforward 0 to 20 (4 to 20mA) setup representing zero to full-scale forward flow, but where both forward and reverse flow need to be represented on a single signal, a bidirectional split range is used instead, commonly 4-12mA for reverse flow and 12-20mA for forward, with 12mA representing zero.
Limitations: it's a single-variable output, one loop, one measurement, unless HART is layered on top. Accuracy can be affected by cable resistance on long runs, and the receiving equipment's analog input resolution ultimately limits precision. Like any analog signal, it can drift slightly over time and benefits from periodic verification against a reference.
HART, and how it relates to current output
HART (Highway Addressable Remote Transducer) is a digital communication protocol that rides on top of the existing 4-20mA loop, using the same two wires, without disturbing the analog signal at all. It works by superimposing a small digital signal over the current loop, so the primary flow reading stays exactly as accurate and continuous as a standard 4-20mA output, while a second, parallel channel of digital data becomes available on the same wiring.
That digital channel is what makes HART genuinely useful. A handheld HART communicator or host system can interrogate the meter for secondary variables, totalizer values, temperature, diagnostics, device status, and can remotely configure ranges or damping, all without running a single extra wire. Because it's backward compatible, a HART-capable meter still works perfectly with older equipment that only reads the 4-20mA signal and simply ignores the digital overlay. That combination, rich diagnostics without new cabling, is why HART became so widely adopted in process industries.
Status output
A status output is a simple binary signal, on or off, rather than a continuous measurement. It's essentially a switch, either a relay contact or an open-collector transistor output, that reports a specific condition rather than a value. Most meters let you assign the status output to whichever condition matters most on that site: flow direction, a high or low flow alarm threshold, empty pipe detection, a general fault or diagnostic alarm, or low battery on battery-powered units. Configuration usually includes choosing normally-open versus normally-closed logic to suit the receiving equipment.
In practice, status outputs feed straight into a PLC digital input for simple interlock logic, stopping a pump on empty pipe detection, or triggering an alarm light or SCADA alert when a threshold is breached, without needing any analog or digital communication capability on the receiving end at all.
Modbus RTU
Modbus RTU is a serial digital protocol, typically run over RS-485 wiring, using a master-slave polling architecture. A single master device, a PLC, RTU, or SCADA system, polls each meter in turn by address, and the meters respond with structured data. Because RS-485 supports multi-drop wiring, dozens of meters can be daisy-chained on a single twisted pair back to one central point, instead of running an individual cable for every meter.
Its real advantage over 4-20mA or pulse is that multiple values, flow rate, totalizer, temperature, diagnostics, alarms, can all be read over the same two wires, rather than needing a separate loop or pulse cable per variable. That makes it a natural fit for multi-meter sites: municipal pump stations, water treatment plants, and industrial sites with several meters reporting back to one control system.
Limitations: Modbus RTU is half-duplex and polling-based, a meter can't spontaneously report a reading, it can only respond when asked. Cable length is limited to roughly 1200 metres without repeaters, and as more devices are added to a bus, the effective update rate for each one drops since the master has to poll them sequentially. There's also no built-in security, no encryption or authentication, so it's typically used on isolated or otherwise-secured networks.
PROFIBUS PA/DP
PROFIBUS comes in two relevant flavours. PROFIBUS DP is the faster, general-purpose variant used widely in factory automation. PROFIBUS PA (Process Automation) is a lower-power variant designed specifically for process industries, capable of carrying both power and data on the same twisted pair, and able to support intrinsically safe installations in hazardous areas.
It supports cyclic and acyclic data exchange with strong diagnostics, and has a long track record in process plants, particularly where a Siemens-based DCS is already in place. You'll mostly find it in chemical, oil and gas, and water or wastewater treatment plants with established PROFIBUS infrastructure, especially where the PA variant's intrinsic safety capability matters.
Limitations: PROFIBUS is gradually being superseded by Ethernet-based alternatives like PROFINET in new installations. PA segments have real limits on cable length and device count depending on the power budget, and bridging between PA and DP, or into a modern controller, usually requires a dedicated coupler or link device, adding complexity most sites don't need for a straightforward flow measurement point.
PROFINET and EtherNet/IP
These are the two dominant industrial Ethernet protocols, PROFINET from the same PI organisation behind PROFIBUS, and EtherNet/IP from the Rockwell/Allen-Bradley ecosystem via ODVA. Both run over standard Ethernet cabling and switches, giving far higher bandwidth and richer real-time data exchange than any of the serial fieldbus options above, plus the convenience of integrating straight into a plant's existing IT-style network infrastructure.
You'll find PROFINET more often in Siemens-centric plants, and EtherNet/IP in Allen-Bradley/Rockwell-centric ones, and increasingly, both are the default choice for new industrial builds and modern municipal treatment plant control systems simply because Ethernet infrastructure is cheap, familiar, and gives access to a device's full diagnostics and parameter set over the same cable used for everything else.
Limitations: the meter needs the specific communication module fitted, which adds cost over a basic analog or pulse device. Network design matters too, real-time control traffic needs proper segmentation from general IT traffic to avoid interference. There's also a degree of ecosystem lock-in, a Siemens plant wanting PROFINET and a Rockwell plant wanting EtherNet/IP means the "wrong" protocol choice complicates integration down the track.
FOUNDATION Fieldbus
FOUNDATION Fieldbus is another process-industry digital fieldbus, conceptually similar to PROFIBUS PA, carrying both power and data over a single twisted pair (the H1 segment) and supporting intrinsically safe hazardous area installations. Its distinguishing feature is a function-block architecture that in some configurations allows control logic to actually execute inside the field device itself, rather than solely in a central DCS.
It's mostly found in large heavy-process industries, oil and gas, refining, and petrochemical plants that standardised on Foundation Fieldbus-based DCS platforms years ago.
Limitations: like PROFIBUS PA, its install base is shrinking in favour of Ethernet-based options for new projects, and it demands specialised engineering knowledge. For the kind of irrigation, municipal, and general industrial water measurement we deal with across Canterbury, it's rarely, if ever, the right fit, most sites here will simply never encounter it.
IO-Link
IO-Link isn't a fieldbus in the traditional sense, it's a simple point-to-point digital connection between a single device and an IO-Link master, running over standard 3-wire sensor cabling. The master then aggregates several IO-Link devices onto the plant's main network. It effectively upgrades a basic sensor into a fully digital, bidirectional connection without needing extra wiring beyond what a standard sensor cable already provides, and supports "plug-and-produce" device swaps where the master automatically reconfigures a replacement device.
In flow metering, IO-Link shows up most often on compact, entry-level mag meters aimed at simpler secondary or utility measuring points, Endress+Hauser's Picomag is a good example, rather than large critical process lines.
Limitations: it's point-to-point back to a master only, not a true multi-drop bus by itself, and cable runs are shorter than RS-485-based options. It's better suited to simple, lower-criticality measuring points than to large or complex process applications, and it's still a relatively new standard in flow metering specifically, so support varies by brand and model.
GPRS and cellular
This is the odd one out, a wireless connection over the mobile phone network via a SIM card, rather than any wired link back to a local PLC or SCADA system. The name GPRS still gets used loosely to describe this whole category, but GPRS itself runs on the old 2G network, and 2G and 3G were both fully switched off across New Zealand by the end of March 2026. What's actually deployed today is 4G LTE, or purpose-built low-power variants like Cat-M1 (LTE-M) and NB-IoT, designed specifically for this kind of infrequent, low-bandwidth telemetry, with deep signal penetration and very low power draw, running on the network infrastructure that's actually still switched on.
It's built for genuinely remote, off-grid metering, sites with no mains power and no existing communications cabling within practical reach.
You'll find this kind of telemetry on purpose-built battery-powered mag meters designed for exactly this scenario. Endress+Hauser's Promag W 800 and Krohne's WATERFLUX 3070 remote version are real examples, both built around cellular modems that have moved on to modern network standards as older ones were switched off. These units log readings internally and push data to a cloud platform or central server at scheduled intervals rather than streaming continuously, specifically to conserve battery, which on some of these units can last up to 15 years.
Limitations: it depends entirely on cellular coverage at the site, which can still be a genuine problem in remote valleys or high country. There's an ongoing SIM/data cost, reporting frequency is typically much lower than a wired connection to preserve battery life, and the battery, however long-lived, still needs eventual replacement. Unit cost is also higher than a basic analog or pulse meter, reflecting the added telemetry hardware built in.
Choosing the right one
For a simple irrigation take with a data logger already handling the compliance reporting, pulse or 4-20mA is often all that's needed, sometimes with HART layered on for diagnostics. For a municipal pump station with several meters reporting into one SCADA system, Modbus RTU is the common middle ground. For a modern industrial plant already built around an Ethernet-based control architecture, PROFINET or EtherNet/IP is usually the natural fit. The fieldbus options, PROFIBUS PA, Foundation Fieldbus, sit at the more specialised end, generally only relevant if a site already has that infrastructure in place. And for anything genuinely off-grid, modern cellular telemetry, 4G LTE, Cat-M1, or NB-IoT, is often the only realistic option at all.
Getting this right matters more than it might seem. The wrong output type doesn't just complicate an install, it can mean a meter that technically works but never actually integrates cleanly into the system it's meant to serve. If you're specifying a new meter and aren't sure which output makes sense for your infrastructure, it's worth talking it through before the order goes in.