Pulsed water meter: All you need to know

A practical guide to pulse outputs, their limits and remote reading

A pulse output does not transmit the meter's reading. Instead, it generates an electrical signal each time a defined volume of water passes through the meter, and another device must count those signals and convert them into consumption. Pulse outputs are widely used on existing water meters, either built in or added as a module, making them a common starting point for remote meter reading projects.

The principle is simple, but its limitations matter. Understanding both helps determine whether remote reading of an existing pulse meter will deliver data that matches the register.

Key takeaways

• A pulse output emits one pulse per fixed volume of water. The pulse carries no units, no meter ID and no absolute index.
• Pulses are generated by a reed switch, a Hall sensor or an optical sensor linked to the meter's register.
• A pulse output, a dry contact and the standardised S0 interface are not the same thing, so electrical compatibility has to be checked before connection.
• A digital pulse total matches the register only with the correct pulse weight, an appropriate initial counter value and reliable pulse detection. Counting and data transmission are two separate questions.
• Encoder-based readings and M-Bus communication can provide more information than basic pulse counting, depending on the meter and its interfaces.
• Pulse converter connects an existing pulse water meter to LoRaWAN or NB-IoT without replacing the meter, as long as the interfaces are compatible.

What a pulse output actually transmits

A pulse output is linked to the meter's register. Each time a defined volume passes, it switches an electrical circuit. The receiving device sees state changes on a wire, not litres. It counts them and multiplies the count by the pulse weight.

The pulse itself carries no additional information. There is no meter identity, no unit, no status information and no absolute index, only the information that another defined volume has passed. Everything else has to be known in advance and configured in the device that counts the pulses.

Reed switch, Hall sensor and optical sensor

Pulse outputs commonly use one of three sensing principles. A reed switch consists of two metal contacts sealed in a small glass tube, closed by a magnet as the register moves. The reed switch needs no power, but the connected counting device must provide a way to detect contact closures. The mechanical contacts can introduce counting errors. When the contacts close, they can bounce, and one closure may appear as several short pulses unless the counting device filters them out (debounce).

A Hall sensor detects a magnetic field electronically. It has no mechanical contacts and therefore no mechanical contact bounce, but as an electronic component it needs a supply voltage.

An optical sensor reads a marked element of the register with light instead of a magnet, so it does not depend on magnetic coupling. Like a Hall sensor, it needs power. Where a sensor needs power and no mains supply is available, battery-powered ACRIOS pulse converters can supply 3.3 V to the connected sensor.

Pulse output, dry contact and S0 are not the same thing

A pulse output is a general description: the meter signals volume by changing the state of an electrical circuit. How it does that varies. A reed switch provides a dry contact: a potential-free switch that only opens and closes. Electronic sensors may use different output configurations, such as an open collector.

S0 is a standardised pulse interface with defined electrical characteristics intended to support interoperability between compliant outputs and inputs. A pulse output on a water meter is not automatically an S0 output, so the meter's electrical specification has to be checked against the converter input before connection.

Pulse weight: what one pulse means

Pulse weight is the volume represented by one pulse. A pulse carries no units, so one pulse can mean 1 litre or 10 litres, and the counting device cannot tell which. The value is specified for the meter or its pulse output module and has to be configured in the counting device.

The conversion itself is simple. At 10 litres per pulse, 125 pulses mean 1,250 litres, or 1.25 m³. If the configured pulse weight is incorrect, the calculated consumption will be systematically overestimated or underestimated, potentially without an obvious indication of the error.

Pulse weight also sets the resolution. The larger the volume per pulse, the coarser the data, and the longer a small leak can run before it produces a single pulse.

In ACRIOS pulse converters, pulse weight and unit are configured per input through the Lua scripting interface, together with debounce where the meter needs it, so readings arrive already in litres or m³.

Pulse, encoder or M-Bus

A pulse is not the only way a water meter can share its reading. With a pulse output, the counting device maintains a separate running total. Matching the meter's register requires the correct pulse weight, an appropriate initial counter value and reliable pulse detection.

An encoder register provides the register value itself in digital form rather than individual pulses. M-Bus is a communication protocol that can carry the meter ID, values, units and status, and several meters can share one bus. These categories are not mutually exclusive: an encoder describes how the meter's register value is obtained, while M-Bus defines how meter data is communicated. More on the protocol in the M-Bus, wM-Bus and RS-485 technology guide.

Where a meter already offers M-Bus or wM-Bus, converters for those protocols may be a better fit. Pulse converter is an option where a compatible pulse output is the only interface available.

How to tell if a water meter has a pulse output

The most reliable source is the meter's documentation and type label. Terms such as pulse output, reed or impulse, together with a stated pulse weight, indicate that an output is available. On the meter itself, a cable or a pulse module attached to it is a visible sign that an output is already fitted.

Some meters are prepared for a pulse module but have none fitted. Which module fits and how it is mounted depends on the manufacturer. Where the markings are unclear, the manufacturer and type number are usually enough to find the right documentation.

Limits of pulse counting

Two separate questions decide whether pulse data can be trusted: whether every pulse is counted correctly at the input, and whether the counted values reach the receiving system.

At the input, errors can come from contact bounce, electrical interference from pulse cables running alongside power lines, and pulses arriving faster than the input can register. Because the pulse carries no reference value, the signal itself cannot correct a missed or extra pulse, and the digital total drifts away from the register.

Transmission is a different matter. ACRIOS pulse converters keep counting during a network outage, store the pulse values locally and send the last three readings once the connection is restored. The cumulative count is preserved, although resending the last three readings does not necessarily provide a complete consumption history for a longer outage.

Whether the converter's total has to match the register depends on the goal. If the digital value must match the register, the starting value must match the register at installation. If consumption from installation is sufficient, the count can start from zero.

Tampering and backflow are further limits. For example, a cut cable stops pulses altogether. A basic single-channel pulse output does not indicate flow direction. Detecting backflow therefore requires a meter or sensing arrangement that can distinguish flow direction.

Connecting pulse water meters to LoRaWAN and NB-IoT

Once the pulse output and its pulse weight are known, an existing pulse water meter can be connected to remote meter reading without replacing it, provided the interfaces are compatible. The converter counts the pulses, converts them into volume and sends the values over a wireless network.

ACRIOS pulse converters exist in LoRaWAN and NB-IoT versions, each with up to four S0 inputs. The main practical difference is the network. LoRaWAN requires access to a private gateway or coverage from a public LoRaWAN network, while NB-IoT uses an operator SIM card and sends data directly wherever there is coverage, with no gateway.

Both versions support event-triggered alarm messages when configured thresholds are exceeded, independent of the regular reading interval, and both can be reconfigured remotely. Battery-powered versions are designed for over 10 years of operation at hourly readings, with real values depending on configuration and conditions, as explained in battery life in IoT: beyond the datasheet number.

For a single pulse water meter where technicians want to compare the digital value with the register on site, a converter with a display is an alternative, as described in ACR-EX in water utilities.

What a pulse output can and cannot tell

Pulse outputs offer a straightforward way to extend remote reading to existing water meters, provided their electrical characteristics, pulse weight and counting requirements are understood. They are well suited to tracking consumption, but they cannot independently confirm the meter's absolute register value or provide the additional information available through some digital interfaces. The right approach therefore depends on what the existing meter supports and what data the project needs.

Glossary of terms used

• Debounce: filtering of short, unstable signal changes so that one contact closure is counted as one pulse
• Dry contact: a potential-free switch that only closes and opens a circuit, without supplying any voltage of its own
• Encoder register: a meter register that provides its value in digital form instead of individual pulses
• Hall sensor: an electronic sensor that detects a magnetic field without mechanical contacts
• LoRaWAN: a long-range, low-power wireless network protocol in which devices send data through gateways to a network server; the network can be public or private
• M-Bus (Meter-Bus): a wired European standard for reading data from meters such as water, gas and heat meters
• NB-IoT (Narrowband IoT): a low-power cellular technology operating in licensed mobile operator networks, suited to devices transmitting small amounts of data
• Pulse weight: the volume represented by a single pulse, as specified for the meter or its pulse output module
• Reed switch: a pair of metal contacts sealed in a glass tube that close when a magnet passes nearby
• S0: a standardised electrical interface for transmitting pulses between a meter and a counting device