by Rian Sullings, Manager Smart Metering and IoT - WaterGroup
I'm often asked how smart water metering solutions can utilise existing mechanical meters. Many people assume that because an older meter is a completely mechanical instrument, it cannot be used as part of a modern digital smart water metering solution. These questions come from a variety of people, many of whom are new to the concepts of smart water metering. What these questions ultimately come down to is "How do you turn a mechanical water meter into a smart water meter?"
The answer is to attach a digital metering device to the mechanical meter, effectively transforming it into an electronic data collecting, wirelessly communicating smart water meter. To do this, you need a smart water metering device and a sensor.
Most common mechanical meters produced over the past 20 years have a form of data output. These outputs are either mechanical or electronic switches which open and close as water passes through the meter. An open/closed signal output by a meter is called a pulse. By converting the physical movement of water flow into the output of a digital signal, even older mechanical meters are compatible with modern data logging and wireless communication systems. This has been a great foresight of the metering industry where provisions were made in the past to cater to future technologies.
Mechanical pulse outputs are comprised of a magnet, either attached to a gear inside the meter or attached to one of the dials on the register, and a corresponding pulse sensor.
The most common pulse sensors for magnetic pulse outputs are reed switches. A reed switch is a very simple sensor made of two conductive metal reeds positioned less than 1 millimetre apart inside a hermetically sealed glass tube. These glass tubes are surrounded by silicon-like adhesives to protect the sensor and a plastic casing to plug or clip it into position on the water meter. The two reeds are soldered to wires which exit out of the plastic casing to enable the sensor to interface with smart water metering devices.
Internal Components of Water Meter Pulse Sensor
When a magnet passes by the sensor, the two reeds make contact together as they are pulled towards the magnet. The diagram below shows how the reed switch closes and opens as the magnetic dial on the meter turns and outputs a digital signal. For each full revolution of the dial, the meter outputs 1 pulse. A pulse is the state change from open to closed, and back to open.
Each pulse that is output by a meter represents a predetermined volume of water passing through that meter. For example a meter may output 1 pulse for every 10 litres of water. The volume of water that each pulse represents is dependent on the positioning of the magnet and sensor which varies between different makes and models of water meters.
Similar to reed switches, Hall effect sensors can also be used to output data from mechanical water meters. Hall effect sensors are transistors with varying voltage outputs that respond to magnetic fields and are effectively an electronic equivalent of the mechanical reed switches.
More advanced pulse sensors utilise induction to detect the position and movement of a target affixed to a dial on the register of the meter. This technology is similar to RFID and NFC which are used in contactless payment systems in bank cards and smart phones.
These inductive sensors are larger and more complex than reed switches and Hall Effect sensors. Internally, they are comprised of an inductive coil which varies a voltage output based on the position of the metal target on the meter, a long-life lithium battery, and an integrated chip to interpret the signals from the coil and output pulses and other signals.
There are some key advantages of inductive sensors such as detecting the direction in which the dial is turning, thereby enabling dedicated data outputs to alert for backward flow through the meter. They can also alert for sensor disconnection, apply calculations to reduce the pulse rate, and output data by more complex protocols such as M-Bus. They are also generally more reliable than reed switch based sensors.
It is important to understand that pulse monitoring is not perfect. Due to issues such as switch bounce and electrical interference, some pulses can be missed or false pulses can occur which can impact the integrity of the data. For example, over a period of time a meter may show 10,000kL of use on the register but a pulse device may record 10,005kL or 9,997kL. To this extent, pulse devices may not be suitable if it is critical for your business case to achieve perfect 100% accuracy and the complete removal of the need for manual meter readings for billing. The pulse logging device should be properly designed to minimise the impact of these issues.
Another pitfall to avoid with pulse based smart metering is that cables can be damaged, especially in outdoor environments. They can be cut by lawn mowers, chewed by dogs, or tripped over by people. Exposed cabling should be kept short and any plugs or cable connectors should be enclosed to prevent tampering.
Retrofit Smart Water Metering Device and Integrated Smart Water Meter
With new digital water meters with integrated wireless communications now available, it is easy to assume that mechanical meters are destined to become a relic of the past. Fortunately, the most common models of mechanical water meter in Australia, the Elster V100 and the Itron TD8, are both pulse enabled. This means that you can leverage your existing assets for many more years to come by transforming them into smart water meters with retrofit devices and unlocking the detailed data required to drive insights and actions to improve efficiency.
Choosing the right retrofit smart water metering device is another topic altogether, however, it is worth mentioning that the cost-benefit of retrofit compared to fully integrated meters should be carefully considered. Fully integrated devices may now be more economical end-to-end than retrofit, largely due to advances in wireless communication technologies and battery life, especially for new metering deployments or where existing mechanical meters are coming to the end of their lifespan. The best solution for you is dependent on your project and your unique requirements.