Both will measure your water take and satisfy ECan consent requirements, but they're quite different instruments, and the right choice depends on your specific situation. This guide compares the two across the factors that matter most.
Before comparing them, it helps to understand the basic operating principle of each type.
Mag meters work on Faraday's law of electromagnetic induction. The meter generates a magnetic field across the pipe, and as conductive water flows through it, a small voltage is induced. The meter measures that voltage and converts it to a flow rate. There are no moving parts, the measurement happens entirely through the pipe wall and the water itself.
Mechanical meters use a rotating impeller or propeller driven by the flow of water. As water passes through the meter, it spins the impeller, and the rotation rate is converted to a flow rate. The measurement depends on the physical movement of components inside the meter.
That fundamental difference, moving parts vs no moving parts, drives most of the practical differences between the two types.
Mag meters are highly accurate across a wide flow range, typically ±0.5% or better under good conditions. They maintain accuracy well over time provided the electrodes stay clean and the installation is correct.
Mechanical meters are accurate when new and in good condition, typically ±2% or better, but accuracy degrades as the moving parts wear. Bearing wear, impeller fouling, and sediment damage all reduce accuracy over time.
Mag meters have no moving parts, which means there's very little to maintain between verification cycles. The main maintenance task is electrode cleaning, in Canterbury's irrigation water, which often carries sediment, organic matter, or mineral deposits, electrodes can foul over time. Regular inspection and cleaning is straightforward and can usually be done as part of a routine service visit.
Mechanical meters require more regular attention. The impeller, bearings, and seals are subject to wear and need periodic inspection and replacement. In Canterbury irrigation conditions, particularly where the water carries sediment or grit, mechanical wear accelerates.
One practical warning sign is worth knowing: if your pump is running but the dial on a mechanical meter isn't turning, and is just oscillating back and forth instead, this usually means debris is stuck in the turbine or paddle. The same oscillating behaviour can also show up if a mechanical meter has been installed in a vertical orientation. Most mechanical meters handle vertical installation fine, but a few models don't, so it's worth checking the manufacturer's installation guidance before assuming a fault.
Under current ECan consent conditions, all flow meter types, mag, ultrasonic, and mechanical, are verified on the same five-year cycle:
Since both meter types now sit on the same verification cycle, the compliance cost of staying verified is the same regardless of which you choose. The real cost difference comes down to maintenance, covered below, rather than how often a verification visit is required.
Mag meters cost more upfront than mechanical meters, but how much more depends heavily on size. Within mag meters themselves, the price gap between sizes is smaller than most people expect: a 50mm mag meter and a 150mm mag meter aren't that far apart in price. Where it really jumps is once you get above 300mm, the cost climbs steeply from there. As a guide, a standard irrigation mag meter can range anywhere from around $4,000 up to $8,000 depending on size, but the sky is the limit once you're into larger pipe diameters.
Mechanical meters are generally less expensive to purchase. Typical pricing starts at around $700 and can go up to $4,500 for a decent 300mm unit. However, when you factor in higher maintenance costs and earlier replacement due to wear, the total cost of ownership over 10 years often favours the mag meter, particularly for larger diameter pipes or higher-use applications.
Sediment and grit, particularly in surface water takes, Canterbury irrigation water can carry significant sediment loads. Mechanical meter impellers are vulnerable to abrasion and fouling from sediment, accelerating wear and reducing accuracy. Mag meters handle sediment well, there are no moving parts to damage.
Low flow rates, at low flow rates, mechanical meters can stall or lose accuracy. Mag meters maintain accuracy across a wider flow range.
Pipe size and debris, regardless of pipe size, the first consideration is whether the source is free of debris. If it is, a mechanical flow meter from 32mm up to 300mm is generally the most cost-effective way to stay compliant. Mag meters come into their own where the water source can carry debris, with no moving parts, any debris present has no effect on the internals of the flow tube. It's also worth noting that most mechanical meters don't display flow rate directly, it has to be calculated or observed by timing volume over time, whereas mag flow meters have digital displays showing flow rate in real time.
Yes, when correctly installed and in good condition. Like all meter types, mechanical meters are verified every 5 years under current ECan requirements. Regular verification is essential to confirm your mechanical meter is still within tolerance, since accuracy can degrade as moving parts wear.
In most cases yes, although some flow meters don't share the same face-to-face dimensions and flange types. This isn't just a consideration when swapping a mechanical meter for a mag meter, meters of the same type can have different face-to-face (flange-to-flange) measurements and flange types too. NZ Flow Group can assess your existing installation and advise on what's involved in an upgrade.
Both can satisfy ECan consent conditions provided they're correctly installed, verified on schedule, and operating within the ±5% accuracy tolerance. All meter types are currently verified on the same 5-year interval.
Flow meter sizing is based on your pump delivery rate and flow velocity through the pipe, not simply the pipe diameter. Getting it wrong, particularly undersizing, affects accuracy and performance. Contact us with your pump delivery rate and pipe size and we'll advise on the right meter.
Tell us about your application, pipe size, flow rates, site conditions, consent requirements. We'll recommend the right meter without pushing you toward a particular product.
Talk to usWe supply both mag and mechanical meters and will give you straightforward advice without pushing you toward a particular product.