The choice is usually framed as cost. It is more often decided by the water, the flow profile and what you intend to do with the readings.
What mechanical does best
- No power at all. A multi-jet mechanical meter needs neither battery nor wiring, which removes the battery replacement round entirely.
- Lowest cost per point. On a large household roll-out where the budget is the binding constraint, this is usually the deciding factor.
- Familiar to installers. Like-for-like replacement on an existing mechanical base needs no new training and no new tooling.
The trade-off is the low end. Below its starting flow a mechanical meter simply does not measure, and as the measuring element wears over years it drifts — almost always in the direction of under-reading.
What ultrasonic does best
- Nothing in the water path. No moving parts to wear, so accuracy stays where it started for far longer.
- Low flow is measured. A wide turndown ratio means drips, night flow and leakage show up in the data instead of disappearing.
- Sediment tolerance. Loaded or scaling water affects it far less than a mechanical measuring element.
The trade-off is power and cost. Ultrasonic needs a battery, and the unit costs more — though the battery is sized to be swapped at the meter change rather than as a separate visit.
Where each is the right answer
- Tight budget, no power plan, straightforward billing → mechanical.
- Low flow matters, the water carries sediment, or the meter is part of a leakage programme → ultrasonic.
- Apartment prepaid with arrears control → either, depending on whether you want the valve and the low-flow detection together.
- Distribution mains and DMA boundaries → dual-channel ultrasonic, where reverse flow and disturbed flow after elbows have to be handled.
A mixed answer is usually the right one
Most real projects use both: mechanical across the household inlets where unit cost dominates, ultrasonic at the points where the reading has to hold up — risers, building inlets, DMA boundaries and industrial intakes. Send the DN at each point and we will say which we would put where, and why.