Single-jet and multi-jet meters

Both are mechanical velocity meters in which water turns an impeller and the impeller's speed is counted as volume. A single-jet meter drives the impeller with one stream; a multi-jet meter splits the flow into several jets around the impeller, which balances the force on it.

Both are common on household and small commercial connections. Knowing which type is fitted helps when reading test results, ordering spares or judging how a meter will cope with the local water.