The water flow though a pipe is measured by using a mechanical flow meter. These work through positive displacement, where the volume of water flowing though in a given amount of time is measured. Domestic areas typically use a piston/rotary positive displacement meters or a nutating disk meter, while commercial areas use turbine flow meters.
Piston meters involve a piston rotating within a chamber with a known volume. Each rotation occurs when a set volume of water passes through this chamber. Through a gear mechanism or a magnetic drive, the rotation is measured and displayed as a flow rate. Nutating disk meters have a disk that wobbles about a vertical axis when water flows through it. Gearing connected to the disk registers the volume flow within an accuracy of 1 percent. Turbine flow meters operate in a similar manner to the piston meter; the flow of water rotates the turbine around an axis which gets displayed as a flow rate. During a steady operation, the rotation speed of the turbine is directly proportional to the fluid velocity. Although less accurate than piston meters at low rates, turbine meters, which do not restrict the flow path, are perfect for the high flow rates needed by water distribution systems, large commercial users and fire protection systems.