Paddlewheel flow meters are mechanical meters that have a freely rotating paddlewheel set in the path of a fluid stream. The flowing liquid or gas causes the turbine to spin upon its axis. The rate of spin will be proportional to the velocity of the flow. Paddlewheel flow meters are simple and reliable. They are able to measure high flow rates with low-pressure loss. Though versatile, paddlewheel meters do best in applications with constant conditions in liquids such as water or lower viscosity fluids.