Transit time meters use two transducers that attach to the outside of the pipe—one upstream and one downstream. These operate by comparing the speed of sound through the liquid in both the upstream and downstream directions. The faster the liquid is flowing, the greater the difference in time, but if the liquid is not flowing, then the speed of sound will be the same in both directions. Because bubbles and solids can reflect the sound, transit time meters work best in clean fluids, such as potable water, coolant, or hydraulic oil. Portable models are available.