CHAPTER 13 - SPECIAL MEASUREMENT METHODS IN OPEN CHANNELS
Besides being used in closed conduit systems, propeller meters are frequently placed at the end of pipes delivering water to open channels (figure 14-6 in chapter 14). When used this way, they are often called open flowmeters. The requirements and maintenance problems inherent to these meters are discussed more thoroughly in chapter 14, which should be read along with the following information.
These meters should be installed in open channels that submerge the pipe exit or have small overflow check structures that assure submergence for the desired discharge range. Meters are available for pipe diameters from 2 to 72 inches (in). These meters need sufficient driving velocity and are likely to be inaccurate below 1.5 feet per second (ft/s). Spiral flow caused by poor entrance conditions from the canal to the supply pipe is a common source of error. Straightening vanes provided or specified by manufacturers should be installed. A poorly developed velocity distribution profile can also cause considerable errors in registration.
The accuracy of propeller meters is generally within +/-2 to +/-5 percent of the actual flow. However, careless setting of the meter in the turnout will cause sizable errors if the meter is not properly positioned (Schuster, 1970). For example, a meter with a 12-in-diameter propeller, suitable for measuring discharges up to 8 cubic feet per second (ft3/s) in a 24-in-diameter pipe, when set with the hub center 1 in off the center of the pipe, showed an error of 1.2 percent. When the meter was rotated 11.5 degrees in a horizontal plane, equivalent to 1/4 in measured on the surface of the 22-in-diameter vertical meter shaft housing, the error was 4 percent, indicating that a small angular misalinement of the meter will cause a greater error than would be caused by a moderate eccentricity.