This manual provides information about the most common large flowmeters used in water treatment and in custody-transfer applications, including the Venturi, modified Venturi, orifice plate, electromagnetic (mag), turbine and propeller, transit-time ultrasonic, vortex, averaging Pitot, and averaging insertable electromagnetic (mag). The discussion of these meters covers basic theory, installation, maintenance, and advantages and disadvantages. General concepts applicable to flowmeters are also discussed, including flow characteristics, installation and performance. The third edition of this manual contains significant updates that aim to provide more specific information on flowmeters. Additional information has been added on data signals, electronic communication information signal outputs, field testing and more.