Abstract
Fly-ash loading of stack gas frequently precludes velocity measurements with a standard Pitot tube due to plugging of the static pressure holes. The use of a double Pitot tube obviates this difficulty, but in the lengths commercially available its use is restricted to stacks of about 30 inches in diameter. Calibration charts are presented demonstrating that an 18-inch double Pitot tube may be fitted with metal tubing extensions as necessary to allow velocity traverses of large stacks with only small departures from velocity meter scale readings.