This report describes the results of a study to determine the effects that various geometric ducting configurations have on the flow profiles and the distribution of particulate in ducting systems of large (> 100 MW) power plants. The program included both laboratory model studies and field testing at large power plants. The measurement of total volumetric flow and particulate emissions at less than full operating capacity was also investigated. The results of flow angularity measurements in large stacks at typical sample port locations, including downstream of induced draft fans, are similarly discussed. Special attention was given to the aerodynamic effects of S-tube/sampling probe interference on velocity measurements with an S-tube in EPA Stack Emissions Measurement Reference Methods 2 and 5. A computerized technique was used to determine the effectiveness of various equal area sampling strategies in providing accurate measurements of three emission parameters: average particulate concentration, total volumetric flow rate, and total emissions. Numerous typical and atypical velocity and particulate concentration profiles were studied. Sampling strategy recommendations are presented.