A round duct traverse is as dependable an accurate as that of a rectangle traverse. There are different steps that need to be taken when reading round ducts. Lets start by going over the formula for finding the area of a round duct. The radius of a circle is half of the diameter. An example would be the diameter being 10 inches making the radius 5 inches. The formula for a 10 inch duct would be Radius * Radius * Pi / 144 = square feet ( 5 * 5 * 3.14 / 144 = .545 sq.ft.). You will need to subtract any insulation from the diameter of the duct just as we did in a rectangle traverse. Now that we know what the area is we can move on to step two, drilling the holes. This is much more simple than rectangle ducts because you only need two holes, one on the top or bottom and one on the side. When taking your velocity readings you should always avoid taking readings in the center of the duct. This area will be very low velocity and will make the readings unreliable. The pitot tube tip must always be pointed into the airstream. Most manuals will tell you that a duct this size will need at least twenty readings, but I find that to be a little to much. I try and take as many readings as I can get in to make the traverse as accurate as possible. In this example I used a total of 16 readings, eight horizontal and eight vertical. Now the third and final step will be to avarge your velocity readings by the number of total readings (16) and multiply that by your duct area(1200fpm * .545 = 654cfm). Make sure you plug the holes after taking your readings with some type of plastic plug.
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