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In a rectangle duct traverse it is important to get an equal number of readings to cover the whole cross
section of the duct. Drilling the correct number of holes and spacing the holes correctly is the most
important part. The first step is finding the area of the duct in square feet. The example I used is a
duct size of 22" wide by 15" high. Find the inside diameter of the duct that the air is flowing through.
We have 1" insulation on all insides of the duct so we have to subtract 2" from both the width and height.
The formula used is width * height / 144 = sq.ft.( 20 * 13 / 144 = 1.8). For a
duct this size we will need to keep our holes 6" apart or less to get maximum accuracy. The first and last
holes should be half the distance of the others holes drilled. With a 20" duct you would drill the first
and last holes 2" from the end and 4" out from each of those holes, for a total of 5 holes. For a duct
this size a minimum number of readings would be 16 but not to exceed 64. I used 5 readings per hole for a
total of 25 readings. There are three different types of instruments that can be used for traverse
readings. Air Balancing Company uses an electronic digital manometer. This device makes traverse measuring
quick and easy. The magnehelic gauge is another device used. This gauge measures in velocity pressure,
so each reading taken will need to be converted to duct velocity. This is done with the equation
Vp = V / 4005 squared. Third is an incline monometer which is used just like
the magnehelic gauge.
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