Understanding Traverse Points in Circular and Rectangular Ducts

 Minimum number of traverse points in circular and rectangular stacks or ducts

Traverse points in Circular stacks or ducts

For circular stacks with diameters greater than 60 cm (24 inches), the minimum number of traverse points required is twelve (12), or six (6) in each of two directions 90° apart, when the duct diameters from disturbances are eight (8) or more upstream and two (2) or more downstream.  For circular stacks with diameters between 30 and 60 cm (12 and 24 inches), the minimum number of sample points required is eight (8), or four (4) in each of two directions 90° apart.  For stacks less than 30 cm (12 inches) in diameter, refer to Method 1A for calculating traverse points. 

Procedure for locating each Traverse Point

The procedure for locating each traverse point along the diameter for a circular stack and then marking the probe assembly or pitot tube is as follows: 
• On a Method 1 field data sheet (data sheet can be computer or calculator generated) multiply the stack diameter by the percentage taken from the appropriate column of Table 2-2
• Add the port nipple length to each value for each traverse point. 
• Convert the decimal fraction to 1/8th (0.125) of an inch for each point (English units only). 
• For stacks  60 cm (24 inches) in diameter, relocate any traverse points that are closer than 2.5 cm (1.00 inches) from the stack wall to 2.5 cm and label them as “adjusted” points.  You may combine two successive points to form a single adjusted point, which must be sampled twice.   
• For stacks, 60 cm (24 inches), do the same, except the adjusted distance is 1.3 cm (0.5 inch). 
• Measure each traverse point location from the tip of the pitot tube, and mark the distance with heat-resistant fiber tape or whiteout correction fluid.

Traverse points in rectangular stacks or ducts

For rectangular stacks or ducts, an equivalent diameter must first be calculated using the 
following equation: 
              2 L W
    De=  ---------
              W + L
where De  = equivalent diameter of rectangular stack 
L= Length of stack
 W = width of stack 
The minimum number of traverse points required for rectangular stacks is nine, or 3 x 3. After the number of traverse points has been determined, the location of each traverse point must be calculated.  The traverse points and their locations are designated as the sample point matrix.  For circular stacks, the stack cross-section is divided into concentric rings of equal area based on the number of traverse points divided by four (4), the rings are bisected twice, and the sample points are located in the centroid (center of mass of each equal area, as shown in Figure. 
For rectangular stacks, the centroids are much easier to determine, as shown in  Figure.