There always been plenty of discussions on the appropriate length of sample pipe/tube to extract a sample from a stack or duct, especially for purpose of Emission Monitoring. I have seen requests for sample probe extending to 1/3rd of the inner diameter of stack or extending to the center of the stack. But the question is how is this length requirement decided.

There are some technical aspects that are important considerations in this discussion.

  • If the sample probe too short it will not be able to provide a representative sample.
  • If the sample probe is too long, it may create mechanical stability issues due to high velocity in the stack

Let us discuss this with respect to what is specified in the USEPA’s 40 CFR Part 60/75. It is specified in effect to say that ….

It is suggested that the measurement point be

no less than 1.0 meter from the stack or duct wall or

within or centrally located over the centroidal area of the stack or duct cross section.

It is also as specified in the CFR,

Centroidal Area means the central area of the stack or duct that is no greater than 1 percent of the stack or duct cross section. This area has the same geometric shape as the stack or duct.

To give a perspective of what this means for our day to day use and applications. Let us consider 1M, 2 M and 3M stack ID for purposes of our calculation. We will calculate the location of the centroid and there by deriving the maximum and minimum length of the sample probe.

Based on the above we can establish a simple rule of thumb that

  • For stacks below 2 M in diameter, we can select a sample probe that will be at least half of the stack ID from the inside wall of the stack.
  • For stacks above 2 M in diameter, we can select a sample probe that is 1 M long from the inside wall of the stack.

The above probe dimensions do not take into consideration the wall thickness, insulation, nozzle heights, etc. which will vary from site to site.

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