Why Are Sampling Ports Required to be 90 Degrees Apart?

Why Are Sampling Ports Required To Be 90 Degrees Apart?

This question often arises regarding the requirement to place sampling ports 90 degrees apart in order to be Method 1 compliant.  This article will explain why this requirement is important to keep in mind during stack construction or alteration.

Quick Overview of EPA Method 1

Diagram 1

  1. The flow through the stack at the port location must be minimally cyclonic.
  2. For Method 1, the stack diameter must be 12 inches or greater or 113 square inches in a cross-sectional area–Method 1A includes ducts that are between 6 and 12 inches.
  3. The sampling plane must be located more than two stack diameters downstream from the nearest upstream disturbance and more than half a stack diameter upstream from the stack exit or next downstream disturbance. (See Diagram 1).
    1. A minimum of two (2) test ports 90° apart must be installed on the sampling plane
    2. Common sense dictates that four (4) ports are preferred to allow for multiple trains and train maneuverability.
    3. Platform width should be greater than 4 feet from the stack to the handrail.


(See the original blog post for more details on the above)  (See the full text of EPA Method 1)

Why is 90° apart important?

In order to demonstrate that the airflow in the stack is minimally-cyclonic, measurements must be taken at traverse points on two (2) axes (x and y).  See Diagram 2.

Diagram 2


As you can see, ports that are located 180° apart would be on the same axis (x or y).  The problem with sampling on the same axis is that there may be a “dead” air pocket in the stack.  If you sample on one axis only, you are missing an important angle for confirming that the airflow is minimally cyclonic.  Most importantly, placing ports 90° apart is required by regulators who will insist on strict adherence to EPA Method 1 for compliance testing.

There are many issues to consider for an emissions test.  Being prepared with the knowledge to properly construct sample ports will save money by preventing excessive stack modification.  Furthermore, understanding and adhering to the guidelines of EPA Method 1 will ensure that sample port, size, placement, and access are not an issue on test day.

For assistance in determining your specific sampling port needs, questions about EPA Method 1, or any other stack testing issues feel free to call Environmental Source Samplers at 1-888-363-0039.  It would be our pleasure to assist you.

Copyright © 2018 by Environmental Source Samplers, Inc.  All rights reserved.