Measurements not at a representative sampling plane skew measurement data. The proper location of traverse points, then, is the most important measurement that a source tester makes. The importance of traverse points has been solidified by the adoption of methods relating to their determination as Method 1.
EPA Method 1 and associated Method 1 equivalents from various air districts are used to provide guidance for the determination of individual source sampling points (traverse points) located in equal areas within an exhaust stack or duct. In order to use the Method unrestricted, the following criteria must be satisfied.
For stacks and ducts less than 12 inches in diameter, use EPA Method 1A and variants. Two sampling planes will need to be identified 8 diameters apart.
A non-parallel flow profile is described as cyclonic. Think of a corkscrew or auger – the flow is spiraling from the equipment through the duct to the stack exit. While not prevalent, cyclonic exhaust streams do exist. A cyclonic flow profile will have an average yaw angle greater than 20 degrees.
A disturbance is any change in piping or items fixed inside a stack or duct that will cause the flow profile to change. Common disturbances are bends, reductions, tees, and even the top of an exhaust.
Upstream and downstream…..practically, both are based on frame of reference. For Method 1, downstream is defined as the ducting between the operating equipment and the test ports. Upstream, then, is the ducting between the test ports and the stack exit. For a vertical stack with a sampling platform and ports, the downstream distance is measured below the ports, while the upstream is measured above.
Instead of trying to meet the minimum spatial requirements of Method 1, try to be closer to the ideal requirements of 8 diameters downstream of a flow disturbance, and 2 diameters upstream. The chance of a cyclonic flow profile decreases as the disturbance distances increase. This is not always possible; sometimes temporary stack extensions are required.
For reference, we have a calculator that helps determine ideal exhaust size.
Alternate site selection criteria may be found for proposed measurement planes that do not meet minimum Method requirements. However, it is still recommended that equipment exhausts be designed or modified to meet the minimum, as the alternative procedure does not guarantee the location is testable.
