Where to Read a Smoke Plume for Method 9
Identifying the correct point on the plume, including steam and water vapor situations
Excerpt from Online Visible Emissions Course · Return to course summary »
Determine the point on the plume
Visible emissions observations should be read at the point of the plume where the opacity is the greatest. Method 9 states that opacity observations shall be made at the point of greatest opacity in the portion of the plume where condensed water vapor is not present.
When water vapor or steam are not present
When a plume does not have water vapor or steam, finding the point to read is straightforward — read at the point of the densest smoke.
Steam plumes — when water vapor is present
Water vapor plumes are not considered emissions and should be excluded from observation data when performing a reading. Vapor plumes usually appear very white and billowy, with mottling and shadowing like a cloud, transitioning to thin and wispy at the point of dissipation.
Steam plumes contain liquid vapor which can affect how opacity is discerned. There are two types of steam plumes: