Which method is used to measure opacity in stacks for emissions monitoring?

The measurement of opacity in stacks for emissions monitoring is primarily conducted using optical measurement techniques that incorporate a light source and a detector. This method works by sending a beam of light through the exhaust emissions from the stack. The amount of light that is transmitted through the emissions and received at the detector is then measured.

Opacity is a measure of how much light is blocked by the particles or pollutants within the exhaust gas. The more opaque the emissions, the more light is absorbed or scattered by the particles present. By quantifying the amount of light that can pass through, operators can assess the efficiency and environmental compliance of the stack's emissions.

Other methods mentioned are not suited for this purpose. Thermal imaging detects heat rather than light and does not provide a direct measurement of opacity. Gas chromatography is designed for analyzing the composition of gaseous mixtures, identifying specific compounds rather than measuring opacity. Ultrasonic testing is primarily used for detecting structural flaws in materials and is not applicable to measuring opacity in emissions. Thus, the optical measurement method stands out as the appropriate and reliable technique for assessing stack opacity in emissions monitoring.