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Emission Measurement Center
Method 9 - Visual Opacity
FREQUENTLY ASKED QUESTIONS (FAQS) for Method 9
When performing a Method 9 observation, what is the correct procedure for calculating an opacity average if one or more 15–second readings are missed (plume rolling, plume obscured by fugitive emissions, distraction, etc.)?If, for any reason, a 15-second reading is not made, a dash should be placed on the data sheet for that 15-second interval, and an explanation should be provided in the comments section of the data sheet. Running averages can include a time-lapse break in the opacity readings when caused by an event that would make taking a valid reading difficult or impossible (for example, plume obscured by fugitive emissions, plume rolling, or an improper background). Running averages should not include a time-lapse break when a valid reading could have been made, but was not (e.g., because the observer had eye strain or was distracted). In calculating running averages, you must still use 24 consecutive readings which are then summed and the total divided by 24 to yield the opacity average. For a more detailed description of the procedure, see Sections 6.1 and 4.3.2, I and J of the QA Handbook here.
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