Visible Emission Opacity Six-Minute Average (Method 9)

The six-minute opacity average an EPA Method 9 observation produces, and why it is the finding rather than the worst moment anyone saw. Twenty-four readings at fifteen-second intervals make one six-minute block, and the block's arithmetic mean is what is compared against the limit. That structure is what makes a short puff and a steady haze different findings. A soot blow producing a very dark plume for thirty seconds contributes two readings to a block of twenty-four, and the average can remain in compliance; a faint plume that never clears contributes twenty-four readings at a lower value and can exceed the same limit. Operators who watch the stack and judge by the worst moment consistently misread which condition is the violation, so both cases are computed side by side here. Method 9's observation conditions are as binding as the arithmetic, and nothing in this calculation can substitute for them. The observer stands at a specified distance with the sun in a specified sector behind them, reads against a contrasting background, and records the conditions -- and a reading taken with the sun in the wrong quadrant, against a bright sky, or through the plume at the wrong angle is not a valid reading regardless of what the observer saw. That is why an operator's own observation is not equivalent to a certified reading. Certification matters and it lapses: observers are certified against a smoke generator and must recertify on a stated interval, and readings by a lapsed observer are not enforceable readings. The sum of the block is entered here rather than the individual readings, because the field form records them and the sum is what a person transcribes -- and transcription is where the arithmetic goes wrong. This computes an average from entered readings; it does not validate the observation conditions, the observer's certification, the applicable limit or its short-duration exceptions (many standards allow one six-minute period per hour at a higher opacity), or whether a continuous opacity monitor's data would govern instead. 40 CFR Part 60 Appendix A Method 9, the applicable subpart, and the reviewing authority govern.

block average = the sum of the readings divided by their count, with 24 readings at 15-second intervals making one six-minute block; the readings at a stated peak a block could carry is the limit times the count over that peak.

EPA Method 9 (40 CFR Part 60 Appendix A) by name -- readings at 15-second intervals, 24 to a six-minute block, and the block's ARITHMETIC MEAN compared against the limit. The block sum and the limit are ENTERED. It does not validate the observation conditions (sun position, background, distance and angle are specified by the method and a reading outside them is not valid), the observer's certification and its expiry, the applicable limit or its short-duration exceptions, or whether continuous opacity monitoring data would govern instead.

One arithmetic mean; no method text is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: readings_sum_pct, reading_count, peak_reading_pct, limit_pct, steady_reading_pct, block_average_pct, margin_pct, readings_at_peak_allowed

• Observation conditions are not validated sun position, background, distance and angle are specified by the methodEPA Method 9
• Certification lapses observers recertify on a stated interval and a lapsed reading is not enforceablethe reviewing authority
• Short-duration exceptions are not applied many standards allow one period per hour at a higher opacitythe applicable subpart