20% opacity
An observer assigns a 20% opacity reading at an observation moment. It describes how much the plume obscures the background. Emission duration is a separate measurement.
Field methods • practical comparisonMethod 9 vs. Method 22:
Method 9 vs. Method 22:
different measurements.
Method 9 estimates how much a plume obscures the background. Method 22 records how long visible emissions occur. Your rule or permit determines which measurement you need.
Compare the methodsSide by side
Compare the required procedures.
| Question | Method 9 | Method 22 |
|---|---|---|
| What is measured? | Visual opacity of emissions. | Duration of visible emissions; frequency can be calculated from emission time. |
| How is it observed? | Momentary opacity readings at specified intervals. | Continuous observation, timing the period and the emissions. |
| Typical record | Opacity values, observation times, source and field conditions. | Observation period, accumulated emission time, source and field conditions. |
| Observer qualification | Requires a qualified Method 9 observer. | Method 9 certification is not required; the observer still needs training in visible-emission detection. |
| Typical application | Opacity determinations from stationary sources under applicable standards. | Fugitive emissions, and visible smoke from flares, where the applicable regulation calls for it. |
| Can one replace the other? | Only when the applicable authority allows it. | Only when the applicable authority allows it. |
Based on EPA Method 9 and EPA Method 22. Check the applicable standard for the required method, observation duration, and reporting criteria.
Understanding the resultsIllustrative Method 9 reading Illustrative Method 22 calculation
What does a 20% result mean?
20% of the time
If visible emissions occur for 120 seconds during a 600-second observation period, the frequency is 120 ÷ 600 × 100 = 20%. This does not mean the plume has 20% opacity.
These are explanatory examples, not compliance determinations or prescribed observation periods.
Before you start
Check your rule or permit.
- Identify the source and rule. Find the method named in the applicable regulation, permit, or monitoring plan.
- Confirm the procedure. Check duration, conditions, recordkeeping, and how the result is evaluated.
- Prepare the observer. Arrange Method 9 qualification when needed, or the detection training required for Method 22.
Both methods depend on a suitable view of the emissions. Lighting, background, observer position, and condensed water vapor can affect what is visible.
Common questions.
Is Method 22 just a yes-or-no check?
No. It tracks emission time during continuous observation. Depending on the applicable requirement, the record may be used to calculate emission frequency.
Do I need smoke school for Method 22?
Method 22 does not require Method 9 certification. It does require an observer trained in detecting visible emissions and the conditions that influence detection.
Can I use Method 22 for an opacity limit?
Method 22 does not measure opacity. Use the method specified by your applicable requirement or an authorized alternative.