Teams ask us all the time whether they should be running Method 21 or OGI. Usually the applicable subpart already decided — but knowing why each exists keeps you from over-spending, or monitoring the wrong way.
Method 21
EPA’s reference method (40 CFR Part 60, Appendix A-7) for measuring VOC leaks with a portable analyzer held at the component. It returns a ppm number — and that number is what your subpart’s leak definition is written against (e.g. 500 ppm valves, 2,000 ppm pumps under NSPS GGGa).
Optical Gas Imaging (OGI)
An infrared camera that lets you see a leak plume. Fast, great for hard-to-reach components and finding big emitters. EPA first allowed OGI as an Alternative Work Practice under 40 CFR 60.18; newer rules (NSPS OOOOa/OOOOb, the refinery and chemical NESHAPs) increasingly build it in.
Side by side
| Method 21 | OGI | |
|---|---|---|
| What it gives you | A documented ppm concentration per component | A visual of the leak; find/fix, not a number |
| Best for | Compliance counts against a leak threshold | Wide sweeps, large emitters, hard-to-reach points |
| Speed | Slower — component by component | Fast — scan many components at once |
| Regulatory basis | Reference method; most leak definitions written against it | Alternative Work Practice (40 CFR 60.18) & built into newer NSPS/NESHAP |
The rule usually decides
Where teams get burned is treating the two as interchangeable. An OGI survey doesn’t automatically satisfy a Method 21 monitoring requirement, and a Method 21 route won’t catch a leak on a component nobody scheduled. Read the controlling subpart first — then pick the tool.
How AIM runs both
OGI to sweep wide and catch the obvious leakers fast; Method 21 to put a defensible number on components when the rule requires a count and a repair clock. The camera finds it; the analyzer proves it — and we track every repair against the regulatory deadline.
Frequently asked
Is an OGI survey enough to meet a Method 21 requirement?
Not automatically. If your subpart is written against a Method 21 concentration and repair threshold, an OGI survey alone usually won’t satisfy it. OGI can qualify where the rule expressly allows it as an alternative work practice or building it in — read the controlling subpart before you swap methods.
Which is required at a refinery under Refinery MACT CC?
Refinery MACT (40 CFR Part 63, Subpart CC) and the NSPS subparts that apply to refinery equipment spell out the monitoring approach and leak definitions. Many refineries run Method 21 for compliance counts and use OGI where the applicable subpart permits it. The rule, not preference, sets the baseline.
What leak thresholds does Method 21 measure against?
It depends on the component and subpart — for example, 500 ppm for valves and 2,000 ppm for pumps under NSPS GGGa. Method 21 (40 CFR Part 60, Appendix A-7) gives the ppm reading; your subpart’s leak definition sets the number that counts.
Can you run both on our units?
Yes. We run OGI to sweep wide and catch large emitters fast, then Method 21 to put a defensible number on components when the rule requires a count — and we track every repair against the regulatory clock.
This is general information, not a regulatory applicability determination. How these methods apply depends on your equipment, permit, and the subpart you’re subject to. Confirm your obligations with a qualified professional or your permitting authority.
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