Two leak detection methods dominate fugitive emissions monitoring: EPA Method 21, which measures gas concentration with a portable analyzer held next to each component, and Optical Gas Imaging (OGI), which uses a specialized infrared camera to visualize hydrocarbon plumes from a distance. Both are accepted under EPA rules, but they detect leaks in fundamentally different ways. Picking the right approach — or, increasingly, combining both — is one of the most important design decisions for any LDAR program.
How Each Method Works
EPA Method 21 (PID/FID)
Approach: Point-by-point screening with a calibrated portable gas analyzer.
Instrument: Photoionization detector (PID) or flame ionization detector (FID) calibrated with a reference gas such as methane.
Procedure: Probe held within ~1 cm of each leak interface; the maximum reading is recorded.
Output: A ppm number for every component in the inventory.
Optical Gas Imaging (OGI)
Approach: Area-scan with an infrared camera tuned to hydrocarbon absorption bands.
Instrument: Mid-wave or long-wave IR camera (commonly mid-wave for methane and light hydrocarbons).
Procedure: Operator sweeps the camera across equipment from several feet away and watches for visible plumes.
Output: Video evidence of leaking components; no ppm value.
What Counts as a Leak
Method 21 Thresholds
Each regulation specifies its own ppm threshold. Common values include 500 ppm (NESHAP refinery rules and many state programs), 2,000 ppm (some pump and connector services), and 10,000 ppm (older standards such as 40 CFR 60 Subpart VV for valves in gas service). The applicable threshold also varies by component type and the chemical service.
OGI Threshold
Any visible emissions on the camera display are treated as a leak. There is no ppm reading; the operator records the location, captures video, and tags the component for repair. Detection sensitivity depends on the camera model, gas type, distance, wind, and background temperature.
Side-by-Side Comparison
Speed and Coverage
Method 21: Slow. A technician must physically position the probe at every monitored interface, log the reading, and move on. Surveying a typical compressor station can take days.
OGI: Fast. A trained operator can scan thousands of components per shift by walking the equipment and watching the viewfinder. Coverage of the same compressor station may take only hours.
Sensitivity
Method 21: A calibrated PID or FID can detect very small leaks well below the regulatory threshold. Numerical readings allow trending and prioritization.
OGI: Generally less sensitive than a calibrated analyzer for very small leaks, particularly in cold weather or high wind. However, OGI catches leaks from areas a probe cannot reach (open-ended lines, pressure relief vents, tank thief hatches, large equipment surfaces).
Cost
Method 21: Lower equipment cost (a portable PID/FID typically costs a fraction of an OGI camera). Higher labor cost over time because monitoring is slow.
OGI: High capital cost (cameras commonly run from tens to over one hundred thousand dollars) plus specialized training. Lower labor cost per inspection at scale.
Documentation
Method 21: Logged ppm values per component create an auditable numerical record that is easy to chart and trend.
OGI: Video clips and still images are powerful evidence of a leak (and of repair confirmation), but they are larger files and harder to summarize statistically without supporting software.
Operator Skill
Method 21: Skill matters (probe placement, calibration, response time), but the procedure is well-defined and repeatable.
OGI: Highly operator-dependent. Image interpretation under varying weather, lighting, and background conditions requires extensive training. Most rules now require documented OGI training and refresher cycles.
Regulatory Acceptance
NSPS OOOOa and OOOOb (Oil and Gas)
OGI is the primary fugitive emissions monitoring method for affected well sites, compressor stations, and processing plants. Method 21 is permitted as an alternative or backup at the operator's option. See our NSPS OOOOa Overview and NSPS OOOOb Deep-Dive for the specific frequency and recordkeeping requirements.
Refinery NESHAP, HON, and Subpart VV
Method 21 remains the default for many older equipment-leak rules covering refineries, chemical plants, and SOCMI sources. Operators can adopt the AWP, but the original Method 21 requirement is still on the books and remains the path of least resistance for legacy programs.
State Programs
State rules vary widely. Colorado Regulation 7, Pennsylvania GP-5/GP-5A, and several other state programs explicitly allow or require OGI for oil and gas sources. Always check the applicable state authority before relying on either method.
When to Use Each Method
Method 21 Is Best For
Refineries, chemical plants, and SOCMI facilities subject to legacy NESHAP and NSPS rules.
Sites where regulators require numerical ppm data to demonstrate compliance.
Investigations where OGI has flagged a leak and the operator wants to quantify it before repair.
Programs with large existing investment in PIDs/FIDs and trained Method 21 technicians.
OGI Is Best For
Oil and gas well sites, compressor stations, and gas plants under NSPS OOOOa/OOOOb.
Equipment with thousands of components where point-by-point monitoring is impractical.
Surveys of pressure relief vents, thief hatches, flare stacks, and other locations a probe cannot reach safely.
Programs that benefit from visual evidence of leaks for training and corrective-action follow-up.
Combining Both Methods
The strongest LDAR programs use OGI and Method 21 together. OGI rapidly identifies where the leaks are; a follow-up Method 21 reading quantifies how big they are and provides a numeric record that supports trend analysis, emissions calculations, and regulatory reporting. AVO (audio/visual/olfactory) inspections by operators round out the program by catching obvious leaks between formal monitoring rounds.
How Software Supports Both Methods
Whichever method you use — or both — the recordkeeping load is substantial. Ecesis LDAR Software centralizes the data trail in one place: component inventories, monitoring frequencies, instrument calibration records, ppm readings, OGI video and still attachments, leak tags, repair work orders, re-inspection deadlines, and the regulatory reports that pull from all of it. Field technicians collect data on the mobile app (online or offline) and the records sync back to the cloud automatically.
Ecesis Software Solutions for LDAR
LDAR Software
Component inventories, monitoring schedules, leak tracking, and regulatory reports.
Air Emissions Management
Stack tests, emissions inventories, and Title V reporting alongside LDAR.
Inspection Software
Mobile checklists for AVO inspections and routine field rounds.
Preventive Maintenance
Auto-generated work orders for leak repairs and re-inspection deadlines.
Document Management
Calibration records, monitoring plans, and OGI video evidence in one place.
Mobile EHS App
Capture readings, photos, and video from intrinsically safe field devices.
