Where EPA Method 21 finds leaks by sticking a probe on one component at a time, optical gas imaging finds them by pointing a special infrared camera at a whole scene and watching invisible gas turn visible on the screen. An OGI camera renders escaping hydrocarbon vapor as a moving plume in its video image, letting a technician scan large amounts of equipment quickly and spot leaks that would take hours to find component by component. This guide explains how OGI works, when it is used as an alternative to Method 21, and how its survey timestamps, location tags, and detected-leak logs integrate into a monitoring records system.
Optical Gas Imaging (OGI) in one line: Optical gas imaging is a leak-detection technique that uses a specialized infrared camera to make escaping hydrocarbon gas visible as a plume in a video image. Because certain gases absorb infrared light at specific wavelengths, the camera renders a leak that is invisible to the eye as a dark or moving cloud on screen, letting a surveyor scan large areas quickly. It is used as an alternative or complement to the component-by-component screening of EPA Method 21.
An optical gas imaging camera exploits a simple physical fact: many hydrocarbon gases absorb infrared light strongly at particular wavelengths. An OGI camera is built to be sensitive to a narrow band of infrared where the target gases absorb, so when a plume of such gas drifts across the camera's view it changes how much infrared reaches the detector from the background behind it. The camera translates that change into an image, and the escaping gas that is completely invisible to the human eye appears as a smoky, moving cloud on the display.
Because it images a whole scene rather than sampling one point, OGI is fundamentally a scanning tool. A surveyor can stand back and sweep the camera across a tank battery, a compressor skid, or a bank of valves and immediately see whether anything is emitting, then move closer to pinpoint the source of any plume. The moving nature of the plume in video is itself a strong cue, because gas drifting and dispersing looks distinctly different from static background, which helps the surveyor distinguish a real leak from visual clutter.
OGI's strength is speed and coverage rather than precise quantification. It excels at rapidly surveying large amounts of equipment and at finding leaks in places that are awkward to reach with a probe, such as elevated fittings, and it gives an intuitive visual of where gas is escaping. What a basic OGI survey does not directly provide is a concentration in parts per million or a mass rate; it shows presence and rough magnitude of a plume. That difference in what the two techniques deliver is central to how OGI and Method 21 relate.
OGI and Method 21 approach the same problem from opposite directions. Method 21 is a contact screening method: it measures a numeric concentration right at each individual component and compares it against a defined threshold, which makes it precise and quantitative but slow, since every component must be visited. OGI is a remote imaging method: it visually scans many components at once for plumes, which makes it fast and good at wide coverage but based on seeing a plume rather than measuring a number. Each has situations where it is the better fit.
Regulations have increasingly recognized OGI as an accepted survey approach, sometimes required outright and sometimes offered as an alternative work practice that a facility may choose in place of Method 21, often with its own conditions on how the survey is conducted. Whether OGI is required, optional, or must be paired with Method 21 depends on the specific rule that applies to the facility and equipment. In some programs a facility runs regular OGI surveys to catch leaks quickly and uses Method 21 for follow-up or for cases the imaging cannot resolve.
In practice the two are often complementary rather than rivals. An OGI camera can survey a site quickly and flag which equipment is emitting, and a technician can then use a Method 21 instrument to obtain a concentration reading on a specific flagged component if a numeric value is needed. Using imaging for broad, fast detection and contact screening for precise confirmation lets an operator get the coverage advantage of OGI without losing the quantitative rigor of Method 21 where the applicable rule or the repair decision calls for it.
An OGI survey produces its own kind of evidence: video or imagery of scanned equipment, the date and time each area was surveyed, the location of the survey, and a log of any leaks detected with their apparent source. For that survey to count toward a compliance obligation, this evidence has to be captured and organized, because the regulatory question is not only whether a survey happened but whether the right equipment was covered on the required schedule and whether detected leaks were then repaired. Loose video files and handwritten notes make that hard to prove.
Feeding OGI results into a records and monitoring system such as Merobix ties the survey into the same compliance framework as the rest of an LDAR program. Survey timestamps and location or GPS tags establish that a given area was covered on a given date, and each detected leak becomes a logged event against the affected equipment that starts a repair clock, just as a Method 21 exceedance would. The imagery can be retained as the supporting evidence for the detection. The result is a defensible record that a required OGI survey was completed and that what it found was acted on.
Centralizing OGI logs alongside Method 21 readings, repair records, and inventory data also makes the whole emissions picture visible in one place. An operator can confirm that every area requiring a survey was covered on schedule, see which locations repeatedly show plumes, and track detected leaks from discovery through repair and reverification regardless of which detection method found them. Bringing imaging surveys into the same records system as contact monitoring is what lets a facility manage detection method mix cleanly and demonstrate coverage and follow-up to an auditor.
Many hydrocarbon gases absorb infrared light strongly at specific wavelengths, and an OGI camera is tuned to a narrow infrared band where the target gases absorb. When a plume drifts across the view it changes the infrared reaching the detector from the background, and the camera renders that change as a visible moving cloud on screen. Gas that is completely invisible to the eye appears as a smoky plume, letting a surveyor scan large areas quickly.
It depends on the applicable regulation. In some programs OGI is accepted as an alternative work practice that a facility may use in place of Method 21, sometimes with its own survey conditions, and in others it is required outright or used alongside contact screening. OGI is faster and better for wide coverage, while Method 21 gives a precise concentration reading. Many operators use OGI for broad detection and Method 21 for confirmation where a number is needed.
A basic OGI survey shows the presence and rough magnitude of a plume rather than a precise concentration or mass rate. It excels at quickly locating leaks across large areas and awkward spots, but by itself it does not report a number in parts per million or kilograms per hour. To obtain a quantitative value, a technician typically follows up with a Method 21 reading or another quantification method on the specific flagged component.
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