Infrared Training Center

Monday, March 31, 2014

A New Look at Air Infiltration in Buildings

gf-mw-air-infiltration-thumbWe can’t see air with a thermal imager, right? Anyone who has completed Level I training knows this very well. A common follow-up question is always why, then, can thermographers still detect air leakage bypasses in buildings?

That’s possible because an infrared camera senses changes in thermal patterns created by air moving over a surface (given there’s a temperature difference) when either infiltrating into (example seen here, image above), or exfiltrating from, a building.

Thanks to a video that my fellow colleague, and ITC Instructor, Ron Lucier recently sent in it looked like, at least for a fleeting moment, that we might have to re-consider this question; can we see air with infrared?  In his video, available below, it certainly appeared that way when using a FLIR GF320 mid-wave infrared camera to view air leakage.  Was it actually detecting air?  How was this possible?  Well not exactly, as we’ll explain, but it was still interesting to watch:

Here’s what Ron had to say about this clip that he captured while on the road training. It appears to show actual air, not just the effects of it, infiltrating under the exterior exit door of the hotel’s conference room.

The High Sensitivity Mode (HSM) of the camera is a continuous image subtraction – a rolling subtraction, frame by frame, of the image so what you are seeing is the difference from one image to another.  The FLIR GF320 camera utilizes this function to enhance small gas leaks by detecting cloud motion; the gas cloud moves but the objects relative to the camera don’t.  In the video I placed the camera on a tripod, dropped the temperature range to its lowest setting (hence the noisy image) and observed air infiltration because of the slight negative pressure inside the meeting room.  It was about 15 F (-9.4 C) outside when I took the video.

The “waves” you see in the 2nd part of the video, shot in HSM mode, are the carpet fibers cooling as air moves across the surface. This mimics what a false gas leak may look like to a GF series camera even though it’s just convective heat transfer that’s taking place.  The pattern went away the following day after 18 inches of snow had piled up on the door, blocking air movement into the building.

Fascinating. So while it’s not actual air that we’re detecting, it provides an interesting, and very different, thermal perspective for us to enjoy. Certainly a pattern that we’re not used to seeing with a typical, long-wave, infrared camera more commonly used by thermographers in building applications and one that I thought you might enjoy.

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