More on CO detectors

Q. I think your April 1-8 column had an error. You wrote that CO is heavier than air. CO has a molecular weight of 28. Air is mostly N2, which also has a molecular weight of 28. Air also has some O2, which has a molecular weight of 32, so the air mixture is heavier than CO. It should be OK to put the CO detector up high, and not near the floor.

A. You make an interesting, and apparently debatable, observation. It led me to research U.S. public agencies and European university studies, which agreed with you about the molecular weight of nitrogen, oxygen and carbon monoxide (CO).

A study by the Fire Marshal’s Public Safety Council, which repeated information from the U.S. Department of Energy and the online scientific group Ask a Scientist, states, “Unlike smoke, which rises to the ceiling, CO mixes with air. Recognizing this, a CO detector should be located at knee-height (which is about the same as prone sleeping height).”

Ask a Scientist couldn’t agree on the height, perhaps because some scientists were analyzing the gases separately, comparing, as you did, the molecular weights of pure, unmixed gases. Others recognized that CO readily mixes with air (which comprises nitrogen, oxygen, argon, carbon dioxide and other trace elements).

Smoke, on the other hand, doesn’t readily mix with air, according to the fire marshals, and rises above it. So if CO that has just left a hot piece of heating equipment mixes readily with air, it will circulate at a higher level, cooling as it mixes with the air.

Mixing with nitrogen, oxygen, argon and carbon dioxide, it becomes denser and heavier than the air it hasn’t yet mixed with, according to the fire marshals. As a heavier air/monoxide mixture, it condenses and will sink.

You may not agree, as those scientists could not agree, but either way, according to them, the mixture will eventually reach a working detector, and as they concluded, the sound of the alarm is the more important issue. The location of the CO detector may vary, based on the flow of air in the home, dependent on how much outside air is getting in and the source.

For example, malfunctioning heating equipment on the basement floor will spew heated CO to the basement ceiling, where it drifts into a stairwell and under the stairway door. At this point, it has been cooling as it circulates. Wafting its way across the first floor, the fire marshals believe, it is most concentrated either near the floor or, as they stated, at knee height.

If the heating equipment is on the same level as the bedrooms, it may circulate, heated, to the ceiling, but unless you kept the door open, it cools and drifts to the floor to get under the door. Either way, I believe, it then builds from the floor up. Be safe, and thanks for your observation.

© 2010 Monte Leeper
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