The Dr. Bill FAQ

What is the Best Location for Carbon-Monoxide Detectors?

A caller wanted to know whether to mount a carbon monoxide (CO) detector high, near the ceiling, or low, near the floor. He was confused because the instructions said to mount it high, but he thought that CO was heavier than air, and would thus tend to pool at low levels. By Bill’s estimation (and that of a later caller), though, he determined that CO gas was lighter by atomic weight than the major constituents of air (Nitrogen and Oxygen), and would indeed rise. Also, Carbon Monoxide expelled by most cases where it would trigger a detector (from a furnace or from a car’s exhaust) would be hot, and thus have even a lower density than the surrounding air.

I recently came across a blurb in Today’s Homeowner about standards for CO detectors. According to it, Underwriters Laboratories specified a standard (#2034) describing what exposure levels to allow before detectors sound an alarm. 2034 specified a level that resulted in a large number of false alarms. 2034-95 allows 15 parts per million (PPM) of CO, which still resulted in false alarms. 2034-98 will allow 30 PPM for 30 days before sounding an alarm.

What the caller was probably thinking about was Carbon Dioxide (CO2), which is heavier by molecular weight.

For reference, the atomic weights of the atoms and molecules involved:

Carbon (C) 12
Oxygen (O) 16
Carbon Monoxide (CO) 12+16=28
Carbon Dioxide (CO2) 12+16+16=44
Nitrogen 14
Air (primarily N2 and O2) about 28 Would have a weight somewhere between 28 (14+14) and 32 (16+16),but closer to 28 than 32 since air is 78.084% Nitrogen and 20.947% Oxygen

The periodic table of the elements is a great resource for atomic weights. Here are a few of the best:

In researching this myself, looking for the tables referenced below, I went through the same motions, and then remembered I had a reference that would give me the answer. I found this little black book called Pocket Ref, by Thomas J. Glover, that has answers for lots of questions you never even considered asking, from computer ASCII codes to airline two-letter codes to material dumping angles to gas densities.

From the little black book, page 9:

Gas grams/liter
Carbon Monoxide 1.2500
Air 1.2928
Oxygen 1.4290
Nitrogen 1.2506
Carbon Dioxide 1.9770

Pocket Ref
Compiled by Thomas J. Glover
Second Edition
ISBN: 1-885071-00-0

Sequoia Publishing, Inc.
P.O. Box 620820 Dept. 101
Littleton, Colorado 80162-0820
(303) 972-4167

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