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Comment by psadauskas | Hacker News Reader

Not directly related to this issue, but I'm skeptical about the Nest Protect in general. It includes a CO detector, but CO is a denser-than-air gas, so stays near the floor while the Protect is mounted on the ceiling to detect smoke. Also, most CO detectors need to be replaced every few years because it wears out. Does this mean you have to buy a new $300 Nest Protect every 5 years?

I have a Nest Thermostat that I love, but the Nest Protect doesn't seem nearly as cool or useful...

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> CO is a denser-than-air gas, so stays near the floor while the Protect is mounted on the ceiling to detect smoke

Research on this topic seems to indicate that CO dissipates pretty evenly:

"Contrary to a significant amount of public opinion, CO did not layer on the floor, float at the middle of the chamber, or rise to the top. In each case, the levels of CO equalized throughout the test chamber. It took longer to equalize when CO was infused at the top of the chamber than the bottom, but levels always became identical with time."

http://www.ncbi.nlm.nih.gov/pubmed/21536403

CO is about the same density as air, but it will rise with warm air. Placing CO detectors on the ceiling is allowed by the NFPA.

Nest protect costs $130 and yes you're expected to replace it after 7 years. It's actually a legal requirement to replace CO detectors after 7 years in many jurisdictions.

The primary gas in the atmosphere is nitrogen, N2 with a molecular weight of 28. The second most prevalent gas is oxygen, 02 with atomic weight 32.

Carbon monoxide, CO at 28 is the same molecular weight as Nitrogen and therefore it diffuses similarly. This is part of what makes it so serious a hazard.

carbon monoxide is less dense than air - according to wikipedia, density at room temperature is 1.15 kg/m³ for CO vs 1.275 kg/m³ for dry air.

The nest protect retails for US$129.00 and has a service lifetime of 7 years.

I recently checked all the smoke detectors at my parents house and found that they were all well past their 10-year replace-by date. (I replaced them, but not with Nest smoke detectors).

Fun little tangent; I don't know how old my smoke detectors are, but the one by my front door gets a little test every now and again. One of my neighbors burns a fireplace on cold days. Invisible (but smellable) quantities of wood smoke pool under the rafters of my porch; the next morning, when I leave through the door, the smoke alarm always chirps for a moment! It amazed me how sensitive it is, when I first figured out what was going on.

The CO sensor in Nest is designed to last 7 years. The Nest has a built in timer that will start chirping after 7 years, forcing you to throw it away.

Most modern CO alarms last 10 years. Nest chose the smaller sensor, which dies sooner. Very poor decision on their part.

For more detail: Nest uses a CO sensor by Figaro, TGS5342. This is a smaller version of Figaro's TGS5042. The CO sensor is filled with a water reservoir, with some air inlets to let gas enter. Over several years, the water evaporates out of those air inlets and the sensor dies.

Nest's smaller sensor has as smaller water reservoir, which means it doesn't last as long as the other alarms with a larger reservoir. First Alert uses the larger TGS5042 sensor, therefore it lasts longer. Kidde uses their own sensor, which also lasts 10 years.

That is very true. I have one of those older CO detectors and the instructions say it should be about a half a foot from the floor. I have no idea why the Nest CO detector wants it mounted 10 feet from the floor. Seems useless.

I believe any combo smoke+CO detector gets a pass to be up high. And isn't CO roughly the same weight as air anyway?

That being said, I wouldn't trust Nest for a safety device where the goal is to have a simple reliable device, which contradicts having a wifi-connected uC in control. I don't know that they have the most engineering-minded culture either, based on the horrid input lag of their thermostat.

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