On Fri, 12 Jun 2015 12:26:15 -0700, (Dave
Platt) wrote:

My understanding is that the sensor in the CO detectors, being
chemically based, does have a limited lifetime. As of 2009, ANSI/UL
specs require that such alarms begin chirping an "end of lifetime"
signal after 5 years of operation... and a couple of weeks after this,
you can no longer turn off the chirp.

The lifetime of the smoke detector is determined by the battery life,
not the sensor life. Kidde and others make them with a 10 year life.
In order to do that, the battery is not replaceable. This requirement
was designed to prevent the all too common smoke detector with a dead
battery installed problem. Kidde uses "sealed battery" as their
buzzwords for non-replaceable battery.
http://www.kidde.com/home-safety/en/us/products/fire-safety/smoke-alarms/
(Click the "10 year battery" box)
Note that all of them are photo-electric, not ionization type sensors.
Photo-electric detectors will detect the smoldering beginnings of a
fire long before an ionization detector. However, once flames start,
both work equally well.

The industry would love to get 10 years for CO detectors, but the
technology is not quite there yet. The currently fashionable
electro-chemical type CO detectors have about a 5 years life, and then
must be replaced. There have been CO detectors sold with replaceable
sensors, but since the sensor is the largest part of the cost, they
were deemed uneconomical. There are CO detectors available that can
last 10 or more years, but they're complex, expensive, and/or power
hogs. There was one patent (that I can't seem to find) that was
essentially a gas chromatograph.

There are also one-time sensors, that change color when exposed to CO,
but do not change back. These have their uses, but not in the home.
The main advantage is that they are far more sensitive than the common
electro-chemical detectors, which require 10 minutes at 400 ppm to
produce a reading.

Some detail on the different type of sensors:
https://en.wikipedia.org/wiki/Carbon_monoxide_detector#Sensors


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558