carbon monoxide alarm diagram
Illustration: Harry Campbell
1. Test/Reset Switch
2. Microprocessor
3. LED operation light
4. LCD display
5. LED alarm light
6. Peak level switch
7. Carbon monoxide sensor
8. High-decibel alarm
9. Battery power supply
At the heart of all carbon-monoxide (CO) alarms is a sensor that detects and measures the amount of carbon in the air. But not all sensors work in the same way. In fact, detectors use one of three technologies, metal-oxide semiconductor, biomimetic, and electrochemical.

All three methods are accurate enough to meet the critical Underwriters Laboratories standard (UL 2034). They're also similar in their sensitivity to humidity, which can interfere with readings and shorten sensor longevity. Where they differ is significant, too: power consumption, lifespan, and cost.

Metal-Oxide Semiconductor

Uses a small, electrically heated bead of tin dioxide (SnO2) to break down carbon monoxide. The electrons it releases lower the sensor's electrical resistance. A microprocessor tracks those minute changes in resistance over time and calculates the CO concentration. Mechanically simple, reliable, and resistant to contamination, these sensors are so power-hungry that they must be plugged in. A battery supply will last a month at most. Life span: Up to 10 years.


Relies on an inexpensive synthetic-gel cell that mimics hemoglobin's absorption rates. The cell darkens in the presence of CO; microprocessors monitor the transmittance level of an infrared beam shining through the sample. Operates under battery power, but because the color-changing characteristics of cells are so difficult to quantify, these sensors aren't available with digital readouts. Life span: Up to 10 years.


Breaks down CO with an acid electrolyte. The released electrons induce a small current in a platinum electrode. A microprocessor monitors the current to calculate the CO concentration. Provides accurate, quantifiable measurements under battery power. Life span: 6 to 8 years.
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