How Carbon Monoxide Detectors Work

You may be surpr­ised to discover that a carbon monoxide (CO) detector really isn't a complicated piece of equipment. Housed inside a plastic chamber approximately 3 inches (7.6 centimeters) in diameter are parts that work in unison to alert a househo­ld when lethal gas is detected. The exact components can vary from one model to another, but here's a run-down of the basic parts:

­In the United States, sensors for home use are calibrated to detect CO concentrations according to Underwriters Laboratories (UL) safety standards. When the sensor detects dangerous levels of the gas, it sends an electronic pulse to the alarm. The higher the concentration, the faster the alarm will respond. For example, the alarm will respond to concentrations of about 70 ppm in as little as an hour but will respond to concentrations of 400 ppm in as little as four minutes [source: UL 2034]. The alarm sounds at about 85 decibels (dB), just a little quieter than a typical lawnmower, but often very shrill.

­CO detectors can use several types of sensors to sense carbon monoxide in the air. Biomimetic sensor technology reproduces CO's effects on hemoglobin in the blood. In a biomimetic sensor, a gel changes color as it absorbs CO. A separate sensor detects the c­olor change and alerts the detector's processor to sound the alarm. Once a biomimetic sensor has changed color, it must be placed in a CO-free environment to reset itself. Biomimetic sensors can also be used on portable CO detector cards — if you see the disc on the card turn dark, there's a dangerous amount of CO in the air.

Metal oxide semiconductor sensors use circuits applied to a silica chip. When CO comes into contact with the circuitry, it lowers the electrical resistance. The processor detects the change and causes the alarm to sound. These sensors require lots of electricity, so they generally plug into wall outlets rather than using batteries.­

Like metal oxide sensors, electrochemical sensors use changes in electrical current to detect carbon monoxide. But instead of using an integrated chip, electrochemical sensors use electrodes immersed in a chemical solution. Electrochemical instant detection and response (IDR) sensors are often used in professional sensing equipment and detect dangerous levels of CO instantly.

­In the United States, emergency rooms treat about 20,000 people for carbon monoxide (CO) exposure each year, and an average of 400 people die from carbon monoxide exposure annually [source: ]CDC]. Most nonfatal exposures happen in people's homes, which is why it's important to have a CO detector to go along with — or as part of — your smoke detector.

Before selecting a carbon monoxide (CO) detector, do a little research. Determine whether your local laws require you to have CO detectors of a particular type or configuration. You'll need to decide whether you'd rather have a battery-operated or plug-in detector. A plug-in unit goes into any wall outlet and can be moved to another room if necessary. But unless y­our plug-in unit has a battery backup, you're unprotected in the event of a power failure. This may not seem like a big deal in the case of temporary outages, but imagine a long power outage during winter. You may turn to a portable space heater or a wood stove to keep your home warm — exactly the sort of fuel-burner that may release CO into the air.

Battery-operated units, on the other hand, are typically attached permanently to a surface in your home. This usually isn't a problem unless your detector mistakes other gases for CO, causing false alarms and prompting you to move the unit. Depending on the size of your home and the number and location of fuel-burning appliances, a combination of types may work best. Follow the manufacturer's instructions for placement, maintenance and scheduled inspection.

The National Fire Protection Agency (NFPA) recommends carbon monoxide detectors in several locations:

The NFPA also recommends using detectors that can be linked so that one alarm sets off all the others, alerting the whole household.

Installing CO detectors is only one step in preventing carbon monoxide poisoning. Other important steps include:

Finally, test your carbon monoxide detector monthly — it's your last line of defense against a malfunctioning stove or heater.