The interior of a carbon monoxide detector. When pressed, the peak button causes the LCD screen to display the highest level of CO recorded since the last reset.

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Inside a Carbon Monoxide Detector

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:

  • A small silicon microchip sends an electronic charge, a sort of "what to do" signal, to the other parts of the detector. The chip is fused to a copper-wired, integrated electronic circuit panel, which is the base for the unit.
  • Light-emitting diodes (LEDs) use lights to let you know how the unit is functioning and whether the battery needs to be replaced. Some detectors also have a liquid crystal display (LCD) panel that shows a readout of CO levels.
  • Wires to the building's electrical system or a battery provide power to the unit.
  • A test/reset button lets you make sure the alarm is working.
  • A detection chamber houses the CO gas sensor.

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­The CO sensor identifies and measures CO gas concentration in the atmosphere in parts per million (ppm). 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.