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Appliance Science: How carbon monoxide detectors work

Carbon monoxide is a silent killer. Fortunately, a carbon monoxide detector can detect even minute amounts in your home, alerting you before it becomes a problem. Here's how they work.

Colin McDonald/CNET

Carbon monoxide is a killer. This colorless, odorless gas is a normal side product of the combustion of things such as the gas or oil that heats your house. Normally, this is only released in very small amounts and is dispersed into the atmosphere through a vent or chimney. However, if a heater is only partially burning fuel or there isn't enough ventilation, it can quickly become a problem, because even small amounts can be fatal. That's why laws were passed that required carbon monoxide detectors to be fitted to most homes, offices and other buildings. Let's look at how these devices detect this stealthy killer.

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Colin McDonald/CNET

How carbon monoxide kills

Carbon monoxide is a stealthy killer. An amount as low as 10 parts per million (ppm) can cause headaches, and 600 ppm levels can quickly lead to unconsciousness and, eventually, death. That's how it kills most people: it puts them to sleep, then kills them while they sleep. The effect of low concentrations mounts over time, with even a very low concentration causing adverse effects if you are exposed for long enough. That's because of the sneaky way it harms the body.

Carbon monoxide blocks the ability of your blood to carry oxygen, replacing the spot that oxygen takes on the hemoglobin in your red blood cells. This iron-rich compound is how your blood carries oxygen around the body, and carbon monoxide bonds to hemoglobin much tighter than oxygen alone. Over time, as more and more hemoglobin is blocked from doing its job by carbon monoxide, your blood just can't carry enough oxygen to keep your tissues going, especially the brain. They begin to fail, causing headaches, nausea, unconsciousness, and eventually, death.

How carbon monoxide detectors work

There are several different ways to detect carbon monoxide, but most home carbon monoxide detectors use an electrochemical sensor. These rely on how the behavior of a chemical changes when carbon monoxide is around.

Inside the carbon monoxide alarm is a small sensor with three electrodes on it. These three electrodes lead into a container of a chemical called the electrolyte. This container is gas permeable, meaning that gas from the atmosphere can pass freely into and out of it. A small voltage is applied between two of the electrodes (called the working and the reference electrodes). When carbon monoxide enters the sensor, it reacts with the oxygen also present in the atmosphere, releasing an electron and creating a small current between the electrodes. Some sensors use electrodes coated with platinum or other metals that catalyze the reaction of carbon monoxide and oxygen, helping it to happen quicker than it would normally.

This small current is what a carbon monoxide detector is looking for. It uses this to measure the amount of carbon monoxide: the more of it there is in the air, the more current the sensor generates. Some sensors show you the carbon monoxide level on a display: a useful feature that helps to reassure that the sensor is working. When the current (and thus the carbon monoxide level) has reached a certain level, it triggers the alarm. Typically, they will sound the alarm immediately if they measure a level of above 400 ppm.

It isn't just a case of raising the alarm once the amount goes over a certain level, though. As well as warning you if the level of carbon monoxide suddenly rises, a carbon monoxide detector should also warn you if it detects a low level over a long period, because this could indicate that the carbon monoxide being produced by a heater or other combustion source is not being dissipated as it should. Typically, a good carbon monoxide alarm will warn you if it detects a level of above 20 ppm for longer than a couple of hours.

This complexity means that carbon monoxide detectors require more power than smoke alarms, so most need to be plugged into a wall socket or wired into a power source. If you want to see what's inside a carbon monoxide alarm, there is an interesting teardown here. They do contain a battery, but this is for running the detector during power outages rather than powering the entire alarm like smoke alarms. Also like smoke alarms, carbon monoxide alarms have a finite lifespan. The chemistry of the sensor also breaks down over time, so most manufacturers recommend that you replace them every five years or so to make sure that they can keep you safe.