How Does a Carbon Monoxide Detector Alarm Work


Carbon monoxide alarms are, without question, the most effective way to detect carbon monoxide in the air and alert you to the presence of this potentially deadly gas. 

Why is carbon monoxide so dangerous for humans? 

Well, because it is potentially lethal in confined and unventilated spaces. If a large amount of carbon monoxide is inhaled it will displace the oxygen in a person’s blood and ultimately deprive the heart, brain, and other vital organs the oxygen they need to function properly. If a large amount of carbon monoxide is breathed in it can cause a person to lose consciousness and suffocate in no time of all. However, the good news is, CO poisoning can be reversed if caught in time. That’s why carbon monoxide detectors are vital and that’s why every homeowner should invest in at least one.

What is carbon monoxide?

If any fuel is to burn efficiently it needs plenty of oxygen. If oxygen is not available, fuel will not burn. Most firefighters use this knowledge to deal with blazes, and that’s why fire blankets, fire extinguishers and sprinklers are designed to remove the oxygen, or heat or fuel from a fire to extinguish a blaze quickly. However, if there is still a small amount of oxygen available, then a fuel will continue to burn, but it won’t burn efficiently and completely. The upshot of this is that fuel will not burn cleanly and produce water and carbon dioxide: instead, a poisonous gas – carbon monoxide is produced -as there is insufficient oxygen to produce carbon dioxide. Once that stage is reached, then things can become dangerous.

Carbon monoxide is a colorless and odorless gas. Therefore, most people won’t notice it when it starts to accumulate in a room or building. A person suffering from carbon monoxide poisoning will initially feel drowsy and will ultimately lose consciousness and die, unless treated. It might be a painless way to die, but the sad truth is it is completely unnecessary.

If carbon monoxide detectors were fitted in every home or workplace where fuel burning appliances are used to produce energy through combustion, then thousands of lives could be saved worldwide. If a carbon monoxide detector is installed in a house, it will start to detect potentially poisonous gases as soon as a heating source starts to produce carbon monoxide. What’s more, an alarm will sound as soon as the gas levels become dangerous. If the alarm sounds, the heating source should be switched off, windows and doors should be opened, and you should then leave the building and call for professional help.  

How do carbon monoxide detectors work?

There are basically two types of carbon monoxide detectors: detector strips, or biometric detectors that you mount on a wall – these inexpensive alarms mimic the way our bodies respond to carbon monoxide, and more expensive electronic alarms that run off a mains power supply or battery pack. Both alarms/detectors share certain basic functions but operate in different ways. Which ever type of detector you choose to use it should comply with British Standard EN 50291 in the UK.

Biomimetic patch detectors or chemical “blob” detectors

These simple detectors are essentially pieces of plastic with a small beige-colored dot on them. If a high level of carbon monoxide is detected in a room, the dot changes color from beige to black. Why does that happen? Well, it a simple chemical reaction. The detector dot is rough and is made of silica gel and impregnated with a catalyst which includes chemicals like palladium and molybdenum salts (such as palladium sulphate and ammonium molybdenate).

When carbon monoxide touches the detector, it is oxidized and thereby turns the beige dot black. The strip also contains a chemical salt made from a transition metal such as iron nickel, or copper. Once the carbon monoxide is removed, this metal salt draws some oxygen from the air and changes the catalyst back to its original chemical form—so the detector spot changes color back to beige again and regenerates itself in the air.

Strip detectors are cheap to produce and are an effective way to provide basic protection from carbon monoxide poisoning. However, they do have several deficiencies and disadvantages:

  • Strip detectors do not activate an alarm, so you must look at them to see if the strip changes color to detect danger.
  • If the build-up of carbon monoxide is quick and sudden, you may not even get the opportunity to check the color change of the detector strip before it is too late
  • Detector strips also need changing regularly – as often as 3 to 6 months, so over time you will probably spend as much as you would if you’d opted for an electronic carbon monoxide alarm.

Electronic carbon monoxide alarms

There are several electronic carbon monoxide detectors on the market, and they all look similar, however, the way they work differs. Colorimetric detectors also have a chemical dot inside like strip detectors described above. A light beam shines onto the dot and an electronic eye or photoelectric cell measures the light reflected. If there’s no carbon monoxide detected, the strip remains beige and lots of light is reflected. When carbon monoxide is detected, the strip turns black, the light shining onto it is absorbed and little or none is reflected. The electronic eye detects the lack of reflected light and a shrill, piercing alarm sounds. 

Metal-oxide detectors have open chambers containing sensors made of metal (tin or platinum) oxide. When carbon monoxide is detected, the metal oxide reacts with it: the carbon monoxide “steals” oxygen from the metal oxide, converting itself into carbon dioxide, turning the metal oxide into pure metal, and producing heat at the same time. An electronic circuit monitors the temperature inside the chamber and sounds the alarm if too much heat is produced too quickly. Detectors like this are often battery-operated.

Electrolytic detectors, works in a similar way to a battery. The terminal called electrodes, which are made from platinum are dipped into a chemical solution called an electrolyte. When carbon monoxide is present, the electrolyte conducts electricity more readily, making a current flow in a detector circuit, and triggering an alarm. Electrolytic detectors are usually the most sensitive and accurate—and therefore also the most expensive. They also need powering from an electrical outlet, instead of batteries. 

Electronic detectors vary considerably in sophistication, and therefore cost. The simplest, and the cheapest ones are either ‘on’ or they’re ‘off’: if carbon monoxide is present in dangerous levels, they trigger an alarm and maybe flash a warning light at the same time. However, you won’t know there might be a problem until the alarm has sounded.

More sophisticated and expensive detectors have a digital display showing the amount of carbon monoxide present as a reading in parts per million ppm (PPM). Detectors like this can alert you to a gradually worsening problem with carbon monoxide exposure by showing a progressively increasing PPM reading. Any reading over about 35PPM (the maximum exposure level permitted in workplaces for any eight-hour period by OSHA, the Occupational Safety and Health Administration) is a cause for concern, but you might want to check out your appliances or your room ventilation if you get any significant PPM reading at all.

How many detectors does your home need?

There might be a temptation to assume you only one carbon monoxide detector in a household, but that would be a mistake. It’s far safer to have one near each appliance, fire, furnace, fuel-burning engine, or other device that could potentially produce CO gas if it malfunctions. So, if you have a gas boiler in one room of your house and separate, standalone gas fires (or coal fires) in other rooms, more than about 10m (30ft) away from your detector, or on other floors of the same building, you need one detector next to each of them

Where to place a carbon monoxide detector

You should ideally have a carbon monoxide detector on each floor of your house and in the same room as any potential source of the gas, such as a boiler, stove, or oven. The detector should be placed around head height on a wall or shelf, although some are also built into ceiling-mounted multipurpose fire alarms. You should check your smoke detector to see whether it is already also fitted with a carbon monoxide detector.

Can a carbon monoxide sensor detect smoke?

Carbon monoxide sensors detect when the carbon monoxide level of gas in the air, reaches dangerous levels. Smoke detectors, on the other hand, detect levels of smoke in a particular area. They do this either via ionisation or photoelectric devices. Ionisation smoke alarms have a small amount of radioactive material between two electrically charged plates which ionises the air – smoke entering the chamber will then disrupt this current and activate the alarm. Photoelectric devices are generally considered better for smouldering fires, as they aim a light source into a sensing chamber at an angle away from the sensor – when smoke enters the chamber, the light is reflected onto the light sensor, triggering the alarm.

There are now devices on the market that promise to detect both CO and smoke. These combination alarms claim to provide the protection of two separate devices in one system, with sensing technologies that work together to detect fires and the odorless, colorless gas of CO.

In theory this makes perfect sense, as carbon monoxide will often build up because of a fire. When a fire burns in an enclosed room, the oxygen in the room is gradually used up and replaced with carbon dioxide – once this has built up, the fuel is prevented from burning, thereby producing carbon monoxide.

The advantages of such detectors are that they require less space to fit than two separate units. They may be particularly useful for those with smaller living areas, where ceiling space is at a premium. To avoid any confusion these combined CO/smoke sensors raise different warning messages for smoke and CO, thus ensuring users don’t get confused.

Are combined CO and smoke detectors as safe and a better bet for home safety than separate units? Well, while there are a few dual units available on the market from the likes of Google and Kidde, most products on the market are specialist CO or smoke detectors, so the balance of proof would suggest that until this relatively new technology has been more rigorously tested, it is probably wisest to invest in separate CO and smoke detectors for now. 

Can carbon monoxide sensors detect gas and propane leaks?

A carbon monoxide detector cannot detect a natural gas leak. Carbon monoxide (CO) is a toxic gas created when fuel is burned in the presence of low levels of oxygen.  A CO detector cannot detect a leak in a propane tank either, so, homeowners could still be at risk should a leak occur.

How to calibrate a carbon monoxide sensor?

All carbon dioxide sensors require calibration. The calibration can be accomplished by calibrating the sensor using the automatic baseline calibration or using a known gas as reference. 

Calibration by use of nitrogen

One of the most accurate methods of CO sensor calibration is by exposing it to pure nitrogen. The idea is to replicate the conditions in which the sensor was originally calibrated during its manufacture. Nitrogen calibration is necessary if levels of CO to be measured will range between 0-400 ppm. Calibration by use of nitrogen requires a tank of pure nitrogen, calibration software, and a sealed calibration enclosure to ensure the accuracy of the process, and consequently, it is an expensive method.

Use of fresh air

The method is a more cost-effective method compared to nitrogen calibration. However, maximum accuracy cannot be guaranteed. It relies on the known fact that outdoor air has about 390 ppm CO concentration. Therefore, the sensor is calibrated at 400 ppm and then 400 ppm is deducted from the newly calculated counterbalance value. Calibration by use of fresh air is best suited for sensors meant to be installed in greenhouses or manufacturing settings since the sensor gets exposed to different levels of CO constantly.

How do you test a carbon monoxide detector?

  • Press and hold down the test button on the front of the device. After a few seconds, you should hear two beeps which signify that the device has entered testing mode.
  • If you are unable to hear any sound or if the alarm is too weak, replace the battery and retest.
  • If after battery replacement the device still doesn’t beep, replace the whole carbon monoxide alarm as soon as possible.

There are also some other guidelines you should follow while testing your carbon monoxide detectors:

  • Like smoke detectors, test carbon monoxide detectors monthly.
  • Clean the carbon monoxide detector thoroughly so that there’s nothing obstructing the entryway to the sensors.
  • While most carbon monoxide detectors are plugged into a wall socket or are hardwired to your house’s electrical system, almost all of them have a backup battery in case of power failure. You should replace your backup batteries once a year.
  • Have an escape plan updated in case the alarm goes off. Carbon monoxide is lighter than air, so it rises. When the alarm goes off, crouch and stay low to the ground while making your way out of the building.
  • Inspect your ventilation systems. Carbon monoxide poisoning happens when there’s a build-up of the gas in an area. Making sure that all the areas in your building are well ventilated. This will drastically reduce the risk of carbon monoxide poisoning.

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