One sailor’s experience highlights that it isn’t just carbon monoxide that can set off your carbon monoxide alarm. Vyv Cox explains why

Four main types of carbon monoxide detectors are available, although only one carbon monoxide alarm, the electrochemical type, is advised for domestic and industrial use.

The optochemical detector comprises a chemical pad that changes colour from yellow to black when exposed to carbon monoxide.

Although the cost of the device is the lowest available the protection offered is also the lowest.

Some small domestic appliances use this technology with a visual display only.

In a biomimetic sensor a gel darkens in colour when it absorbs carbon monoxide in a similar way to that of haemoglobin.

Batteries overheating during charging produce hydrogen and can set off a carbon monoxide alarm

Batteries overheating during charging produce hydrogen and can set off a carbon monoxide alarm. Alcohol vapours will too, but will damage the sensor. Credit: Graham Snook/Yachting Monthly

The colour change can be detected by an infra-red light source and detector, sounding an alarm.

This type of sensor is the most reliable but is expensive.

It tends to be used in large establishments such as hotels, hospitals and the like where air fresheners and disinfectants are in regular use.

The metal oxide semi-conductor type utilises electrodes in a tin dioxide ceramic heated to 400 degrees C.

The presence of carbon monoxide lowers the resistance of the cell, which can be detected by an electronic circuit, sounding an alarm.

This type relies upon mains electricity, or shortlived batteries, and is gradually being phased out in favour of the electrochemical type.

Electrochemical sensors are widely used in industry and domestically, giving reliable service for up to 10 years with low power requirements.

This type will now be examined in more detail.

Diesel heaters can emit carbon dioxide through faulty exhaust installations

Diesel heaters can emit carbon dioxide through faulty exhaust installations

Electrochemical sensors are available for a wide range of toxic gases, including carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen dioxide and many others.

Gas drawn into the sensor passes over a catalysed electrode that causes an electrochemical reaction producing a current that can be detected by the instrument.

Ideally, sensors designed for specific gases will only detect that particular one, but that is difficult to arrange without the introduction of expensive components.

The internal design of a carbon monoxide alarm is very similar to one for hydrogen sulfide and some dual-purpose sensors are intended for the detection of either of these gases.

In this case the sensor can detect either gas but cannot differentiate between them.

Substance-specific carbon monoxide sensors incorporate an activated charcoal filter that removes hydrogen sulfide and other acid gases before they reach the electrode.

Gas ovens can be a source of carbon monoxide. Credit: Graham Snook/Yachting Monthly

Gas ovens can be a source of carbon monoxide. Credit: Graham Snook/Yachting Monthly

Further confusion arises due to the process by which the substance-specific carbon monoxide sensor detects this gas.

During the detection process the carbon monoxide reacts with water vapour to produce carbon dioxide, thereby releasing free hydrogen thus: CO + H2O -> CO2 + H2

Unless the sensor has been specifically designed not to detect hydrogen, the electrode will also detect this gas.

In practice, most carbon monoxide sensors in general use will activate when the device detects 150ppm of CO for 30 minutes.

Continues below…

The same sensor will activate when it detects 300ppm of hydrogen for the same length of time.

Battery charging in a closed yacht scenario will easily produce in excess of 300ppm of hydrogen and thus the device will be activated.

In Rob Stone’s case (see below), it appears that a battery was at fault which, with continued charging, would have led to excessive gassing of hydrogen and oxygen, hence the activation of the detector.


Vyv Cox is a retired metallurgist who spends six months a year in the Med, living aboard his Sadler 34, mostly at anchor

Hydrogen sulfide can be detected by smell at very low levels indeed, only 1-2ppm, although in stronger concentrations the gas has either no smell or a sickening, sweet odour.

Carbon monoxide sensors recover rapidly from hydrogen exposure but in the case of some other substances recovery can take considerably longer – days in some cases.

High concentrations of alcohol or solvent vapours can even cause permanent damage.

Cleaning the sensor should not be carried out using any such compounds and sensors that suffer direct exposure to diesel vapour should be replaced.

Carbon monoxide alarm tips

A carbon monoxide alarm offers considerable personal protection for very little outlay.

  • Test the battery regularly and replace if necessary, though the test function doesn’t check the sensor.
  • CO detectors for use in a boat should comply with EN 50291.
  • The Boat Safety scheme says: ‘All cabins with a fuel burning appliance should have a CO alarm fitted.’ This includes gas stoves, diesel heater outlets and oil lamps.
  • Be aware that hydrogen gassing from over-charged batteries may cause a CO meter to activate.
  • Investigate battery gassing for reasons such as excessive charging, faulty charge controllers, ageing batteries, as well as checking for sources of carbon monoxide.
  • If necessary vent the battery compartment by opening hatches, wafting air and similar measures.
  • Full information on recommended types and placing can be found at

‘In our experience’

Rob Stone shares his experiences.

Jeanneau SO 50DS

Rob Stone’s Jeanneau SO 50DS. Credit: Rob Stone

My wife and I have been sailing extensively a Jeanneau Sun Odyssey 50DS around the Med since March 2016.

The Jeanneau is currently berthed in Cartagena, Spain. I returned to the UK for some work, and subsequent cruising plans were then curtailed by the pandemic.

I recently shared my experiences on one of the sailing forums with battery failure and detection by a carbon monoxide detector, as I had heard mention that CO detectors could be triggered by H2S gas.

A battery on a boat

Detector app alerted Rob to battery failure. Credit: Rob Stone

We had a recent incident in which our CO detector operated a couple of times for no apparent reason and with no smell of H2S.

The detector we have on board can be accessed via a phone app and it did indeed show it was a high level alarm and not a fault activation.

On checking our batteries, one of the domestic cells (Varta lead acid) was too hot to touch and you could hear and feel it boiling!

Needless to say we had a very lucky escape from a potential fire or explosion.


Enjoyed reading Why an activated carbon monoxide alarm may indicate a battery charging issue?

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