CARBON MONOXIDE ALARMS
Why they indicate battery charging issues
Four main types of carbon monoxide detectors are available, although only one, 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. 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.
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 sensor is very similar to one for hydrogen sulfide and some dualpurpose 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.
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. 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 right), 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.
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.