What you should know about Gas Alarms

Gas Explosion on Boat

The consequences of a liquid petroleum gas (LPG) leak on board a boat can be disastrous. Every year gas leaks cause deaths and serious injuries, which could have been prevented with the installation of a user-friendly and cost-effective gas alarm system.

LPG is not poisonous but it is highly explosive and can lead to devastating damage if it is allowed to build up.

How are LPG levels measured?

LPG levels are measured using the 'Lower Explosive Limit' (LEL). This means that when the concentration of gas reaches 100 per cent LEL, it becomes explosively dangerous - and if you light a match, it would explode. Most gas alarms are calibrated to sound an alarm at around 10 per cent LEL, well before the gas becomes explosively dangerous.

A strong smell of gas can often be noticed when the concentration is only 1 or 2 per cent LEL because gas suppliers add a harmless but strong-smelling chemical to the gas. Despite the strong smell, this level is not dangerous and so the gas alarm will not usually sound. This is useful to know because you do not want a gas alarm system giving repeated false alarms which could lead to the crew mistrusting the device and ignoring a genuine alarm or even worse switching off the gas alarm altogether!

What vessels are required to fit a Gas Alarm?

Whilst gas alarms are mandatory on commercial vessels, charter yachts and sailing school boats, there are no regulations requiring an owner to fit an alarm on a privately-owned vessel. However, many marine insurers may have specific clauses in their insurance policy wording that require an owner to fit a gas alarm if a gas cooker or gas-powered heating system is installed. Do check your insurance policy as your cover could be invalidated.

How do gas sensors work?

Most LPG sensors are made from sintered metal oxide that is heated. When gas passes over the sensor, it increases in electrical conductivity. The second part of the sensor is enclosed and is not exposed to the gas. This is used as a control reference and the difference between the two is used to trigger the alarm

What else could set the alarm off?

LPG has widely been used as a sustainable, CFC-free replacement for refrigerators and aerosols. It is also used in gas lighters, cordless hair care appliances, and other portable equipment.

So over-zealous use of deodorant or hairspray could lead to a build up of LPG and trigger the same alarm as a gas leak from your cooker. The sensors can also detect other flammable hydrocarbons, such as paint thinners and solvents, as well as petrol and diesel fumes.

It is therefore good practice to mount a gas sensor away from areas that could contain these false sources, such as engine bays or lockers used to store chemicals, cleaning agents, spare fuel cans and outboard engines.

Faulty alarms

The alarm could also sound to warn you of a fault. But how do you know if it is a genuine fault and not a gas leak?

A good gas alarm should make it plainly obvious if it is alerting you to a fault rather than a potentially dangerous gas situation. Most alarms use separate lights to signal a fault, but the better products use a combination of lights and an audible warning that is different to the normal alarm sound.

Thank you to Peter Spreadborough of Calor Gas Centre Southampton and David Stoppard of Marine Systems Engineering for the material used in the preparation of this article.


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