Choosing a Gas Alarm

View the selection of Gas Alarms available from the Marine Shop

Gas explosions are never small. They don’t happen often, but when they do they make headline news.As most people know, Liquefied Petroleum Gas or LPG (Propane or Butane) is heavier than air and will collect in the bilge or the bottom of a locker. It only takes a small volume of gas to produce an explosive mixture. When the concentration of gas in air reaches 100% of the lower explosive limit (LEL) (The LEL of Butane is about 2% gas 98% air) the mixture will ignite if you strike a match or if there is a spark from your electrical system. Gas alarms should detect LPG before the concentration reaches 25% of LEL. To put that in perspective, a normal nose will detect a very strong smell of gas when the concentration is about 5% of LEL, that’s before most of the alarms will be triggered.

Even though the risks of a gas explosion are small, it attracts a mountain of paperwork in the form of Legislation, codes of practice etc. In most cases, however, the conclusion of the investigation is that a gas system is perfectly safe if installed and maintained correctly.

A gas alarm is mandatory on commercial vessels, charter & sailing school boats and it makes sense to fit one to a cruising boat. An alarm will not stop a gas leak, but it will alert you to its presence. It should not be used as a substitute for a good installation or good housekeeping practices. It is always good practice to turn the gas off at the cylinder when it’s not in use.

How do the sensors work?

A LPG sensor is made from a sintered metal oxide that is heated. On detecting gas passing over it increases in electrical conductivity. Another part of the sensor is enclosed and not exposed to the gas. This is used as a control and the difference between the two is used to trigger the alarm.

What else will set the alarm off?

LPG has been used as a CFC free replacement in refrigerators and aerosols. It is also used in gas lighters, cordless curling tongs, gas lights and other portable appliances. So an over zealous use of an underarm deodorant or hairspray may lead to a build up of LPG with potentially the same results as a leak from the back of your cooker. The sensors will also detect other flammable hydrocarbons such as paint thinners and other solvents, not forgetting the obvious ones petrol, diesel and Alcohol.

Another reason for the alarm going off is to warn of a fault in the system. The problem is how do you know if it’s a fault or a gas leak? All of the alarms tested have lights to warn of a fault, some use a combination of lights and audio warnings. Either way some thing is not right and needs attention.

David Stoppard of Marine Systems Engineering says “there is a case for fitting the alarm with the cheapest replacement sensors as they will need replacing from time to time. Continued false alarms only lead to miss trust in the alarm system and have lead to crews disconnecting it to avoid the annoyance.”

When a Gas Alarm sounds

If your alarm sounds follow the action plan list on our emergency action notice.

Fitting a Gas Alarm

A Gas Alarm will require a fused electrical supply. Although 24v alarms are available the vast majority sold are 12v. The control module should be located in a prominent area, around the chart table with other instruments is a common location.

You may want the Alarm powered when the boat is left unattended. This has the advantage of alerting you to a problem as soon as you return to boat. The disadvantage is that the alarm draws electrical power and you may be left with flat batteries. Another option is to have the alarm wired in so it comes on when you turn on the batteries. This saves battery power, but there is a risk that the battery switch may cause a spark and ignite any leaked LPG. But if the cylinder is turned off in a properly drained locker, that shouldn’t be a problem.

There are single or multi sensor alarms available. The codes of practice suggest that one is fitted per cabin that has a gas appliance. It would make more sense to fit one also in areas in the boat that gas is likely to collect, for instance, install a sensor head in the space under the gas locker or in a space where gas pipe joints are not easy to inspect, in which case a multi sensor system would suit.

David Stoppard says “the ideal place to install a sensor is at the lowest point gas is likely to collect. The sensor should not be installed in an enclosed space away from a source of gas, directly below or above cooker, next to a hatch, porthole or extractor fan. Also they should not be installed where dirt and dust can block the sensor and stop it working or where it is likely to get wet.”

Find out more about fitting the Dualwatch Marine Gas Alarm

Other Safety devices

It makes sense that if there are no leaks in your gas system, then the risk of explosions is almost non existent. All installations should have a method of checking the soundness of the gas system. Although the minimum requirement is to have just a test point, an Alde bubble leak tester is a quick and simple method of checking your gas system. It can be carried out every time you turn your gas on and does not require any specialist knowledge or equipment.

There is no substitute for a good gas installation. Always consider using a professional gas engineer. A list of marine gas installers can be found here

British Standard / CE

There are no British or European Standard for the performance of a marine gas alarm. Most manufactures therefore use the Domestic gas alarm standard BS EN50194:2000 and applied it where possible to the marine environment. The CE mark usual means the alarm has been test for electrical safety and radio frequency interference. It does not mean it has been tested for performance in detecting gas.

Read the recent Practical Boat Owner group test report on LPG Alarms - 3Mb

Gas Alarms available to buy from the Marine Shop

SF Detection Ez Sense Gas Alarm

SF Detection Single Gas Alarm

Marinecare Dualwatch Marine Gas Alarm

Pilot Gas Alarm

Nereus WG1000 / WG2000 Gas Alarm