Most people are aware that both Butane and Propane LPG (Liquefied petroleum gas) are heavier than air, and can collect in the bilge if there’s a leak. When the concentration of gas to air rises to 2% (the lower explosive limit or LEL), the mixture will ignite if you strike a match to light the cooker or if there is a spark from your electrical system.
Gas alarms should detect LPG before the concentration reaches 25% of the LEL. To put things in perspective, our sense of smell is so strong that we notice gas when the concentration is only 0.5% to 1% of the LEL.
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The Dualwatch Alarm
 Marinecare have designed and manufactured what we feel is the best LP Gas and Carbon Monoxide detector for all leisure marine craft, the Dualwatch combined alarm. From the internal workings through to the external casing, everything has been designed with boats in mind.
The marine environment as we all know is an extremely harsh environment with corrosion, moisture, vibrations and even high humidity levels all taking a toll on your equipment. With this in mind Dualwatch has been designed to a high standard, a safety product must be of a high standard to ensure that it works when required as required.
The alarm panel incorporates separate LED's to give a visual warning of a gas leak or the presence of CO whilst an audible alarm is sounded when either gas is detected. Dualwatch is the only alarm on the market where the CO sensor is integrated into the alarm panel, with the provision for one or two remote and easy to fit plug in LP Gas sensors.
Find out more about the Dualwatch Marine Gas Alarm
Carbon Monoxide
A build up of carbon monoxide (CO) may not be as dramatic as an LPG explosion, but it can be equally deadly. It has no smell, taste or colour and can come from flues, exhausts and poorly maintained cooking and heating appliances. Even adverse weather conditions have caused fatal leaks. Remember it’s not only your boat that can produce carbon monoxide. Engines and generators of surrounding boats in a busy marina can also be a deadly source. Carbon monoxide can kill quickly when it leaks in large quantities. If it builds up over time, it can initially cause headaches, nausea and dizziness, which are often incorrectly diagnosed as flu or seasickness.
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How do the sensors work?
The LPG sensor is made from a sintered metal oxide, which, on detecting gas passing over it, increases in electrical conductivity. This is used to trigger the alarm. The carbon monoxide operates on the fuel cell principle. It involves the electrochemical oxidation of carbon monoxide to produce carbon dioxide. As the carbon monoxide is oxidised, electrons produced at an electrode generate a small current, which is directly proportional to the amount of carbon monoxide present.
Fitting the control panel
 I helped Jim, from Marinecare, to fit a Dualwatch Alarm to Steve Flick’s Sunlight 30, Manushi. The carbon monoxide sensor is built in to the control panel, so the panel should be ideally located 1.5m away from the cooker to prevent it from setting off accidentally. The layout of Steve’s boat meant we couldn’t get it quite that far away, but Jim felt that there shouldn’t be any problems. Jim marked the position using the template supplied, making sure that there was at least 25mm at the rear of the panel and no wires in the way. After drilling four pilot holes, Jim cut along the inside of the line to get a firm fit. The power and sensor cables were pushed through the cut out and the control panel was fixed with four screws.
Wiring Up
 As LPG is heavier than air, the sensor must go as low as possible. We found a clean, dry section of bilge close to the cooker. As the sensor head comes with self adhesive pads, filling was easy. Manushi’s fuel tank is directly underneath the chart table so we fixed the cable down the side of the tank housing, drilling 8mm holes where necessary. Once the cable was in place, the three colour coded plugs were pushed into the sensor.
The kit has 6m of sensor cable, which was more than adequate for this installation. You may want the Dualwatch 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 65mA and you may be left with flat batteries. Another option is to have the alarm Dualwatch 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. Steve wished to be able to turn the Dualwatch Alarm off, as he is away from the boat for long periods and does not want to return to dead batteries. The auxiliary power socket by the main switch panel was a suitable place to connect into the power supply, and a 3A fused connection was made. The kit came with 3m of power cable, which was shortened to suit the installation.
Operating and testing
 When first connected, the LPG sensor stabilised and the red alarm light lit up for a few seconds. This initially takes about three minutes. The carbon monoxide sensor is tested by holding in the test button for 30 seconds. The LPG sensor needs a whiff of gas from a lighter to trigger the alarm. Fan fresh air into the sensor to stop it.
Find out more about choosing a gas alarm
Buy the Marinecare Dualwatch Marine Gas Alarm from the Marine Shop 
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Choosing an Alarm 
