Gas Detectors

Many gases are toxic and can be harmful to humans and animals.  Others can at even extremely low concentrations form a lethal mixture that will explode when ignited. As such they pose an arguably even greater danger than fire to anyone onboard.

For this reason, gas detectors are mandatory equipment for many commercial boats and highly recommended to be fitted on boats with enclosed spaces that carry LPG or petrol.

A gas detector is a device that detects the presence of gas in an area and is used to detect a gas leaks or fuel fumes. Different types of detectors detect combustible, flammable and toxic gases. Originally, detectors were produced to detect a single gas, but modern units may detect several toxic or combustible gases.

Marine gas detectors can provide an early warning before gas concentrations reach the concentration required for an explosion, the Lower Explosive Level (LEL). Typically such units are sensitive enough to warn you when gas concentrations reach 20% of the LEL. As a sensor’s detection exceeds a preset alarm level, the alarm will be activated. 

Most have internal buzzers and warning lights.  Some have additional switched outputs for connection of external devices such as buzzers, sirens and flashing lights. Audible external alarms are usually louder than the internal buzzers and can more easily be heard above engine noise. External visual signals could be flashing spreader lights or anchor lights. It is also possible to route gas alarm signals to some security systems, which may even gives owners SMS alerts.

Some alarms interface with a control system to allow a process to be automatically shut down. For LPG users, some units will automatically shut off an electrically operated gas solenoid valve fitted close to the gas bottle. A fringe benefit of this feature is that these usually also allow the LPG solenoid valve to be turned on and off by remote control.

Gas detectors can be powered by a self contained battery or be hard wired into a larger system, e.g. a 12V or 24V dc boat wiring system.

Better types of gas detectors have remote sensors that can be placed in the areas where gas vapours accumulate or even enable monitoring several areas by supporting more than one sensor. Liquefied Petroleum Gas (LPG) and petrol vapours are heavier than air and will settle in the lowest areas of your boat - usually the bilges. Sensors should therefore be fitted low down, although they should be kept above levels likely to be flooded.

The use of a bilge blower does not ensure that explosive levels of gas are automatically removed. Fuel leaks can create new concentrations as fast as the blower clears them.

Having a gas detector installed also does not fully guarantee safety from gas poisoning or explosions.  Gas detectors have their limitations and knowing how they work will allow you to judge how much to trust them, how to maintain them and what other vigilance and measures are required.

It must be noted that significant variations occur in the composition of gases such as LPG, depending on their source.  The resultant LEL can therefore vary significantly from sample to sample, making it difficult to accurately define the design parameters for gas detectors.  Ambient conditions such as temperature and humidity as well as the proximity of chemical contaminants can affect the function of gas detectors too. However, in practice the low alarm threshold of 20% of LEL provides a significant factor of safety. 

How do the gas sensors work?  Whilst there are various types of sensors available, commonly a Pellistor is used.  Also known as a catalytic bead sensor, is just wire of platinum with a catalyst on it’s suface with enough current passing through it to heat it up to about 500C. When an explosive gas is present, it will burn on the hot catalyst causing the resistance of the wire to increase. The change in resistance can be measured and thus trigger a signal.  These sensors can be damaged or destroyed if submerged in water or exposed to excessively high gas concentrations.  Care must thus be taken when testing sensors by flooding them with gas from a lighter.  Sensors must be protected by taping them up when painting or working with substances that give off strong fumes in their proximity. 

Most gas detectors have a built-in self test for the internal electronics which usually also automatically tests the sensor(s) and connecting wires.  Note that these tests do not test the sensitivity of the sensor.  That can only be done using a test gas of calibrated concentration.  The cost of getting a professional to conduct this test is probably more than it would be to just replace sensors on a regular basis, unless multiple sensors have been installed.

Special mention should be made of carbon monoxide detectors.  Detectors for LPG and petrol fumes usually do not detect carbon monoxide. Carbon monoxide is usually found on boats as a result of engine exhaust gas leaks and is an odourless and tasteless but vicious killer.  Investing in a good quality CO detector cannot be recommended too highly.