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Selecting Vapor Detectors


M-2A Fuel Vapor Monitor monitors gasoline and propane. It includes one sensor.


Order the MS-2 Second Station/Replacement Sensor to monitor a second location.

By Tom Burden

What they do

Fume detectors monitor the concentration of dangerous gasses in the air and sound alarms to warn of a potentially dangerous situation. They help prevent explosions or fires from concentrations of gasoline vapor or propane gas. Fuel vapor detectors will react to some—but not all—combustible vapors like cooking fuels, hydrogen, solvents, and certain cleaning compounds.

How They Work

A special sensor wire has a small electrical current passing through it when the unit is turned on. The presence of combustible hydrocarbon vapors causes a change in the wire’s resistance, triggering the alarm. Fuel vapor sensors commonly sound the alarm if the concentration of explosive gasses in the air reaches 10 to 20 percent of what is required for an explosion (the Lower Explosive Limit), giving you time to fix the problem and save your boat and crew.

Gasoline and Diesel Fumes

If you are among the boaters who think the bilge blower will eliminate dangerous vapors prior to starting a gasoline engine, think again. Fuel leaks can create fumes as quickly as the fan can clear them out, so running a blower doesn’t always eliminate the risk. For peace of mind, we offer fuel fume detectors with automatic bilge blower activation, which keep your boat safe while it is unattended and will sound an alarm if the blower can’t keep up with the fumes.

Propane and CNG

Propane is by far the most popular onboard heating and cooking fuel because it is efficient, cheap and readily available wherever you go. Propane is heavier than air and settles below deck, in enclosed spaces like storage lockers and the bilge, where an electrical spark can ignite it. Detecting propane and CNG requires separate monitors. CNG is lighter than air and requires detectors to be mounted high, within 9" of the ceiling.

What to Look For

Automatic bilge blower activation: Smart fuel fume detectors will set off an alarm, and start the bilge blower to extract harmful fumes, reducing the danger of explosion, especially when nobody is on board the vessel.

Audible and visible alarms: Look for gasoline vapor/propane gas detectors with both audible and visual alarms. Test buttons for the alarm are helpful in determining the functionality of the detector. A detector that properly detects fumes but doesn’t trigger an alarm is useless.

Sensitivity: This is a bit of scientific mumbo-jumbo but very important so we’ll try to explain it in layman’s terms: You don’t want an alarm to go off a second before the concentration of harmful gasses reaches explosive level. Hence, gasoline, propane and CNG detectors are pre-programmed to alert you early, at 10 to 20 percent of the Lower Explosive Limit (LEL).

Sensor mounting position: Propane sensors don’t like water, but they should be mounted low and near the appliance using propane (e.g. stove or heater). Gasoline sensors live in a dreary spot, just above slosh height in the bilge but no lower than the height of the starter solenoid in the bilge. CNG sensors, on the other hand, need to be mounted high, because this gas is lighter than air.

Battery vs. hardwired: Detectors without power are pretty useless, therefore we recommend hardwired units that are always on and don’t need to have batteries replaced periodically. A power-indicator light that shows the detector is fully operational is also quite useful.