best location for gas alarm

[Ric]

I've just bought a little combined CO/Gas alarm. How sensitive are these things? If there is a gas leak somewhere, the bulk of the gas will tend to sink as it is heavier than air but given the convection mixing in a small boat I'd imagine there would be traces of gas at all levels. So where do people put them in their boats? I'm thinking of putting it in the locker under my oven, given that that the oven is the most likely cause of a leak. What's the consensus?

I'm less concerned about the CO aspect of the alarm, and given that CO is lighter than air the best position for CO detection would seem to be at a higher level.

Incidentally, if gas is detected in your boat, what do you do about it (apart from turining off electrics etc)? How do you empty the gas from the boat given that it is likely to be concentrated in the bilge?

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[Talbot]

seems a bit strange for this sensor to be combined, or does it have 2 sensors going to the one instrument. IIRC the gas sensor should not actually be in the bilge as salt water can ruin/cause false alarms, but it does need to be as low as possible in the area where gas leaks are most likely. If you do not have a flame failure device, then it should be relatively near the cooker. The CO sensor needs to be as high as possible, but is only really relevant if you have gas heating, or are using the oven a lot (or have other gas appliances that are on permanently e.g. fridge or water heater).

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[MainlySteam]

Regarding removing leaked LPG from the boat, the practice, should you be unfortunate enough to have to do it, is to pump it out with the manual bilge pump which if of the common diaphragm type will remove it - assumes one has a manual bilge pump, of course.

John

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[LORDNELSON]

or............

... or bail out the air from the bilge with buckets, I'm told it looks a bit strange but works! Bear in mind that exhaust gas leaks can set off LPG alarms. The sensor on my sniffer is on a tranverse line from the oven but on the centerline of the boat just under the floorboards; we have very flat bilges and this is about the lowest practicable place. If the sniffer is switched off or the if it smells gas the gizmo operates a solenoid valve that switches off the gas supply at the gas tank.

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[StephenW]

Suggest it is not put within an enclosed locker but is directly under the cooker to enable the free flow of gas (if it should free-flow!) to trigger the device. Same prnciple as siting a smoke detector i.e no restriction of flow around the device. I have mine fixed at virtually floor level immediately below the cooker

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[peterb]

The density of any gas depends on its molecular weight (MW). Air is a mixture, in round figures of about 80% nitrogen and 20% oxygen. The MW of oxygen is 32 and the MW of nitrogen is 28, so that the equivalent MW of the mix is about 28.8. The MW of carbon monoxide is 28, and of butane is 58. So CO has just about the same density as air, while butane is about twice as dense. However, if CO is being generated in the cabin then it's probably coming from an improperly oxygenated flame, so will be hot and likely to rise. Butane will certainly fall, but don't think of it as being like water. The highest concentration may be in the bilge, but if there is any sort of dangerous concentration then it should be detectable anywhere in the lower part of the cabin.

The usual advice if butane is detected is not to turn electrics off. Sometimes (particularly with dc circuits) the spark on breaking the circuit is greater than that on making it. Baling the butane out with a bucket will work, but only if there has been a really massive leak. Otherwise, just turn the gas off at the cylinder and ventilate the boat as much as possible.



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[pheran]

I assume you have seen the thread on GAS INSTALLATION below.

<hr width=100% size=1><font color=blue>A convert to steel boats and displacement cruising - the only way to go</font color=blue>

 

[charles_reed]

I remember

fitting a gas alarm in the bilges of the little trailer-sailer I had from 1978 thro' to '90.
We were tied up about 3 boats out from the quay in Weymouth, when the alarm started to go off for no apparent reason at irregular intervals.

After about the 5th false alarm, I worked out that about 6-8 minutes after one of the cars parked on the quay started up, so did the gas alarm.

Since than I've made a practice of doing without, having found my sense of smell a perfectly adequate substitute.

You need to change it about every 3 years, the "sniffer" deteriorates, so it needs to be fairly easily accessible and I'd suggest it's in the bilge area close to the cooker (as hydrocarbon gases are heavier than air keep it low and as most leaks are from LPG cookers).
prevention being better than cure I prefer ALL my cooker burners to have flame-failure devices and to use genuine armoured (not reinforced hose) for the flexible cooker to copper pipe LPG supply.

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