How do I test my gas alarm

[rog]

I've read through the various recent posts concerning gas and gas alarms and, although my alarm sounds during initial warm up, what I would like to know is how do I properly test that the sensor's actually doing it's job - compared to testing my fire alarms, say, where we can easily use smoke.
I don't even understand how the sensor thing really works - I know it heats up a little and I can only guess there's some sorta electrical resistance change if there's gas about.
I also see it suggested the sensor is regularly changed but is this because it deteriorates through age or through use - if the latter what sorta hours use per annum is envisaged as we're not using our boat much at present, unfortunately, and the cable to the thing is rather hard to access.
Any reference or clarification would be appreciated.

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[halcyon]

Mate who used to make them used a gas lighter without a flint to test the system, just sprayed a little gas gas at the sensor head.
Main problem is corrosion due to seawater etc, in the sensor elements.

Brian

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[pheran]

I use the gas stove lighter thinggy from the galley. Just the slightest whiff should set it off.

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

 

[rog]

Thanks both very much - why didn't I think of it - it's so obvious
Happy New Year

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[rog]

Thanks both very much - why didn't I think of it - it's so obvious
Happy New Year

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[peterb]

If you don't have that sort of lighter, then use an old washing up liquid bottle. Wash out, take off top, insert blob of cotton wool with few drops of petrol on it. To test alarm, uncap bottle and squeeze towards the alarm.

Of course, to encourage regular testing you need to appreciate that the alarm is sensitive to virtually any flammable vapour. Take a mouthful of your favourite spirit (gin, whisky, rum, etc), then breathe on the sensor; alarm should operate.

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[MainlySteam]

Any methane propelled earosols sprayed into the bilge or in the general area of the sensor will set them off too. Often accidently!

John

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[SteveA]

The only problem I can see with these recommendations is that they are not testing the sensitivity of the sensors - the sensing heads can get soured by all sorts of other chemicals which can make them less sensitive to flammable gas - which could mean the alarm wouldn't sound until too late.
I don't know if there is anyone producing a test gas for the leisure market but in my line of work we use a gas mixture of 2.5% Methane, 20.9% Oxygen with the balance Nitrogen. This gives an LEL of 50% for which any gas alarm should have reacted.

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[MainlySteam]

I do agree with you Steve. A friend of mine had the facilities to make up his own test mixtures and explored his sensor - he found that the gas concentration to set it off was more concentrated than the explosive range!

I do not test mine with known mixtures, but its sensitivity is adjustable and I try to set it as sensitive as possible without nuisance alarms from engine running hydrocarbons (it is located close to the engine) and (confidently????) assume that is below the minimum explosive concentration for LPG. If I use a spray can to any extent in the boat, especially one containing hydrocarbon product as well as propellant then it always alarms when the gases have time to settle to the bilge.

John

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[snowleopard]

if you're a non-smoker, waving a tin of contact adhesive around will set it off as i found out when fixing my head lining!

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[jkim1]

the chap who suggests using the test gas at 50% lel is correct but if you can obtain the gas which is +- 20% you must also be careful to put it to the head at the correct rate and ensure that the head is not pressurised. Its a job requiring specific kit for specific sensors. I have done a bit of looking around and to all intensive purposes gas detectors unless run continuallyare not worth having and then should be check calibrated every 3 months and whenever they go off or are turned off. If the above are not followed the effectiveness cannot be 100%. The very act of turning on the gas detector (if there were to be gas in the boat ) would or rather could blow the boat up. All very bad isnt it.
One solution is to have a bilge vent fan which is switched by a switch which is suitable for use in explosive atmospheres and run it vented overboard for a few minutes before using any other kit. Another is to use a portable gas detector and sample the air or possibly gas in the bilge before energising any equipement and getting the portable detector checked out on a regular basis. But on a more practical basis turning off the gas on the bottle and buying one of those regulators with a leak checking pressure gauge and a velocity check valve is probably the most effective solution. I work with smart gas detectors that cost £1000 each and sometimes they give false alarms athlough so far have always failed safe thank God. One wonders what sort of quality you are going to get for about £250 for a twin unit. It may be better to throw it away every year and rely on the manufacturers guarantee at least you will have someone to sue then if the lot goes belly up and catches fire

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[MainlySteam]

Further to the practical basis suggested, I always make it a rule to open the seacocks in and sniffing the bilges as the first task when arriving at and entering the boat and certainly before turning the power on (the boat is always left electrically dead). In that way I hope for the possibility, while doing that, of smelling the odorant if there has been a leak before any sources of ignition exist. I do assume though, but do not know, that if gas had leaked when the boat was unattended (very unlikely if the cylinder has been isolated), then in a bilge the odorant would likely have suffered from odorant fade.

John

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