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Author Topic: Why do smoke alarms always go off at 3:00 am?  (Read 6844 times)

Offline Geezer

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Why do smoke alarms always go off at 3:00 am?
« on: 28/06/2010 23:41:05 »
I'm sure the people that make these things have them rigged so they only give false alarms at 3:00 am. How do they manage to do that?

Last night they got really clever. The smoke alarm started beeping every minute or so at 3:00 am (of course) and I was trying to remember how to detach the $%&^#^%! thing from the ceiling while standing on a chair without actually putting the lights on.

I was seriously considering applying a ball peen hammer to it, when Mrs G pointed out that it was the carbon monoxide detector at the other end of the room that was complaining. Its seven segment display was flashing Lb. What the heck has pounds got to do with it? I threw it into my neighbor's yard and went back to bed.

 


 

Offline LeeE

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Why do smoke alarms always go off at 3:00 am?
« Reply #1 on: 29/06/2010 14:10:28 »
Its seven segment display was flashing Lb.

Lb = Low Battery.

Alarms like these have to alert you when the battery is low so that it doesn't run down completely and become u/s without you realising.
 

Offline SeanB

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Why do smoke alarms always go off at 3:00 am?
« Reply #2 on: 29/06/2010 17:53:01 »
3PM is probably the time when the house is coldest, as the heating is off, no lights are on and all doors are closed to the kiytchen. It then looses heat and as the battery in the alarm gets colder it's terminal voltage drops, as well as the chemical reaction rate inside decreases, increasing the battery internal resistance. When the detector does it's regular battery test by powering up from sleep and checking the smoke chamber and measuring battery voltage this causes the voltage to drop below the threshold for low battery. This then sounds the low battery warning, and the extra draw then keeps the battery voltage depressed.

Best is to put good quality batteries in the unit, alkaline Duracell, Energiser or Rayovac, not el Cheapo off brand items. You also should replace them every 2 years, unless you buy the lithium smoke alarm batteries, which will last 10 years in service.

 

Offline Geezer

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Why do smoke alarms always go off at 3:00 am?
« Reply #3 on: 29/06/2010 19:23:10 »
Thanks SeanB.

You know, you could be right about the low temperature being the final straw. So there really could be some correlation with time. And I thought they were just conspiring against me.

We might have to promote this topic to general science!

BTW - I figured out what Lb meant and replaced the batteries. The "chucking out the window" part was a bit of poetic license ;D
« Last Edit: 29/06/2010 19:25:08 by Geezer »
 

Offline Stefanb

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Why do smoke alarms always go off at 3:00 am?
« Reply #4 on: 15/07/2010 04:11:26 »

 

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