Is there life after death?

[Chris Hogg]

For a smoke alarm...well, actually one of those heat alarms that you use in the kitchen that's less sensitive to steam and cooking stuff. Safelincs Ei3103RF

, two years old. In my elderly mother's kitchen. Started beeping a couple of weeks ago. Once every 30 seconds or perhaps every minute. I didn't time it. Replaced the battery (9 volt) with new. Seemed OK. Beeping again today. Dammit! Replaced the battery *again* using the second battery in the twin-pack. Didn't cure the beeping. "But it's a brand new unused battery, right type and everything! I know, I'll take the battery out; that'll shut it up!" Did it heck!. Carried on beeping as before!!

How can that be? Do these things have some sort of storage capacitor in them that keeps supplying a little charge until it too finally goes flat? I'm puzzled!

[Jethro_uk]

Don't (some) smoke detectors rely on a radioactive source ? Which decays.

[Huge]

Yes, but not for power! It's a minute amount of Americium 241. About

200 ug if my mental arithmetic is correct.

Not significantly in a human lifetime; it's half-life is 432 years.

[Tim Streater]

0.28 ug according to Winky. Which is an alpha-emitter (so not dangerous if left alone) and which decays to Neptunium-237, radioactive but with a half-life of 2.41 million years, so also not dangerous.

[Huge]

OK, ta. That's what happens when you're doing 3 things at once.

[David]

Mains power with battery backup, or battery power only?

I've just had to change the battery in a mains smoke alarm with cherpes and that kept on chirping with no mains and no battery so there must be a capacitor or something in there.

Check the voltage of your batteries just in case.

Cheers

Dave R

[Chris Hogg]

Battery only. Nice to know I'm not the only one whose alarm beeps on zero power! Thanks.

[Dave W]

There might be a small rechargeable battery inside, normally kept charged by the battery, but keeps the unit working while you change the battery. Not for long though.

[Big Les Wade]

So why do they tell you to replace them after seven years or so? I always assumed it was because the radioactive source had decayed.

[Tim Streater]

These things work by detecting the alphas given off by the americium. Smoke absorbs these so when the level off alphas drops the detection circuit assumes a fire. Possibly after 7 years the level has dropped enough to start giving false alarms. But I'm just hypothesising here.

[Etaoin Shrdlu]

We've had the same one for something like 25 years. I know it still works, because it goes off about once a year when the missus's hair dryer grille is full of dust, or when the toaster is full of crumbs :-)

[Huge]

Not with a 432 year half-life, it won't.

[Chris Hogg]

How does the detector work, and what's *its* lifetime? I imagine it eventually gets choked with a layer of dust and grime, which must reduce its sensitivity.

[The Natural Philosopher]

essentially the radioactivity provides a leakage path through the air, until smoke buggers up the ions.

[Brian-Gaff]

Don't take this the wrong way, but are you absolutely sure it was this device which is beeping? are there any other devices around the house. One can easily be fooled by reflected audio when its quite high in pitch and short lived. Brian

[Chris Hogg]

That is a real possibility and one that I've considered, especially as I wear two hearing-aids so my hearing, and its directional capabilities, isn't the best. But two of us both thought it was the kitchen alarm that was beeping. Proof will be tomorrow when I visit again. If it's stopped beeping, it was the kitchen; if not, it's somewhere else.

[Chris Hogg]

Stopped beeping now. Must be a storage capacitor in there somewhere.

[ARW]

Pressing the test buton with no battery usually stops it pretty damn quick.

[ARW]

Well if you were selling smoke alarms then why not:-)?