Is there life after death?

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

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

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

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

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

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

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.

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.

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.

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 :-)

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

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.

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

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

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.

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

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

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