# OT/NT How so smoke detectors work?



## Semper Vaporo (Jan 2, 2008)

Well... Hang on... I know how they detect smoke and other products of combustion, but I have one that seems to have an additional capability.

I have 4 smoke detectors. One in the basement, and the three others are all within 6 feet of each other, only separated by open doorways.

One of them is chirping, indicating that it is in need of a new battery. Just a couple of months ago I replaced the batteries in all four of them and I test them at least monthy by pressing the test button on the front. They all still test properly, but apparently one of them now has a bad battery.

But for the life of me, I cannot figure out which one it is. When I am sitting in my easy chair in the living room, I can hear a "Chirp" about every 30 to 45 seconds. But when I get my carcass up out of my easy chair and wander toward the hallway where the 3 units are mounted, the chirping ceases.

I went to the hallway, faced the smoke detector over the bathroom door, with the master bedroom to my left and the 2nd bedroom (storage room) to my right and each of those smoke detectors is within about 4 feet of my head. I stood there for 5 minutes waiting for one of them to chirp.

Nothing, nada, psyzltche, narry a chirp, peep, tweet nor bellow!

I headed to the basement and on the way I heard another chirp, but due to the high frequency of the tweet I was unable to determine if it was in front of me or behind! I went to the basement and stood down there for another 5 minutes and again, no sounds from any of them.

I returned to the hallway, but by then my bladder decided it had had enough and I headed on into the bathroom. Whilst there, I again, heard a little cheep, but again no way to determine from whence it came. I stayed a few minutes in the bathroom and timed the chirping period and found that it was almost exactly 30 seconds...

That is until I poked my head out the bathroom door, then the chirping ceased altogether.

I really do not want to replace all the batteries again so soon after having done so.

PLEASE, SOMEBODY, explain how a smoke detector with a low battery is capable of detecting a human seeking to find it. PLEASE?


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## Gary Armitstead (Jan 2, 2008)

C.T., 

Sure it's NOT a cricket? Just askin'.


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## chuck n (Jan 2, 2008)

My only suggestion to to take a battery out one detector at a time until the chirping stops. On second thought Gary may be correct and you have a cricket. I hear them in my house occasionally and when I go into the room where they are the chirping stops.


Chuck


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## Semper Vaporo (Jan 2, 2008)

HA! I FIXED IT!!!

Nope, not a cricket!

I went and sat down in the living room and got all comfy.

CHIRP!

I grabbed a new 9-volt battery from the cabinet and put it in the detector in the hallway and put the replaced one back on the shelf and returned to the living room and got all comfy again.

CHIRP!

I took the battery from the shelf and replaced the battery in the 2nd bedroom with it. Put that battery on the shelf and returned to the living room and got all comfy.

GOT IT! No chirps for the last 10 minutes.

I just threw that battery away!

I still need to know how it detected I was near enough to maybe determine which one was chirping... seems to me like the technology could prove useful in burglar alarms and such.


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## Spule 4 (Jan 2, 2008)

The sensors also have a service life. Testing them with the button is fairly useless. Do it with a wood match that has been blown out. Not my opinion, but something told to me by a fire protection consultant that serves on one of the NFPA committees.


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## bottino (Feb 7, 2008)

I have had this problem off and on for years. I finally took three of five detectors down. Now my house is for sale, and I need to get them up and working. My electrician says if I have him come to make them work, the first thing he will do is take them down one at a time, blow canned air through them to get rid of the dust, and replace all the batteries. Basically, he was telling me to do that first. They are interconnected, so they know when a battery is going down. So I will do the blow air, replace all batteries first.
Good luck. They drove me crazy. I destroyed one trying to get it down. 
Paul


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## George Schreyer (Jan 16, 2009)

smoke detectors work by ionizing a particle of anything that gets into the detector housing with a low intensity radioactive source (the beta particles cannot escape the detector housing). If there is dust inside, it can freak out the detector.


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## Spule 4 (Jan 2, 2008)

Some are photo-sensing also. Depends on type, but yes, dust can set them off, along with other things like over-cooked bacon.


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## Ray Dunakin (Jan 6, 2008)

Interesting. I should blow some canned air through our detectors, they probably have a pretty good accumulation of dust by now. 

Had to replace a bunch of them several years ago when I smoldering jack o'lantern filled the house with smoke and not a single alarm went off. I don't know if they were just too old, or maybe had too much dust.


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## George Schreyer (Jan 16, 2009)

The ones that use a radioactive source will also experience loss of sensitivity over time as the source decays. I don't know what radioisotope is used nor how long it will actually last.


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## Ray Dunakin (Jan 6, 2008)

Maybe smoke detectors should come with an expiration date.


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## Greg Elmassian (Jan 3, 2008)

The radionuclide used in ionization smoke detectors is an oxide of americum-241, which is bonded to a metallic foil and sealed in an ionization chamber.
Americium-241 is a man-made radioactive metal, first discovered during the Manhattan project. Americium-241 is formed spontaneously by the beta decay of plutonium-241.

Americium-241 emits alpha particles and low-energy gamma rays. The half life of americum-241 is about 432 years. I would submit that failures in smoke detectors are not from the radiation "wearing out".









Regards, Greg 







http://www.epa.gov/radtown/terms/index.html#alphahttp://www.epa.gov/radtown/terms/termghi.html#gamma


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## Spule 4 (Jan 2, 2008)

Posted By Ray Dunakin on 23 May 2010 11:56 PM 
Had to replace a bunch of them several years ago when I smoldering jack o'lantern filled the house with smoke and not a single alarm went off. I don't know if they were just too old, or maybe had too much dust. 



Ionizing type detectors take MUCH longer to detect a smoldering fire Vs. photo. But cost is a factor....


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## Torby (Jan 2, 2008)

My dad used to answer such questions from his over-inquisitive, and only, son by saying, "Nicely." 

But I think, in this case, I'd say, "Poorly." 










(Amazing how you only see the meaning-changing typo AFTER you hit the submit button.)


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## Spule 4 (Jan 2, 2008)

Anyone else want the Totally Rad Smoke Detector 3000? 

http://www.myvideo.be/watch/6553653


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