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TheMG
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TheMG

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Ionization smoke detector decay, what is true?

From what I understand, an ionization smoke detectors works by using a small radiation source to ionize the air in the detector, causing a small electrical current to flow. As smoke enters the detection area, it obstructs this flow of current and the drop in current sets off the alarm.

There is contradictory information as to what exactly happens as the radioactive material decays as the detector gets older. Some sources say the detector will become less sensitive, others say it will actually become more sensitive, to the point where it will eventually give increased nuisance alarms and possibly randomly go off.

Which is true?

Logic says that the latter would be true, since with the decay of the radioactive material would cause a reduction in the electrical current in the detector.

Note that I am not seeking reasons not to replace old smoke detectors, nor am I suggesting anyone not replace their detectors regularly. I am merely seeking the truth for the sake of knowing the truth.

Killa200
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join:2005-12-02
TN

Killa200

Premium Member

I agree with you that logic would seem to say it would get more sensetive of the environment as the radioactive source decays. The only thing that throws me is with the americium-241 in an Ionizing smoke detector having a half life of 432 years, when does the thing exactly get too sensitive?

dolphins
Clean Up Our Oceans
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join:2001-08-22
Westville, NJ

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I don't know the answer to your question but I do have 2 alarms that have become increasingly sensitive. The one in my kitchen and the one at the top of my second floor staircase start beeping whenever my oven reaches about 400 degrees. The oven is clean and there are no physical signs of smoke when operating.

cdru
Go Colts
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join:2003-05-14
Fort Wayne, IN

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MVM

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said by TheMG:

Logic says that the latter would be true, since with the decay of the radioactive material would cause a reduction in the electrical current in the detector.

As the Am decays, it would reduce the amount of ionization of air, decreasing the current between the electrodes. However I don't think enough Am would decay over just 10 years to contribute significantly to the reduced ionization. My guess is that after 10 years, environmental conditions may do more to increase the sensitivity. Dust, airborn grease, corrosion, etc can settle on the electrodes decreasing the current creating more nuisance alarms. Or at least that's my theory.

Whatever the reason, NFPA recommends replacing them every 10 years. At less then $10 for a basic model, that's less than a dollar a year for protection.

Cho Baka
MVM
join:2000-11-23
there

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Cho Baka to Killa200

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said by Killa200:

IThe only thing that throws me is with the americium-241 in an Ionizing smoke detector having a half life of 432 years, when does the thing exactly get too sensitive?

Just to play it safe, I replace mine every 250 years, whether it needs it or not.

rex0
join:2002-02-10

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This is mostly speculation on my part the am 241 does have a 432 year half but it's alpha decay into np 237 over the service years of a smoke detector could influence the output of ionization energy that the chamber is looking for as the np 237 builds up.
If you calculate the decay constant after 10 years somewhere around 1.5% of the am 241 has decayed into np 237. When you consider the actual quantity is just under 0.3 ug it could be enough to change the output energy.

Either that or they say replace after 10 years because the other components are running 24/7/365 and might simply be more likely to breakdown.
8744675
join:2000-10-10
Decatur, GA

8744675 to Killa200

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I have a wired smoke alarm that keeps emitting the 'low battery' beep every minute or so, but only when the house gets cold. Even with a new backup battery, it starts beeping on cold mornings. Now I'm trying to figure out how to replace it without having to buy a whole new unit and base, and rewire it into the box.

Other battery operated alarms I've had seem to do the same thing after a couple years.
Bobcat79
Premium Member
join:2001-02-04

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Bobcat79 to dolphins

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to dolphins
said by dolphins:

I don't know the answer to your question but I do have 2 alarms that have become increasingly sensitive. The one in my kitchen and the one at the top of my second floor staircase start beeping whenever my oven reaches about 400 degrees. The oven is clean and there are no physical signs of smoke when operating.

Gently vacuum your smoke detectors, and I bet your false alarms go away.
Bobcat79

Bobcat79 to rex0

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to rex0
said by rex0:

Either that or they say replace after 10 years because the other components are running 24/7/365 and might simply be more likely to breakdown.

They say to replace them every 10 years to protect the profits of the smoke detector manufacturers.

Chuck22
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join:2003-11-10
Salt Lake City, UT

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said by 8744675:

Even with a new backup battery, it starts beeping on cold mornings.

I had exactly the same problem. It turned out that it was not the battery in the beeping unit, but a battery in the "first" unit which was in a different room and which did not beep was bad/dead. When I replaced that battery, then the other ("beeping unit") became silent.

Nick_L
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join:2003-01-22
Pittsburgh, PA

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said by 8744675:

I'm trying to figure out how to replace it without having to buy a whole new unit and base, and rewire it into the box.

Nine times out of ten, you cannot. Most smoke alarms are not designed to have a replaceable chamber. You must replace the whole unit. Additionally, you must replace the unit with the same (or listed compatible) model or linked alarms will not sound correctly.
ncbill
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join:2007-01-23
Winston Salem, NC

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Just replace the unit after a decade.

I upgraded my hard-wired AC-only ionization units to ionization/photoelectric AC w/ battery backup units.

Put in lithium 9V batteries - when those need replacing, it's time to replace the detectors again.

tmh
@myvzw.com

tmh

Anon

said by ncbill:

Put in lithium 9V batteries - when those need replacing, it's time to replace the detectors again.

I have 3 units that run on lithium batteries. The longest they lasted was four years. Nowhere close to the claimed 10 year battery life.

tschmidt
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said by Bobcat79:

Gently vacuum your smoke detectors,

I think Bobcat hit a home run. Problem is more likely dirt build up then half life issues.

»www.epa.gov/rpdweb00/sou ··· ion.html

»en.wikipedia.org/wiki/Americium

Personally have not noticed any aging problems with our hardwired ionization smoke detectors. We built our house in 1982. Got around to replacing them in 2007 with battery backup units.

/tom

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

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Jack_in_VA

Premium Member

said by tschmidt:

I think Bobcat hit a home run. Problem is more likely dirt build up then half life issues.

»www.epa.gov/rpdweb00/sou ··· ion.html

»en.wikipedia.org/wiki/Americium

Personally have not noticed any aging problems with our hardwired ionization smoke detectors. We built our house in 1982. Got around to replacing them in 2007 with battery backup units.

/tom

I contracted ADT to maintain our Fire Alarm System in the plant. One test they did monthly was an actual test of the detectors. A given area was tested each month so that all would be physically tested once per year.

They used a smoke wand. It seems that a detector can act like it's ok but be absolutely blind to smoke. Yes dirt and grime does effect them.
TheMG
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TheMG

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said by Jack_in_VA:

They used a smoke wand. It seems that a detector can act like it's ok but be absolutely blind to smoke. Yes dirt and grime does effect them.

Which is why when I test my detectors I use a wooden match by blowing out the match and then holding the smoldering match about a foot under the detector so that smoke rises up into it.

The test button doesn't tell you anything other than the buzzer is functioning.

tmh
@verizon.net

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Anon

to TheMG
said by TheMG:

Logic says that the latter would be true, since with the decay of the radioactive material would cause a reduction in the electrical current in the detector.

Note that I am not seeking reasons not to replace old smoke detectors, nor am I suggesting anyone not replace their detectors regularly. I am merely seeking the truth for the sake of knowing the truth.

From Wikipedia (»en.wikipedia.org/wiki/Sm ··· detector)

The NFPA strongly recommends the replacement of home smoke alarms every 10 years. Smoke alarms become less reliable with time, primarily due to aging of their electronic components, making them susceptible to nuisance false alarms. In ionization type alarms, decay of the 241Am radioactive source is a negligible factor, as its half-life is far greater than the expected useful life of the alarm unit.


John Galt6
Forward, March
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John Galt6 to tmh

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said by tmh :

I have 3 units that run on lithium batteries. The longest they lasted was four years. Nowhere close to the claimed 10 year battery life.

The batteries have a 10 year shelf life...not a usage life of 10 years.

dolphins
Clean Up Our Oceans
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join:2001-08-22
Westville, NJ

dolphins to Bobcat79

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said by Bobcat79:

Gently vacuum your smoke detectors, and I bet your false alarms go away.

No false alarms since vacuuming. Thanks for that tidbit of info.
Bobcat79
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join:2001-02-04

Bobcat79

Premium Member

said by dolphins:

said by Bobcat79:

Gently vacuum your smoke detectors, and I bet your false alarms go away.

No false alarms since vacuuming. Thanks for that tidbit of info.

When mine get dirty, they start going off when the humidity is high, e.g., when I take a shower. That's how I know they need cleaning.
gardengal
join:2008-10-09
Lunenburg, MA

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I had a false alarm awhile ago that I couldn't figure out. I changed the battery and it has been fine since. Maybe the battery needed vacuuming
Bobcat79
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join:2001-02-04

Bobcat79

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I vacuum my smoke detectors when I change the batteries.