On 7/30/2020 6:42 PM, Gil wrote:
On 7/30/2020 1:49 PM, Fred McKenzie wrote:
In article ,
Â* Mercellus Bohren wrote:

do they ever need to be completely replaced?

The fire alarm people say yes.Â* Apparently there is some radioactive
material involved, that deteriorates over several years.

Fred

*Ionization smoke alarms* require replacement every ten years. As
mentioned by Fred, the radioactive particle in them deteriorates to the
point where the alarm function becomes very slow to activate. Follow the
manufacturers advice. They're not that costly to begin with if you
spread it out over the ten year life. Cheap insurance.

Whatever it is that is the basis for a 10-yr replacement period for
ionization smoke detectors, it doesn't seem likely to be owing to the
decay of the isotope. Am-241 has a half-life of 432.2 yrs.

t_half=432.2;
lambda=log(2)/t_half;
exp(-lambda*t_half) % make sure no typos, etc., ...
ans =
0.5000
exp(-lambda*10) % after 10 years
ans =
0.9841


has only lost 2% of initial activity. That wouldn't seem enough of a
sensitivity loss to me.

I looked at a NFPA study that referenced a Dallas project that tracked
new installations for a period of 10 years that claimed only 27% were
still operational after that time.

However, it did not provide comprehensive data on the causes of the
failures other than most were simply not replacing batteries or taken
down when renters moved or otherwise destroyed. An apparent other
failure mode of significance had to do with not cleaning -- I suppose if
were in kitchen or the like grease buildup and all could do it, but
again it didn't explain what needed cleaning or what the source of not
being clean was.

But source decay wasn't listed at all and if it were the problem, none
would be operational as all would decay at the same rate and have same
relative source strength (presuming the initial population all of same
vintage).

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