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Re: [Phys-l] Ionization type smoke detectors.

Ed, thank you for the informative explanation of the background
science behind the operation of ionization smoke detectors.

David Bowman

Below my post is the original post submitted by Ludwik. I believe
the error in the passage that Ludwik quoted is the sentence that
reads, "the alpha radiation is absorbed by smoke particles."

The gaseous products of combustion do not have terrifically more
stopping power for alphas than ordinary air. The ion current does
not diminish because the alpha radiation is absorbed.

Products of combustion contain a fair number of conglomerated atoms
(products of incomplete combustion) and also a fair number of free
radicals (molecules with an unpaired electron), and also a fair
number of ions.

All of these combustion products are pretty good at combining with
the ionized air (nitrogen and oxygen) that the alphas produced. So
it is not that the combustion products stop the alphas; rather, they
stop the ionized air.

But why does this "scavenging" of ionized oxygen and nitrogen stop
or reduce the ion current? The current being monitored by the
smoke-detector electronics is the current of ionized oxygen and
nitrogen moving between the electrodes of the ionization cell. In
this situation, the electron or hole current in the electronics can
be viewed as the "image current" of the charges moving across the
electrode gap. The size of the measured current is determined not
only by the number of moving charges, but also by the velocity of
the moving charges.

When the products of combustion (POCs) enter the smoke detector
chamber and the ionized oxygen and nitrogen become attached to
these POCs, there are still lots of ions present, but the ionized
oxygen and nitrogen, now attached to the POCs, move much more slowly
in the applied electric field because the effective mass of each ion
has increased tremendously. So the ions are still there, but their
mobility has gone down dramatically, and that means the measured ion
current goes down dramatically, and the alarm is sounded.

Ludwik mentioned CO2. CO2 is a POC in most cases, but it is not a
tremendous scavenger for ionized O2 and N2. Presence of CO2 can set
off an ionization smoke detector, but it takes a fair amount. The
products of incomplete combustion (PICs) which are larger molecules
and conglomerates of molecules are much better ion scavengers.
These PICs are mostly what is slowing down the ion current in the
chamber. Likewise, cooking gas (methane or propane) can trigger the
alarm, but not as readily as PICs. Propane, being a larger more
polarizable molecule would be better than methane at scavenging ions
and tripping the alarm. I once had a smoke detector near a
bathroom, and water vapor from the shower would sometimes set it
off. I eventually moved the detector further from the bathroom

Companies that do inspections of buildings to make sure the fire
extinguishers, smoke detectors, exit lights etc. are in working to check the smoke detectors at actually detecting
something rather than simply pushing the test button on the alarm.
Rather than using a fire, they have a spray can of Freon. The
various Freons are pretty good at scavenging the ionized O2 and N2,
and a short squirt of Freon should set off an ionization smoke
detector in a hurry.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817