Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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

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 door.

Companies that do inspections of buildings to make sure the fire
extinguishers, smoke detectors, exit lights etc. are in working order...
like 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

-----Original Message-----
From: Ludwik Kowalski
Sent: Wednesday, September 27, 2006 9:07 PM

According to:

" . . . The alpha particles emitted by the Am-241 collide with the
oxygen and nitrogen in air in the detector's ionisation chamber to
produce charged particles called ions. A low-level electric voltage
applied across the chamber is used to collect these ions, causing a
steady small electric current to flow between two electrodes. When smoke
enters the space between the electrodes, the alpha radiation is absorbed
by smoke particles. This causes the rate of ionisation of the air and
therefore the electric current to fall, which sets off an alarm. . . .

1) Why is the current reduced sufficiently when a small fraction of CO2
molecules is mixed with air?

2) Would an alarm also be triggered by presence of unburned (from a
leak) cooking gas in air?