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

Here's a rather simple experiment for a student. Put a smoke detector ina pressure cooker attach a bicycle pump (one w/ an integral P. gauge) and see if they can get it to "go off". Ideally they are designed so doubling the density would be quite enuff. If not, then better to have a long hose for remote pumping! Tho, I suspect the old humongous ones can easily withstand 100#/in^2. That'd be ~ quadrupling the density?

I don't think they detect CO, CH4, or CO2, the electron density is ~ 1/3 greater CO2 / O2, not enuff. Furthermore, the ionization energy for C is 11.3 vs. 13.6 eV for O. Which leads to another experiment using a home made flow chamber. The web has a spec. sheet for the smoke detector's IC. One may be able to measure the current. If not, it's an easy matter to make an electrometer using an op amp instead of the IC.

bc, who is reminded he used a "hot" polonium source to measure the ion current in liquid and directionally frozen argon.


Brian Whatcott wrote:

At 08:07 PM 9/27/2006, you wrote:

According to: http://www.uic.com.au/nip35.htm

" . . . 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?


Because the stopping power (mass per cross section area) of a
soot particle is greater than O2 or N2 ??


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

Ludwik Kowalski


I'm guessing not on the basis of the same parameter.



Brian Whatcott Altus OK Eureka!

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