3/16/2011
Why Ionization Smoke Alarms Might Fail
Dean Dennis
Ddennis1111@gmail.com
The concept of the Ionization alarm was invented by Swiss physicists Ermst Meili and Walter Jaeger in
the late 1930’s to detect gases in mines. This concept is important to note because gases consist of
small particles. Yesterday’s ionization alarm is basically today’s ionization smoke alarm. It is the
nature of smoke that makes this a problem. Smoke agglomerates as it moves away from its source,
transforming into a larger particles. This can be a problem for the ionization alarm since its technology
uses a chamber that is best at detecting small particles. So opening up an oven door while cooking,
using a nearby toaster or other cooking activities which results in a release of numerous very small
particles are events the ionization alarm chamber might sense and sound the alarm. However, a
smoldering fire, or fire not near the vicinity of the alarm can be a problem. When smoke is in the
process of cooling and moving away from its source, the ionization alarm can fail due to larger particles
and fewer charged small particles. Compare this to when smoke particles enter the chamber of a
photoelectric (light-beam) smoke alarm. Smoke entering this chamber is entering a chamber that uses
a laser beam as a sensor. The photoelectric beam “sees” the smoke particles and sounds an alarm.
This is not the case with the ionization alarm. To understand its probability for failure we need to
understand what happens when smoke enters the ionization chamber.
The ionization alarm has a chamber that utilizes two plates that are connected to a battery. These
plates are separated by about a centimeter in distance. Since these plates are connected to a battery,
one of the connected plates has a positive charge, and the other a negative charge. In between the
plates is air (primarily oxygen and nitrogen molecules). At the bottom of one of the plates is a small
sealed amount of radioactive material called Americium 241 which is in a state of rapid radioactive
decay. This radioactive material transforms itself at the rate of 39 billion times per second, and in the
process is constantly knocking the electrons off the oxygen and nitrogen molecules. This process is
called ionizing the air which gives the ionization alarm its name. Now, as the electrons get knocked off
they move to the positive plate and the electron-free oxygen and nitrogen molecules move to the
negative plate. This movement creates a current which is electronically monitored by the ionization
alarms circuit board. When there is a predetermined disruption, a signal is sent which triggers the
alarm. So, where a photoelectric alarm actually has a light source that detects physical smoke
particles, the ionization alarm utilizes a current that must be disrupted by particles. This current is
disrupted when enough particles (a function of both size and density) flood the space in the ionized
chamber and start attaching to the molecules that are seeking an electron. This disruption neutralizes
the flow of the electrons going to the positive plate, and the electron-free oxygen and nitrogen
molecules going to the negative plate. The ionization alarm’s electronics recognize this drop in current
then the alarms sounds. Since we think of the ionization alarm as a smoke detector, you would like to
think that smoke is what is most effective at tripping the alarm, but that is not necessarily the case. In
fact, as mentioned before, cooking activities, exposure to shower steam or even heavy paint fumes,
might trigger the alarm. This alarm technology relies upon a predetermined particle flow, or current,
between two plates. This is why the ionization alarm has so many inherent nuisance alarm problems,
nearly eight times as many as the photoelectric alarm. So, a number of things can trigger an ionization
alarm, but what about smoke?
Smoke may or may not trigger an ionization alarm and here is why; think of a fire as the blast from a
shotgun. If the ionization chamber is near the blast, many particles will flood the chamber, attach to
the ionized air thereby disrupting the current and the alarm will sound. Now let’s picture the
ionization chamber a distance away from the blast. As the particles move away from the blast, or
source, they spread out and become fewer. Now as these fewer particles enter the ionization chamber
they may only bond with a few of the ionized air particles and this might not be enough to disrupt the
current to sound the alarm. In fact, this is what can and does happen. In testing done by the National
Institute of Standards and Technology there have been documented instances where ionization alarms
fail to sound even when a room is filled with smoke. Keep in mind that when we can easily see smoke
it is because the particles have agglomerated and become large enough to see. Paradoxically, when
we see smoke there are less small dense particles, which is what the ionization needs to trigger.
Sometimes the ionization alarm will not sound until toxic gases are so dense and heated that a
flashover is about to occur. Sadly, this is often too late and people may have already died from smoke
inhalation. This possible delay in alarming is due to the technological design of the ionization alarm.
Many people that have researched smoke alarms believe that ionization alarms can never be a reliable
smoke alarm. Some researchers refer to them as a heat detectors. Today, ionization alarms are only
prevalent in residential settings. Commercial properties, years ago, started installing photoelectric
alarms. It is for these reasons that enlightened fire officials are recommending photoelectric alarms
for residences. Photoelectric alarms provide a safety measure against the full spectrum of fires that
ionization alarms cannot provide. This is why the largest fire organization in the World, the
International Association of Fire Fighters (300,000 members) only endorses photoelectric alarms. For
years they have told homeowners to not only change their batteries, but to also change their smoke
alarms to photoelectric alarms.