SMOKE DETECTORS...

John writes....

So, I was taking a ridiculously hot shower the other day and found that I actually set off the fire alarm- not with my hotness- but with all of the steam the hot water was producing. I knew smoke detectors were sensitive, but why would it confuse steam with smoke? How exactly does a smoke detector work? I fear I may have blinded the electric eye in mine.

There are two types of smoke detectors. One type is the photoelectric detector that uses a beam of light to see minute particles in the air, and the other is the ionization detector.

The photoelectric detector uses a beam of light and a photo sensor. When smoke (or other particulate as you observed) passes into the light beam, some of the light reflects onto the sensor sending the unit into alarm.

Ionization detectors use an ionization chamber. Inside is a small amount (perhaps 1/5000th of a gram) of americium-241. The radioactive element americium has a half-life of 432 years, and is a good source of alpha particles. The alpha particles generated by the americium ionize the oxygen and nitrogen atoms of the air in the chamber. To "ionize" means to "knock an electron off of." When you knock an electron off of an atom, you end up with a free electron (with a negative charge) and an atom missing one electron (with a positive charge). The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent.

When smoke enters the ionization chamber, it disrupts this current -- the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn.

Most household detectors are of the ionization type. They are generally less expensive and are better at detecting flames. The photoelectric detector has the advantage of detecting smoke better so if something is smoldering, the photo detector is more effective. If it is flaming, the ionization detector is better. For the best protection, you should have both.

Yes, the detectors will detect smoke, steam, and even dust(Bonny will often set the detector off with her hotness). On construction sites, when we are remodeling an existing structure, the first thing we do is disable the system or cover the detectors in the area that we are working so as not to set off the alarm. And, yes, it is possible, though not probable that you could have "blinded the eye" of your detector. The best way to tell is to push the test button on your detector. If you really want to make sure, light up one of those cigars your always puffing on and hold it under the detector. Some fire marshals will not pass a system by using the test buttons though many do. There is a product that is a spray can of simulated smoke to test alarms. The one time I installed a system in a federal building, it was specified that each detector had to be tested with real smoke. The alarm company had a fancy device that used a stick of incense to deliver smoke for testing.