Smoke Detectors

[jcole]

Could someone explain to me why you would use the photoelectric smokes over the ionization smokes? This was asked to me recently by a builder and I couldn't answer him. I told him the expense was the reason why I didn't use them but I think he was wanting to know what makes the photoelectric more valuable. Can anyone help?

 

[bphgravity]

Here's a statement from the NFPA:

Most smoke alarms use one of two common sensing systems for detecting a fire.

Ionization-type smoke alarms have a small amount of radioactive material between two electrically charged plates, which ionizes the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating the alarm.

Photoelectric-type alarms aim a light source into a sensing chamber at an angle away from the sensor. Smoke enters the chamber, reflecting light onto the light sensor; triggering the alarm.

Ionization vs. photoelectric

Photoelectric alarms respond slightly faster to smoldering fires; ionization alarms respond slightly faster to flaming fires. Since, as a practical matter; you can not predict the type of fire that will occur; the slight difference is irrelevant. Either type of alarm will detect nearly every type of fire quickly. Some manufacturers offer dual-chamber alarms that use both sensor systems.

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[JohnE]

I went to a seminar conducted J. Fleming, Deputy Cheif in the Boston Fire Dept. and he provided overwhelming evidence that ionization detectors do not save lives. In a smoldering fire (99% of deaths are while people are sleeping, which is almost always caused by smoldering fire), ionization detectors go into alarm 15 - 60 minutes AFTER photo's. i.e. too late.

 

[jcole]

Thanks for the replys.