Ionisation Recommended

For minimum protection the number of smoke alarms to be fitted will depend on the type of home you live in:

• If your home is on one floor, at least one smoke alarm, preferably of the optical type, may be enough to provide you with early warning of a fire.
• If your home has more than one floor, at least one smoke alarm should be fitted on each level. In this case a combination of Optical and Ionisation smoke alarms, preferably interconnected, will give the best protection.
• A smoke alarm should be fitted every 7.5 meters of hallways and rooms
• A smoke alarm should be located within 3 metres of bedroom door.
• Preferably all smoke alarms should be interconnected.

For maximum protection the number of smoke alarms to be fitted will depend on the type of home you live in:

• A smoke alarm should be fitted in every room (except kitchen, bathroom and garage)
• A smoke alarm should be fitted every 7.5 meters of hallways and rooms.
• A smoke alarm should be located within 3 metres of bedroom door.
• Preferably all smoke alarms should be interconnected.

There are two main types of smoke detectors: ionization detectors and photoelectric detectors. A smoke alarm uses one or both methods, sometimes plus a heat detector, to warn of a fire. The devices may be powered by a 9-volt battery, lithium battery, or 120-volt house wiring.

Ionization Detectors

Ionization detectors have an ionization chamber and a source of ionizing radiation. The source of ionizing radiation is a minute quantity of americium-241 (perhaps 1/5000th of a gram), which is a source of alpha particles (helium nuclei). The ionization chamber consists of two plates separated by about a centimeter. The battery applies a voltage to the plates, charging one plate positive and the other plate negative. Alpha particles constantly released by the americium knock electrons off of the atoms in the air, ionizing the oxygen and nitrogen atoms in the chamber.
The positively-charged oxygen and nitrogen atoms are attracted to the negative plate and the electrons are attracted to the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The drop in current between the plates triggers the alarm.

Ion ventricle can be classified into internal ion ventricle (reference ionization ventricle) and external ventricle (testionization ventricle), and their ionization radiations are provided by the same source. Under certain voltage, the internaland external ion ventricles are at a balance of the work electric current. Refer to Figure 1, point A is the electric currentbalance point of ionization in the clean air. While Figure 2 denotes that balance point moves to the point A?after enteringsmoke, the movement from point A to point A?leads to the change of the balance voltage of the collection electrode andthe change is enough to trigger alarm circuit to realize the destination of fire alarm.

Occasionally, you will walk into a store and a bell will go off as you cross the threshold. If you look, you will often notice that a photo beam detector is being used. Near the door on one side of the store is a light (either a white light and a lens or a low-power laser), and on the other side is a photodetector that can "see" the light.

When you cross the beam of light, you block it. The photodetector senses the lack of light and triggers a bell. You can imagine how this same type of sensor could act as a smoke detector. If it ever got smoky enough in the store to block the light beam sufficiently, the bell would go off. But there are two problems here:

1.It's a pretty big smoke detector.
2.It is not very sensitive.

There would have to be a LOT of smoke before the alarm would go off -- the smoke would have to be thick enough to completely block out the light. It takes quite a bit of smoke to do that.
Photoelectric smoke detectors therefore use light in a different way. Inside the smoke detector there is a light and a sensor, but they are positioned at 90-degree angles to one another, like this:

In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor: