Inside the Ion/Photo smoke alarm, there are two stand-alone smoke detectors: An Ion type smoke detector and a Photo type smoke detector. Each one has its own smoke detection chamber. Their purpose is to detect smoke, but really, they are both particles-in-the-air detectors.
The Ion detector is most sensitive to small particles, the ones that are so small they can't be easily seen. It has a little bit of radioactive material. When air enters the Ion's detection chamber, the air is ionized by the radioactivity. The air, then, conducts a small electrical current between two electrodes inside the chamber.
When particles enter the chamber, they block some of the electrical current. Because the current is reduced, the Ion detector calls that reduction "smoke", and sounds the alarm.
The photo detector is most sensitive to large particles, the ones that are large enough to easily reflect light. The photo's detection chamber is black so that inside it, the chamber won't reflect light. There's a light emitting diode (LED) and a light detection transistor. Separating the LED and the transistor is a black outcrop, preventing the light from the LED from reaching the transistor.
When particles enter the chamber, light is reflected off the particles. The reflected light goes around the black outcrop inside the chamber and is received by the photo transistor. Because light reaches the transistor, the detector calls that smoke, and sounds the alarm.
There's nothing to show which detector has sensed the particles, the alarm will sound if either the Ion detector or the Photo detector sense particles in the air.
It is true, a combination Ion/Photo Smoke Alarm will reduce false alarms. You are having false alarms, so that truism doesn't work in your situation.
There are three possibilities for why there are false alarms, and right now, none of them can be ruled out:
- Something in the environment is causing the smoke alarms to detect particles. This can be electrical interference or some other outside influence. This could take weeks on-site in order to confirm, using delicate instruments. Very costly.
- There is a bad manufacturing run of smoke alarms or bad design of smoke alarms. This very rarely happens, but I have known it to happen. Usually, when the manufacturer figures it out (finally) they'll recall all the bad detectors. Before there is a recall, though, replacing the smoke alarms with another model of alarm made by the same manufacturer, or changing the manufacturer could eliminate the false alarms. Before there is a recall, however, making the change is your cost.
- Particles in the air really do exist during that time of night. Having replaced a thousand two detectors, I found that Ion detectors are more prone to random alarms than Photo detectors. The cost of a Photo smoke Alarm (single detector) is less than a combination Ion/Photo Smoke Alarm (dual detector).
I recommend that you replace the combination smoke alarms that you have with a different manufacturer's Photo only smoke alarms. The reason is:
- Outside interference might not meddle with the replacement smoke alarms
- If there's a bad run or bad design, a different manufacturer won't have the same problems
- There's a reduced chance that small nighttime particles will be cause false alarms