We had what seemed to be an overly sensitive smoke detector that would even go off when boiling water. Got sick of it and bought a new one that said on the packaging that it didn’t respond to steam. Sounded great except that a week later we discovered it didn’t respond to smoke either (burnt some cooking, lots of smoke) checked the battery, all that, it should have gone off with the smoke.
In: Engineering
Older ones have a radioactive sample inside releasing alpha particles. These are large particle that easily get blocked by particles in the air. They have a detector on the other side sensing the particles. When smoke gets in the middle, it blocks the particles. However steam will also block the particles, so set it off too.
There are two basic kinds – one uses a beam of light and the other uses small sample of radioactive material.
u/KillerOfSouls665’s answer kind of combines the operation of two systems together, so they aren’t wrong, but they aren’t right either.
The light based ones use beam of light going ‘nowhere’ and have a light-seeing sensor placed somewhere else. Normally the light goes nowhere so the sensor can’t see it and that’s fine. If smoke particles get up into the detector they scatter the beam of light and now some of the light will hit the sensor which completes a circuit to trigger the alarm.
The radiation based ones have two small plates that *almost* touch each other. The radiation the material releases changes the air molecules in a way that allows electricity to travel through the air. So energy leaves one plate and jumps to the other plate and a circuit is formed that’s great. When smoke gets into the air it stops the process that allows the air molecules to carry an electric charge, so no energy jumps from plate to plate and the circuit is broken. The broken circuit triggers the alarm.
[As Per the NFPA](https://www.nfpa.org/education-and-research/home-fire-safety/smoke-alarms/ionization-vs-photoelectric)
The YouTube channel “Technology Connections” has an excellent video on the matter.
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