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I have been doing some research on the differences between ionization and photoelectric smoke detectors, and main differences in their capabilities, seems to be their ability to detect different kinds of smoke. I read that ionization detectors are better at sensing fast-flaming fires while photoelectric detectors are better at detecting smoldering fires.

I can understand why the first detects fast-flaming fires better: because those fires have less smoke, and the ionization detectors are more sensitive (more smoke is necessary to scatter the light in photoelectric ones). However, I do not understand why an ionization detector is less sensitive to smoldering fires. Shouldn't it be equally sensitive to a large amount of smoke too?

Eames
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    Hi! Could you structure your question a little more, mainly by adding empty lines to denote paragraphs? Also, "I read that" **always** screams for *where* you've read that. – Marcus Müller Nov 26 '17 at 15:46
  • @MarcusMüller Sure – Eames Nov 26 '17 at 15:49
  • Probably because a “smoldering fire” may not always produce lots of smoke - one can lift up a bed of leaves covering smoldering embers which has produced almost no indication of smoke and it bursts into flame as the air / oxygen hits it... – Solar Mike Nov 26 '17 at 16:55

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The smoky fires send to have larger smoke particles reaching the smoke detector before smaller particle products of combustion. The larger particles seem to be more quickly detected by photoelectric sensors. However the actual detector performance is also influenced by the detector sensitivity and other design details. The performance of various detector designs is influenced by efforts to avoid nuisance alarms from cooking, water vapor from showers and other factors. Evaluations made by testing agencies seem to be based mostly on testing rather that theoretical evaluation of the designs. Since serious injury and death seem to result more often from smoke inhalation than flame, detection of large smoke particles is given priority by some experts. Perhaps there is a study somewhere that factors in the detection capability of human and animal noses.

Reference: NFPA and pages linked there.