Detector Sensitivity Is Stated as Smoke Obscuration Per Unit Length
Smoke detection sensitivity is quoted as obscuration: the fraction of a light beam lost over a stated path length, written as a percentage per foot or per metre. A typical spot smoke detector alarms somewhere near \( 2\% \) to \( 4\% \) obscuration per foot, and an aspirating system can be set below \( 0.01\% \) per foot. The unit matters because it is a per-length figure: obscuration is defined against a path, so quoting a bare percentage is meaningless, and comparing two devices means comparing them at the same path length. It also converts a marketing phrase into arithmetic. A device claiming to be a thousand times more sensitive is claiming an alarm threshold three orders of magnitude lower in the same units, which is a checkable statement about the smallest fire it can see. The usual confusion is treating obscuration as a concentration; it is an attenuation, so a thin haze over a long path reads the same as a thick one over a short path. After this Concept you can compare two detectors' sensitivities honestly and state the smallest smoke condition each will alarm on.
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Smoke detection sensitivity is quoted as obscuration: the fraction of a light beam lost over a stated path length, written as a percentage per foot or per metre. A typical spot smoke detector alarms somewhere near \( 2\% \) to \( 4\% \) obscuration per foot, and an aspirating system can be set below \( 0.01\% \) per foot. The unit matters because it is a per-length figure: obscuration is defined against a path, so quoting a bare percentage is meaningless, and comparing two devices means comparing them at the same path length. It also converts a marketing phrase into arithmetic. A device claiming to be a thousand times more sensitive is claiming an alarm threshold three orders of magnitude lower in the same units, which is a checkable statement about the smallest fire it can see. The usual confusion is treating obscuration as a concentration; it is an attenuation, so a thin haze over a long path reads the same as a thick one over a short path. After this Concept you can compare two detectors' sensitivities honestly and state the smallest smoke condition each will alarm on.
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