Particulate Entry Lag in Spot-Type Smoke Detectors.
Particulate Entry Lag in Spot-Type Smoke Detectors.
(1141 K)
Cleary, T. G.; Chernovsky, A.; Grosshandler, W. L.;
Anderson, M.
Fire Safety Science. Proceedings. Sixth (6th)
International Symposium. International Association for
Fire Safety Science (IAFSS). July 5-9, 1999, Poitiers,
France, Intl. Assoc. for Fire Safety Science, Boston,
MA, Curtat, M., Editor(s), 779-790 pp, 2000.
Keywords:
fire research; fire safety; fire science; computational
fluid dynamics; smoke detectors; test methods; signals;
detection time; detection response
Abstract:
It is well documented that alarm signals from spot-type
smoke detectors (ionization and photoelectric) are
delayed when the threshold value has been achieved
outside the detector housing as a result of convective
transport of smoke through the detector to the sensing
volume. This delay must be understood in order to
properly design detection systems, improve fire modeling
where detection is a focus, and interpret analog
detector signals coming to a central panel that
processes the information. Previously researchers have
modeled the time-lag as a first order response with a
characteristic time proportional to the inverse of
velocity and a constant of proportionality defined as a
characteristic length. Here, a two-parameter model is
presented. The model parameters are a dwell time and a
characteristic mixing time. Both parameters are
correlated with velocity using a power-law equation.
Detector response experiments were performed in the Fire
Emulator/Detector Evaluator (FE/DE). Ceiling mounted
analog output detectors were exposed to near step
changes in smoke concentration over a wide range of flow
velocities. Detector signals were compared to laser
extinction measurements slightly forward of the detector
housing. Over a flow velocity range from 0.02 m/s to 0.6
m/s, the time for detectors to achieve the maximum
response ranged from seconds to 100's of seconds. The
data were used to fit model parameters that exhibit
strong velocity dependence at low velocities.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899