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|Challenges in Estimating Smoke Detector Response|
Issue 56: Challenges in Estimating Smoke Detector Response
By James A. Milke, Ph.D., P.E., FSFPE
An engineering approach for estimating the response of smoke detectors to flaming fires using temperature rise, obscuration and velocity is included in Annex B of NFPA 72.1 Given that neither ionization nor photoelectric smoke detectors respond to conditions represented by any of these three parameters, inherent errors might be anticipated when using these guidelines for estimating the response of smoke detectors.2
While it's easily recognized that smoke detectors do not respond to thermal or velocity conditions, it may not be so apparent that light obscuration is also irrelevant. Contemporary photoelectric smoke detectors respond based on the scattering of light caused by smoke particles.3 The response of smoke detectors incorporating ionization and photoelectric technologies depends on the distribution of particle sizes and concentration of particles in the smoke, as presented in Table 1.4
Table 1. Relationship of Smoke Particle Characteristics and Smoke Detector Sensitivity
The relationship between optical density (i.e. light obscuration) and the size and concentration of smoke particles is:
Comparing the proportionalities in Table 1 with that in equation (1), the disconnect between light obscuration, light scattering and ionization chamber response is readily apparent.
Heskestad and Delichatsios suggested values of the optical density that coincided with smoke detector response in their experimental program. These optical densities are noted in NFPA 721 and can be converted into the obscuration levels included in Table 2.
Table 2. Obscuration levels for response of smoke detectors to flaming fires [%/f (%/m)]
Data of smoke parameters and smoke detector response from five large-scale experimental programs were reviewed (see Table 4). A wide variety of fuels were included in the programs.
Table 4. Summary of Experimental Programs
While the wide variation in obscuration levels is due to the lack of a relationship between light obscuration and the detection mechanisms, another factor is the transient nature of the experiments. The transient variation in light obscuration during the shredded paper test in the UMD/UL experimental program is depicted in Figure 2. Ionization and photoelectric smoke detectors were in close proximity to each other on the ceiling of the test room. The "I" and the "P" indicate the obscuration levels in the vicinity of the exterior of an ionization smoke detector ("I") and a photoelectric smoke detector ("P") when they activated.
If only the obscuration levels at the time of smoke detector response
were reported, a conclusion could be developed to suggest that an
appropriate threshold obscuration level to depict the response of a
photoelectric smoke detector is 2.5%/ft. However, the first time the
light obscuration reached 2.5%/ft (8%/m) in this experiment was
approximately 80 seconds after ignition. The photoelectric smoke
detector responded at approximately 140 sec after ignition, with the
maximum obscuration level prior to activation being well in excess of
The smoke detector responses in the five programs strongly depend on
the characteristics of the smoke and in some cases the detector
technology. As such, proposing a single set of guidelines to estimate
detector response is very difficult. Guidelines are proposed using 80th
percentile values of the smoke parameters measured at the time of
response for combinations of mode of combustion (flaming vs.
non-flaming) ventilation (none, 6 ACH and 12 ACH) and type of detector
technology (ionization vs. photoelectric).
For flaming fires, the 80th percentile values of the obscuration
level at the time of detector response in the experimental programs are
presented in Figure 3.
Suggestions on guidelines for estimating smoke detector response using temperature rise and velocity are included in the research report.11,12
James A. Milke is with the University of Maryland
1st Quarter 2011 - Detection of Forest and Wildlife Fires -- NEMA
1st Quarter 2011 - Fixed Fire Fighting Systems in Road Tunnels -- Andreas HÃ¼ggkvist
Winter 2009 - Fire Detection and Alarm Systems in Building Under Construction -- NEMA
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