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Challenges in Estimating Smoke Detector Response
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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


Light scattering (LS)
Ionization chamber (MIC)

* ni, and di are the number count (density) and particle diameter for a small range of particle size "i", referred to as a "bin" in the UL study.1


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)]


Material Photoelectric Ionization
Wood Crib 3.39 (10.7) 1.14 (3.69)
Cotton fabric 1.83 (5.88) 0.12 (0.39)
Polyurethane foam 10.9 (31.5) 10.9 (31.5)
PVC 20.6 (53.1) 20.6 (53.1)


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


Experimental Program Ventilation # tests
Kemano6 None 12
Naval Research Laboratory (NRL)7,8,9 None, 12 ACH 41
Home Smoke Alarm Project (NIST)10 None 32
Smoke Characterization Project (UL)4 None 33
University of Maryland and Underwriters Laboratories (UMD-UL) 11,12 None, 12 ACH 88

In all of the experimental programs, smoke detectors activated at a wide range of obscuration levels. As an example, obscuration measurements in the vicinity of the smoke detectors at the time of response of ionization and photoelectric smoke detectors from the NIST project are indicated in Figure 1.

  Figure 1. Obscuration Levels at the Time of Smoke Detector Response10


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 2.5%/ft (8%/m).

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).

Figure 2. Light Obscuration in Shredded Paper Test11,12

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.

Figure 3. 80th Percentile Values of Light Obscuration at the Time of Smoke Detector Response11,12


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



  1. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2010.
  2. Schifiliti, R. and Pucci, W., "Fire Detection Modeling, State of the Art," Fire Detection Institute, 1996.
  3. Custer, R, , Meacham, B., and Schifiliti, R., "Design of Detection Systems," SFPE Handbook of Fire Protection Engineering, 4th Ed., National Fire Protection Association, Quincy, MA, 2008.
  4. Fabian,T., Gandhi,P., Patty, P and Chapin, J., "Smoke Characterization Project: Technical Report," Fire Protection Research Foundation, Quincy, MA, April 2007.
  5. Heskestad, G. and Delichatsios, M., "Environments of Fire Detectors – Phase 1: Effect of Fire Size, Ceiling Height and Material," Measurements Vol I (NBS-GCR-77-86), Analysis Vol II (NBS-GCR-77-95), National Institute of Standards and Technology, Gaithersburg, MD, 1977.
  6. Su, J. Crampton, G., Carpenter, D., McCartney, C. and .Leroux, C. "Kemano Fire Studies - Part 1: Response of Residential Smoke Detectors, Research Report," National Research Council Canada, Ottawa, 2003.
  7. Gottuk, D., et al., "Identification of Fire Signatures for Shipboard Multi-criteria Fire Detection Systems," Naval Research Laboratory, Washington, 1999.
  8. Rose-Pehrsson, S. et al., "Multi-Criteria Detection Systems Using a Probabilistic Neural Network," Sensors and Actuators, B 69, 325-335, 2000.
  9. Wong,, J. et al., "Results of Multi-Criteria Fire Detection System Tests," Naval Research Laboratory, Washington, 2000.
  10. Bukowski, R., et al., "Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings," NIST TN 1455-1, National Institute of Standards and Technology, Gaithersburg, MD, 2008.
  11. Mowrer, F., Milke, J., and Gandhi, P., "Validation of a Smoke Detection Performance Prediction Methodology, Volume 2. Large-scale room fire tests," Fire Protection Research Foundation, Quincy, MA, 2008.
  12. Mowrer, F., Milke, J., and Gandhi, P., "Validation of a Smoke Detection Performance Prediction Methodology, Volume 3. Evaluation of Smoke Detector Performance," Fire Protection Research Foundation, Quincy, MA, 2008.

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