Comparisons of two different commercially available photoelectric smoke detector models found significant differences in how the detectors responded to black smoke from flammable liquid fires, according to a recent study.

Results of the tests, using UL 268-type fire tests, showed that the differences were due to the fact that the smoke chamber on one smoke detector design (Type A) used an antistatic acrylonitrile butadiene styrene plastic and the other detector (Type B) used a standard, non-antistatic ABS for its smoke chamber. The tests were reported in the October 2005 issue of Fire Technology. Manufacturers of Type A and Type B detectors were not identified in the paper.

“Throughout the entire flammable liquid fire test, the signal level from the antistatic ABS detector tended to be well correlated with the actual smoke density in the room, whereas signal levels from the non-antistatic ABS detector dropped sharply during the post-flaming phase and lost correlation with the continued high smoke densities present in the fire room,” said lead author James R. Qualey, principal systems engineer at Colarusso, Qualey and Assoc. Steve Penney of Tyco Safety Products co-authored the study.

In the flammable liquid fire tests, both detector types reached their alarm thresholds in about the same time, roughly 50 seconds. But their behaviors diverged significantly after the flaming phase of the fire ended, typically at about 100 seconds into the test.

Further investigation determined the cause of the puzzling behavior to be differing plastic types in the smoke chamber components, Qualey said.

Qualey said they can't prove it, but they hypothesize that during the initial flaming phase of the hydrocarbon fire, the ceiling jet drives smoke-laden air into the smoke chamber, causing the initial response. During this period, the surfaces of the non-antistatic smoke chamber may accumulate a significant surface charge, perhaps due to air friction.

“If the surface charge were of opposite polarity to that of the smoke particles, it is possible that the surfaces of the smoke chamber might attract suspended smoke particles, thus effectively clearing the air inside the scattering volume of the smoke chamber and resulting in a decrease in signal level,” he said.

Researchers double-checked their results by putting a Type A antistatic ABS smoke chamber in the Type B detector. The results reversed.

“We thoroughly showed it was the plastic making the difference,” Qualey said.

Qualey presented the results two years ago at the Fire Suppression & Detection Research Application Symposium in Orlando and was a little surprised that some smoke detector manufactures were unaware of this phenomenon.

“It was news to some manufacturers, like the manufacturer of the Type B model,” he said.

Degraded performance of the standard non-antistatic ABS Type B detector was apparent only in its response to dense black smoke produced by the flammable liquid fire test. In the smoldering smoke, wood fire and paper fire tests, which used cellulose-based fuels to produce gray smoke, little if any difference in response was observed between Type A and Type B detectors.

“Prior to the Qualey paper, the smoke detector industry did not have a quantified understanding of the benefits and impact that utilizing an antistatic material had on the smoke entry characteristics of smoke detectors,” said Bruce Fraser, director of Industry Relations, SimplexGrinnell, a unit of Tyco Fire & Security. “The paper gave us a scientific understanding of why our previous practice was important.”

Fraser said Simplex products will continue to use antistatic material in the construction of smoke detector chambers.