Performance of Home Smoke Alarms - National Institute of ...

Thus, within the range typical ofionization smoke alarms, the responseis within the limits of the UL 217 test.Residential photoelectric alarmstypically alarm at transmittances greaterthan 0.84 in the UL test. While theFE/DE smoke deviates outside the MIClimits at a transmittance of about 0.9,this observation is not by itself a causefor concern.Laser Light Transmittance(1.52 m pathlength)0.80.850.90.95The deviation between UL and FE/DEtest smoke results at lower lighttransmittance values suggests a smokesize distribution difference between thetwo smolder smokes. Since the UL11001790807060MIC output (pA)Figure 8. UL grey smoke (smolder smoke) sensitivitylimits and FE/DE datasmoke box is a re-circulating system, it can take between 12 min to 17 min to reach a lighttransmittance value of 0.8. The FE/DE is an open, single-pass flow system and the smoketransport time for these calibration tests is on the order of 30 s. Therefore, potentially moresmoke aging (i.e. coagulation growth ofthe smolder aerosol) can take place inthe smoke box relative to the FE/DE.Assuming that fresh smolder smokes areidentical in the smoke box and theFE/DE, additional coagulation growth inthe smoke box will tend to increase lightextinction while decreasing the MICoutput for a fixed amount of smoke.This could explain MIC differencebetween the two sources at lowertransmittances. The difference betweenrelative levels of obscuration and lightscattering of fresh and aged smoldersmokes should be less pronounced.There are two reasons for thissupposition. First, coagulation growthtends to incorporate the smallestparticles into bigger particles. TheLaser Light Transmittance(1.52 m pathlength)0.60.650.70.750.80.850.90.95110090807060MIC output (pA)Figure 9. UL black smoke sensitivity limits andcorresponding FE/DE datasmaller particles scatter little light, and contribute almost nothing to obscuration, thus they arenot missed. Second, the growth of particles that scatter significant amounts of light will tend to505040403030