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ION VS. PHOTO - IS THERE A “QUALITATIVE” DIFFERENCE FOR LIFE SAFETY?. JOSEPH M. FLEMING DEPUTY CHIEF BOSTON FIRE DEPT. FOR CAL STATE FIRE MARSHAL TASK GROUP APRIL 2011. SOME BASIC INFORMATION -ION.
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JOSEPH M. FLEMING
BOSTON FIRE DEPT.
CAL STATE FIRE MARSHAL TASK GROUP
An ionization chamber is very simple. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation. The alpha particles generated by the americium ionize the air in the chamber. The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage (opposites attract, just like with magnets). The electronics in the smoke alarm sense the small amount of electrical current that these electrons and ions moving toward the plates represent.
When smoke enters the ionization chamber, it disrupts this current -- the smoke particles attach to the ions and neutralize them. The smoke alarm senses the drop in current between the plates and sets off the horn
In the normal case, the light from the light source on the left shoots straight across and misses the sensor. When smoke enters the chamber, however, the smoke particles scatter the light and some amount of light hits the sensor: The sensor then sets off the horn in the smoke alarm.
Photos are about 10X less susceptible to “small” nuisance smoke.
Ionization is approx 20 times more sensitive to particles with 0.2 micrometer diam. (toast), than particles with 1.0 micrometer diam. (smoldering).
A = beam photo B = spot photo C = ionization
“When either ionization or photoelectric smoke alarms are located outside bedrooms and on each level of a house, they provide adequate warning to allow occupants to evacuate through their normal egress routes in most residential fire scenarios”. (NIST Review of alarm Studies, Fire Journal 1993.)
This same study was referenced by NIST at the CPSC Meetings to plane the new NIST project and is still used by NIST in public presentation. (It is also referenced by BRK in their letter to the Council.)
Either ion or photo provide adequate escape time (average of 17 minutes)
Higher sensitivity provided more escape time (esp. for smoldering)
Heat provided little life saving potential
Forced air heat spread smoke into closed bedrooms; fires in closed bedrooms were lethal to occupants
Response of 2nd level alarms inadequate for 1st level fires (every level requirement)
alarms at each end of a long hallway
Ion faster on flaming, photo faster on smoldering
No difference in ceiling or wall (<12in from ceiling) mount
Bukowski, R.W., Christian, W.J. and Waterman, T.E., alarm Siting and Sensitivity Requirements for Dwellings, available at http://smokealarm.nist.gov/
Los Angeles FD (1963 and 1978)
Bloomington MN FD (1969)
Japan Housing Corp (1974)
FMRC (apartments 1974)
Indiana Dunes (Phase 2, 1976)
Minneapolis MN FD (1978)
Australian Dept. of Housing and Construction (1979)
In all, 10 studies in 4 countries over 20 years involving 206 experiments with real alarms in real homes with real fuels reached consistent conclusions
Bukowski, R. W., Studies Assess Performance of Residential alarms,
NFPA Journal, 87, 1, pp 48-54, (1993).
N/I means prior to 1991 but, not included in NIST Study.
Why is Conclusion 4 more important
than Conclusion 1?
THIS WOULD APPEAR TO CONTRADICT 8 PREVIOUS TESTS WITH SIMILAR CONDITIONS (I.E. TEST THAT SMOLDERED MODERN FURN. >30 MINS) FROM 5 DIFFERENT COUNTRIES OVER A 30 YEAR PERIOD THAT FOUND ION INADEQUATE FOR SMOLDERING,
- DOES IT?
“A report from the Commerce Department’s National Institute of Standards and Technology (NIST) today stated that both types of commercially available home smoke alarms (also called smoke “alarms”) consistently provide people enough time to escape most residential fires.” - NIST Press Release
Photo(3-4% O/ft), Ion 2 - (17-19% O/ft), Ion 3 - (20-22% O/ft)
“The information obtained in these tests will provide a basis for numerous public fire safety
messages and will quantify the protection provided by residential smoke alarms. Videos will be
made available for use in public safety announcements and educational materials.” (From page 9 of NIST Smoke Alarm Report.)
TO MY KNOWLEDGE THE VIDEOS HAVE NEVER BEEN MADE AVAILABLE. - JAY FLEMING
What NIST called a flaming fire was an “ultra-fast” fire. (This would be rare while occupants sleeping.)
For cooking, the most common “normal” flaming fire, the photo was slower than on but still provided 10 mins. ASET.
Ion failed in many smoldering tests. (Even though NIST did not measure tenability along paths of egress.)
Note: During the NIST Smoke Alarm Tests the
flaming fire was started by an electronic “match”
(not a propane burner) and the heating element in
the smoldering tests was placed within material
and no cotton was laid on top – Jay Fleming
FIRE DEATHS WERE DECREASING BEFORE WIDESPREAD USE OF ALARMS AND CONTINUED TO DECLINE AFTER “MARKET SATURATION”.
% OF FATAL FIRES WITH
% OF HOMES WITH ALARMS
% OF FIRES WITH WORKING ALARMS
FROM 1994 – 2001
% OF FATAL FIRES WITH WORKING SMOKE ALARMS INCREASED 100%
% OF HOMES WITH SMOKE ALARMS INCREASED 2%
% OF FIRE WITH WORKING SMOKE ALARMS INCREASED 12%
This trend should have signaled a problem by the mid 80’s.
FROM 1980 - 2001 THERE IS NO CHANGE - ALARMS HAVE MADE NO APPARENT DIFFERENCE IN RISK, AS DEFINED BY NFPA.
In the late 70’s approximately 6,200 people dies per year in homes.
According to the NFPA:
If no one had alarms residential fatalities = 4,230.
If everyone had alarms resid fatalities = 2,430.
Actual ave for 1999-2001 = 3,140 fatalities per year.
According to the NFPA, fatalities would have decreased by approx 2,000 people per year without any smoke alarms! (2/3 of total.)
Data from NFPA Smoke alarm Study 11/04.
I do not understand why this is still even being debated? Even the manufacturers acknowledged the advantage of photoelectrics years ago. – Jay Fleming
What about now?
“The typical industry-controlled code or standard is formulated by a committee elected or appointed by a committee elected or appointed by a technical society or similar group. Many of the committee members are drawn from the manufacturers to whom the code is to be applied. Others are drawn from engineering consulting firms and various Government organizations. However, since near unanimous agreement in the committee must generally be obtained to set requirements or to change them, the code represents a minimum level of requirements that is acceptable to industry.”
“In a subtle way, the use of industry codes or standards tends to create a false sense of security. Described by code committees and by the language of many codes themselves as safety rules, they tend to inhibit those legally responsible for protecting the public from taking the necessary action to safeguard health and well being. Many states and municipalities have incorporated these codes into their laws, thus, in effect delegating to code committees their own responsibility for protecting the public.”
Photoelectric alarms might reduce by 1/2 the # of people dying in fires, when the alarm works. (This would be a 20% reduction.)
Photoelectric alarms might reduce the number of disabled alarms due to nuisance alarms. (Assume problem reduced by 3/4 - 15% reduction)
It seems reasonable to assume that switching from ionization to photoelectric technology could save 900 lives (.35 * 2,565) per year!
This number could be higher, if # of fatalities that occur when no smoke alarm present is over-estimated. (Many Chief’s assume that if occupants died then the smoke alarm wasn’t there - good PR opportunity.