Ion vs. Photoelectric: A Historical View
Smoke detection arrived in the United States in 1951 when Cerberus GmbH of Switzerland signed a contract with the manufacturer CO-Two for distribution of the Cerberus ionization smoke detector. Following is a partial historical account of fire detection development in the U.S. since then, including some key bulleted dates.
1958 — A fire at Our Lady of the Angels, a Catholic elementary school in Chicago, claimed the lives of 92 children and three teachers. The tragedy inspired a massive call for better and more sensitive fire detection.
1967 — The National Fire Protection Association (NFPA) published a guide for residential fire detection, which recommended a heat detector in every room wired to a control panel. No mention of smoke detectors was made in the document.
1969 — BRK Electronics of Aurora, Ill., received a UL Listing for a battery-operated smoke detector (SS69H) utilizing a resistance-grid sensor and heat detectors. The device, which could be mounted on a wall or ceiling, was actually listed as a “control panel.”
1970 — Statitrol Corp. developed and sold a battery-operated ionization smoke detector. This was followed by AC-operated photoelectric smoke detectors from Pyrotector and Electro Signal Labs (ESL).
1974 — NFPA 74 (“Standard on the Installation, Maintenance and Use of Household Fire Warning Equipment”) was issued, opening the market for residential smoke detection with single-station detectors that no longer had to be wired to a control panel.
The ability to sell UL-Listed, single-station smoke detectors led to a flood of brands entering the market, including GE, Gillette, Norelco and First Alert. All of these products were ionization-type detectors. The percentage of homes outfitted with smoke detectors rocketed from a miniscule number in 1970 to greater than 50% by 1980.
The manufacturers of photoelectric type detectors were hungry to get a share of the market. Their approach: show that photo detectors are better at detecting smoldering fires. They did this by staging demonstrations where cotton was placed on a light bulb and allowed to smolder. Hence began the controversy between the two device types.
At the urging of many stakeholders in the 1970s, UL decided to combine the commercial test standards that governed ionization and photoelectric technologies. The objective was to combine UL 168 (Commercial Photoelectric Smoke Detectors) and UL 167 (Commercial Ionization Smoke Detectors) into one standard that would cover single and multiple-station detectors.
The toughest task was to include large-scale fire tests that both types of sensors could respond to within prescribed time limits. UL 168 had never had full-scale fire test requirements. The new standard was to be UL 217, Single and Multiple-station Smoke Alarms.
In 1974, UL ran fire tests to determine the ability of each type of detector to pass. Six test scenarios were attempted with different materials: shredded paper, polystyrene, gasoline, wood, cotton and polyurethane. Both types of sensors responded well enough to the polystyrene, gasoline and polyurethane fires; however, there were detection issues with the other three materials.
The conclusion, as reported by UL, was unambiguous: “ … neither detector does well in all types of fires.” This only added more proverbial fuel to the growing dispute between the technologies. Thus, the tests were significantly modified so that each type of technology could pass the shredded paper and wood brand tests. Doing so allowed one standard to be released with the understanding that “neither detector does well in all types of fires,” according to a UL report.
Throughout the 1970s manufacturers of each technology battled for market share. White papers were published and skewed demonstrations performed. Lawsuits amounting to millions of dollars were pursued when smoke detectors did not “alarm fast enough.” And most significantly, confusion reigned among the fire protection community as well as with end users.
The 1980s witnessed the release of a detector that had both an ionization sensor and a photoelectric sensor in one unit. It attempted to solve the controversy except that its high price point hampered the device’s market share potential. Comparatively, by the late 1980s the basic ionization models had fallen below $10.
In the 1990s the requirements for a detector in each bedroom was added to UL 217, as well as the requirement that all hardwired detectors had to have battery backup. Still no one had developed an inexpensive detector that quickly detects both smoldering and flaming fires.
Among the objectives of a smoke characterization study conducted by UL in 2007 were to develop recommendations to UL 217 and allow for the development of new smoke-sensing technologies. As a result of UL’s project, the following was identified for future consideration: “Requiring the use of combination ionization and photoelectric alarms for residential use in order to maximize responsiveness to a broad range of fires.”
The reason for this recommendation was that “Some of the evaluated flaming and non-flaming test scenarios triggered one but not both photo and ion alarms within the alarm response time criteria specified in UL 217.”
Hence, the controversy perpetuates today.
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