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Smoke Detectors

Smoke detectors are faster acting than heat detectors. They are frequently used in fast-acting automatic fire detection systems that incorporate an extinguishing agent release function to protect high value or highly combustible storage and work areas. Computer rooms, aircraft storage and repair areas, explosive processing areas, and telephone equipment rooms are frequently protected in this way.

Smoke-actuated detectors may be of the photoelectric type used in spot, beam, or duct designs or the ionization type, which is applied in the spot or duct design. The principle of operation is the same, regardless of design.

PHOTOELECTRIC SMOKE DETECTORS.\ Most modern photoelectric detectors of the spot type use the light-reflection principle to detect




Figure 7-11.\Typical arrangement of photoelectric smoke detector components.

smoke. The diagram in figure 7-11 shows a typical arrangement of functional parts. A pulsed light beam from a light-emitting diode (LED) with its associated optics is projected across the interior of a blackened chamber that may contain smoke to be detected. A photocell, with its optics, looks toward the projected beam along a line perpendicular to the beam. When smoke enters the chamber, the smoke particles reflect a small portion of the light beam toward the photocell, which provides a voltage to be amplified and causes an alarm. The light source may be monitored ahead of the smoke chamber and regulated to prevent variation of the light intensity from causing erratic detector behavior.

In detectors of the beam type, the light source and photocell are mounted near the ceiling on opposite sides of the protected room. When smoke obscures the light below a predetermined value at the photocell, an alarm results.

Detectors of the duct type are intended for detecting smoke in an air-handling system. A detector of this type is mounted directly on the outside of an air duct or nearby with a sampling tube extending about three quarters of the way across the inside of the duct. The air flows into the smoke detection chamber mounted on the outside of the duct, and back into the duct through a return tube, having a hole or holes directed downstream. As long as there is airflow in the duct, a portion of that air continuously flows through the detection chamber.

IONIZATION SMOKE DETECTORS.\A small amount of radioactive material ionizes the air inside a chamber that is open to the ambient air. A measured, small electrical current is allowed to flow through the ionized air. The small, solid particle products of combustion that enter the chamber as a result of fire interfere with the normal movement of ions (current), and when the current drops low enough, an alarm results. A two-position switch to control sensitivity may be provided. A detector of this type is shown in figure 7-12.




Figure 7-12.\Ionization Smoke detector.

Modern ionization detectors have additional means to improve stability and immunity to atmospheric effects. A reference chamber is vented to the outside through a small orifice, which does not readily admit smoke particles. Temperature, humidity, and pressure changes are sensed by both the reference chamber and the smoke chamber, and their effects on alarm sensitivity are eliminated by electronic balancing.

The major difference between detectors of the spot and duct types is the method of moving the smoke into the detection chamber. The spot type detects or relies on convection of air in a room. The duct type is intended. for detecting smoke in an air-handling system and is mounted directly on the outside of an air duct or nearby with sampling and return tubes extending completely across the duct.

TESTING SMOKE DETECTORS.\Before testing detectors that are connected to auxiliary functions, such as release of a fire extinguishing agent, release of fire doors, or fan shutdown, disconnect or bypass the auxiliary functions (unless the test is specifically intended to test these features). Before the test, notify the fire department and persons where the audible signals can be heard.

Most PHOTOELECTRIC detectors have a built-in test feature. In some models, a test light source actuated by a key-operated test switch or by a magnet held near a built-in reed switch causes light to reach the normally dark, smoke-sensing photocell in a quantity approximately the light of an average smoke test. In other detector models, the smoke simulation is performed by inserting a reflective surface into the smoke chamber so that the actual source light is reflected to the smokesensing photocell. Test at least one detector in each initiating circuit (zone) monthly. Follow a rotation schedule so that all detectors are tested semiannually.

Test failures or false alarms may result from an excessive accumulation of dust or dirt caused by an adverse environment. Blow out the smoke chambers with low-pressure air. (Partial disassembly of the detectors and disconnection of detectors' power, following the manufacturer's instructions, are required.) Since the photocell is normally dark, disassemble and clean it in a darkened area to minimize the photocell recovery time after cleaning before repowering the detectors. Allow approximately 30 minutes for recovery after reassembly of the detectors before reconnecting power.

Disconnecting power by unplugging one detector may also disconnect power from the other detectors further from the power source. Inform the fire department before or during any extended testing period.

Special equipment that may be required for cleaning consists of a low-pressure air source for blowing out dust and a suction cup for chamber cover removal.

If the cleaning does not correct the false alarms or failure to alarm, return the detectors to the manufacturer for repair.

Test at least one IONIZATION detector in each initiating circuit (zone) monthly. Follow a rotation schedule so that all ionization detectors are tested semiannually, following the manufacturer's instructions. Any detectors that produce false alarms between semiannual tests or do not test satisfactorily should be checked for sensitivity, following the manufacturer's instructions and using test equipment available from the manufacturer or other sources. An aerosol synthetic smoke is available from some manufacturers for testing their detectors.

Unsatisfactory tests or erratic operation may indicate a need to remove accumulated dust or dirt. The frequency of cleaning should be based on results of regular tests and local conditions. Clean, check, and test operation and sensitivity, following the manufacturer's instructions. For loose dust deposits, blow the area with lowpressure air after removing a protective cover. For more stubborn deposits, disassemble and clean, using a liquid recommended by the manufacturer. Recheck sensitivity and adjust if necessary after cleaning and drying thoroughly.


Some smoke detectors of this type produce an electrical shock that may not be severe enough to cause injury directly but could cause a fall from a ladder. Some manufacturers, because of such possible injury to personnel or damage to the detectors, do not recommend servicing by anyone other than factory-trained personnel. Personnel in the customer service departments of most manufacturers can give advice on the telephone for specific problems. Be prepared to give the equipment model number and other pertinent information.

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