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FT 39AUTOMATIC ALARM SYS

AUTOMATIC FIRE DETECTORS(Lifting Air Detector)

OBJECTIVE

1. To explain the fundamentals of lifting air detectors their function and operation.

REFERENCE

2. a. Manual of Fireman ship Book 9.b. Fire College Notes.

CONTENTS

3. Modern Development of Detectors:

a. Lifting Air Detectors.

Combined Heat and Smoke Detector

4. A laser produces a beam light which does not spread out. It travels throughout itspath as a narrow pencil beam. Special gallium arsenide lamps produce a suitable beam forfire detection purposes although not a proper laser beam.

5. The detector projects a light beam across the detected space just below the ceiling.

It is then reflected back by a corner cube mirror on to a suitable photo-electric cell. A firebelow the beam produces air turbulence which causes the reflected beam to dance andproduce an alternating current output from the PE cell. Certain output frequencies can beidentified as being caused by a fire situation and the alarm is caused to sound.

6. Smoke reduces the light falling on the PE cell and this also can be caused to soundthe alarm.

7. It has now been found possible to employ the same detection technique withoutusing a laser beam or a corner cube reflector. The Beam Master detector produced byChubb uses an infra-red beam to sense both the absorption effect of smoke and airturbulence produced by a fire.

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e. One such semi-conductor responds to the oxygen content. Any situationwhich affects this content will cause a resistance change. The presence of aflammable atmosphere will reduce this oxygen content and by connecting theTagushi detectors are less reliable than the catalysis type but can be very muchcheaper. Their tack of reliability arises from the fact that smoke or steam may causea reduction in the oxygen content and therefore excite a response.

f. Particular situation in which a gas detector could have application are mines,flammable gas and liquid storage areas, mobile home and for the protection ofengine rooms in ships and similar installations where ventilation may be restricted.