Associated Fire Protection

Heat Detectors:
Of all the fire detection devices, heat detectors are the least expensive and have the lowest false-alarm rate. Most heat detectors operate on simple principles, making them versatile and reliable. Since they are not among the fastest response systems available today, they are usually used in conjunction with other forms of fire detection.

Fixed Temperature Heat Detectors:
These heat detectors are designed to operate when a predetermined temperature is reached. They are available in many temperature ranges depending on a facility's individual needs. Many of these detectors use metals that are designed to expand or melt as the temperature rises. In a Fusible Element detector, solder is used to hold a switch in a circuit open. When this solder melts, the switch swings into place, completing the circuit, and activating the alarm. In a Bimetallic detector, two metals with different expansion rates are bonded together in a strip. When the strip is heated, one metal expands faster than the other, causing the strip to bend. This closes the circuit, activating the alarm.

Rate of Rise Detectors:
These detectors are designed to operate when the temperature increase exceeds a predetermined rate. This prevents false alarms due to regular temperature fluctuations, because air is heated more rapidly in a fire. A sealed chamber filled with air contains a diaphragm switch and a small vent. As the temperature rises, the air expands, and some escapes out of the vent. If the air expands too quickly for the excess to escape out of the vent, then the added pressure will trigger the switch, activating the alarm.

Smoke Detectors:
Smoke detectors operate more quickly than heat detectors, but are more prone to false alarms. In facilities where there is a large amount of particulate in the air, other detection devices are recommended. There are two main groups of smoke detection devices: ionization smoke detectors, and photoelectric smoke detectors.

Ionization Smoke Detectors:
These detectors contain a small amount of radioactive material that is capable of ionizing the air around it. This material is kept in a chamber between two electrodes in a circuit. When the air is ionized, it becomes conductive. Smoke particles attract the ions, reducing the conductivity of the air in the chamber. The alarm activates when the air drops below a certain level of conductivity. Ionization smoke detectors are useful in moderate or high hazard areas where flaming is likely to occur.

Photoelectric Smoke Detectors:
These detectors operate on a principle of light obscuration. A beam of light reflects back from an emitter to a receiver. When this beam is obstructed by particles of smoke, the alarm sounds. Photoelectric detectors are faster than ionization detectors, but are more prone to false alarms due to particulate from cooking, smoking, or dust. They are most often used in large, open areas, where there is little ambient pollution to obscure the light. They are particularly useful in areas prone to smoldering fires.

Gas-Sensing Fire Detectors:
Aside from smoke and heat, fires produce a number of gases that can be easily detected. There are two main types of gas-sensing fire detectors: semiconductor detectors, and catalytic element detectors. Semiconductor gas detectors use semi conductors whose conductivities are effected by oxidizing and reducing gases. When the conductivity changes, the alarm sounds. Catalytic element detectors use catalysts that increases the oxidation rate of combustible gases. This causes the heat to rise quickly, and the detector then functions as a heat detector.

Flame Detectors:
Flame detectors are sensitive to the radiant energy of light. They respond quickly, but are prone to false alarms in certain environments. They are usually used in high hazard areas, where materials burn quickly or with explosive flames. Since flame detectors sense light energy, precautions must be taken to prevent their view from being obstructed. Some flame detectors sense infrared and ultra violet light instead of visible light to help avoid false alarms.

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