An automatic fire alarm in a building's security system is a heat detector that responds to the heat from a fire by setting off an alarm. Some heat detecting fire alarms rely on a bimetallic strip as the temperature sensor. This strip responds to heat by closing a normally open electrical circuit to activate the alarm.
How It Works
The bimetallic strip in a fire alarm is made of two metals with different expansion rates bonded together to form one piece of metal. Typically, the low-expansion side is made of a nickel-iron alloy called Invar, while the high-expansion side is an alloy of copper or nickel. The strip is electrically energized with a low-voltage current. When the strip is heated by fire, the high-expansion side bends the strip toward an electrical contact. When the strip touches that contact, it completes a circuit that triggers the alarm to sound. The width of the gap between the contacts determines the temperature that will set off the alarm.
Bimetallic-strip heat detectors have some significant drawbacks. The strip bends slowly at its activation point rather than snapping closed. Alarm sensors based on bimetallic strips are also prone to false alarms from vibrations or jarring, particularly if subjected to non-fire heat that's close to the strip's set activation point. Other arrangements using bimetallic elements offer better performance.
Newer bimetallic fire detectors incorporate bimetallic snap discs instead of strips. The disc in its unstressed condition assumes a concave shape. As the disc gets hotter, the stresses from the uneven expansion of its metals cause the disc to change its curvature, snap into a convex shape and close an electrical switch that sounds the alarm. This type of detector is less prone to false alarms because of the disc's instantaneous positive snap action. Both the bimetallic strip and bimetallic snap disc detectors automatically reset themselves as temperatures return to normal.
Bimetallic strips and snap discs respond best to slowly-developing smoldering fires where temperatures rise gradually to the point where the bimetallic element reacts. They often are combined with pneumatic fire detectors that respond to rapid rises in temperature from a fast-developing fire. There's also a bimetallic combination fire detector that responds to both fast-developing and slow smoldering fires. This type has an aluminum outer cylinder surrounding closely-spaced copper contacts. When temperatures rise fast, the shell expands fast to close the copper contacts. As temperatures rise slowly, the shell expands gradually but at a greater rate than the copper, eventually closing the contacts at the preset temperature.