Fire
and Smoke Detection
There are three
primary types of smoke detectors; ionization, photoelectric and laser
detectors. A smoke alarm system can use one or both types of smoke detection
in combination with a heat detector to protect against fire.
Ionizing sensors contain a small amount of radioactive
material. It is housed in an ionization chamber, which is basically
two metal plates a small distance apart. One of the plates carries a
positive charge, the other a negative charge. The radioactive material
normally emits particles between the plates The particles collide with
the air in the ionization chamber to produce charged particles. The
positively charged metal plate attracts the negatively charged ions
(electrons) and the negatively charged plate attracts the positively
charged ions. The steady flow of ions creates a small but steady current.
The current is disrupted when smoke enters the ionization chamber. This
reduces the flow of electricity between the two plates in the ionization
chamber, and when the electric current drops below a certain threshold,
the alarm is triggered. Also, hot air entering the chamber changes the
electric current level, which triggers an alarm. Ionization technology
is faster at reacting to fast flaming fires that give off little smoke
such as gasoline and other chemical fires.
Photoelectric sensor technology relies on an electric
current that produces a beam of light. When the beam of light is interrupted,
an alarm sounds. Photoelectric smoke detectors contain a light source
and a light-sensitive electric cell. Smoke particles in the detector
deflect the light onto the photoelectric cell, thus generating a current
and triggering the alarm. Photoelectric detectors are better at detecting
slow burning, smoky fires such as furniture and building materials.
Heat
Detection
A heat detector is a device that detects the heat from a fire and triggers
an alarm and/or a water sprinkler system. Heat detectors do not detect
smoke and will not meet fire codes that require a smoke alarm. Two general
types of heat detectors exist; fixed-temperature and rate-of-rise. Fixed-temperature
heat detectors trigger an alarm when the temperature at the detector
reaches a pre-set limit, usually 135 or 200 degrees. Fixed-temperature
detectors are typically used in attics, boiler rooms, or garages. Rate-of-rise
heat detectors will trigger an alarm if the temperature increases at
a rapid rate, typically 15 degrees per minute. Rate-of-rise heat detectors
also have a fixed-temperature detector included. They can be used in
areas where smoke detectors are not suitable such as kitchens, or work
shops.
Carbon
Monoxide (CO) Detection
There are two different kinds of carbon monoxide detectors; electric
current and battery operated. CO detectors using electric current typically
employ a solid-state sensor which purges itself and re-samples for CO
on a periodic basis. This cycling of the sensor is the source of its
increased power demands. Detectors powered by batteries typically use
a passive sensor technology which reacts to the prolonged exposure to
carbon monoxide gas. The two primary types of carbon monoxide detectors
are biomimetic and semiconductor sensors.
Biomimetic units
have a gel that changes color when exposed to CO. This sensor uses synthetic
hemoglobin that mimics the reaction of natural hemoglobin in the presence
of CO. The biomimetic sensor's synthetic hemoglobin discharges CO at
approximately the same rate as the human body, so it needs the same
amount of time to clear itself of CO. This detector has a light beam
which shines through the gel and senses the change in color, activating
the device. These detectors require that the sensor pack be replaced
every two years and are sensitive to cooking or other fumes.
Semiconductor CO
sensors utilize a special semiconductor that changes its resistance
when exposed to CO. The detector senses this change and triggers an
alarm. Both the semiconductor and the biomimetic detectors are highly
reliable. By design, the biomimetic sensor can only detect CO. The second
generation of semiconductor sensors are now more selective and can ignore
the concentration of other common household gases.
CO detectors should
be placed near the top of a wall or on the ceiling. Avoid corner mounts
since the normal movement of air creates a dead spot in the corner of
rooms. Also to avoid false alarms, do not mount a CO detector near a
source of naturally occurring CO such as a furnace.
Other
Detectors
- Freeze
Detector - Undetected furnace failure can lead to frozen
and burst pipes, and serious expense. A freeze detector can safeguard
against extensive damage due to undetected freezing.
- Water
Sensor - Water sensors protect you against the damage
of water leaks that go undetected for hours and even days.
Commercial Security |
Residential
Security
