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Fire Detection
Q: How Many Smoke Detectors Do I Need?
NFPA recommends that you have one smoke detector on every level of your
house, as well as one near all of the sleeping areas in your home. For
example, if your sleeping areas are nestled together in a wing of the
house, one smoke detector in the hallway would probably sound loud
enough to awaken anyone sleeping. However, if your sleeping areas are on
seperate ends of the hallway, you will want to invest in a second smoke
detector to protect that separate bedroom.
The best way to make sure that you have enough smoke detectors is to
listen to them yourself. If there are any areas in the house where the
sound from an activated smoke detector is faint, muffled, or difficult
to hear, or if anyone in your family does not wake up when a smoke
detector sounds, you probably need additional smoke detectors.
Remember, even a working smoke detector can't get you out of danger's
way -- be sure that you have and practice a home escape plan!
Q: Where Should I Put Up Smoke Detectors In My House?
Your smoke detector should be positioned in your house ideally on the
ceiling, or on the wall, between 4-6 inches away from the ceiling-wall
intersection. Be sure to place your smoke detector:
On the ceiling, at least six inches away from the wall.A fire can often
"trap" pockets of air where the wall and the ceiling meet --
smoke might never reach the smoke detector in this "dead air
space" 20 feet away from "sources of combustion
particles" (stoves, furnace, water heater).
Away from the kitchen. A smoke detector too close to the kitchen might
frequently signal false alarms.
10 feet away from bathrooms or other damp, humid areas. The steam can
often set off false alarms.
In areas where the temperature is between 40 degrees Farenheit and 100
degrees Farenheit. Colder or warmer temperatures (like the temperatures
in an attic) might set off false alarms and also shorten the life of the
smoke detector's battery.
In dusty, dirty, or greasy areas.
Near air vents, ceiling fans, or other drafty areas. Drafts can blow the
smoke away from the smoke detector, preventing the smoke detector from
sounding.
More than one foot away from flourescent lights.
If the ceilings are sloped, peaked, or gabled -- 3 feet from the highest
point of the ceiling.
Where the manufacturer recommends its placement. Read the instructions
to be sure that you are placing the unit in the absolute best location.
Q: Should I Use
Battery (stand-alone) or fixed Smoke Detectors (connected to electrical power)?
Both fixed smoke detectors (that are connected to electrical power as
part of a control system) and stand-alone battery-powered smoke detectors
have benefits and drawbacks. Battery powered smoke detectors must have
regular maintenance, including frequent testing and battery replacement.
However, they are easy to install and relatively inexpensive. Fixed smoke detectors are much easier maintenance-wise, but must be hard-wired
into your house. The Newport News Fire Department also recommends that
you select an electric smoke detector that has a battery back-up; in
rare cases, an electrical fire has been known to short out an electric
smoke detector, silencing the alarm.
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Q: Why Is My Smoke Detector Chirping?
If you hear a faint "chirping" or "beeping"
noise approximately every 30 seconds, your smoke detector is probably
trying to tell you something: It's hungry! Some smoke detectors are
built to sound a warning alarm when the battery power begins to run low.
By chirping, the smoke detector is trying to get your attention and
convince you to put a new battery inside it.
Q: How Do I Test My Smoke Detector?
Your smoke detector's manual should give you detailed
instructions on how to test your smoke detector. Usually, this is done
by pressing a button in the center of the smoke detector. If you press
this button down for a few seconds and it beeps, than the battery should
still be working. If the smoke detector does not beep, than replace the
battery and test again.
Q: Are There Smoke Detectors for the Hearing Impaired?
People who are hard-of-hearing also need the protection that
smoke detectors provide. Smoke detectors for the hearing impaired are
smoke detectors that have a strong strobe light attached to the
circuitry. When the smoke detector sense the smoke, the strobe light
begins blinking.
If you are installing one of these smoke detectors, follow the
instructions on where to locate this detector. You might want to install
the smoke detector inside the appropriate bedroom, rather than in the
hallway, so that the light will be stronger when anyone is at their most
vulnerable: when they are asleep. Also, you might need more than one
hearing-impaired smoke detector for the home.
Remember, too, that you'll need a hearing-impaired smoke detector when
you are sleeping away from home. Hotels and motels usually have
hearing-impaired smoke detectors available upon request.
Q: Why Won't My Smoke Detector Let Me Cook? (and Other Problems
with False Alarms.)
If you frequently have false alarms while you are cooking, your
smoke detector isn't hinting to you that your cooking is bad. The most
common cause for frequent false alarms is a bad placement of the smoke
detector. Ionization smoke detectors are designed to alarm when ever a
small amount of "unclear" air passes through its sensors; it
cannot tell the difference between thick acrid smoke, dusty air from the
workroom, or steam from the bathroom.
Q: What types of Smoke Detectors are available for residential
use ?
- 9-volt Battery Powered Smoke Detector
These smoke detectors are battery-operated. They are usually the
cheapest type of smoke detector.
- Advantages: Easy to install, move, and to maintain.
- Considerations: You must check the battery of the smoke
detector frequently, at least once a month.
- Multi-Year Lithium Battery Powered Smoke Detector
These relatively new smoke detectors come with a special battery
that lasts for years. The battery does not need to be replaced as
frequently as 9-volt powered smoke detectors.
- Advantages: Less maintenance required, due to the longevity of
the lithium battery.
- Considerations: The battery should still be tested frequently.
- Electric Smoke Detector
Electric smoke detectors are hard-wired into the home and are
connected into the electrical wiring system.
- Advantages: Because they do not use batteries, electrical
smoke detectors are easier to maintain, and the smoke detector
is never disabled because of a lack of a battery. Electric smoke
detectors with a battery back up are the safest.
- Considerations: The smoke detector should still be tested
frequently. Consider purchasing an electric smoke detector with
a battery backup, or purchase a seperate battery-operated smoke
detector for extra protection. (Electrical fires might
short-circuit the electrical system -- and the electrical smoke
detector.)
Q: What are the
classical Smoke Detectors ?
Ionization Detector
Ionization Detector works by forming an electrical path inside the smoke
detector with a very small amount of radioactive material. When smoke
enters the detector, the smoke molecules attach themselves to the ions
and change the electrical current. This change in the current triggers
the alarm.
Photoelectric Detector
Photoelectric Detector works by containing a light source (like a small
light bulb) and a photocell that is activated by light. Usually, the
light never reaches the photocell. However, when smoke fills the smoke
detector, the smoke reflects the light towards the photocell. The
photocell than triggers the alarm.
Q: What is the difference between smoke detectors?
Both of these types of smoke detectors are designed to detect smoke in
the air. The ionization detector responds slightly quicker to flaming
fires (like draperies), with smaller particles of combustion, while the
photo-electric detector responds quicker to smoldering fires (like a
cigarette.) However, both types of smoke detectors have to pass the same
test to be certified as a UL smoke detector, and both will warn you in
almost all cases of fires.
Q: What are the Novel types of Smoke Detectors ?
Projected Beam type
Projected Beam Smoke detector comprises two elements : a light
source that transmits a beam of light and a detector that incorporates
one or multiple light sensors (photoelectric cells).
The smoke obscures the beam of light directed at the detector.
The location of this system should be carefully selected so as to allow
a clear line of sight between the light source and the detector.
Aspirating (VESDA)
This type of system aspirates the smoke from various locations into a
tube where the smoke is analyzed electro-optically by a line of sight
transmitter-receiver set.
Q: What's the difference between Smoke detectors and Heat
Detectors ?
The difference between smoke and heat detectors is obvious by the
name. Smoke detectors are more sensitive to sensing smoke; heat
detectors are more sensitive to changes in temperature. They both detect
fire typical parameters, however they are not interchangeable.
A smoke detector warns of fire when it senses the products of
combustion (soot) in the atmosphere. As anyone who has a smoke detector
too close to the kitchen knows, a smoke detector goes off very quickly,
even in small amount of smoke. There are slight differences between the
two types of smoke detectors, but they both operate by detecting the
products of fire combustion.
Heat detectors warn of fire when the temperature in the area
around the smoke detector reaches a certain level. Heat detectors do not
notice smoke. Some studies show that heat detectors may not provide
early enough warning to aid escape from a life-threatening fire.
However, a heat detector could be valuable additional protection in
areas such as kitchens and attics, where smoke detectors are not
recommended. They are not recommended for the use in bedrooms or
sleeping areas.
In summary, a combination of heat detectors and smoke detectors in your
home would cover you comprehensively. However, the heat detectors should
compliment, not replace, your smoke detectors: Smoke detectors activate
usually before the flames and heat are sufficient to operate heat
detectors -- and these extra seconds might be critical to your family's
safe escape.
Q: What types of Heat Detectors are available?
Fusible Link
Employs a detection element that melts at a predetermined
temperature. Is usually used in compressor and equipment buildings and
open areas around production equipment and wellheads .
Pneumatic Heat sensor
The heat detector is a pneumatic type that detects abnormally high
temperature along a length of tubing. It is usually used around engines
and turbines, glycol re-boilers and pressure vessels.
Rate of Rise Detectors
Solid state detectors that detect abnormally rapid temperature rise.
They are usually placed in closed areas, used in store rooms, warehouses
and other areas in living quarters.
Fixed Temperature Detectors
Detect abnormally high temperatures, usually bulbs containing a
chemical fluid that boils at a predetermined temperature , rupturing the
bulb and activating the alarm signal means.
Q: What is an Optical Flame Detector?
The Optical detector is an electronic device containing
electro-optical sensors that are sensitive to electromagnetic radiation
in the UV, VIS, IR spectral bands.
The Optical detector "sees" the fire by detecting the
electromagnetic radiation emitted by the combustion products.
Q: How does an optical flame detector detect a fire?
Every combustion process emits 3 types of energy:
1. Heat energy - detected by heat detectors
2. Radiation energy - detected by optical detectors
3. Product energy - contained in unburned products and unstable products
of combustion.
The fastest method to detect the above energies is the Optical
analysis of the radiation emitted in a combustion process.
The Optical detectors detect the radiation energy emitted by the
chemical processes that take place in a fire.
Q; What types of Optical detectors are available?
There are several types of optical flame detectors, including the
various spectral bands that are emitted by the fire products such as UV,
VIS (including Video), IR, Multiple IR:
a. UV type.
b. IR - single sensor type.
c. UV/IR - dual sensor type.
d. IR2 - dual IR sensor type.
e. IR3 - Triple IR sensor type.
f. CCTV - Image processing type.
The flame radiation spectral pattern, being unique, allows several
spectral ranges to be employed simultaneously in the various detection
devices.
Optical Flame Detectors usually employ several optical sensors,
working in specific spectral ranges (usually narrow bands) that record
simultaneously the incoming radiation at the selected wavelengths. The
signals recorded by each sensor are analyzed according to a
pre-determined technique that includes one or more of the following:
1. Flickering frequency analysis.
2. Threshold energy signal comparison.
3. Mathematical ratios and correlations between various signals.
4. Comparator techniques (and-gate techniques).
5. Correlation to memorized spectral analysis.
The modern Optical Flame Detectors employ several of the above
mentioned techniques in order to provide enhanced reliability and
accuracy.
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