What is the real difference between
The following is quoted directly from the System Sensor FAQ page on this
subject at
http://www.systemsensor.com/html/guides/A05-1003.pdf
This white paper is an excellent treatise on the subject. It has useful
information for DIYers and experienced techs alike. The quoted passage is 2
pages of a 21-page document.
"There are two basic types of smoke detectors in use today; ionization and
photoelectric. The sensing chambers of these detectors use different
principles of operation to sense the visible or invisible particles of
combustion given off in developing fires.
Ionization Smoke Detector Operation
A typical ionization chamber consists of two electrically charged plates and
a radioactive source (typically Americium 241) for ionizing the air between
the plates. (See Figure 1.) The radioactive source emits particles that
collide with the air molecules and dislodge their electrons. As molecules
lose electrons, they become positively charged ions. As other molecules gain
electrons, they become negatively charged ions. Equal numbers of positive
and negative ions are created. The positively charged ions are attracted to
the negatively charged electrical plate, while the negatively charged ions
are attracted to the positively charged plate. (See Figure 2.) This creates
a small ionization current that can be measured by electronic circuitry
connected to the plates ("normal" condition in the detector).
Particles of combustion are much larger than the ionized air molecules. As
particles of combustion enter an ionization chamber, ionized air molecules
collide and combine with them. (See Figure 3.) Some particles become
positively charged and some become negatively charged. As these relatively
large particles continue to combine with many other ions, they become
recombination centers, and the total number of ionized particles in the
chamber is reduced. This reduction in the ionized particles results in a
decrease in the chamber current that is sensed by electronic circuitry
monitoring the chamber. When the current is reduced by a predetermined
amount, a threshold is crossed and "alarm" condition is established.
Changes in humidity and atmospheric pressure affect the chamber current and
create an effect similar to the effect of particles of combustion entering
the sensing chamber. To compensate for the possible effects of humidity and
pressure changes, the dual ionization chamber was developed and has become
commonplace in the smoke detector market.
A dual-chamber detector utilizes two ionization chambers; one is a sensing
chamber that is open to the outside air. (See Figure 4). The sensing chamber
is affected by particulate matter, humidity, and atmospheric pressure. The
other is a reference chamber that is partially closed to outside air and
affected only by humidity and atmospheric pressure, because its tiny
openings block the entry of larger particulate matter including particles of
combustion.
Electronic circuitry monitors both chambers and compares their outputs. If
the humidity or the atmospheric pressure changes, both chambers' outputs are
affected equally and cancel each other. When combustion particles enter the
sensing chamber, its current decreases while the current of the reference
chamber remains unchanged. The resulting current imbalance is detected by
the electronic circuitry. (See Figure 5.) There are a number of conditions
that can affect dual-chamber ionization sensors; dust, excessive humidity
(condensation), significant air currents, and tiny insects can be misread as
particles of combustion by the electronic circuitry monitoring the sensors.
Photoelectric Smoke Detector Operation
Smoke produced by a fire affects the intensity of a light beam passing
through air. The smoke can block or obscure the beam. It can also cause the
light to scatter due to reflection off the smoke particles. Photoelectric
smoke detectors are designed to sense smoke by utilizing these effects of
smoke on light.
Photoelectric Light Scattering Smoke Detector
Most photoelectric smoke detectors are of the spot type and operate on the
light scattering principle. A light-emitting diode (LED) is beamed into an
area not normally "seen" by a photosensitive element, generally a
photodiode. (See Figure 6.) When smoke particles enter the light path, light
strikes the particles (Figure 7) and is reflected onto the photosensitive
device causing the detector to respond.
Photoelectric Light Obscuration Smoke Detector
Another type of photoelectric detector, the light obscuration detector,
employs a light source and a photosensitive receiving device, such as a
photodiode (see Figure 8). When smoke articles partially block the light
beam (Figure 9), the reduction in light reaching the photosensitive device
alters its output. The change in output is sensed by the detector's
circuitry, and when the threshold is crossed, an alarm is initiated.
Obscuration type detectors are usually of the projected beam type where the
light source spans the area to be protected.
Smoke Detector Design Considerations
Smoke detectors are based on simple concepts, but certain design
considerations need to be observed. They should produce an alarm signal when
smoke is detected, but should minimize the impact of an unwanted signal
which can arise from a variety of causes. In an ionization detector, dust
and dirt can accumulate on the radioactive source and cause it to become
more sensitive. In a photoelectric detector, light from the light source may
be reflected off the walls of the sensing chamber and be seen by the
photosensitive device when no smoke is resent. The entrance of insects,
dirt, drywall dust, and other forms of contamination into the sensing
chamber can also reflect light from the light source onto the photosensitive
device.
Electrical transients and some kinds of radiated energy can affect the
circuitry of both ionization and photoelectric smoke detectors and be
interpreted by the electronic circuitry to be smoke, resulting in nuisance
alarms. The allowable sensitivity ranges for both types of detectors are
established by Underwriters Laboratories, Inc. and all are verified by their
performance in fire tests. Regardless of their principle of operation all
smoke detectors are required to respond to the same test fires.
Considerations in Selecting Detectors
The characteristics of an ionization detector make it more suitable for
detection of fast flaming fires that are characterized by combustion
particles in the 0.01 to 0.4 micron size range. Photoelectric smoke
detectors are better suited to detect slow smoldering fires that are
characterized by particulates in the 0.4 to 10.0 micron size range. Each
type of detector can detect both types of fires, but their respective
response times will vary, depending on the type of fire.
Because the protected buildings normally contain a variety of combustibles,
it is often very difficult to predict what size particulate matter will be
produced by a developing fire. The fact that different ignition sources can
have different effects on a given combustible further complicates the
selection. A lighted cigarette, for example, will usually produce a slow
smoldering fire if it is dropped on a sofa or bed. However, if the cigarette
happens to fall upon a newspaper on top of a sofa or bed, the resulting fire
may be characterized more by flames than by smoldering smoke.
The innumerable combustion profiles possible with various fire loads and
possible ignition sources make it difficult to select the type of detector
best suited for a particular application.
For more information, see NFPA 72-1999, paragraphs A-2-3.6.1.2, A-2-3.6.1.1,
A-2-3.6.1.2(a), and A-2-3.6.1.2(b).
NFPA 72 requirements also dictate that alarm notification appliances
(including smoke detectors with built-in sounders) produce the 3-pulse
temporal pattern fire alarm evacuation signal described in ANSI S3.41.
(Audible Emergency Evacuation Signals)
Situations Where Other Types of Detectors May Be Used
In certain circumstances where standard smoke detectors are unsuitable,
special-purpose detectors, such as flame detectors, heat detectors, and
other detection devices may be used. The application of these special types
of detectors should be based on an engineering survey and used in accordance
with the manufacturer's installation instructions provided.
Smoke Detectors Have Limitations
Smoke detectors offer the earliest possible warning of fire. They have saved
thousands of lives in the past and will save more in the future.
Nevertheless, smoke detectors do have limitations. They may not provide
early warning of a fire developing on another level of a building. A first
floor detector, for example, may not detect a second floor fire. For this
reason, detectors should be located on every level of a building. In
addition, detectors may not sense a fire developing on the other side of a
closed door. In areas where doors are usually closed, detectors should be
located on both sides of the door.
As already indicated, detectors have sensing limitations. Ionization
detectors are better at detecting fast, flaming fires than slow, smoldering
fires. Photoelectric smoke detectors sense smoldering fires better than
flaming fires. Because fires develop in different ways and are often
unpredictable in their growth, neither type of detector is always best. A
given detector may not always provide significant advance warning of fires
when fire protection practices are inadequate, nor when caused by violent
explosions, escaping gas, improper storage of flammable liquids such as
cleaning solvents, etc."
Above quoted passage belongs to System Sensor.
--
Regards,
Robert L Bass
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Bass Home Electronics
2291 Pine View Circle
Sarasota · Florida · 34231
877-722-8900 Sales & Tech Support
http://www.bassburglaralarms.com
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