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Ionization verses Photoelectric...

Discussion in 'Security Alarms' started by Marc, Jan 12, 2005.

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  1. Marc

    Marc Guest

    What is the real difference between these type of smokes?
    I've always used photoelectric for my residential installations.. and am curious on the real differences.
    should one be used instead of the other in homes?

  2. What is the real difference between
    The following is quoted directly from the System Sensor FAQ page on this
    subject at
    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

    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

    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-, A-2-,
    A-2-, and A-2-

    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.


    Robert L Bass

    Bass Home Electronics
    2291 Pine View Circle
    Sarasota · Florida · 34231
    877-722-8900 Sales & Tech Support
  3. Frank Olson

    Frank Olson Guest

    What is the real difference between these type of smokes?
    I've always used photoelectric for my residential installations.. and am
    curious on the real differences.
    should one be used instead of the other in homes?

    I'm curious. If you don't know what the difference is, why are you using
    one over the other??

    In answer to your question, photo-electrics are recommended for use in a
    residential setting.
  4. Marc

    Marc Guest

    Actually it was what I was trained to use.. smokes like the system sensor
    p/e is what we have always used... the question came about when I went to a
    job where the guy installed his own system and i said upgrading the smokes
    and i showed him what we use and he said he wanted ionization not

  5. mikey

    mikey Guest

    Mr Plagiarism seems to be turning over a new leaf.
    Good on ya, Robert.
  6. Ionization style smoke detectors are more responsive to smaller, electrically charged particles present near flaming fires.

    Photoelectric spot-type smoke detectors are more responsive to the larger, lighter colored particles of smoldering fires.

    What is the real difference between these type of smokes?
    I've always used photoelectric for my residential installations.. and am curious on the real differences.
    should one be used instead of the other in homes?

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