Now I would
like to discuss each of the four factors listed above in terms of the
impact that they have on the margin of safety, i.e. people's ability to
successfully escape a fire.
- Amount of Smoke inside the Detector (Duo)
that I know assume that all smoke is the same, in regards to triggering
smoke detectors. This is not actually true. Different smoke can have the
same optical density, a measure of how much light the smoke obscures,
and yet have different typical particle sizes and color. Certain factors
can affect the particle size and color of smoke.4
- 1. Plastics
tend to produce larger particle smoke than cellulose material produces,
- 2. Smoldering
fires tend to produce larger particles than flaming fires,
- 3. "Aged"
smoke, i.e. smoke that has moved some distance away from the fire tends
to have larger particles than the smoke from the same fire that is still
near the fire.
to the fact that there are different kinds of smoke, these different kinds
of smoke can affect ionization and photoelectric detectors differently,
since these detectors operate on different principles. These principles
are summarized in the NFPA Handbook.5
smoke detector has a small amount of radioactive material that ionizes
the air in the sensing chamber, rendering the air conductive and permitting
a current flow the air between the two charged electrodes. This gives
the sensing chamber an effective electrical conductance. When smoke
particles enter the ionization area, they decrease the conductance of
the air by attaching themselves to the ions, causing a reduction in
ion mobility. When the conductance is below a pre-determined level,
the detector responds."
detector operates on a light scattering principle. They contain a light
source and a photosensitive device arranged so the light rays normally
do not fall onto the device. When smoke particles enter the light path,
light strikes the particles and is scattered onto the photosensitive
device, causing the detector to respond."
can impact on a detectors response time. As a rule ionization detectors
are most sensitive to smaller particles and photoelectric detectors are
more sensitive to larger particles. In addition photoelectric detectors
tend to show a decreased sensitivity to dark smoke. (This is due to the
fact that dark smoke absorbs rather than refracting the light, which the
photoelectric detector relies upon.) As a consequence different kinds
of smoke will cause different detectors to respond at different levels
of smoke inside the detector, i.e. Duo.
A chart which
graphically displays the changing sensitivities of photoelectric and ionization
detectors ionization detectors over the ranges of particle sizes is Figure
Figur12: A represents a photoelectric detector utilizing a "scattered
light principle, a spot detector, B represent a photoelectric
detector utilizing "obscuration", a beam detector, and C represents
an ionization detector, a spot detector. It should also be noted
that this chart assumes that the total mass of particulate stays
constant for a given volume. This causes the number of particles
to decrease as the size increases. It is actually the decrease
in the number of particles that cause the ionization detector
to become less sensitive to large particle smoke.
helps illustrate the relative loss of sensitivity of the ionization detector
as the average particle size becomes larger or as the number of particles
decreases. In fact it reinforces the statement that if you "Double the
radius of the average particle you have only one quarter of the effect
on an ionization detector".7 In addition it helps explain why an ionization
detector that is extremely sensitive to the small particles that are often
given off by cooking may not be very sensitive to the larger particles
given off by smoldering fires. It also helps illustrate the relative low
sensitivity that the photoelectric has to small particles, which helps
explain why it is much less susceptible to nuisance alarms than ionization
detectors. A chart which graphically displays the changing sensitivities
of photoelectric and ionization detectors over the ranges of smoke color
is Figure 28.
addition to re-inforcing the information illustrated by Figure 1, the
Chart in Figure 2 illustrates the photoelectric detectors relative insensitivity
to dark particle smoke.
the information on Figures 1 and 2, one could conclude that; 1) Ionization
detectors are more sensitive than photoelectric detectors to flaming fires,
which tend to have smaller particles, and that 2) Photoelectric detectors
are more sensitive than ionization detectors to smoldering fires, which
tend to have larger particles. This is exactly what Heskestad2 found in
his study as indicated in Table 1.
OPTICAL DENSITIES (OD/ft) & % OBSCURATIONS (%obs/ft)
FOR VARIOUS FIRE SOURCES
Table 1 clearly
indicates that detectors can go off at much higher levels of obscuration
than the rating on the back, typically 1%-2%, would indicate. For the
fire scenarios this is particularly true for the ionization detectors.
The photoelectric detectors not only responded to fire scenarios much
more consistently but since the time of this study in the mid-seventies
the photoelectric detector has improved its response to fires because
it has greatly improved its smoke entry characteristics. This factor will
be discussed in the next section.
- Smoke Entry Resistance or "Length" (L)
"L is a
characteristic length scale of the detector geometry (not necessarily
related to a physical scale) which certainly may depend on the direction
of flow relative to the detector, but is independent of the properties
of the smoke. Consequently, L is a quantity characteristic of the detector
itself, whereas the characteristic optical density, Duo, depends on
the property of the smoke as well as the detector design (including
sensitivity setting). … As L increases (entry of smoke to detection
chamber becomes more difficult), the sensitivity to the smoke must increase
to be able to provide the same type of response."
At the time
Heskestad was conducting his test, the typical L factors for the detectors
in his test were 6ft for the ionization detectors and 20.9-86.7 ft-1 for
the photoelectric detectors. Due to changes in smoke detector design and
technology these numbers for today's detectors are different. The typical
L factors for today's detectors were measured by researchers in Finland
in 19929. They were 10.0-12.0 ft-1 for the ionization detectors and 8.5-26.5
ft-1 for the photoelectric detectors. This change in L Factors is important
since many of the studies used to justify today's testing and installation
standards were conducted with detectors having the old L Factors.
Three - Rate of Smoke Build-Up (d(Du/dt))
is easy to understand. If the environmental obscuration is doubling every
30 seconds as opposed to every 300 seconds, then it just makes sense that
there will be a greater discrepancy between Duo and Dur for the fire with
the faster rate of smoke production. This increase in the response delay
between the environmental obscuration and the internal obscuration the
rate of smoke production impacts on margin of safety. In addition to this
increasing in environmental obscuration at detection time, the rate of
smoke development impacts on the margin of safety in other ways. For any
given amount of time, to allow for occupant reaction and egress, a higher
rate of smoke development will cause a worse environment during egress
than a lower rate of smoke development.
Four - Velocity of Smoke Near the Detector (V)
of smoke flow impacts on a detector's response in two ways. Low velocity
smoke flow affects the ease of entry of smoke into the detector chamber.
Another way that smoke flow velocity can affect detector response is by
impacting on the smoke aging phenomena. By smoke "aging" I am referring
to the fact that as smoke particles cool and travel from the fire source
they start to "stick together" forming larger and fewer particles.10 The
fact that "aged" smoke has fewer particles per unit volume cause the ionization
detector to be less sensitive to "age" smoke.
affect, which is increased at lower velocities, should be accelerated
by doorways, which have a creation distance between the ceiling and the
top of the door. The time that it takes to "fill up" the upper part of
the room of origin before it starts to flow through the doorway will provide
extra time for the smoke to "age" relative to a situation where there
is a smooth ceiling between the fire and the smoke detector. This should
cause detectors, particularly the ionization detector, to have a decreased
sensitivity to smoke when the detector is located outside the room of
origin. This is often the case since most building and fire codes only
require detectors to be located in hallways of residential occupancies.
Conclusion Drawn from Previous Data
presented so far is important in and of itself for an investigator to
consider. What I would like to discuss in the next section is a logical
syllogism that arises from this information.
from smoldering fires, smoke from fires involving plastics, and aged
smoke can be characterized in general as "large particle" fires.
detectors are least sensitive to "large particles" fires
Ionization detectors may not operate in time if they have to detect:
smoldering fires, fires involving plastics, and "aged smoke fires.
is particularly important since smoldering fires tend to occur when people
are sleeping. To quote from a 1985 NFPA Fire Journal article, 11
discovery, typically associated with fires that occur at night when
everyone is asleep, also tends to be a characteristic of smoldering
fires caused by discarded smoking materials. These smoldering fires
are the leading cause of US fire fatalities and detectors are ideally
designed to deal with them."
one could make a similar conclusion concerning the photoelectric detector
and fast flaming fires, but improvements in technology and the short time
periods involved in fast flaming fires makes it appear that the data does
not support that syllogism. However, there appears to be a lot of data
supporting the "ionization syllogism". I would now like to refer to the
conclusions of three studies, which seem to support the conclusion in
the "ionization syllogism".
Smoke Detector Studies
A study was
conducted in 1978 in England12 to study the effectiveness of fire detectors
installed in bedrooms and corridors of residential institutions. An illustration
of some of the results is Table 2.
fires were started using a cigarette placed between pads of fibrous cotton
upholstery wadding. A polyurethane mattress was covered with cotton sheets.
The flaming fires were started with crumpled newspapers, primed with ethanol,
that was placed under the side of the armchair nearest the bed.
Some of the
conclusions of the researchers in this study were the following:
- The tests
were carried out nominally in still air conditions; but variable low
velocity air currents did exist and were observed to affect the flow
of smoke to some extent, particularly in the corridor, for smoldering
fire, or after flow through cracks.
- The half-hour
fire resistant door assembly, when closed, formed an effective barrier
to the heat produced during the complete burnout of the fully furnished
room. However, it did not prevent the escape of sufficient smoke to
cause rapid smoke logging of the corridor escape route. The precise
degree of leakage from the room via all routes is crucial. The velocity
stream of smoke from a flaming fire through the cracks in the door was
only double that produced by the smoldering fire and door open. This
velocity stream is at least an order of magnitude greater if the door
is open and the fire is flaming.
the conditions of ignition from flames, the ionization chamber type
detector exhibited a greater sensitivity to the smoke produced than
the photoelectric system. However, the rate of generation of smoke was
so great that the extra time given by the ionization chamber as a result
may be of little practical use.
chamber type detectors, in the room of origin and the corridor, did
not, in the smoldering fire tests, provide adequate warning that the
escape route was impassable or that conditions in the room were potentially
hazardous to life.
detectors gave much earlier warning in the smoldering fire tests but
those in the corridor did not guarantee sufficient warning of conditions
in the room when the door was closed, but did warn of potential smoke
logging of the corridor.
in Australia reached similar conclusions in 198613. They investigated
smoke detectors ability to detect smoldering fire in a typical residential
dwelling. The smoke used in the test was generated from hardwood smoldered
on a hot plate and artificial smoke meant to copy the smoke from smoldering
fires as well as high smoke evolution which could arise in an arson fire.
Their conclusions were the following:
and ionization detectors sited in bedrooms with the door partly ajar
provide adequate detection of smoldering fires only when it originates
in the same room, and generally provided poor escape time from smoke
originating in other areas of the dwelling.
detectors sighted in the hallway are more effective for detecting smoldering
smoke than ionization detectors, providing adequate escape time for
most conditions of size and location of the smoke sources.
detectors sited in the hallway generally provide inadequate escape times
unless smoke movement into the hallway is slowed down by narrow door
openings, causing a slower loss of visibility, or unless they are sited
close to the smoke source.
1991 Norwegian researchers14 placed smoke detectors inside and outside
the room of origin. The flaming fires were started with a Methenamine,
1588 source. The smoldering fires were started with a glowing cigarette
placed on a textile. They reached the following conclusions.
- The ionization
detectors detected smoke from a smoldering fire much later than optical
(photoelectric) detectors. When the particular conditions during the
fire development are taken into consideration there are reasons to indicate
that this detection principle would not provide adequate safety during
this type of fire.
- In many
countries like Norway, 90-95% of the smoke detectors installed in homes
are ionization types of detectors. Here, smoldering fires are often
caused by smoking and those who have installed such detectors are satisfactorily
safe providing measures are made to prevent smoldering fires from starting.
This means smoking in bed must be avoided. If such homes are to purchase
new detectors, the recommendation is that the optical smoke detector
- For individual
room surveillance, such as in hospitals and hotels, optical (photoelectric)
detectors should always be used. Even though these detectors are slightly
less responsive when detecting smoke from flaming fires in a room, this
time margin should be related to the greater safety optical (photoelectric)
detectors provide when smoldering fires occur. The advantage of ionization
smoke detectors during flaming fires is only about a 15-20 second earlier
warning. This margin will only be decisive for the loss of human life
in extraordinary circumstances.
Assumptions Investigators Often Make Concerning Detectors
are four assumptions, in my opinion, that investigators often make concerning
smoke detectors that may not always be correct:
sometimes assume that if these smoke detectors did not respond that
there was insufficient smoke early enough that there was insufficient
sometimes hypothesize that if the smoke detectors did not respond until
the smoke reached a dangerous level that the fire must have been growing
at such a fast rate that even though it responded quickly the occupants
did not have enough time to escape. I know of at least a couple investigators
who used this logic to assume that accelerants were involved since the
occupants were not alerted until the smoke was already at a level that
sometimes assume that if the occupants could not evacuate safely that
the occupants were not able to respond to the alarm. This could be due
to the fact that they did not hear the alarm or that they were physically
incapable of speedy evacuation.
- 4. In
some cases, investigators assume that there was no smoke detector. They
assume that if there had been a working smoke detector that the occupant
would have evacuated. This conclusion is often supported by the fact
that a smoke detector cannot easily be located due to overhauling.
believe that the basis for these hypotheses, is the assumption that a
small amount of smoke will always trigger a smoke detector. This assumption
based on the common experience that most people, in which the smoke detector
in their home triggers in response to minute amounts of cooking smoke,
even cooking odors that are invisible, or steam. In this case, our common
sense is misleading, particularly in regards to ionization detectors.
As stated earlier, the sensitivity of ionization detectors is inversely
related to the size of the smoke particle, assuming a constant mass/volume.
Smoke from: fires involving synthetics, fires that start in the smoldering
mode, and fires that start remote from the detector, will tend to have
larger particles and therefore possible delayed response from ionization
me make it clear at this point that I am not saying that these hypotheses
are not valid for many fire that are investigated. I just want to point
out that they are not the only explanations for detectors not providing
enough warning. No hypotheses or conclusions should be made concerning
why smoke detectors did not respond in time until the factors discussed
in this paper are considered.
properly investigate fires, particular fatal residential fires the investigator
should be aware of and consider the types of factors discussed in this
paper. They can do this by doing the following.
collect the involved detector(s) as evidence. If the investigation warrants
it can be sent out for testing. Too often the detector involved is destroyed
or lost during the overhaul stage of the fire scene.
the type of the detector. A simple way of identifying ionization detectors
is the radioactive symbol or reference to microcuries that might appear
on the back of the detector.
whether the fire was fast flaming, small particle, or smoldering, large
the location of the detector, relative to the location of the fire.
How many doors are between the detector and the fire? Where the doors
open or closed?
the impact that open windows or HVAC systems might have on the flow
of the smoke.
- If the
detector was disabled, consider how close the detector was to potential
nuisance alarms. If no survivor is alive to help determine why it was
disabled it may be helpful to talk to adjacent apartment or townhouse
occupants, who probably have the detectors installed in the same location.
To help find
the detector it may be helpful to look at adjacent apartment or townhouses.
If constructed at the same time or if they have the same landlord there
is a possibility that the location of the detector(s) in the adjacent
living unit can provide clues to the location in the unit of fire origin.
In the absence of the clues it should be assumed that they located where
the local codes require them to be located. More than once I have been
able to find the detector and the battery in the debris laying on the
floor right under the spot on the ceiling where I assumed the detector
was located. Even though the plastic had melted the metal parts of the
detector were still recognizable.
Most of the
work of a fire investigator is involved with determining the cause and
origin of a given fire. In particular, an investigator must determine
if a fire was incendiary. I admit that few of these factors discussed
in this paper deal directly with this work. However, while they may not
help determine the cause of the fire they could be critical in helping
to determine the cause of death or injuries.
can then be utilized by local or state fire marshals to modify and improve
building and fire codes. Without this type of data code officials have
trouble justifying code changes. For example, assume that investigators
find that in many case a sleeping occupant with a closed bedroom door,
either was overcome before the smoke could reach the hallway detector
or did not hear the detector in the hallway. This information can be used
to justify requiring interconnected detectors in every bedroom. If investigators
find that in many cases of smoldering fires that the ionization detector
is operating too late or not at all then this information could be used
to justify changes in testing and selection of detectors.
I hope this
information proves useful to those who read this paper. I would appreciate
any information that could be provided to me concerning the factors that
this paper discusses.
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