Reading of Fire Gases.

Movement-

The movement of fire gases (smoke layer) is a clear indication of the effect the ‘air

flows’ are having on the smoke layer. A static cold black smoke layer indicates that

the smoke layer has become detached from the original ignition source in the fire

compartment. A static grey smoke layer indicates seepage from the closed up fire

compartment and both scenarios denote that the airflows are not causing the flames

at the seat of the fire to burn up through the smoke layer because they are either

detached or isolated by a closed door.

Pulsating black smoke emitting on ‘over pressure’ from an exit port (window/ door) of

a fire room compartment indicates that air is getting to the seat of the fire, but there

is some back pressure in the smoke layer, creating the illusion of the smoke

pulsating, following the overcoming of the back pressure the next visible sign will be

the pulsations getting less and less and then a flame front reaching the exit port.

Ignitability-

The fire gases showing may have already ignited prior to arrival and are free flaming

from an exit port (window/door) which confirms their ignitability. The colour and

horizontal position of the neutral plane is also a good indicator of ignitability.

The ignitability of fire gases bottled up inside a fire room compartment can be

assessed by observing the fire gases exiting on ‘over pressure’ from the fire

compartment, upon opening of the door to see if they ignite on contact with air in the

corridor/outside, indicating whether they are at the thermal ignition temperature or

not.

Temperature-

The temperature of the fire gases is a big indicator of their fire development and

explosive potential. The fire gases exiting are normally either warm/hot or relatively

cold.

If the fire gases are warm/ hot upon opening up the fire room compartment, then it is

safe to assume the air intake, now entering via the open doorway is getting to the

seat of the fire on the ‘air track’ and burning up through the flame front towards the

exit port where the firefighters are positioned.

Conversely, if the fire gases are relatively cold then you can diagnose that if they are

grey static smoke in a corridor, this is because of seepage from the adjacent fire

room compartment. However, if the fire gases are black cold smoke emitting

approximately 450mm above the floor level from a closed up fire room compartment

following the opening of the door and they have very little movement, this indicates

that there is no flame-up at the seat of the fire and the fire gases are travelling away

from the ignition source and the two are becoming detached from each other.

Colour-

The colour of the smoke indicates the comparative amount of fuel and air that these

fire gases actually contain, basically grey smoke denotes more air than fuel mixture

on the flammable range.

Black smoke denotes more fuel than air and is more likely above its upper explosive

limit and indicates a rich mixture that is oxygen deficient.

White smoke (steam) denotes the evaporation moisture contained within

combustible materials e.g. white smoke emitting from a shingle tile roof is indicating

flames are impinging on the roof tiles either externally or internally and the next thing

to show following total evaporation of the moisture will be high energy fire gases

(black smoke) that may subsequently ignite and involve the entire roof space.

Horizontal position of the neutral plane-

The horizontal position of the neutral plane gives a visual indication of the position of

the fire gases (smoke layer) on the inverted flammable range chart (see below) and

if there is a smoke layer inside a closed up compartment approximately 450mm

above the floor level this indicates a rich mixture above its upper explosive limit.

A cold black static smoke layer in a halfway horizontal position inside a fire room

compartment following the opening up of the compartment indicates the neutral

plane is in the vicinity of its ideal mixture and most likely will contain high-energy

combustible gases and these circumstances can be diagnosed to indicate the

realistic possibility of a ‘cold smoke explosion’.

A cold grey static smoke layer in a halfway horizontal position in a corridor outside a

closed up fire room compartment indicates that the neutral plane could be

somewhere between the lower explosive limit and its ideal mixture and within its

flammable range.

A warm grey smoke layer moving with force, exiting the building/ structure on ‘over

pressure’ indicates the air intake is getting to the seat of the fire, but the ‘lean

flashover’ stage may not have yet been reached in the ceiling area above the seat of

the fire, because of the open plan nature of the building/structure.