Detectors that respond to smoke

Question 1

smoke detectors use one or more of the following

principles.

Answer 1

n Ionisation chamber smoke detectors
n Optical smoke detectors
n Multi-sensor detectors

Question 2

Draw a diagram of an Ionistation detector

Answer 2

Radiation source
Electrodes
negative Ions
Positive Ions
Amplifier
Circuit

Question 3

Describe how an Ionisation detector works

Answer 3

an open chamber within the detector contains a small radioactive source.
The source, usually Americium 241,
emits alpha particles and low energy gamma rays. This radiation causes the air in a chamber within the detector to become ‘ionised’,
promoting the flow of electricity between two electrodes
In the event of fire, smoke will enter the chamber interfere with the flow of electricity,
When the current falls below a predetermined
level, the detector activates the fire alarm

Question 4

name two main types of optical smoke detector

Answer 4

light scatter

obscuration

Question 5

draw a diagram and label a light scatter smoke detector

Answer 5

light source
photo electric cell
to alarm

Question 6

describe the light scatter detector

Answer 6

The light source and the photoelectric cell are
mounted in a lightproof housing
designed to allow smoke to flow into it unimpeded.
In the ‘non-fire’ condition light from the light
source (usually a light emitting diode) does not fall
on to the photoelectric cell.
When smoke particles enter the housing
some light is deflected upwards onto the photoelectric cell.
the cell will either create an electrical current in
the detector circuit or allow more current to flow
through it (depending on the type of cell being
used).
The small increase in current is normally amplified
by a transistorised circuit in order to energise a
relay which controls the alarm.
The detector is pre- set so that the alarm is given when the smoke density reaches a predetermined level

Question 7

describe a light obscuration detector

Answer 7

the
smoke obscures the light. The resultant reduction
in the intensity of light falling onto the photoelectric
cell causes an alarm signal to be raised

Question 8

which is more common light scatter or obscuration

Answer 8

Most
optical smoke detectors now work on the
light-scatter principa

Question 9

Describe combustion gas detectors

Answer 9

Combustion gas detectors are point type detectors,
which respond to the gases produced by a fire, e.g.carbon monoxide.
Carbon monoxide may spread
through certain forms of construction and thus
combustion gas detectors could operate at a considerable
distance from the fire, including on floors other than the fire floor. Care should be taken
that this does not present misleading information to
firefighters or others responding to a signal.
Users of combustion gas detectors should be aware
that the electro-chemical sensors in combustion
gas detectors have a finite life and should be
replaced in accordance with the manufacturers’
recommendations.

Question 10

what are multi sensor smoke detectors

Answer 10

more than one sensor is employed. This can
improve the efficiency of the detector over a wider
range of fire characteristics. It can also bring about
a significant potential for a reduction in the number
of false alarms generated by the detector.

Question 11

what is an aspirating system

Answer 11

In an aspirating smoke detection system, air samples are drawn by a pump or fan through holes in pipes running through the protected area.
The air is exhausted into a central unit containing a smoke detector.
The smoke detector, which may be of
either the optical or ionisation type responds to the
presence of smoke.
Each air entry hole in the system
is considered as though it is a separate smoke
detector

Question 12

Aspirating systems of smoke detectors are good in what situations

Answer 12

Equipment in enclosed cabinets

Ducting

Question 13

Where would a linear beam detector be used

Answer 13

high ceilings
aircraft hangars
cathedrals
tunnels
ducts

Question 14

Describe a linear beam detector

Answer 14

A beam detector consists of two units: a
transmitter producing a pulsed beam of infra-red
light generated by a gallium arsenide light emitting
diode (LED); and a monitor receiver tuned to
accept that frequency.
The two units are mounted at opposite ends of the
space to be protected.
The infra-red beam produced by the transmitter is
analysed by the receiver photo sensor for loss of
strength caused by smoke obscuration
When it exceeds a pre-set level the
‘smoke’ or ‘heat’ LED is illuminated and a fire
signal is transmitted.

Question 15

what type of detector is a linear beam system

Answer 15

optical

Question 16

describe a video detector system

Answer 16

Closed circuit television cameras monitor
an area, the signals from each camera being
analysed electronically. The analysis detects the
presence of smoke from the obscuration of part of
the camera’s field of view. Detection by this means
depends on the illumination of the field of view
either by normal lighting or by specially installed
infra-red light sources.

Question 17

The detection of fire by smoke detectors is dependent

on a number of factors

Answer 17

smoke concentration,

size, and shape of smoke particles

Question 18

In the early stages of most fires the smoke
particles are small, but as the fire develops they
tend to conglomerate to form larger particles. which detector does best at the types fire

Answer 18

The ionisation detector is generally more sensitive
to the smaller, normally visible, smoke particles.
This makes it particularly useful in the early stages of relatively clean burning fires (e.g. of wood and
paper).
It will not, however, always operate in the presence
of cold’ smoke. The optical detector is more efficient in situations where the protected risk is
likely to give rise to dense smoke (i.e. larger particles)
in the earlier stages of a fire as in some burning
plastics. Multi-sensor detectors using both
technologies tend to balance out the disadvantages

Question 19

In the main, earlier detection can be

obtained with a

Answer 19

smoke sensitive system than with

a heat sensitive one.