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in O NBS TECHNICAL NOTE 839

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

FIRE DETECTION

THE STATE-OF-THE-ART
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, ( 3 ) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences 2 — Applied Radiation 2 — Quantum Electronics s — Electromagnetics a — Time

and Frequency 3 — Laboratory Astrophysics 3 — Cryogenics s. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

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Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCffiNCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

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divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

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organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
FIRE DETECTION: THE STATE-OF-THE-ART

Richard L. P. Custer and

Richard G. Bright

Fire Technology Branch

Programmatic Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

y\0 . o 3°!

Sponsored by

Aerospace Safety Research and Data Institute Lewis Research Center

Cleveland, Ohio 44135

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued June 1974
National Bureau of Standards Technical Note 839

Nat. Bur. Stand. (U.S.), Tech. Note 839, 119 pages (June 1974) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:839). Price $1.55
Table of Contents

Page

Summary 1

1. Introduction 3

2. Fire Signatures and Detection Methods 4

2.1 Aerosol Signatures 4

2.2 Energy Release Signatures 6

2.3 Gas Signatures 7

3. Fatality Causal Agents .... 8

3.1 Effects of Aerosols 8

3.2 Effects of Heat 9

3.3 Effects of Toxic Gases 9

3.3.1 Carbon Monoxide 9

3.3.2 Carbon Dioxide 10

3.3.3 Hydrogen Chloride 10

3.3.4 Other Gases 10

3.4 Effects of Oxygen Depletion 11

3.5 Additive and Synergistic Effects 11

4. Classification of Detectors 12

4.1 Geometric Classification 12

4.2 Restoration Classification 12

4.3 Alarm Contact Circuit Classification 12

5. Detector Operating Mechanisms 13

5.1 Convected Energy Detectors 13

5.1.1 Fixed-Temperature Detectors ...... 13

5.1.1.1 Eutectic Metal Type 13

5.1.1.2 Glass Bulb Type . . . 14

5.1.1.3 Continuous Line Type 14

5.1.1.4 Bimetal Type 15

5.1.1.4.1 Bimetal Strip. .... .15

5.1.1.4.2 Snap Disc. ....... 15

5.1.2 Rate-of-Rise Detectors .16

5.1.2.1 Pneumatic Type 16

5.1.2.2 Thermoelectric Detectors . . . .17

5.1.2.2.1 Spot Type 17

5.2 Aerosol and Gas Detectors 18

5.2.1 Ion Chamber Type 18

5.2.2 Photoelectric Type .20

5.2.2.1 Light Attenuation Operation. . .20

5.2.2.2 Light Scattering Operation . . .22

5.2.3 Solid State Type 22

5.2.4 Resistance-Bridge Type. 23

ili
Page

5.3 Laser Beam Fire Detection 23

5.3.1 Laser Heat Detection Theory 24

5.3.2 Laser Smoke Detection Theory 24

5.3.3 Laser Detection Systems 25

5.3.4 Operational Experience . . . . . . . . 25

5.4 Radiant Energy Detectors 26

5.4.1 Infrared Type 26

5.4.2 Ultraviolet Type 28

5.4.3 Combination Ultraviolet-Infrared

Type , 28

5.5 Submicrometre Particle Counting Detectors. . 28

5.5.1 Particle Ionization Type 28

5.5.2 Condensation Nuclei Type 29

5.6 Fiber Optics in Detectors 30

5.7 Ultrasonic Detectors 3

5.8 Combination Detectors. . 31

5.8.1 Rate-of-Rise Fixed-Temperature Type. . 31

5. 8.^Resistance Bridge - Ion Chamber

Type 31

6. Detector Subsystems 32

6.1 Alarm Sounding Devices 32

6.2 Trouble and Supervision Circuits 3 2

7. Ambient Conditions Affecting Detector Response. . 33

7.1 Background Levels of Fire Signatures .... 33

7.2 Heating and Air-conditioning Effects .... 34

8. Reliability of Detection Devices 36

8.1 Heat Detectors 36

8.2 Light Sources and Photocells 37

8.3 Batteries .37

8.4 Field Experience 38

9. Maintenance of Detectors 38

10. Performance Standards and Acceptance Criteria

for Detection Devices . 38

10.1 U.S. Standards and Acceptance Criteria. . . 39

10.2 Foreign Standards and Acceptance Criteria . 42

10.3 Trends in Standards and Test Criteria ... 44

11. Field Tests of Detector Performance 45

12. Code Requirements for Fire Detection 47

12.1 State Requirements 47

12.2 Local Requirements 48

IV
Page

12.3 Model Building Code Requirements 48

12.4 Tne Life Safety Code Requirements 49

12.5 Mobile Home Requirements 49

12.6 Federal Housing Administration

Requirements 49

13. Conclusions 50

14

Acknowledgments 51

15. References 52

16. Bibliography , 60

17. Tables

I. Thermal Particulate Point in Air 78

II. Effect of Carbon Monoxide Exposure 79

IIIEffects of Carbon Dioxide Exposure 80

IV. Effects of Oxygen Depletion 81

V. Ratings of Fixed-Temperature Detectors ... 82

18. Figures

1 Properties of a Smouldering Cotton Fire. . . 83

2 Influence of Coagulation On Smouldering

Wood Smoke 84

3 Particle Size Distribution of Various Types

of Smoke . . . . , 85

4 Line Type Fire Detection Cable 86

5 Line Type Fire Detection Cable Using a

Glass Semiconductor. . 87

6 Bimetal Strip Heat Detector 88

7 Bimetal Snap Disc Heat Detector 89

8 Pneumatic Type Heat Detector 90

9 Ionization of Chamber Air Space 91

10 Effect of Aerosol in Ionized Chamber .... 91

11 Unipolar Ion Chamber 92

12 Bipolar Ion Chamber 92

13 Unipolar and Bipolar Ion Chambers in

Series ...... 93

14 Configuration of a Dual Ion Chamber

Detector 94

15 Beam Type Light Attenuation Smoke

Detector 95

16 Spot Type Light Attenuation Smoke

Detector 96

17 Light Scattering Smoke Detectors 97

18 Simple Schematic of Detector Using a Solid

State Gas Sensor 98
Page

19 Enlarged Sectional View of a Solid State Gas

Sensor

20 Photocell Masking for Heat Detection. . . . . 100

21 Corner-Cube Mirror. ......* 101

22 Typical Layout of Laser Detector 102

23 Heat and Smoke Detection With Laser Beam. . . 103

24 Fresnel Lens System 104

25 Servo-Control of Lens System for Laser Fire

Detector 105

26 bcnematic or Servo-Controlled Laser Fire

Detector. 106

2 7 bcnematic or Condensation Nuclei Particle

Detector. 107

2 8 Rate-of-Rise - Fixed-Temperature Detector

Using a Bimetal Element 108

29 Rate-of-Rise - Fixed-Temperature Detector

Using an Eutectic Metal . 109

30 Rate-Compensation Detector. ... 110

VI
List of Symbols and Abreviations

°C - Celsions

cm - Centimetre

CO - Carbon Monoxide

C02 - Carbon Dioxide

°F - Fahrenheit

F.M. - Factory Mutual Research Corporation

HC1 - Hydrogen Chloride

HCN - Hydrogen Cyanide

HF - Hydrogen Fluoride

H«S - Hydrogen Sulfide

H^O - Water

m - Metre (s)

N.F.P.A. - National Fire Protection Association

NH~ - Ammonia

N0

2 - Nitric Oxide

2

i - Oxygen

Ods ft. - Obscuration Per Foot

O.D. m - Optical Density Per Metre

OH - Hydroxy1 Ion

ppm - Parts Per Million

PVC - Polyvinyl Chloride

s

- Second (s)

SO, - Sulfur Dioxide

U.L. - Underwriters Laboratories

Vll

Fire Detection: The State-of-the-Art

by R.L.P. Custer and R.G. Bright

The current state-of-the-art in fire detection technology is reviewed

considering the nature of fire signatures, detection modes used, test

methods, performance requirements and code requirements for fire

detection. Present trends in standards development and recommendations

for future work are included. An extensive bibliography is provided.

Key Words: Fire detection; fire detection code requirements; fire

detector testing and standards; fire detectors; fire signatures.

SUMMARY

The purpose of this work is to provide a review of

the data regarding the outputs from the combustion process

that might be used to detect the presence of a fire and

to discuss existing fire detector technology. The term

fire signature is defined as any product of a fire which

changes ambient conditions and thus has the potential

for use in fire detection.

Fire signatures, causes of fatalities, and the

classification of detectors are briefly defined to

establish the background for a detailed discussion of

detector operating mechanisms. The operating principles

of each detector type are discussed in terms of function,

threshold of operation and application.

Consideration is also given to such problems as

audibility of alarm sounding devices, trouble circuitry,

supervision, the effects of ambient conditions on detector

operation, and reliability.

A comparison is made of performance standards and

acceptance criteria both in the United States and in

various foreign countries. This includes a brief discussion

of the trends in the areas of testing procedures and

acceptance criteria. A brief treatment of the value of

and restrictions on the results of field tests is also

included.

The expanding body of code requirements pertaining to

the use of detectors is reviewed and examples of specific

requirements discussed.
It is concluded that with present state-of-the-art

hardware, detection of a fire can be achieved within

milliseconds of inception. It is therefore possible,

though not always cost effective, to provide nearly any

response time desired. False alarms, including both

equipment failure and signals from background aerosols

such as cigarette smoke and cooking fumes, represent a

major problem to be overcome. It is suggested that

multi-mode detectors with discriminating logic circuits

can provide some help in this area. Further work

should be encouraged to develop the hardware

to detect fire signatures which are not presently being

used, such as carbon monoxide. It is also concluded that

much work needs to be done to up-grade and standardize

the test procedures and performance requirements for

detection devices.

A bibliography is provided.
I . Introduction

Hostile fire is a major drain on our national resources.

Although the fire loss figures are conservative due to

uncertain and inadequate collection methods , some figures

collected by the National Commission on Fire Prevention

and Control (1)

^

can be useful in presenting the relative

magnitude of the problem. In the U.S. property losses alone

were reported to be 2.7 billion dollars in 19 72. Nearly

12,000 lives are lost each year with the majority resulting

from the effects of smoke and toxic gases before being exposed

to the flames. It has also been estimated that of those

persons exposed to fire, 300,000 will be injured and survive.

Perhaps 50,000 of these survivors will spend 6 weeks to 2 years

in hospitals or as out-patients while undergoing treatment

and reconstructive surgery. When the costs of burn treatment,

productivity loss , insurance operations and fire department

operations are added to the direct fire loss figures the

total cost of fire approaches 11.4 billion dollars.

Any fire, no matter how large it may become, begins

as a small fire. In its earliest stages most fires are

innocuous and are easily controlled. The noxious products

are minimal and generally confined to the immediate vicinity

of the point of origin. The earlier in its history of

development that a fire can be detected, the better are

the chances of escape for those persons in potential danger

and the sooner suppression methods can be brought to bear

on the fire.

The purpose of detection is two- fold, it can reduce

life loss and it can reduce property loss. Where human

lives are at risk, time becomes an important factor. Time

is needed to alert the occupants and time is needed for

them to reach an area of refuge. Throughout this time

period it is essential that an escape route be passable.

Where the risk to life is mimimal and property loss reduction

is the consideration, longer detection times are often

tolerated for the sake of minimizing needless alarms. Often

the sprinkler system with its water flow alarm doubles as

the detection system. Sprinkler systems as presently used

are often slow to respond to smouldering fires —those

which produce large quantities of toxic products.

Appropriately selected and properly installed, fire

detection devices can have a major effect on losses of

life. A study by McGuire and Ruscoe (2) of the circumstances

surrounding 342 dwelling fire deaths in Ontario, Canada,

indicated that the use of smoke detection could result in a

[1] Numbers in ( ) refer to the list of references beginning

on page 52.

3
could result in a 41 percent saving of life; the use of

thermal detectors could save 8 percent.

The purpose of this report is to review data regarding

the outputs from the combustion process and the present

state-of-the-art in fire detection technology. The work

deals with the nature of fire signatures, the operating

principles of the detection elements, performance testing

and acceptance criteria, and code requirements.

2 . Fire Signatures and Detection Methods

From the moment of its initiation, fire produces

a variety of changes in the surrounding environment. Any

product of a fire which changes the ambient conditions is

referred to as a "fire signature" and has the potential

for use in detection. The production of smoke for example,

will result in a decrease in visibility. Not all fire

signatures, however, are practical for detection purposes.

To be useful, a fire signature should generate a measurable

change in some ambient condition. The magnitude of that

change must be greater than the normal background variations

for the condition. The magnitude of the change in an

ambient condition is the signal from a fire signature,

and the background level with its normal variations is

referred to as the noise. All other factors being equal,

such as hardware costs and detection time with respect to

some predefined hazard level, the preferred fire signature

will be that which can generate the highest signal to

noise ratio in the earliest period of the fire development.

The best signatures are those which are associated exclusively

with fire in a wide variety of fuels. Fuel-specific

signatures , such as the release of hydrogen chloride (HC1)

from polyvinyl chloride (PVC) combustion, may be of use

in detectors for specialized applications but are of little

use for general purpose functions. Individual fire signatures

are discussed below and the operational principles of

their associated detection mechanisms are discussed in a

later section.

2.1 Aerosol Signatures

The process of combustion releases into the atmosphere

very large numbers of solid and liquid particles which

range in size from 5xl0~4 micrometres to 10 micrometres.

These particles suspended in air are called aerosols . The

aerosols resulting from a fire represent two different

fire signatures. Those particles which are less than

0.3 micrometres do not scatter light efficiently and

thus are classified as invisible. Those which are larger than
0.3 micrometres scatter light and are therefore classified

as visible. The invisible aerosol signature is generally

referred to as the "products of combustion" and the visible

aerosol signature as "smoke". The detectors which respond

to these signatures are generally referred to as products

of combustion and smoke detectors respectively. The term

"products of combustion" has been widely used as described

above but can be ambiguous in that aerosols, energy, and

gases are, in fact, all products of the combustion process.

For purposes of this discussion the terms invisible and

visible aerosols will be introduced to insure clarity.

Invisible aerosol is the earliest appearing fire

signature noted to date. Heating of materials during

the pre-ignition stage of a fire produces submicron

particles ranging in size from 5x10"^ to lxlO -3 micrometres.

These particles are generated at temperatures well below

ignition temperatures. The temperature at which submicrometre sized particles are generated from materials

is defined as the thermal particulate point. Thermal

particulate points determined by Van Luik (3) for some

common materials are shown in Table I. These invisible

aerosols are generated in very large quantities. A 0.098

ounce (2.8 g) sample of bond paper burned in air in a room

7.5 feet (2.28 m) square with an 8.9 foot (2.71 m) ceiling

produced a maximum particle concentration of 1.3x10°

particles per cubic centimetre (3.6xl0 10 part. /ft3

)

As heating of a material progresses toward the

ignition temperature, the concentration of invisible aerosol

increses to the point where larger particles are formed

by coagulation (4). As this process continues, the particle

size distribution becomes log normal with the most frequent

sizes in the range between 0.1 and 1.0 micrometres. The

smaller particles, less than 0.1 micrometers disappear

either by coagulation or by evaporation, and the larger

particles, greater than 1.0 micrometer, are lost through

the processes of sedimentation following Stokes' Law.

Aerosols in this size range are remarkably stable (4) and

contain particles in both the visible and invisible aerosol

signature range. This "ageing" of aerosols has been reported

by Van Luik (3) and Scheidweiler (5) . The influence of

time on the size distribution of aerosols is shown in

Figures i and 2. The production of visible aerosols can

occur prior to ignition and is usually initiated at

temperatures several hundred degrees higher than the thermal

particulate point (3)

The aerosol size distribution from smouldering and

flaming combustion of various materials has been determined

by Scheidweiler (5) . The results are shown in Figure 3
and it can be seen that in some cases smouldering fires

produce more large particles than flaming fires. It is

important, however, to note that for both fire types, the

maximum relative particle concentration appears to be

in the range of particle sizes smaller than 0.3 micrometres.

This indicates that invisible aerosol signals can provide

early detection- in the immediate vicinity of either fire

type. The detection modes best suited for the invisible

aerosol signature are the condensation nuclei type for

the smaller invisible aerosols and the ion chamber type

for the larger. Photoelectric units with light sources

having a major spectral component in the near ultraviolet

and bluegreen wavelengths and a suitable photocell should

also respond to the larger invisible aerosols since the

best scattering of energy occurs when the particle diameter

approaches the wavelength of the incident radiation.

2.2 Energy Release Signatures

Throughout its entire course, fire is constantly

releasing energy into the surrounding environment. This

energy release produces several useful fire signatures.

The earliest energy signatures detectable with

hardware existing at the time of this publication are the

infrared (IR) and ultraviolet (UV) signatures. With the

exception of acetylene and other highly unsaturated

hydrocarbons, the infrared emissions from hydrocarbon

are particularly strong in the 4.4 micrometre region due

to CO2 and in the 2.7 micrometre region due to water vapor

and account for nearly all the emitted energy (6) . Since

the infrared component of sunlight, a potential noise or

false signal source, is reduced in these regions by

absorption due to atmospheric CO2 and H2O, a high signalto-noise ratio is obtained. The CO2-H2O radiation signature

can be used effectively for detection but there is the

possibility of noise from man-made infrared sources.

Another signature which affects the infrared signal from

a flame is the modulation of the energy output level due

to flame flicker (7) . This flicker is characteristic of

flames and has a frequency range of 1.5 to 15 hertz.

The total infrared signature has been used in the

detection of both smouldering and flaming fires but has

the disadvantage of having a wide range of noise levels

from solar and man-made sources. Detectors using both

the CO2-H2O and flicker signatures have an excellent

signal-to-noise ratio.
The ultraviolet fire signatures appear in flames as

emissions from OH, C0~ and CO in the 0.27 to 0.29

micrometre region (8) . Although the signal to noise

ratio of the UV signatures, are less than those of the

IR signatures, they have been effectively used for flame

detection especially where magnesium or its alloys may

be involved since burning magnesium has a strong signal

in the 0.28 micrometre region (8)

.

Convected thermal energy from a fire causes an increase

in the air temperature of the surrounding environment.

The time required for release of sufficient energy to produce

a significant convected energy signal varies from less

than one minute for rapidly developing fires to hours or

even days for slowly developing or smouldering deep seated

fires. In comparison to invisible aerosol and radiant

energy signatures, the convected thermal energy signatures

often appear well after life threatening conditions have

been reached from excessive aerosol and/or toxic gas

concentrations

.

2.3 Gas Signatures

During a fire, many changes occur in the gas content

of the atmosphere, not only in the area of fire origin

but often in areas far removed from the fire. For the

most part these changes consist of the addition to the

atmosphere of gases which are not normally present. These

changes can be called evolved gas signatures. One change

which often takes place is the reduction of oxygen content

which can be called the oxygen depletion signature

.

Many gases are evolved during a fire. Some examples

are H2O, CO, CO? > HC 1/ HCN , HF, H

2 S , NH- and nitrogen oxides.

Most of the evolved gas signatures are fuel-specific and

are not associated with a sufficiently large number of

fuels to be used for general purpose detection. One gas,

carbon monoxide, is present in nearly all fire situations

and thus, the CO signature may be of use in fire detection.

The rate of CO production varies considerably with the

type of fuel, the amount of ventilation and whether the

fire is of the smouldering or open flaming type. Work

by Bruce (9) showed rapid increases in CO content in room

fire tests. The evolution of CO was related to

2 depletion,

and in one test, the concentration of CO rose to over 17

percent in about 7,5 minutes* High concentrations of CO can

also be reached in rooms remote from the fire. This was shown

by Shorter et al (10) . Concentrations of CO in a closed

second floor bedroom remote from the fire, reached 1.28 percent

in 18 minutes after ignition. The same CO value was reached
at 12 minutes in an open bedroom. The CO signature may have

its best application in detection of slow burning and

smouldering fires. Data obtained by the Los Angeles- Fire

Department (11) indicated that CO buildup preceded hazardous

temperatures in slow burning fires while the opposite was

observed in rapidly burning fires. A complete review of all

evolved gas signatures is beyond the scope of this report.

For a detailed study, the reader is referred to the work of

Ives et al (12) . Although C02 is also non fuel-specific,

the signal-to-noise ratios are generally too low for the

effective use of C02

as a fire signature due to the presence

of ambient C02

.

3. Fatality Causal Agents

When exposed to the by-products of a fire, the human

organism is in a hostile and often fatal environment. The

psychological and physiological effects of exposure to these

products both individually and in various combinations are

discussed below.

3.1 Effects of Aerosols

Aerosols derived from a fire produce several effects on

humans. Much of the aerosol consists of carbon particles

upon which may be adsorbed a number of irritant compounds

such as organic acids, aldehydes or HC1 (12) . Studies with

dogs exposed to kerosene aerosol consisting largely of carbon

soot, produced no noticable damage and most of the soot was

coughed out within 24 hours (13) . This work further indicated

that the aldehydes in wood aerosol were the significant factor

in causing respiratory tract swelling and irritation.

Inhalation of these products makes breathing difficult and

may be accompained by severe coughing and a burning sensation

in the chest and throat. The eyes generally suffer irritation

which makes keeping them open difficult. In addition to the

physiological effects, absorption and scattering of light from

visible aerosols can reduce visibility to near zero in some

cases. These effects combined with fear, anxiety and loss

of orientation often result in panic behavior (14) . Irrational

actions associated with panic have resulted in persons becoming

lost, being trapped in dead-end corridors or closets, and

in extreme cases merely sitting down and waiting to be overcome

Visible aerosol concentration is measured in terms of percent

reduction of light per foot of travel path. For persons

familiar with their surroundings, the Japanese (15) set '9.2

percent per foot (0.13 OD/m) * as physiological and psychologica

tolerance level and for unfamiliar persons 3 percent per foot

(0.04 OD/m)

* Optical Density/meter
3.2 Effects of Heat

The most obvious effect on humans of the thermal energy

released by a fire is that of the burn injury. This usually

occurs when the victim is unable to escape contact with

flames or hot gases. Experiments with animals have shown

that death occurs after 2 minutes exposure at 212°F (100°C)

and at 30 minutes when exposed to 140°F (59 °C) (16) . Although tests conducted by Dupont (17) , using firemen,

showed that the subjects' ability to voluntarily endure

heat varied from 2 minutes at 300°F (147°C) to 45 seconds

at 740°F (389°C) , it is likely that these are extreme

levels of tolerance. In addition to surface burns, inhalation

of heated gases and aerosols often produces thermal

damage resulting in sloughing of the trachea lining and

hemorrhaging in the respiratory tract (18) . Exposure to

elevated temperatures may also cause shock due to pooling

of blood at the body surface as the body attempts to cool

off.

3.3 Effects of Toxic Gases

The toxic gases evolved from a fire are often present

in sufficient quantities to produce adverse effects on the

human organism. The effects of exposure to individual

gases are discussed in the following paragraphs.

3.3.1 Carbon Monoxide

Carbon monoxide is present in nearly all hostile fire

situations and is an important factor in losses of life.

Zikria et al (13) (19) , working with the clinical examination

records and autopsy reports of fire victims in New York

City, found that CO poisoning rather than respiratory tract

damage was the significant factor with victims having post

burn survival times less than 12 hours. The threshold

limit value for exposure to CO for an 8 hour working day

is 50 parts per million (ppm) (20) . In fires with conditions

of restricted ventilation, CO concentrations of 138,000

ppm have been recorded (21) . The physiological effects

of exposure to CO are shown in Table II (22) . CO acts to reduce

the oxygen carrying ability of the blood by combining with

hemoglobin to form carboxyhemoglobin. Hemoglobin is the

oxygen transporting component of blood. The various effects

of CO exposure include, in order of occurence, headache,

dizziness, dimness of vision, nausea, increased pulse and

breathing rates, confusion and loss of orientation,

unconsciousness, reduced pulse and breathing rates,

convulsions and, ultimately death. It is clear that

exposures to even moderate levels of CO can severely

restrict a person's ability to function in a fire
situation especially when combined with the adverse effects

of aerosols and elevated temperatures. Work conducted by

the Johns Hopkins Applied Physics Lab indicates that, in

the 106 fire fatalities studied, 50 percent involved CO

poisoning as the cause of death and an additional 30 percent

were due to CO poisoning complicated by the effects of

coronary disorders and/or the use of alcohol (2 3-A)

.

3.3.2 Carbon Dioxide

Carbon dioxide (C0

2

) is one of the products of complete

combustion and is commonly associated with both flaming

and smouldering fires in amounts exceeding the threshold

limit value of 5,000 ppm (20). In the bloodstream, C02 combines with hemoglobin and acts to stimulate breathing

rate. It may also act as an asphyxiant by displacing oxygen.

The early symptoms of CO2 intoxication are dizziness and

shortness of breath. This is followed by mental excitement

and often maniacal or irrational behavior. Concentrations

of 9 percent (90,000 ppm) can be fatal in 4 hours (2 4)

due to C02-hemoglobin complexing. Although concentrations

up to 6.75 percent (21) have been recorded in bedding test

fires, the principle effect of even low concentrations is

that of raising the breathing rate which increases the

intake of other toxic substances such as CO. The effects

of increasing CO- concentration are shown in Table III (22)

.

3.3.3 Hydrogen Chloride

Hydrogen chloride is not found in fires with cellulosic

materials but is often associated with the burning of many

common plastics such as the vinyls. The threshold limit

value is 5 ppm and a noticeable irritation of the mucous

membranes occurs at 35 ppm. Concentrations of 1,000 to

2,000 ppm produce severe reactions which may be fatal (2 5)

.

3.3.4 Other Gases

Many other gases are evolved from burning materials

in small but potentially dangerous quantities. Tests,

by Gross et al (26) , of both flaming and smouldering

combustion of materials used in interior finishings for

aircraft produced measurable quantities of toxic gases.

These are discussed briefly below. Polyamides and

modacrylics produced hydrogen cyanide. Hydrogen sulfide

(H2S) was produced from chloroprene. Sax (2 8) notes that

H2S is also produced from burning hair, leather, wool and

some dyes as well as some synthetic fabric finishes.

Polysulfone plastic was found to produce sulfur dioxide

(SO2) . Sax notes that SO2 also can be produced from

burning rubber and wood. Gross et al (26) also detected

10
hydrogen fluoride, nitrogen oxides and ammonia. The gases

listed above represent a small percentage of the chemical

species present in fire gases. Although usually present

only in small sub- lethal quantities these gases can produce

additive and synergistic effects within the total system

of fire products. These effects will be discussed in a

subsequent section.

3.4 Effects of Oxygen Depletion

Although' oxygen depletion is usually confined to the

immediate area of a fire, its effects can be felt in remote

areas of the buildings involved with either a large open

burning fire or, if the building is tightly sealed,

smouldering fires of long duration. The physiological

effects of oxygen depletion are shown in Table IV.

3.5 Additive and Synergistic Effects

When dealing with mixtures of toxic materials,

consideration must be given to additive and possible

synergistic effects. Lacking specific information,

a first approximation of the hazardous condition created

by a mixture of toxic substances can be made by assuming

that the effects are at least additive (27) . The threshold

limit of a mixture is exceeded when the following expression

exceeds unity:

C + C2 . . . Cn

T T2 Tn

where C is the observed concentration of any component

of the mixture and T is the established threshold limit

for that component for the exposure time to reach a

predefined effect or lack of effect. It is not, however,

safe to assume that combined effects are strictly additive.

It has been demonstrated that the total effects of mixtures

of toxic materials can be much greater than the additive

case. This is defined as a synergistic effect. Pryor

et al (24) studied the synergistic effects on the mortality

of mice. Control groups of 10 animals were exposed for

4 hours to a mixture of 16 percent Q>2, 750 ppm CO and 30

percent CO2 and all survived. The experimental groups

were exposed to the same mixture but with added small amounts

of NO2 / SO2 or HCN. It was found, for example, that the

addition of 10 ppm HCN to the base mixture resulted in

the death of 2 mice during the 4 hour test. A concentration

of 150 ppm HCN in air was required to achieve the same

mortality rate. Considering the effects of increased

breathing rates associated with elevated CO2 and CO and

11
decreased O2 combined with the irritant effects of fire

generated aerosols and the possible synergistic effects

of small amounts of other toxic gases, it is clear that

persons exposed to these conditions can quickly be rendered

helpless. Thus, when detection is employed for the purpose

of limiting life loss, the use of devices which respond

early in the fire' history before occupied areas become

untenable is clearly indicated.

4 . Classification of Detectors

Fire detectors may be classified in several ways.

Classification can be made on the basis of placement of

functional characteristics of detectors and the operating

principle. Classification by placement and functional

characteristics as described in this section and

classification by operating principle is described in

section 5.0.

4.1 Geometric Classification

Detectors may be classed by the geometry of the area

they cover. Spot detectors are devices whose detecting

element responds to conditions at a single point. A line

detector senses conditions along a continuous linear path.

Volume detectors are those which monitor conditions within

a specified volume and will respond to signals anywhere

within that volume.

4.2 Restoration Classification

Detectors may also be classified by the way in which

they are placed in service following operation. Restorable

detectors are those which can be restored to operative

condition following a fire. Those may be either of the

self-restoring or the manually restoring type. Nonrestorable

detectors have sensing elements which are destroyed by

the fire.

4.3 Alarm Contact Circuit Classification

The type of alarm contact circuit can also be used

to classify detectors. An open circuit detector has

contacts which are normally open and close on alarm while

a closed circuit type opens the contacts or breaks a circuit on alarm. A transfer circuit type opens one circuit

and closes another when the alarm conditions are reached.

12
5. Detector Operating Mechanisms

The following is a discussion of the various operating

i^echanisms presently used to sense the presence of fire

signatures.

5.1 Convected Energy Detectors

Heat detectors respond to the convected thermal energy

of a fire. They may respond either at a predetermined

or fixed temperature or a specified rate of temperature

change. In general/ heat detectors utilize some physical

or electrical change which occurs in a material when exposed

to heat

.

5.1.1 Fixed-Temperature Detectors

Fixed-temperature detectors are designed to alarm

when the temperature of the operating element reaches a

specified point The air temperature at the time of

operation may be higher than the rated temperature due

to the thermal inertia of the operating elements. Fixed

temperature heat detectors are available to cover a wide

range of operating temperatures. This is necessary so

that detection can be provided in areas which are normally

subjected to high ambient (non-fire) temperatures. The

temperature ratings for heat detectors are listed in Table

V.

5.1.1.1 Eutectic Metal Type

Eutectic metals, alloys of bismuth, lead, tin and

cadmium which melt rapidly at a predetermined temperature,

can be used as operating elements for heat detection. The

most common such use is the fusible element in an automatic

sprinkler head. Fusing of the element allows water to

flow in the system which triggers an alarm by various

electrical or mechanical means.

A eutectic metal may be used in one of two ways to

actuate an electrical alarm circuit. The simplest method

is to place the eutectic element in series with a normally

closed circuit. Fusing of the metal opens the circuit

which triggers an alarm. The second method employs an

eutectic metal as a solder to secure a spring under tension.

When the element fuses, the spring action is used to close

contacts and sound an alarm. Devices using eutectic metals

cannot be restored. Either the device or its operating

element must be replaced following operation.

13
5.1.1.2 Glass Bulb Type

Frangible glass bulbs similar to those used for sprinkler

heads have been used to actuate alarm circuits. The bulb,

which contains a liquid and a small air bubble, is used

as a strut to maintain a normally open switching circuit.

When exposed to heat the liquid expands, compressing the

air bubble. When the bubble is completely absorbed, there

is a rapid increase in pressure, shattering the bulb and

allowing the contacts to close. The desired temperature

rating is obtained by controlling the size of the air bubble

relative to the amount of liquid in the bulb.

5.1.1.3 Continuous Line Type

As an alternative to spot- type fixed temperature

detection, various methods of continuous line detection

have been developed. One type of line detector uses a

pair of steel wires in a normally open circuit. The

conductors are insulated from each other by a thermoplastic

of known fusing temperature (28) . The wires are under

tension and held together by a braided sheath to form

a single cable assembly (see Figure 4) . When the design

temperature is reached, the insulation melts, contact is

made, and an alarm is generated. Following an alarm,

the fused section of the cable can be replaced to restore

the system.

A similar alarm device utilizing a semiconductor

material and a stainless steel capillary tube (29) has

been developed for use where mechanical stability is a factor. The capillary tube contains a coaxial center

conductor separated from the tube wall by a temperature

sensitive glass semiconductor material (see Figure 5)

.

Under normal conditions, a small current (i.e., below alarm

threshold) flows in the circuit. As temperature rises

the resistance of the semiconductor decreases allowing

more current flow and the resulting voltage drop triggers

an alarm. These detectors have been used successfully

for detection of fires in aircraft engine cells.

An electrical transmission line device has been

suggested for use as a heat detector (30) . An electrical

pulse is sent along an electrical transmission line. The

line terminates with an impedance load which is the same

as the characteristic impedance of the line being used.

If the line is intact, it will appear to have infinite

length and the pulse will not return to the transmission

point. If the line is damaged by fire, the resulting

discontinunity will reflect the pulse. The time required

14
for the pulse to return can be used to determine the

location of the discontinunity (fire) . 5.1.1.4 Bimetal Type

When a sandwich of two metals having different

coefficients of thermal expansion is heated, differential

expansion causes stresses in the assembly which are resolved

by bending or flexing towards the metal having the lower

expansion rate. The low expansion metal commonly used

is Invar, an alloy of 36 percent nickel and 64 percent

iron. Several alloys of manganese-copper-nickel/

nickel-chromium-iron or stainless steel may be used for

the high expansion component of a bimetal assembly. Bimetals

are used for the operating elements of several types of

fixed temperature detectors. These detectors are generally

of two types, the bimetal strip and the bimetal snap disc.

Bimetal detectors actuate when the element is heated and

flexing action closes a normally open circuit.

5.1.1.4.1 Bimetal Strip

Devices using bimetal strips place the strip directly

in the alarm circuit. As the strip is heated it deforms

in the direction of its contact point (see Figure 6) . The

width of the gap between the contacts determines the operating

temperature. The wider the gap, the higher the operating

point. One drawback to this type of device is its lack

of rapid positive action. The gradual bending of the element

as it is heated may result in false alarms from vibration

or jarring as the rated temperature is approached (2 8)

,

for example , during periods of transient high ambient

temperatures which are below the alarm point.

5.1.1.4.2 Snap Disc

The operating element of a snap-disc device is a bimetal

disc formed into a concave shape in its unstressed condition

(see Figure 7) . As the disc is heated the stresses developed

cause it to reverse curvature and become convex. This

provides an instantaneous positive action which allows

the alarm contacts to close. The disc itself is not usually

part of the electrical circuit. Snap-disc devices are

not as sensitive to false or intermittent alarms as the

bimetal strips described above.

A different application of the thermal expansion

properties of metals is found in the rate compensation

detectors which use metals of different thermal expansion

rates to compensate for slow changes in temperature while

15
responding with an alarm for rapid rates of temperature

rise and at a fixed maximum temperature as well. For a

further discussion of this device, see the section on

Combination Rate-of-Rise-Fixed-Temperature Detectors

.

All thermal detectors using bimetal or expanding

metal elements have the desirable feature of automatic

restoration after operation when the ambient temperature

drops below the operating point.

5.1.2 Rate-of-Rise Detectors

One effect which a fire has on the surrounding

environment is to generate a rapid increase in air

temperature in the area above the fire. While fixed

temperature heat detectors must wait until the room ceiling

temperature reaches at least the designed operating

point before sounding the alarm, the rate-of-rise

detector will function when the rate of temperature change

exceeds approximately 15°F (8.33°C) per minute. Detectors

of the rate-of-rise type are designed to compensate either

mechanically or electrically for normal changes in

ambient temperature which are expected under non-fire

conditions. The various types of rate-of-rise heat

detectors are discussed below.

5.1.2.1 Pneumatic Type

The expansion of gas when heated in a closed system

can be used to generate the mechanical forces needed to

operate alarm contacts in a pneumatic fire detection device.

A completely closed system presents problems in that false

alarms can occur strictly from slow changes in ambient

temperature as the pressure exerted on the operating mechanism is related only to the absolute change in temperature,

regardless of the rate of temperature change. The pneumatic

detectors in use today avoid this problem by venting the

pressure which builds up during slow changes in temperature.

The vents are sized so that when the temperature changes

rapidly, such as in a fire situation, the pressure change

exceeds the venting rate and the system is pressurized.

These systems are generally sensitive to rates of temperature

rise exceeding 15°F (8.33°C) per minute. The pressure

is converted to mechanical action by a flexible diaphragm.

A generalized schematic of a pneumatic system is shown

in Figure 8

.

Pneumatic heat detectors are available for both line

and spot applications. The line systems consist of metal

tubing in a loop configuration attached to the ceiling

16
of the area to be protected. Except where specifically

approved, Underwriters' Laboratories requires that lines

of tubing be spaced not more than 30 feet (9.1 m) apart and

that no single circuit exceed 1000 feet (304.8 m) in length.

Zoning can be achieved by insulating those portions of

a circuit which pass through areas, from which a signal

is not desired.

A pneumatic heat detector is available which will

respond both to small changes in the average temperature

over a large area and rapid changes in a segment as short

as 1/4 inch (0.6 cm) (31) . This is accomplished using a sealed metal tube containing granules of a metal hydride

which releases hydrogen gas at high temperature with a

noble gas (helium) filling the void spaces at a pressure

of one atmosphere. When exposed to low heat levels over

a complete zone, the noble gas in the interstitial spaces

expands closing mechanical contacts in much the same manner

as an ordinary pneumatic system. When a small segment

reaches a temperature of about 1500°F (807°C) , the

hydrogen is released rapidly from the metal hydride in

sufficient volume to increase the internal pressure and

operate the alarm contacts. Unwanted alarms due to small

changes in ambient temperature can be controlled by adjustment

of the alarm contacts. The system is self- restoring. Once

the heat has been removed, the hydrogen gas recombines

with the metal granules to form the hydride.

For spot applications and in small areas where line

systems might not be able to generate sufficient pressures

to actuate the alarm contacts, heat collecting air chambers

or rosettes are often used. These units act like a spot

type detector head by providing a large volume of air to

be expanded at a single location.

The pneumatic principle is also used to close contacts

within spot detectors of the rate-of-rise - fixed-temperature

type. These devices are discussed below.

5.1.2.2 Thermoelectric Detectors

Various thermoelectric properties of metals have been

successfully applied in devices for heat detection. The

properties used are the generation of a voltage between

bimetallic junctions (thermocouples) at different temperatures

and variations in rates of resistivity change with temperature

(27) . 5.1.2.2.1 Spot Type

Spot type devices which operate in the voltage generating

17
mode, use two sets of thermocouples. One set is exposed

to changes in the atmospheric temperature and the other

is not. During periods of rapid temperature change, associated

with a fire, the temperature of the exposed set increases

faster than the unexposed set and a potential is generated.

The voltage increase associated with this potential is

used to operate the alarm circuit.

5 . 2 Aerosol and Gas Detectors

The presence of aerosols and gases produced by a fire

can be detected in several ways. Detection of these fire

signatures is of great interest due to their adverse effects

on the occupants of buildings. The operating elements

of these detectors are described below.

5.2.1 Ion Chamber Type

The ion chamber detector reacts both to the visible

and the invisible components of the products of combustion.

It responds best to particle sizes between 0.01 and 1.0

micrometres (32) . The ionization chamber has been used

for many years as a laboratory instrument for detecting

microscopic particles. Thirty years ago Dr. Ernst Meili,

a Swiss physicist, developed an ionization chamber device

for the detection of combustible gases in mines (33) . The

major breakthrough in the field resulted from Dr. Meili'

invention of a cold-cathode tube which would amplify the

small signal produced by the detection chamber sufficiently

to trigger an alarm circuit. This reduced the electronics

required and resulted in a practical detector. In most

models today, the cold-cathode tube has been replaced with

solid state circuitry which further reduces the size and

cost.

The basic detection mechanism of an ionization detector

consists of an alpha radiation source in a chamber containing

positive and negative electrodes. Radiation sources are

commonly Americium 241 or Radium 226i. The strength of

the sources range from 0.05 to 80 microcuries. The

alpha radiation in the chamber ionizes the oxygen and nitrogen

molecules in the air between the electrodes causing a small

current (10~H amps) to flow when voltage is applied (see

Figure 9)

When aerosols enter the chamber, they reduce the

mobility of the ions reducing the current flow between

the electrodes (see Figure 10) . The resulting change in

the balance of the electronic circuit is used to trigger

an alarm at a predetermined level of aerosol in the chamber.

18
By proper placement of the alpha source, two types

of chambers may be produced. These are unipolar and bipolar

(32) chambers. A unipolar chamber is created by using

a tightly collimated alpha source placed close to the negative

electrode thus ionizing only a small part of the chamber

space (see Figure 11) . With this configuration, most

of the positive ions are collected on the cathode leaving

a predominance of negative ions flowing through the chamber

to the anode. The bipolar chamber has the alpha source

centrally located so that the entire chamber space is subject

to ionization (see Figure 12) . The unipolar chamber is

theoretically a unipolar and a bipolar chamber in series (see

Figure 13) . The relative merits of the two types of chamber design

have been discussed by Johnson (33) , who indicated that the

unipolar chamber has approximately three times the sensitivity

of the bipolar configuration. It has been proposed that

this is due to the fact that there is less loss of ion

carriers by recombination or neutralization of ions of

opposite signs which occurs in the bipolar chamber. This

results in a higher signal-to-noise ratio and a stronger

alarm signal to the amplifier circuit.

The alarm signal in an ion chamber detector is generated

by a voltage shift at the junction between a reference

circuit and the sampling chamber. The voltage shift results

from a current decrease in the sampling chamber when products

of combustion are present. The reference circuit may be

either electronic, which compensates for power supply

variations and component aging, or a second ion chamber

only partially open to the atmosphere (see Figure 14)

These circuits are referred to as single chamber and

dual chamber, respectively. The dual chamber has an

advantage in the reduction of false alarms due to changes

in ambient conditions. The reference chamber will tend

to compensate for slow changes in temperature and humidity.

It should be noted that some ion chamber detector

designs are subject to changes in sensitivity with varying

velocity of air entering the sampling chamber. Some

designs will loose sensitivity as velocity increases and

others will shift sufficiently in the more sensitive

direction to trigger a false alarm. Care must be taken

to choose the appropriate design for the area to be

supervised.

Tests by Putnam and Parker (34) indicate that ion

chamber detectors are not suitable for use in applications

where high ambient radioactivity levels are to be expected.

19
The effect of radiation is to reduce the sensitivity. They

also indicate that false alarms can be triggered by the

presence of ozone.

Ion chamber detectors are available for both industrial

and domestic use, Models are produced for both single

station and system applications. Power supply requirements

vary from 240 and 120 volt AC to battery units using 9

to 13.5 volts DC.

5^-2.2 Photoelectric Type

The presence of aerosols generated during the combustion

process affects the propagation of light as it passes through

the air. Two effects of the aerosol/air mixture can be

utilized to detect the presence of a fire. These are

attenuation of the light intensity integrated over the

beam path length, and scattering of the light both in the

forward direction and at various angles to the beam path.

Theoretical explanations of these effects can be found

in References 34 and 35.

5.2.2.1 Light Attenuation Operation

The theory of light attenuation by aerosols dispersed

in a medium is described by the Lambert-Beer Law. It states

that the attenuation of light is an exponential function

of the beam path length (1) , the concentration of particles

(c) and the extinction coefficient of the particles (k)

This relationship is expressed as follows (35)

I = I e"kcl

o

where I is the final intensity at length 1 and I is the

initial intensity at the light source.

Smoke detectors which utilize attenuation consist

of a light source, a collimating lens system, and a

photosensitive cell (see Figure 15). In most applications,

the light source is an incandescent bulb. Two new light

sources are currently being considered for use in

photoelectric aerosol detectors. Light emitting diodes

(LED's) have the potential for providing a reliable long

life source of illumination with low current requirements.

Pulsed LED's can generate sufficient light intensity

for use in detection equipment. Another promising light

source is the solid state injection laser. Using

milliwatt or microwatt power sources, these laser chips,

less than 1/10 inch (.25 cm) in diameter, could provide long

20
life light sources. Present problems with cooling,

however, limit their use.

The photosensitive device may be either a photocurrent,

photovoltaic or photoresistive cell. The photovoltaic cells

are usually selenium or silicon cells which produce a voltage

when exposed to light. These have the advantage that no bias

voltage is needed but, in most cases, the output signal

is low and an amplification circuit is required. These

units alarm when the photocell output is reduced by

attenuation of the light as it passes through the smoke in the

atmosphere between the light source and the photocell.

Photoresistive cells change resistance as the intensity

of the incident light varies. Cadmium sulfide cells are

most commonly employed. These cells are often used as one leg

of a Wheatstone bridge and an alarm is triggered when

the bias voltage shift in the bridge circuit reaches a

predetermined level related to the designed light attenuation

desired for alarm.

In practice, most light attenuation or projected beam

smoke detection systems are used to protect large open areas

and are installed with the light source at one end of the

area to be protected and the receiver (photocell/relay

assembly) at the other end. In some applications, the

effective beam path length is increased by the use of

mirrors. Projected beam detectors are generally installed

close to the ceiling where the earliest detection is possible

and false alarms resulting from inadvertent breaking of

the beam are minimized.

Although most systems employ a long path length and

separation of the light source and the receiver, there

are spot type detectors which operate by light attenuation.

One such unit, uses a 7.8 inch (0.19 m) light path with

a sealed reference chamber and an open sampling chamber

(37) . Presence of smoke in the sampling chamber results

in a voltage reduction from the selenium photocell which

is measured by a bridge circuit containing the photocell

from the reference chamber (see Figure 16)

.

There are several problems associated with projected

beam detection. Since these devices are essentially line

detectors , smoke must travel from the point of generation

into the path of the light beam. This may take time and

allow the fire to develop headway before the alarm is sounded.

In addition, owing to the long path lengths often used,

considerable smoke must be generated in order for sufficient

attenuation to be achieved. This problem can be reduced

somewhat by the use of multiple beams or reflecting mirrors.

21
Finally, continuous exposure to light can damage or accelerate

the aging of photocells, resulting in increased maintenance

and possible system failure (38)

5.2.2.2 Light Scattering Operation

Scattering results when light strikes aerosol particles

in suspension. Scattered light reaches its maximum intensity

of about 45° from the path of the beam in both the forward

and backward directions and the intensity is at a minimum

in the direction perpendicular to the beam path (3 8)

The intensity of scattered light is also related to particle

size and the wavelength of the incident light. This intensity,

as described by Rayleigh's (36) (37) theory for particles

up to 0.05 times the wavelength of the incident light,

is directly proportional to the square of the particle

volume and inversely proportional to the fourth power of

the wavelength. The theory of scattering for larger particles

(0.8 times the wavelength of light) has been defined

by Mie. These theories of light scattering are valid only

for isotropic spherical particles and are very complex.

Aerosol from a fire is a non-homogeneous mixture of particles

which are often neither spherical nor isotropic and scattering

intensities must be determined empirically for each aerosol

mixture. References 34 and 35 contain a good general treatment

of scattering theory.

Smoke detectors utilizing the scattering principle

operate on the forward scattering of light which occurs

when smoke particles enter a normally dark chamber or labyrinth,

The presence of smoke will increase the forward scattering

of light from 10 to 12 times (38) , but the intensity of

the scattered light will decrease as the angle between

the beam path and the photocell increases. The photocells

used in these detectors may be either photovoltaic,

photocurrent or photoresistive. Typical component

configurations are shown in Figure 17. These units are

of the spot type and may be used as single station devices

with self-contained power supply and alarm or as part of an

integrated system with remote power supply, alarm and

zone-indicating hardware.

5.2.3 Solid State Type

A recent entry into the detection field uses a bulk

N-type semiconductor composed of metal oxides such as tin

oxide, zinc oxide and ferric sesquioxide (39) . Originally

developed as a gas leak detector, the operating element

responds with a large decrease in resistance when exposed

to reducing or combustible gases, such as hydrogen, carbon

monoxide, methane, propane, alcohol, volatile oil, and

22
acetylene. In the simplest application the operating element

is placed in series with an alarm device and the power source

and in the quiescent condition acts as a high resistance to

block the flow of current to the alarm circuit (see Figure 18)

.

The detector element must be heated in order to operate. A

diagram of the detector element is shown in Figure 19. Units

using this type detector element have the advantage of being

self-restoring when the alarm conditions are no longer

present. Although many detectors which use this sensor

are on the market, their value as a fire detector has not

been fully established. Potential problems may exist regarding

contamination of the sensor and its expected life span.

Also, since the sensor responds to a variety of gases which

are not fire signatures, false alarms can be a major problem.

These units can go into alarm when exposed to furniture

polishes, bug sprays, cleaning fluids and a variety of

other commonly used items. Detectors using this sensing

element are used in spot type applications. This solid

state sensor presently is a controversial device and its

adequacy as a fire detector has not been completly established.

5.2.4 Resistance-Bridge Type

Detectors using the resistance-bridge principle respond

to the invisible aerosol and gaseous products of combustion.

The sensing element consists of a high purity glass plate

on which is deposited a high resistance grid. When this

grid is exposed to products of combustion, the resistance

decreases due to adsorption of materials on its surface.

This change is used to initiate an alarm. One design uses

two resistance grids in a bridge circuit. One grid is

exposed to the atmosphere while the other is only partially

exposed. In this configuration the partially exposed grid

acts to compensate for slow changes in ambient conditions

and the bridge remains balanced. Under fire conditions,

the resistance of the exposed grid drops faster than the

partially exposed grid. The resulting potential in the

bridge circuit is used to trigger an alarm. Resistance grid

detectors have had difficulty with false alarms from

moisture and other airborne contaminants.

5.3 Laser Beam Fire Detection

D.I. Lawson (40) has proposed the use of laser beams

passing through a building close to the ceiling as a means

of detecting the presence of a fire. The laser detector

is used in much the same manner as a collimated incandescent

light beam but has several major advantages. The distances

over which an incandescent source may be used are restricted

due to diffusion of the light beam. The laser consists

23
of a coherent monochromatic light beam in which all the

energy is propagated in phase. This allows the beam to

be focused on a point at greatly increased distances (114 8

feet [350 m] ) (41) . Thus the laser beam can respond to

fire signals over a larger area. In addition to smoke,

the laser system will also respond to heat.

5.3.1 Laser Heat Detection Theory

The laser detects the presence of heat by reaction

to changes in the index of refraction of the air along

the path of the beam. The refractive index of air varies

about one part per million with each rise in temperature

of 1.8°F (1.0°C) . This change in refractive index can

cause variations in the velocity of the laser beam. If

the beam from a laser is split optically into two beams

which are separated vertically and recombined at a receiver,

the vertical variations in refractive index will result

in path differences between the two beams. The output

of a photocell receiver will vary from a minimum to a maximum

with cancelling or reinforcement as the beams recombine.

This was investigated by Lawson (46) and it was discovered

that the variations from a small fire would result in a fluctuating signal on the order of 8 hertz for this system.

This, it was felt, was too low to be discriminated from

background noise due to building vibration and other sources.

Lawson called this an " Interferons trie System".

Variations in the position of a single beam system

due to changing index of refraction will also move the

spot on the photocell. O

1 Sullivan, et al (42) investigated

the use of a checkered mask over the receiver photocell

with the openings approximately the size of the laser spot

(Figure 20) . Under quiet (non-fire) conditions the spot

was assumed to remain over one opening in the mask. Under

fire conditions, the spot fluttered and moved across the

mask resulting in a fluctuating signal at the receiver

output. The frequency of this fluctuation under fire

conditions for this system was found to be on the order of

40 to 80 hertz while the ambient flutter frequency ranged from

to 20 hertz. Using this approach it is possible to

discriminate the fire signal from the background noise.

5.3.2 Laser Smoke Detection Theory

The laser beam responds to the presence of visible

products of a fire in the same manner as an ordinary light

beam. The presence of particulant in the beam path causes

scattering and results in attenuation of the transmitted energy

This is monitored by a photocell. Comparison of the energy

24
of the received signal with that of the transmitted signal

can be used to indicate the presence of smoke (43)

.

5.3.3 Laser Detection Systems

The state-of-the-art in laser detection systems has

been presented by Ghosh (44) . The system described is designed

to respond to both heat and smoke. The basic system consists

of a 1 mW He-Ne laser focused with a telescope on a cornercube mirror (Figure 21) which returns the beam to the receiver.

The corner cube configuration will reflect the beam at 180°

regardless of the position of the mirror. This overcomes

the vibration and aleignment problems associated with plane

mirrors. A typical layout is shown in Figures 22 and 23.

The beam returned from the corner-cube mirror passes through

an interference filter and a half-silvered mirror. Half of

the beam falls on the checkerboard masked photocell to detect

heat using an amplifier tuned to 70 hertz and half falls on

a photocell system to determine the energy level of the

returned beam for comparison with the output of the laser to

detect smoke (Figure 23)

.

To allow for flutter of the beam and still use small

photocells, one proposed design uses a modified circular

Fresnel lens placed ahead of two small photocells (Figure 24)

.

One photocell is at the focal point of the even numbered strips

of the lens and one at the focal point of the odd. Thus the

total output of the two photocells is independent of the

position of the spot on the lens.

Ghosh also describes a system using servo-control of the

position of the spot on a four quadrant photocell as shown in

Figures 25 and 26. Glass plates (Pi and P2 ; Figure 25) are

placed within the optics of the telescope (Li and L2; Figure

25) . These plates are rotated by servo-motors which are

controlled by the outputs from each quadrant of the photocell

through "difference" amplifiers to maintain alignment of the

beam (see Figure 26) . The output from the "difference"

amplifiers is used to drive another amplifier tuned to 70 hertz

This latter amplifier is used to detect heat when a 70 hertz

signal is generated at the quadrant cell. Variations in the

spot location on the quadrantcell are fed to a summing

amplifier and used to measure attenuation of the beam energy

as an indication of the presence of smoke.

A 10 second delay is built into the system to allow

for momentary breaks in the beam and other transient signals ,

5.3.4 Operational Experience

Although there are no commercially available laser

25
detection units presently on the market, several pilot

installations have been made by the Joint Fire Research

Organization in factories, warehouses, cable tunnels and

a simulated coal mine gallery (41)

.

Tests for response to heat and smoke were made with

both direct beam" (no return mirror) and returned beam

devices over path lengths of 246 feet (75 m) to 114 8 feet

(350 m) . Position of fire and smoke sources and ambient

wind velocity were varied.

Response to fire in a cable tunnel ranged from 10

to 20 seconds for an 11 inch (28 cm) diameter container

of methanol. Smoke in the same tunnel was detected in

25 seconds. In cases where fire or smoke was not detected,

high air velocity was thought to be the factor.

Smoke tests in a simulated mine gallery with various

velocities and source locations resulted in detection times

ranging from 24 seconds at 2 feet per second (0.75 m/s)

to 3 minutes and 12 seconds at a velocity of 9 feet per

second (2.75 m/s) at the worst location.

In general, the conclusions reached were that detection

time is directly related to air velocity, ambient light

can have an adverse effect on smoke detection, the laser

system is practical as a heat and smoke detector in large

buildings, and it will operate effectively in tunnels with

the expected air velocities present.

5.4 Radiant Energy Detectors

Several methods can be used to detect fires by sensing

the radiant energy from smouldering or flaming combustion.

The spectra used are in the infrared and ultraviolet bands.

These bands have wavelength ranges of 0.7 to 14 micrometres

and 0.001 to 0.4 micrometres respectively.

5.4.1 Infrared Type

Infrared detectors basically consist of a filter and

lens system to screen out unwanted wavelengths and focus the

incoming energy on a photovoltaic or photoresistive cell

sensitive to the infrared. Infrared radiation can be detected

by any one of several photocells such as silicon, lead

sulfide, indium arsenide and lead selenide. The most commonly

used are silcon and lead sulfide. These detectors can respond

to either the total IR component of the flame or flame

flicker in the frequency range of 1.5 to 10 hertz (45)

or 4 to 15 hertz (46)

.

26
Interference from solar radiation in the infrared

region can be a major problem in the use of infrared

detectors receiving total IR radiation in that the solar

background can be twice that of a flame signal from a fire

of 500 square centimetres. This problem can be partially

resolved by choosing filters which exclude all IR but that

in the 2.5 to 2.8 micrometre and/or 4.2 to 4.5 micrometre

ranges. These represent absorption peaks for solar radiation

due to the presence of CO2 and water in the atmosphere

(45) . In cases where the detectors are to be used in normally

dark applications, such as in vaults (46), this filtering

is not necessary. Another approach to the solar interference

problem is to employ two detection circuits. One circuit

is sensitive to solar radiation in the 0.6 to 1.0 micrometre

range and is used to indicate the presence of sunlight.

The second circuit is filtered to respond to wavelengths

between 2 and 5 micrometres. A signal from the solar sensor

circuit can be used to block the output from the fire sensing

cell, giving the detection unit the ability to discriminate

against false alarms from solar sources. This is often

referred to as a "two color" system (45) . For most

applications, flame flicker sensors are preferred in that

the flicker or modulation characteristic of flaming combustion

is not a component of either solar or man-made interference

sources. This results in an excellent signal-to-noise ratio.

These detectors use frequency-sensitive amplifers whose

inputs are tuned to respond to an alternating current signal

in the flame flicker range (1.5 to 15 hz)

.

Flame flicker detectors are designed for volume supervision

and may use either a fixed or scanning mode. The fixed units

continuously observe a conical volume limited by the viewing

angle of the lens system and the alarm threshold. The

viewing angles range from 15° to 170 c for units from various

manufacturers. One scanning device has a 400 foot (122 m)

range and uses a mirror rotating at 6 revolutions per minute

through 360° horizontally with a 100° viewing angle. The

mirror stops when a signal is received and alarms after a

15 second delay to screen out transients (46)

.

Infrared detectors provide volume surveillance by

rapidly responding to the designed alarm level anywhere

within their range of vision. In addition, they will

respond to reflected or reradiated infrared signals

originating in areas which might be shielded from direct

observation (47) . There are also detectors of this type

designed to respond to passing sparks or flame fronts in

piping (48)

.

27
5.4.2 Ultraviolet Type

The ultraviolet component of flame radiation is also

used for fire detection. The sensing elements may be solid

state such as silicon carbide (49) or aluminum nitride

(50) , or gas-filled tubes in which the gas is ionized by

UV radiation and becomes conductive, thus sounding the alarm

(7) . The operating range of UV detectors is in the 0.17

to 0.30 micrometre region and in that region they are

insensitive to both sunlight and artificial light. The

UV detectors are also volume detectors and have viewing

angles from 90° or less to 180°.

5.4.3 Combination Ultraviolet- Infrared Type

The combination of UV-IR sensing has been applied to

applications in aircraft (51) and hyperbaric chambers (49)

fire protection. These devices alarm when there is a

predetermined deviation from the prescribed ambient UVIR balance. The aircraft application (51) uses the UV-IR

discrimination for fire warning in conjunction with a

continuous wire overheat detector, the analysis being

performed by an on-board minicomputer

.

5.5 Submicrometre Particle Counting Detectors

During the earliest stages of combustion, in the

pyrolysis or precombustion stage, large numbers of submicrometre sized particles are produced. These particles fall

largely in the size range between 0.005 and 0.02 micrometres

(52) . Although ambient conditions normally find such

particles in concentrations from several thousand per cubic

centimetre in a rural area to several hundred thousand

per cubic centimetre in an industrial area, the presence

of an incipient fire can raise the submicrometre particle

concentration sufficiently above the background levels

to be used as a fire signal. In the area of its flame,

for example, a match can produce 10H particles per cubic

foot (10' part/cc) (52) . Production of these particles

can fill a volume lO^ times the volume of the material

lost in producing them to a concentration 10 times a normal

ambient of 4x10° to 2x10 particles per cubic foot (2x10^

to 10^ part/cm^) (Van Luik, F.W., Jr., Environment One

Corp. Private Communication)

.

5. 5. 1 Particle Ionization Type

A devce which determines submicrometre particle concentration by measuring the variation in electrical charge due to the

28
presence of ionized particles has been described by Hum

(53). Air samples, filtered to remove coarse material,

are drawn through a chamber where the submicrometre particles

are exposed to a negatively polarised ion field generated

by a radioactive source. An ion separator removes the

positive primary ions from the field and the remaining

negative primary ions are allowed to interact with the

particles. The sample passes through a primary ion

remover so that only the negatively charged particles

(secondary negative ions) continue through the detector.

Tne sample then passes between two concentric polarized

tubes where the secondary negative ions (particles) are

electrostatically precipitated giving up their charge to

the outer tube. This results in a variation in potential

across the tubes. Since each particle carries one charge,

these variations are directly proportional to changes in

particle concentration of the air sample. With a background

of 4.2x10^ particles per cubic foot (1.5xl0 5 part/cm^),

it is possible t.o detect a change of 100 particles.

Tne device is able to detect fires as small as a two-squareinch piece of tvping paper or heating of one-half inch of

PVC wire insulation to 572°F (300°C)

.

With an appropriately chosen alarm threshold, devices

operating in this mode can be effective fire detectors.

5.5.2 Condensation Nuclei Type

Condensation nuclei are liquid or solid submicrometre

(0.001 to 0.1 micrometres) particles which can act as the

nucleus for the formation of a water droplet (52) . By use

of an appropriate technique, submicrometre particles can be

made to act as condensation nuclei on a one particle-one

droplet basis and the concentration of particles is measured

by photoelectric methods. A mechanism for performing this

function has been described by Skala (54) and is shown

schematically in Figure 27. An air sample is drawn through

a humidifier where it is brought to 100 percent relative

humidity. The sample then passes to an expansion chamber

where the pressure is reduced with a vacuum pump. This

causes the relative humidity to rise above 100 percent

resulting in condensation of water on the particles. The

droplets quickly reach a size where they can scatter light.

The dark field optical system in the chamber will allow

light to reach the photomultiplier tube only when the water

droplets are present to scatter light. The output of the

photomultiplier tube is directly proportional to the number

of droplets (i.e., the number of condensation nuclei) present,

The system uses a mechnical valve and switcning arrangement to allow sampling from up to 4 detection zones with

29
as many as 10 sampling heads per zone. Each zone is sampled

once per second for 15 seconds. All 4 zones are sampled

each minute.

The system is nominally set to alarm at concentrations

exceeding 2.2x10^ particles per cubic foot (8x10^ part/cc) , although it is possible to select different thresholds

for each zone depending on the background noise and the

sensitivity required. It is also possible to have the

sensitivity vary for different conditions with time of

day. The system design is such that with the maximum sample

travel distance from the most remote sampling head, fire

will be detected within 2 minutes of the time the products

of combustion first reach the head.

5.6 Fiber Optics in Detectors

Fiber optics is the science which deals with the

transmission of electromagnetic radiation through transparent

filaments (55) . Fiber optic systems consisting of bundles

of oriented fibers have been found to have good transmission

characteristics in the visible and the near infrared. Kapany

(56) reported transmittance of 50 percent in the visible

region over a length of 7 feet (2.4 m) and over 50 percent

for oxide glass in the 0.3 to 6.0 micrometre region and 50

percent in the 1.0 to 12.0 micrometre region with an

arsenic-sulphur glass. One fire detection system for use

with aircraft engine spaces (55) uses the characteristics

of flexible fiber optics to transmit IR and visible radiation

signals from the potential fire site to the aircraft cockpit.

At the receiving end, the signals transmitted by the fiber

optics are continuously viewed by a cadmium selenide

photoconductive cell. Used in conjunction with a circuit

for ultraviolet detection, the system can discriminate

fire from overheat conditions. When an alarm is triggered,

the observer may make visual confirmation of conditions

in the fire area by moving the photocell assembly and viewing

the aligned fiber optics through a suitable objective lens

system.

5.7 Ultrasonic Detectors

The heat which results from the presence of a fire can

be detected using an ultrasonic receiver-oscillator (57) (58) , The ultrasonic oscillator sets up a stable standing wave

pattern in the area to be supervised. Movement of air in the

room, caused by the convection of hot gases from a fire, disturbs the standing wave pattern. This disturbance is monitored by an ultrasonic receiver and is used to trigger an

alarm. These systems are a spin-off of intrusion detection

technology and would only be applicable when the protected

30
premises are unoccupied. A problem exists regarding the

discrimination of a fire alarm from an intrusion alarm.

5.8 Combination Detectors

Several devices are available which use more than

one operating mechanism and will respond to multiple fire

signatures with a single unit. The combination detectors

may be designed to alarm either from any one of the fire

signatures or only when all the signatures are present

at predetermined levels.

5.8.1 Rate-of-Rise Fixed-Temperature Type

Several heat detection devices are available which

operate on both the rate-of-rise and fixed-temperature

principles. The advantage of units such as these is that

the rate-of-rise elements will respond quickly to rapidly

developing fires while the fixed temperature elements will

respond to slowly developing smouldering fires when the

design alarm temperature is reached.

The most common type uses a vented hemispherical air

chamber and a flexible diaphragm for the rate-of-rise

function. The fixed-temperature element may be either

a bimetal strip (see Figure 2 8) or a leaf spring restrained

by an eutectic metal (see Figure 29) . When the designed

operating temperature is reached, either the bimetal strip

nas flexed to the contact point or the eutectic metal has

fused, releasing the spring which closes the contacts.

A second device which can be classified as combination

rate-of-rise - fixed-temperature is the rate-compensation

detector. This detector uses a metal cylinder containing

two metal struts. These struts act as the alarm contacts

and are under compression in a normally open position (see

Figure 30) . The outer shell is made of a material with

a high coefficient of thermal expansion, usually aluminum,

while the struts, usually copper, have a low coefficient.

When exposed to a rapid change in temperature, the shell

expands rapidly, relieving the stress on the struts and

pulling them closed. Under slowly increasing temperature

conditions both the shell and struts expand. The contacts

remain open until the cylinder, which expands at a greater

rate, has elongated sufficiently to pull them closed. This

closure occurs at the device's fixed-temperature rating.

5.8.2 Resistance Bridge - Ion Chamber Type

One method of reducing false alarms with some of the

more sensitive types of aerosol detectors, is to combine

31
operating elements which function in different modes in

a gated circuit. An approach has been to use the

resistance-bridge concept combined with an ion chamber.

Each detection element has its own bridge circuit which

triggers one electronic gate. When a fire signal is

present from both gates (bridges) , the combination detector

goes into alarm. -

6 . Detector Subsystems

In addition to the sensing element, two major items

must be included as part of a detection system. These

are the alarm devices and the trouble circuitry.

6.1 Alarm Sounding Devices

Once initiated by the detection element, an alarm

must be indicated to the occupants of the area concerned.

This may be accomplished by means of lights, horns, bells

or buzzers. Chimes may be used in special occupancies

such as hospitals. The primary alarm mode in most applications

is auditory. Present audible devices are required to produce

a sound pressure level of at least 85 decibels (dB) . This

is determined with respect to a reference level which

corresponds roughly to the threshold of hearing. Pure

tones are not generally used for alarms as they are not

as audible as mixed tones of the same intensity. High pitched

sounds are undesirable in that they cannot be discriminated

as well by the human ear against background noises. In

addition the high frequency response of the human ear

decreases as the age of the listener increases. It has

been suggested (see Section 9) that the requirements for

sound level be expressed as dBA. This represents the "A"

scale on a sound survey meter which has its frequency

response characteristics shaped to account for the frequency

response of the human ear.

An important consideration in the use of audible alarms

is that of the background noise. Not only its intensity

but the range of frequencies represented must be included.

In order to be discriminated, an alarm sounding device

must produce a noise level higher than that of the background.

A difference of 15 dBA over ambient is considered the

minimum for waking people from normal sleep (59) . In addition,

alarm devices which produce noise in the same frequency

spectrum as the ambient may not be distinguished though

the dB level may be in excess of 15 dBA over the background.

6.2 Trouble and Supervision Circuits

Trouble and supervision elements are included with

32
detection systems to sound a signal, distinctly different

from that of a fire alarm, when a component failure or

circuit interruption renders the system or any unit detector

inoperative without sounding the fire alarm. An example

of this is the trouble circuitry for the incandescent bulb

in a light scattering aerosol detector. In a device where

the photocell would normally be dark, failure of the bulb

would not trigger an alarm, thus the need for supervision.

In systems having many spot type detectors which are normally

open switches, an end-of-the-line resistor in parallel

with the circuit maintains a small supervisory current

flow in the system. Breaking the circuit continuity cuts

the supervisory current and sounds the trouble alarm.

Detectors using battery power supplies have supervision

of the battery voltage. When voltage reaches a predetermined

level, the alarm sounds intermittantly indicating a trouble

condition.

7 . Ambient Conditions Affecting Detector Response

Ambient conditions have a strong influence on the

choice, placement and response of detectors. Improper

choice of detection mode or improper placement can lead

to problems ranging from no alarm or delayed alarm, when

a fire occurs, to excessive false alarms.

7.1 Background Levels of Fire Signatures

When choosing a detector for a specific location,

consideration must be given to the background levels of the

signatures to which the detector might be exposed under nonfire conditions. The use, for example, of a UV or IR

detector in a location where gas or arc welding is commonplace will generate false alarms. This is not a failure

of the detector but rather it is a response to the presence

of the signature and signal strength for which the unit

was designed. Detectors responding to invisible aerosol

fire signatures are especially prone to false signals from

such sources as cooking fumes, cigarette smoke and automobile

exhaust fumes. A survey of false calls from automatic

detection systems in Great Britain (60) revealed an overall

ratio of 11:1 false calls to real calls for all types of

detection including sprinkler alarms. The same survey

showed that ambient conditions such as extraneous heat

and smoke, extremes in ambient temperature, moisture (snow,

rain and steam) and high velocity air movement were responsible

for 25.9 percent of the false calls.

It is not probable that false alarms due to localized

and transient changes in the ambient levels of fire signatures

can be completely eliminated. However with sufficient

33
information about ambient variations in different

occupancies, the number could be considerably reduced.

A program to establish a data base for variations in ambient

conditions is presently underway in Great Britain (61)

.

This study will include data on aerosols, temperature,

humidity, radiation, air velocity, and vibration. It

is possible that- raising the threshold of alarm for certain

fire signatures in specific occupancies, could reduce the

incidence of false alarms without sacrificing the design

goal of the installation. Difficulties arising from

transient false signals might also be reduced through the

use of multiple signature detectors and integration of

the signals such that an alarm will sound either at

a high threshold of any one signature or a lower threshold

with multiple signatures.

7.2 Heating and Air-conditioning Effects

Heating and air-conditioning systems in buildings

can exert several effects on the placement and operation

of detection devices. These effects result from forced

or convected air movement and the development of thermal

inversions. Detectors sensing radiant energy signatures

are not effected by these factors. Devices responding

to the convected energy signatures are only slightly

affected since the convected energy of a fire which is

sufficiently large to actuate these units, easily overpowers

ambient circulation patterns. The most important effects

of heating and air-conditioning are on the movement of

the products of combustion which make up the aerosol and

gas signatures. Detectors sited without regard to air

flow and thermal effects may be slow to respond, or in extreme

cases may miss the fire signal completely.

Forced hot air systems are widely used in many

occupancies. Air flow patterns must be considered in

placing detectors and are related to the overall air flow

in the building between the supply registers and the cold

air return. In a two story dwelling, for example, each

room can have its own supply register and the cold air

returns may all be on the first floor or in the basement.

Under these conditions, cool air descending from above

and warm air rising from below creates air currents in

the stairwell which move products of combustion to the upper

floors. Detectors placed at the top of a stairwell can

sense the aerosol and gas signatures originating in the

areas below. Air flow up the stairwell is not always uniform.

Changes in direction such as that at the ceiling of the

second floor can produce a stagnant area where there is

little or no movement. Aerosol or gas detectors placed in

this area may be slow to respond due to lack of sufficient

34
air velocity to carry products through the detector housing

to the sensing element.

It is not uncommon with forced air systems to find

detectors placed too close to the air stream issuing from

a supply register with the result that the units are actually

being continuously purged when the heating system is in

operation. Often detectors are placed in the return air

ducts in order to detect aerosol drawn into the system

and are used to shut down circulating fans or convert the

system to an exhaust mode. Consideration must be given

to the effects of dilution of aerosols by air drawn from

areas unaffected by the fire. Often intolerable conditions

can exist elsewhere in the building long before a strong

signal can reach the detector.

Apartment and office buildings often utilize heating

systems which serve a single unit. In apartments, the

building corridors may serve as a make-up air supply through

undercut or louvered doors with exhaust through kitchen

and bathroom ventilators. Noxious products of combustion

originating in other areas of the building are often drawn

into apartments by this means. Detectors placed appropriately

in the corridors can be used to sound a general alarm.

The air flow to kitchen and bathroom exhausts should be

considered when siting detectors within an apartment. In

offices where make-up air is drawn from outside the building,

corridors often function as a return air plenum. Air flow

within rooms in such cases may tend to be toward the corridor

doors

.

Gravity hot air systems are often found in older

buildings. When in use, they depend largely on vertical

supply ducts and natural openings between floors for

circulation. Usually the air flow patterns are from supply

registers at floor level to the ceiling and down natural

openings to the lowest level. Lacking the impetus of fans

for diffusion of the supply air, the paths of air flow

are often restricted leading to "dead" spaces and eddy

effects. Detailed siting surveys should be conducted prior

to installation of detectors where these systems are used.

Hot water/steam radiator systems are also common in older

buildings. These systems are gravity dependent and have

less effect on the transport of aerosols than forced air

systems. Movement will be confined to two general regions:

the convection cell developed around the radiator itself,

and the vertical circulation *rom floor to floor by natural

openings in the structure. As in the case of gravity hot

air, care must be taken to seek out the eddy areas and

dead spots and to avoid them. Radiant heating using electric

coils in the ceiling is becoming common in certain areas.

35
The use of radiant heating of the ceiling type can produce

a problem. When the system is in operation, a layer of

hot gases at the ceiling can prevent the combustion products

from reaching ceiling-mounted detectors. In such cases,

detectors may have to be wall-mounted perhaps 12 inches

below the ceiling (N. Swanson, Security Engineering Co.,

Inc., Private Communication).

Complete building air-conditioning systems will generate

air flow patterns which are, in general, similar to those

of the forced hot air system. Two additional problem areas

can be found with the use of cooling systems. In the dry

areas of the country, air cooling is accomplished using

evaporation techniques. Air cooled by this method has

a high relative humidity. The effects here are to enhance

the agglomeration of smoke particles and trap them in the

moist air. The warm dry air rises to the ceiling and a

sharp boundary condition is created. Cool moist smoke

rising to the ceiling can be trapped at this interface.

Unless the air is agitated by a rapidly burning fire,

ceiling-mounted detectors, even in the fire area, may not

respond to the aerosol signal at all (Pat Phillips, A.E.C.

Las Vegas, Private Communication).

Although some specific detector siting information is

available in the National Fire Protection Association (N.F.P.A.)

Standard 74 (62) and in various manufacturers' application

guides, little design data is available to assist in

understanding the effects of comfort heating and cooling

systems on detector location. Optimization of detector

location for heat-on, heat-off and cooling-on, coolingoff conditions should be based on studies of air flow

conditions in buildings and existing heating and air

conditioning technology.

8. Reliability of Detection Devices

Although detailed reliabiity data are lacking for

most detection devices, some general statements can be

made regarding certain critical components based on field

or laboratory experience and manufacturers' literature.

8.1 Heat Detectors

Heat sensing detectors are generally the most reliable

types in terms of component failure since these devices

respond directly to the presence of heat by a physical

change in the detector operating elements. Heat detectors

may fail due to mechanical damage or abuse after

installation or by failure of components or circuitry in

36
peripheral equipment such as power supplies or alarm

indicating equipment.

Detection devices for fire signatures other than heat

employ electronic circuitry of varying complexity to sense

the presence of a fire signature and to monitor the output

of the sensing element. The reliability of the device

is related to the reliability of its components as they

are used in each type of circuit.

8.2 Light Sources and Photocells

The lamps used in photoelectric type smoke detectors

are critical to detector operation. The operational lifetime

of incandescent bulbs used in photoelectric detectors ranges

from about one month to about 37 months (63) . Lamp life

can be increased by operation at reduced voltages. Lowering

the operating voltage reduces filament evaporation, one

of the causes of failure. Vibration and shock particularly

with fragile, aged filaments often lead to lamp failure.

Power surges and power failures are also significant factors.

It appears that an average life of 3 years in continuous

service can be expected with present bulbs. The problem

of bulb life might be solved through the use of light emitting

diodes (LED). Manufacturers' data on LED's indicates a

possible life span on, the order of 10 ^ hours approximately

100 years. At this time, barring traumatic damage, the

light intensity would be reduced by one half. These

light sources are mechanically stable and should be less

prone to damage from vibration.

The sensitivity of photocells used in detectors may

have a tendency to drift somewhat with aging. The usual

method of compensation for such changes is through the

use of compensation photocells in various configurations.

These cells act as a reference to maintain balance in the

circuit.

8.3 Batteries

The use of batteries as the primary power supply for

single station detectors has several problems which can

effect detector reliability. The present requirements

for battery-operated devices call for a 1 year lifetime

and an audible trouble signal lasting 7 days when some

critical voltage is reached. The type of battery being

used can affect the operation of the detector. Alkaline

batteries have a constantly decreasing voltage curve as

they wear out. Detectors using these batteries require

periodic sensitivity readjustment to maintain the designed

alarm threshold. Failure of a home owner to readjust the

37
detector could result in delayed detection or render the

unit inoperative. Mercury batteries have a constant voltage

throughout most of their life and can be used to control

the sensitivity-drift problem. At the end of their life

however, mercury batteries undergo a rapid drop in voltage.

This will present two problems. First, the sensitivity will

decline rapidly and second, it is possible that the reduced

power available may shorten the operating time of the alarm

horn or, in some cases, prevent its operation.

8 . 4 Field Experience

Some field experience is available which is of interest.

Several instances have been reported of power transformer

failures in 110 VAC operated detectors. These failures

have, in some instances, resulted in an alarm when the

smoke from the failing transformer was detected. It is

not known whether these failures have initiated a fire,

but the possibility should not be overlooked. Failures

of solid state components have also occurred due to electrical

transients which have overpowered the built-in transient

protection.

9. Maintenance of Detectors

Maintenance problems also affect detector reliability

particularly in photoelectric and ionization types. Accumulations of dust and films on the bulbs, lenses and photocells

will reduce the intensity of light within the detection

element. The effect of this varies with the type of detector.

Projected beam type photoelectric detectors will become

more sensitive with contamination increasing the possibility

of false alarms. Light scattering detectors, on the other

hand, will become less sensitive as light intensity is

decreased. Ionization detectors are also effected by

contamination. Deposition of dust and films inside the

ion chamber will decrease the current flow across the chamber

and raise the sensitivity. This can result in an increase

in the false alarm rate. Collections of dust, particles

of lint and other large airborne contaminants can often

be trapped in the protective screens or li^ht shields of

smoke detectors. This can block smoke entry and prevent

or delay an alarm. Proper cleaning and maintenance is

important to retain the designed operating characteristics

of these detectors.

10. Performance Standards and Acceptance Criteria for

Detection Devices

Definitive performance standards and acceptance criteria

are not currently available for all types of detection

38
devices. Work is presently underway by national and international standards groups to define required performance

levels and develop appropriate test methods. The existing

standards will be discussed in the following paragraphs

and comparisons will be made where appropriate.

10.1 U.S. Standards and Acceptance Criteria

Testing of detection devices for approval is presently

being carried out in the United States by two organizations;

the Underwriters 1 Laboratories, Inc. (U.L.) and the Factory

Mutual Research Corp. (F.M.). Each organization publishes

a list of approved or listed devices.

U.L. presently tests detection devices in 3 ways,

by conformance to published standards, by conformance to

non-published internal or interim methods, or by verification

of manufacturers' claims about the capabilities of their

devices. The actual test series to which a device is subjected

may include one or all of the above approaches. Although

U.L. has several standards covering the peripheral hardware

associated with detector power supplies, alarm devices

and other equipment only two, U.L. 168-19 71 (6 4) and U.L.

521-1971 (6 5) , apply to the performance of the detection

element itself.

U.L. 168-1971 applies to the testing of photoelectric

type detectors. Beam type detectors are required to alarm

in the presence of visible gray aerosol at the following

levels indicated in percent obscuration per linear foot

of beam length:*

Total Obscuration

Length of Beam (percent ft_ l) (O.D. m"1 )**

(ft) (m)

22 (6.71) 2.0 (0.029)

22-44 (6.71-13.4) not more than 36.0 (0.513)

for total beam

44 (13.4) 1.0 (0.014)

* No alarm is to be sounded for obscuration of 4 percent ft""l

(0.057 O.D. m"1 ) over the entire beam length [i.e., 0.4

percent ft" 1 (0.006 O.D. irT 1 ) for a 10 ft. (3.05 m) length]

** O.D. m"1 is optical density per metre.

39
Spot type photoelectric units must alarm at an obscuration

between 0.4 and 4 percent ft"" 1 (0.006 and 0.057 O.D. m"1

) . Spot type units are tested for sensitivity and normal

operation in a box containing an air circulation system (see

Section 10.3) and a smoke density meter. Smoke is generated

by smouldering cotton lamp wick within the box and the

obscuration level recorded at the moment of alarm. In order

to consider differences in response to gray and black aerosol,

this standard requires that listed photoelectric detectors

respond to black aerosol before the obscuration reaches

10 percent ft" 1 (0.143 O.D. nr 1 )

.

In order to evaluate the unit's reliability when exposed

to various deleterious conditions, sample units are tested

for response after exposure to high humidity [85 percent

at 86°F (29.7°C) '], temperature extremes (32°F - 120°F)

(0°C - 48.4°C) , corrosive atmospheres (CO2/SO2 , H2S and

salt spray) , application of paint, and extremes of supply

voltage (85 percent and 110 percent of nominal) . Sensitivity

following those tests must be within 50 percent of normal.

Units are also subjected to a jarring test for mechanical

stability.

U.L. 168 also requires the electrical supervision

of circuits or components which will render the unit

inoperative when they fail (e.g., tube or bulb filaments,

interlocks, etc.). Failure of supervised elements must

actuate a trouble signal which is separate and distinct

from the fire alarm signal. Failure of non-supervised

components must result in alarm or have no appreciable

effect on detector operation. The electrical circuitry

and any switch or relay contacts are tested for failure

under current and voltage overloads and switching components

are tested for endurance through 6000 cycles of operation.

Heat detectors must meet the requirements of U.L.

521, Fire Detection Thermostats- 19 71 (65) . This standard is

used for approval of fixed- temperature, rate-of-rise, and

rate compensation detectors of both spot and line type.

The performance requirements are similar to U.L. 16 8 with

respect to corrosion, temperature, humidity, vibration

and overload tests.

The designed fixed-temperature ratings are verified

in an oil or water bath and an air oven test by raising

the temperature at 1°F (.55°C) per minute from a temperature

below the designed operating point until the alarm point

is reached. Uniformity of response must be between 5 and

8 percent of nominal depending on the desired rating.

40
Rate-of-rise types must not alarm when exposed to rates

of 12°F (6.6°C) per minute or less up to 135°F (56.60°C)

but must alarm at a rate of 15°F (8.33°C) per minute.

Sensitivity to fire exposure is expressed in terms

of spacing limitations under standard test fire conditions

such that an alarm is sounded within two minutes following

ignition or prior to the operation of standard 160°F (70.4°C)

automatic sprinkler heads on 10 foot (3.05 m) spacing. Ratings

for 10, 12.5 and 15 foot (3.05, 3.81 and 4.57 m) spacings

can be achieved in an air oven test with the temperature

change programmed to simulate the actual full scale fire

tests. For larger spacing, the full scale test must be

run.

Factory Mutual Research Corporation (F.M.) also has two

published standards pertaining to detection devices, Class

No. 3230-3250, "Smoke Actuated Detectors for Automatic

Fire Alarm Signaling" (66) and Class No. 3210, "Thermostats for Fire Detection" (67) . These standards are similar

to their U.L. counterparts with some notable exceptions.

Both contain considerably less detailed information concerning

the tests such as corrosion, vibration and electrical overload

endurance.

Class No. 3230-3250 deals with all types of "smoke"

detectors while U.L. 16 8 applies only to photoelectric

types. While the minimum sensitivity in terms of an

obscuration of 4 percent ft-1 (13.12 percent m" 1 ) (0.17

O.D. m~l) is the same, there is no required maximum limit

on sensitivity. It is felt that for certain applications,

such as cleanrooms , very high sensitivity would be

desirable. F.M. also tests "smoke" detectors for adverse

effects of high velocity air flow, an important factor in

duct supervision applications and with certain designs of

ion chamber detectors.

F.M. Class No. 3210 maintains closer tolerances than

U.L. 521 on the ratings of fixed temperature detectors,

allowing only a 3 percent variation from the nominal figure.

Rate-of-rise detectors must respond to changes between

15°F (8.33°C) and 25°F (13.75°C) per minute for F.M.

approval while U.L. 521 requires 15 °F (8.33°C) per minute.

Factory Mutual detector spacing requirements are obtained

in the same manner as U.L. 521 with the exception that

the temperature rating of the sprinkler head used is comparable

to that of the thermal detection unit being tested. U.L.

tests only with heads rated at 160°F (70.4°C) even though

the detector may be rated at 135°F (56.7°C) .

41
As is the case with U.L., F.M. depends, in addition

to their published documents, on internal standards and

verification that the product meets the intent of its design.

In both F.M. standards it is stated that for household

warning use, not all requirements of their standards need

be met, but approved units must have comparable sensitivity

and reliability and meet the requirements of N.F.P.A. 74-

1972 (62)

.

N.F.P.A. Standard 74-1972, "Household Fire Warning

Equipment," combines information dealing with acceptance,

performance and installation. The basic requirement for

acceptance is that all equipment be "approved". This

standard has the only performance requirements pertaining

to batteries as a primary power source. Batteries for

use in detectors must provide a one year life including

normal operation and any periodic testing. A "low battery"

trouble signal sounding at intervals of one minute or less

for seven days is also required. Batteries used for

secondary or back-up functions must assure 24 hour operation

following a power failure and failure of power followed

by restoration must not sound an alarm. This standard

also requires that an alarm sounding device produce at

least 85 decibels at a distance of 10 feet (3.04 m)

.

The Department of Housing and Urban Development, in

the Guide Criteria for Operation BREAKTHROUGH, has published

acceptance criteria for self-contained (single station)

combustion products detectors of both the photoelectric

and ion chamber types (6 8) . Acceptable detectors are

required to alarm at an optical density of 0.01 per foot

[2 percent ft -1 obscuration (0.029 O.D. m-1) ] . The alarm

sounding device must have a loudness level of 85 dBA +

5 dBA at 5 feet (1.52 m) . Data should be submitted by

the manufacturer to indicate the stability of detection

over a one year period and a reliability analysis to predict

the expected life of the units. All units should meet

the intent of U.L. 168-1969 where applicable.

10.2 Foreign Standards and Acceptance Criteria

For purposes of comparison, representative examples

of foreign detection standards will be briefly discussed.

Primary consideration is given to the performance aspects

of standards from Britain (69) , France (70) , Australia

(71) , New Zealand (72) and Japan (15) (73)

.

The British Standard (BS 3116, Part I) for spot type

heat detectors uses an air oven to test the response to

varying rates of temperature change. No discrimination

42
is made in the rating tests between the fixed temperature

and the rate-of-rise types. Approved detectors are assigned

to one of three groups based on a test-generated response

curve relating response time to rate-of- air- temperature

rise. All the detectors in a given group will have similar

response characteristics to a given set of thermal conditions

The groups are designated as Group 1, Group 2 and Group

3 in order of decreasing sensitivity. This allows the

engineer to choose a detector with the desired response

time for a given set of expected fire conditions. None

of the detectors are subjected to an actual fire test.

The section of BS 3116 pertaining to smoke detection is

presently under development and the proposed requirements

will be discussed in a later section.

Heat detectors in France are subjected to the operating

temperature tests as described in BS 3116-1959. Rate-ofrise detectors must not be subject to alarms at a rateof-rise of 12°F (7°C) per minute. The sensitivity of

ionization detectors is tested using the BS 3116 air oven

with the circulation velocity reduced to 49.2 feet per

minute (0.25 m/s) (see Section 10.3). A dry flat sheet of

pure cellulose weighing 0.06 ounce (2 g) is placed on a

hot plate as the aerosol source. Detection must be achieved

before the sample is destroyed (usually less. than 1 minute).

The test is repeated with progressively smaller samples

until the limit of sensitivity is reached. The French

standard does not cover photoelectric or other detection

modes

.

The performance test used in the Australian Standard,

AS A122-1968, evaluates heat-actuated detectors under three

rates of temperature rise, 40°F (22°C) , 4°F (2.2°C) and

1°F (0.55°C) per minute. Detectors are classed as either

normal temperature, those operating between 135 °F and

190 °F (56.7°C and 86.9°C) , or high temperature operating

between 190°F and 270°F (86.9°C and 130. 9°C) . All heat

detectors must operate within 2.5 minutes when exposed

to a rate-of-rise of 40°F (22°C) per minute.

Fixed temperature detectors tested by New Zealand

Standard NZSS 2139:1967, are tested both in a liquid bath

and an air oven. In the liquid bath, the detector must

not alarm during 7 days exposure to a temperature 10 °F

(5.5°C) below its design operating temperature, and at

its design temperature the heat detector must operate in

60 seconds or less. Air oven exposure at a rate-of-rise

of 10°F (5.5°C) per minute must cause an alarm within

3 minutes of reaching the designed operating temperature.

43
The Japanese testing procedures have been discussed

by Watanabe (15) . Operational tests of heat detectors

are carried out in a circulating air oven. The response

time is tested as a function of exposure to a heated air stream

having a designated velocity and temperature as well as

at a constant rate of temperature rise under natural

convection (73) . Fcr example, a rate-of-rise spot-type

detector of Class 1 rating, must respond in 30 seconds

to a heated air stream with a velocity of 83.8 feet per

minute (70 cm/sec) and a temperature of 36°F (20°C) above

room temperature. It must also operate in 4.5 minutes

with a constant rate of temperature rise under natural

conditions of 18°F (10°C) per minute. The threshold of

non-operation is expressed in a similar manner.

The Japanese test method for aerosol detectors of

the ionization and photoelectric types uses a modified

Aachen chamber (15) . The aerosol source is pyrolyzing

filter paper. The aerosol density is measured using

percent obscuration per foot for photoelectric types and

using a combustion products meter for ion chamber units.

The aerosol is circulated through the chamber to assure

a homogenous mix. A typical Class 1, nondelay type,

photoelectric unit must respond in 30 seconds to 2.3 percent

obscuration per foot [7.5 percent m~l (0.033 O.D. m~l)

]

at 120 feet per minute (60 cm sec"1

) (see Section 10.3) and

no alarm in 5 minutes at 0.76 percent per foot [2.5 percent

m~l (0.011 O.D. m~l) ] at the above air flow velocity. The

response sensitivity of ion chamber units is measured against

a combustion gas measuring instrument or a parallel plate

ionization chamber. Aerosol detection units are approved as

Class 1, Class 2 or Class 3 in order of decreasing sensitivity.

Full scale fire tests of detectors, although considered

beneficial are not used in Japan due to problems of reproducibility.

10.3 Trends in Standards and Test Criteria

At the present time many testing agencies and standards

authorities are in the process of developing new test methods

and criteria and revising those which are outdated.

In the United States, Underwriters' Laboratories, Inc.

is presently developing standards for ion chamber detectors

and electrically operated single station fire alarm devices.

The present tread with U.L. is to combine separate test

standards into a single standard.

Work is being conducted at the National Bureau of

Standards to study the behavior of smoke detectors exposed

to various types of smoke, rates of smoke production, and

44
rates of smoke movement. Part of the NBS program is directed

toward the development of an interim set of performance

standards based on these variables. It has been determined

Ly the authors that testing aerosol detectors under high

velocity conditions in test chambers can result in acceptance

of units which will be slow to alarm or perhaps not alarm

at all under actual fire conditions. This is a serious

problem in applications where only one detector might be

used as, for example, in a single family dwelling. This

so called smoke entry problem relates more to the detector

housing design than to the sensing element itself. Proper

evaluation of an aerosol detector should then include

testing for response to the designed aerosol signature

level at a variety of air flow velocities extending at least

to a low velocity of 15 feet per minute (0.76 m s"^)

The trends in foreign standards for detection devices

are similar to those in the United States. An attempt is

underway to develop a set of uniform standards throughout

Europe. The thermal detector standards will most likely

follow BS 3116. Smoke detection standards are in a state

of flux. The test scheme proposed by the Technical

University at Aachen (74) suggests an application test

where detectors are installed in a room and exposed to

a wide variety of fires. The report of testing for a particular

detector then would indicate for which risks it could properly

be used. Included in the Aachen scheme is a "blinding"

test which is being developed to evaluate the effects

of bright lights on photoelectric detectors. Hosemann

(75) suggests that before supraregional or truly international

tests can have meaning, a standard ionization test chamber,

a standard obscuration (beam type) test instrument and

a standard light: scattering instrument must be developed.

These instruments would be used to evaluate the performance

of different detectors with the same detection mode exposed

to the same smoke. Hosemann also suggests the development

of a uniform test aerosol for use with these instruments.

It would appear from the present trends in detection

standards development that future acceptance criteria will

be based on detector performance under a wide variety of

fire and smoke conditions. If this trend continues, it

will facilitate the choice of the appropriate device for

any expected fire situation.

11. Field Tests of Detector Performance

Many field tests and full scale laboratory tests have

been conducted with fire detectors (76) (77) (78) (79)

(80) (81) . The results of these tests can give valuable

45
information regarding the relative response characteristics

of different detectors under varying fire conditions. It

should be noted, however, that the performance of a detector

in one set of conditions can be vastly different from that

of the same detector under different conditions. In addition,

wide variations in response time can be noted with different

brands of smoke detectors operating on the same principle

under conditions of slowly moving smoke. This is primarily

related to cabinet design and may not be noticed in tests

if the velocity of the moving smoke is greater than 20

to 25 feet per minute (6.09 to 7.62m min~l) . The factors

which effect the response of detection devices are many.

Some examples are rate of fire growth, volume of the fire

area, type of fuel, distance of detector from the fire

and the air velocity and flow patterns in the building.

The results of detector tests should be evaluated only

in terms of the specific detection hardware used, the nature

of the fire exposure, the arrangement of the detectors

and the configuration of the test facility.

Although the results of any one test series cannot

establish all the properties of detection devices or

establish a fixed order of preference among detector types,

a broad review of the results can generate some general

statements. Probably the fastest detection devices for

open-burning fires are the radiant energy detectors. Tests

in hyperbaric chambers (82) (83) indicated that detection

could be achieved with times between 20 milliseconds and

18 seconds. Tests of thermal and smoke detection in a

dwelling fire (76) showed that, with a smouldering upholstery

fire, rate-of-rise and 135°F (56.6°C) fixed-temperature

detectors respond at approximately the same time (118-119

minutes) . The temperature at the time of alarm was 140°F

(59.4°C) . A similar test was conducted with 2 photoelectric

detectors included. Alarm was received from the photoelectric

unit in the fire room at 38 minutes. This test terminated

at 50 minutes with no alarm from the thermal detectors.

Another test in this series used a rapidly developing fire

in a trash barrel in the test room. This time the rateof-rise detector alarmed at 2 minutes, while the 135°F

(56.6°C) fixed-temperature unit responded at 5 minutes

and the photoelectric detector in 8 minutes. Based on

these and other tests, it appears that rate-of-rise detection

is the fastest means for thermal detection of rapidly growing

fires. In another test using an upholstery fire in the

living room of the test building, the photoelectric detector

in the dining room, separated by the hall and a stairway

alarmed in 33 minutes (the photoelectric in the fire room

actuated in 21 minutes) while the rate-of-rise unit in

46
the fire room responded in 1 hour and 40 minutes. Here

the advantage of smoke detection in a smouldering fire

is obvious.

Comparisons of response time between photoelectric

and ion chamber detectors will vary depending on the location

of the detectors with respect to the fire and the nature

of the fuel. At close range, the ion chamber will alarm

ahead of the photoelectric while at some distance away

the photoelectric unit will be faster. This is due to

the increase in particle size of the aerosol due to coagulation

as it ages with time and distance from the fire (77) . The

ion chamber is more sensitive to the small particles found

close to the fire while the photoelectric detector responds

faster to the larger more stable particles at a distance.

Little test data are available for full scale tests

of detectors such as the laser, the submicron particle

counters and the ultra-sonic detectors. These types are,

in most situations, faster in response to fire than thermal

detection.

Before choosing a particular detection device it is

advisable to determine the type of fire expected and the

response time needed. Following this, a survey of the

test results with similar exposures can be of great

assistance in choosing the appropriate detector.

12. Code Requirements for Fire Detection

In past years building and fire codes made little

or no mention of requirements for fire detection in buildings.

The chief concern centered around evacuation alarms activated

by manual pull stations. Recently code officials at the

local and state levels, as well as the model code authorities,

have begun requiring detection in various types of occupancies

,

notably multi-family residential. The extent of this move

to require detection is so great that a detailed treatment

and analysis of all requirements is beyond the scope of

this work. However, the trends can be noted by discussing

some selected examples.

12.1 State Requirements

At the state level, several states are requiring smoke

detection. Ohio requires smoke detection in industrialized

(modular or factory produced) One, Two and Three Family

Dwellings (86) . Detectors are to be placed in the vicinity

of, but outside, all sleeping rooms. Detectors may be

of any type except thermal-only. The State of Nevada has

47
similar requirements but requires ionization detectors

specifically. In addition to residential requirements,

proposed changes to the state code of Ohio would alter some

fire resistance requirements for buildings which have either

an automatic sprinkler system or an approved air pressurization

and smoke detection system. These proposed amendments would

require the use of smoke detection in conjunction with

air pressurization in hospitals, nursing homes and rest homes.

12 .Local Requirements

Many cities and local jurisdictions are requiring

smoke and/or heat detection. The city of Acton, Mass.,

for example, requires heat detection in multiple family

dwellings of more than 6 units or boarding and rooming

houses with more than 10 occupants. Heat detectors are

required in utility rooms, heating rooms and similar service

areas. In addition, all new single family dwellings and

multiple dwellings of six or less units must have smoke

and heat detection devices (84) . Many cities are also

requiring smoke detection in dwelling units of apartment

buildings. The city of Dallas, Texas requires the use

of approved detectors of products of combustion other than

heat in each dwelling unit in apartment buildings (85)

.

12.3 Model Building Code Requirements

The model building codes which are adopted by many

local jurisdictions are also generating detection

requirements. The BOCA (Building Officials Conference

of America) Basic Building Code (86) , for example, now

requires a minimum of one smoke detector for visible or

invisible particles in each new dwelling unit of the

multi-family and one and two family types. The Uniform

Building Code (UBC) (87) requires detection of products

of combustion other than heat in each dwelling unit of

an apartment house as well as in detached dwellings. UBC

also allows the use of heat or smoke detectors to initiate

the return of elevators to the main floor. Detectors

are required in elevator lobbies to prevent elevator doors

from opening when untenable conditions exist. Smoke detectors

are also required by UBC in air handling systems in the

main supply duct to shut down the system when smoke is

present.

UBC has adopted UL-168 (Smoke Detectors, Photoelectric

Type) and is considering the use of HUD's performance

criteria for smoke detectors. The HUD requirements would

apply to detection for dwellings.

48
These are examples of detection requirements presently

adopted by 2 model building codes. Other model codes are

presently in the process of developing detection requirements,

and it is likely that all model codes will soon have new

or expanded requirements

.

12.4 The Life Safety Code Requirements

The Life Safety Code (N.F.P.A. 101) (62) is widely

adopted throughout the United States and Canada. The 1973

changes to this code contain greatly expanded detection

requirements and will have substantial impact. An automatic

detection system which responds to products of combustion

other than heat is required in all corridors of new hospitals,

nursing homes and residential custodial care facilities

except on sleeping room floors where each sleeping room

has detection and detectors are provided at corridor smoke

partitions. A complete automatic fire detection system

can substitute for a manual alarm system where required

in residential, business and mercantile occupancies. New

sections have been added to Chapter 9 (Education Occupancies)

to cover various types of child day care facilities. Detectors

responding to products of combustion other than heat are

required in instances where sleeping facilities are present

except in fully sprinkled child day care centers. An

interim amendment covering detection in One and Two Family

Dwellings is included in the 1973 edition of NFPA No. 101.

The use of single station type detection units is allowed

for the dwelling and child care requirements.

12.5 Mobile Home Requirements

The revisions to NFPA 501B-1973 (Mobile Homes) require

that at least one listed smoke detector, which may be single

station, be installed in each mobile home outside each

sleeping area (88).

12.6 Federal Housing Administration Requirements

The Federal Housing Administration (FHA) has proposed

revisions to the Minimum Property Standards (MPS) for MultiFamily (89) , Housing for the Elderly and Handicapped (90)

and Nursing Homes (91) which would require smoke detection

in all "elevator buildings" (5 or more stories) . In multifamily housing, the detectors would be in the living units,

at fire doors in the fire wall, and in elevator lobbies.

The elevator would bypass a floor on which the lobby detector

has operated. In non-elevator buildings at least 1 smoke

detector would be placed in each living unit. Smoke detection

requirements in Housing for the Elderly and Handicapped and

49
Nursing Homes are similar to Multi-Family except that

patient room doors left open would close automatically on

activation of a detector within the room. The FHA requirements are not presently in effect but will be in the near

future. FHA contemplates extension of smoke detection

requirements to One and Two Family Dwellings one year after

the revision of the MPS's for Multi-Family and Care-Type

Housing.

13. Conclusions

With the present state-of-the-art detection technology,

fire detection can be achieved within milliseconds of inception

Thus it is possible, though not always cost effective, to

provide nearly any level of response time desired. It is also

clear that the widespread use of detection can significantly

reduce both life loss and property loss.

Several areas of the detection picture, however, need

improvement in order to provide data which will allow effective

engineering judgements to be made in selecting the appropriate

detector for specific applications.

In the area of false alarms, a problem associated

particularly with smoke detection, the proposed work on

the nature and variability of ambient levels of fire

signatures can provide the necessary data to design detectors

such that false or unwanted signals will be minimized. One

promising approach may be through the use of multi-mode

detectors requiring signals from several fire signatures

before a fire alarm is initiated.

Changes need to be made in the testing and approval

procedures to provide engineering data for approved detectors.

Data should be provided which accurately describe the performance of approved units over a wide range of smoke sources,

air velocities, rates of heat evolution, and installation

configurations. These data can permit better engineering

of detection systems with respect to the expected fire

exposure and response time criteria. By limiting approval

testing to a narrow range of fire signatures, ambient

conditions and installation configurations, or testing

against a standard device such as an automatic sprinkler

head, many detectors which have desirable characteristics

for specific applications may be excluded from the market

place while others which may have undesirable properties,

such as insensitivity to certain fire signatures, may be

accepted. This can result in detector failures due to

lack of engineering data concerning the capabilities of

a listed or approved unit.

50
In conjunction with the development of expanded test

procedures, work should be done to establish threshold

hazard values for determination of appropriate alarm levels

of fire signatures. This is important where life safety is

the prime consideration. This is particularly significant

with regard to aerosol and gas signatures and for the

development of threshold requirements for multi-signature

detectors.

Also there is a need to continue to investigate methods

of sensing additional fire signatures. Carbon monoxide,

carbon dioxide and oxygen depletion signatures may be of

value in multi-mode detectors. In addition, it is possible

that spin-offs from air pollution detection and laboratory

analysis technology can be transferred completely or in part

to advance the technology of fire detection.

14 . Acknowledgments

The authors greatfully acknowledge the assistance

of Ms. Nora Jason and the staff of the Fire Information

Reference Services of the National Bureau of Standards

for their invaluable aid in the procurement and

compilation of the reference and bibliographic materials.

51
15. References

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America Burning. U.S. Government Printing Office,

Washington, D.C. (1973)

.

2. McGuire, J.H. and Ruscoe, B.E., The Value of a Fire

Detector in the Home. Division of Building Research,

National Research Council of Canada, Ottawa, Fire

Study No. 9 (1962)

.

3. Van Luik, Jr., F.W., Characteristics of Invisible Particles Generated by Pre Combustion and Combustion. 77th

Annual Meeting of the National Fire Protection Association,

St. Louis, Mo. (May 17, 1973).

4. Rodebush, W.H., General Properties of Aerosols. Chapter

4, Handbook on Aerosols, U.S. Atomic Energy Commission

(1950) .

5. Scheidweiler, A., New Research in Fire Detection

Technology. International Symposium (1968)

.

6. Comeford, J.J. , The Spectral Distribution of Radiant

Energy of a Gas-Fired Radiant Panel and Some Diffusion

Flames. Combustion and Flame, Vol. 18, 125-132 (1972).

7. Wagner, J.P. , A Survey of the Principal Operational

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Research Abstracts and Review, Vol. 13, No. 2, 95-

113 (1971)

8. Maurodineanu, R. and Boiteux, H., Flame Spectroscopy.

John Wiley and Sons, Inc., New York (1965) .

9. Bruce, H.D., Experimental Dwelling Room Fires. Forest

Products Laboratory, Madison, Wis., Report No. 1941

(1953) .

10. Shorter, G.W., McGuire, J.H., Hutchinson, N.B., and

Legget, R.F., The St. Lawrence Burns. Quarterly of the

National Fire Protection Association, Vol. 53, No. 4,

300-316 (1960) .

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Dwellings. Quarterly of the National Fire Protection

Association, Vol. 56, No. 3, 201-246 (1963).

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12. Ives, J.M., Hughes, E.E. and Taylor, J.K., Toxic Atmospheres Associated with Real Fire Situations. Nat.

Bur. Stand. (U.S.), Washington, D.C., NBS Rep. No. 10807

(1972) .

13. Zikria, B.A., Ferrer, J.M. and Floch, A.B., The Chemical

Factors Contributing to Pulmonary Damage in Smoke Poisoning. Surgery, Vol. 71, No. 5, 704-709 (1972).

14. Stevens R.E., Church Oyster Roast Fire Panic. Quarterly

of the National Fire Protection Association, Vol. 49, No.

4, 277-285 (April 1956) .

15. Watanabe, A., Testing Methods of Fire Detectors in

Buildings. Symposium on Automatic Fire Detection,

London (March 1972)

.

16. Bond, H., Ed., Fire Casualties of the German Attacks.

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.

17. Dupont, H.B., How Much Heat Can Firemen Endure? Fire

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24. Pryor, A.J., Johnson, D.E. and Jackson, N.N., Hazards of,

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29. Fire Protection of Concorde. Fire International, Vol.

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33. Johnson, J.E., Concepts of Fire Detection. Engineering

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36. Wampfler, G.L., Light Transmission and Light Scattering

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38. Haessler, W.M., Smoke Detection by Forward Light

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45. Barrett, R. , Optimum Design in Infrared Flame Detectors.

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49. Eggleston, L.A., Fire Safety in Hyperbaric Chambers.

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53. Hurn, M.J., Fire Detection Using a Sub-micron Particle

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56
65. Fire Detection Thermostats. Underwriters* Laboratories,

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72. Heat Actuated Fire Detectors. Standards Association

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75. Hosemann, J. P., Considerations in Testing Smoke

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57
77. Rueger, R.A. , Home Fire-Alarm Tests. Bloomington, Minn.

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80. Official Report on the Fire Alarm Tests, Malar Baths,

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81. Carter, D.I., Fire Extinguishment and Protective

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82. Eggleston, L.A., Fire Detection in Hyperbaric Chambers.

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84. ACTON (Town) Building Code. Section 13 8, Acton, Mass.

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77
TABLE I

THERMAL PARTICULATE POINT IN AIR

(From Van Luik (3)

)

Material

Bakelite

PVC Insulation

Acrylan Carpeting

Wool

Paper

Pine Board

Polystyrene

Polyethylene

Motor Oil (SAE30)

Temperature

°F °C

380 191

290 142

340 169

360 180

500 257

320 15 8

710 373

410 208

310 153

78
TABLE II

EFFECT OF CARBON MONOXIDE EXPOSURE

(After Claudy (22))

Percent

Concentration PPm Time Effect

0.02 200 2-3 hr. mild headache

0.08 800 45 min. mild headache

2 hour death possible

0.32 3,200 10-15 min. dizziness

30 min. death

0.69 6,900 1-2 min. dizziness

10-15 min. death

1.28 12,800 2-3 breaths unconscious

1-3 min. death

79
Percent

Concentration PPm

0.5 5,000

3.0 30,000

5.0 50,000

10.0 100,000

TABLE III

EFFECTS OF CARBON DIOXIDE EXPOSURE

(After Claudy (22)

)

Effects

increase depth of breathing

breathing rate doubles

300% increase in breathing rate

possible death even with

sufficient atmospheric oxygen

80
TABLE IV

EFFECTS OF OXYGEN DEPLETION*

Percent

21-17

Time

Indefinate

17-14 2 hr.

14-11 30 min.

9 5 min.

6 1-2 min.

Effect

Respiration volume decreases,

loss of coordination and difficulty in thinking

Rapid pulse and dizziness

Nausea, vomiting and paralysis

Unconsciousness

Death within a few minutes

* These figures are only approximate as there are some

variations in the literature.

81
TABLE V

RATINGS OF FIXED-TEMPERATURE DETECTORS

(From UL-521 (64) )

Rating

Ordinary

Intermediate

High

Extra High

Operating Range

135-165°F (57-73°C.)

175-225°F (79-106°C.)

250-300°F (120-147°C.)

325-375°F (161-188°C.)

Maximum Ceiling

Temperature

100°F (37°C.)

100-150°F (37-65°C.)

150-225°F (65-106°C.)

225-300°F (106-147°C.)

82
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Input Voltage

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Figure 18. Simple Schematic of Detector using a Solid State Gas Sensor

(Figaro Engineering, (38))

98
Figure 19. Enlarged Sectional View of a Solid State Gas Sensor,

(Figaro Engineering, (38))

99
Photo -cell

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mask to cu. cxtroncous light

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Figure 20. Photocell Masking for Heat Detection.

(From Lawson, (42))

100
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Figure 24. Fresnel Lens System.

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110
NBS-U4A (rev. 7.73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-839.

4. TITLE AND SUBTITLE

Fire .Detection: The State-of-the-Art

2. Gov't Accession

No.

7. AUTHOR(S)

R. L. P. Custer and R. G. Bright

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Aerospace Safety Research and Data Institute

NASA Lewis Research Center

Cleveland, Ohio

15. SUPPLEMENTARY NOTES

3. Recipient's Accession No. NASA CR-134642

5. Publication Date June 1974

6. Performing Organization Code

8. Performing Organ. Report No.

10. Project/Task/Work Unit No. 490-3373

11. Contract/Grant No.

13. Type of Report & Period Covered Final 1973

14. Sponsoring Agency Code

16. ABSTRACT (A 200-word or /ess factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The current state-of-the-art in fire detection technology is reviewed

considering the nature of fire signatures, detection modes used, test

methods, performance requirements and code requirements for fire detection.

Present trends in standards development and recommendations for future work

are included. An extensive bibliography is provided.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Fire detection; fire detection code requirements; fire detector testing and

standards; fire detectors; fire signatures.

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