Problem with the use of a flame-flicker forest fire detector

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Problem with the use of a flame-flicker forest fire detector McGuire, J. H.

NATIONAL RESEARCH OOUNOIL

CANADA

D I V I S I O N OF BUILDING

RESEARCH

A

PROBIB4

WITH

THE

USE

OF A

FLAME-FLICrnR

FOREST PIRE DrnEOTOR

by

J. H.

MaGuire

Internel Report No.

277

of the

Division

o f

Building

Research

OTTAWA August

1963

NATIONAL

RESEARCH

COUNCIL CANADA

DIVISION OF

BIKIXIXG FJ>SMRCB

A PROBLW

WITH THE

USE OF A FLAME-FLICKER FOREST FIRF: DEVPECTOR

by

J.

H.

MoGuire

Internal

R e p o r t

No.

277

of -the

Division of Building Research

OTTAWA

The Division a t t e m p t s

t o

a s s i s t ,

through i t s F i r e Researoh S e c t i o n , i n f o r e s t f i r e problsms whioh a r e compatible w i t h

i t s

own i n t e r e s t s . The work now - r e p o r t e d was c a r r i e d o u t i n response t o a r e q u e s t f o r a s s i s t a n c e

f r o m

t h e

NRC

Associate

Committee on F o r e s t F i r e P r o t e c t i o n . Laboratory t r i a l s u s i n g a f l a m e - f l i c k e r technique a s a p o s s i b l e means of d e t e o t i n g f o r e s t f i r e s under poor v i s i b i l i t y o o n d i t i o n s were promising b u t l i m i t e d f i e l d t r i a l s i n d i c a t e d s e r i o u s d i f f i c u l t i e s from r e f l e c t e d

s u n l i g h t .

The

a u t h o r i s

a

r e s e a r c h o f f i c e r w i t h t h e F i r e Research S e c t i o n

o f

t h e D i v i s i o n and 9 s concerned w i t h a number of physios problems involved i n t h e

d e t e c t i o n , development and extinguishment of f i r e s .

Ottawa

August 1963

N. B. Butcheon A s s i s t a n t D i r e c t o r

FLAME-FLICKE3R EOmS!l! FIRE DETECTOR

A f i e l d t e s t of a f l a m e - f l i c k e r f o r e s t f i r e detector

on a very h o t and b r i g h t day gave an u n d e s i r a b l y high l e v e l of f a l s e responses. I t i s b e l i e v e d t h a t t h i s phenomenon r e s u l t e d from r e f r a c t i o n , by thermal a i r c u r r e n t s , of s o l a r r a d i a t i o n r e f l e c t e d from t h e t a r g e t a r e a .

The

DBR

F i r e Research S e c t i o n was approached by t h e NRC Associate Committee on F o r e s t F i r e P r o t e c t i o n f o r a s s i s t a n c e i n d e t e c t i n g f o r e s t f i r e s under c o n d i t i o n s of poor v i s i b i l i t y . Detection f a c i l i t i e s a r e most d e s i r a b l e when one o r more f i r e s a r e i n p r o g r e s s , when *he l i k e l i h o o d of f u r t h e r f i r e s i s h i g h , and when v i s u a l d e t e c t i o n is v i r t u a l l y impossible because of smoke haze which could be b l a n k e t i n g a n a r e a of hundreds of square m i l e s ,

The scope of t h i s r e p o r t i s confined t o r e p o r t i n g

a n

a t t e m p t t o produce a simple device ca7able of d e t e c t i n g f o r e s t f i r e s ; no review is made of p o s s i b l e a l t e r n a t i v e techniques.

PRINCIPLE OF OPERATIOR

If f i r e i s t o be d e t e c t e d a t a d i s t a n c e , t h c most obvious approach t o t h e problem i s t o u t i l i z e an i n f r a - r e d s e n s i n g element. A l e a d s u l p h i d e p h o t o - c e l l i s a convenient choice f o r t h i s purpose because i t i s a v e r y s e n s i t i v e device and i t s response peaks a t almost e x a c t l y t h e wavelength of maximum emission of a gOO°C flame

( 2 . 5

microns),

To s i m p l i f y t h e a p p a r a t u s , it was decided t h a t t h e a n g u l a r cover of t h e d e t e c t o r should be s u b s t a n t i a l s o t h a t t h e s i m p l e s t s c a n i i n g techniques could be employed. Because o f t h i s d e s i p requirement, t h e l i m i t q t i o n s of t h e a p p a r a t u s become dependent on t h e l i k e l i h o o d of f a l s e responses due t o s o l a r r a d i a t i o n . Figure 1 g i v e s t h e r a t i o of t h e emission f r o m t h e sun p e r u n i t a r e a a t a s p e c i f i e d wavel.ength, t o t h a t

f r o m a s u r f a c e a t a t e m p e r s t u r e of 1150°K ( r e p r e s e n t i n g a f l a m e ) . I n t h e r e g i o n concerned (abov-G 2 t o

3

rnicrans) t h e sun i s a b o u t 100 times a s b r i g h t a,? a _flame,

F a l s e r e s p o n s e s w i l l be a s s o c i a t e d e i t h e r w i t h s p e c u l a r o r d i f f u s e r e f l e c t i . o n of s o l ~ r r a d i a t i o n from t h e t a r g e t a r e a . The c a s e of s p e c u i a r r e f l e c t i o n i s r e a d i l y a n a l y z e d by u s i n g t h e e q u i v a l - e n t r a d i a t i n g s u r f a c e c o n c e p t (1). If t h e m i r r o r i n t h e t a r g e t a r e a i s t a k e n a s t h e e q u i v a l e n t r a d i a t i n g s u r f a c e r e p r e s e n t i n g t h e s u n , t h e l e v e l o f r a d i a t i o ~ from it p e r u n i t a r e a w i l l a g a i n be 100 t i m e s t h a t from t h e same area of s u r f a c e a t a t e m p e r a t u r e of l150°1C ( r e p r e s e n t i n g a f i r e ) . It f o l l o w s , t h e r e f o r e , t h a t i f a f a l s e r e s p o n s e i s t o be avoided and t h e s e n s i t i v i t y o f t h e d e t e c t o r i s s e t t o d e + e c t a f i r e of minimum s p e c i f i e d p r o p o r t i o n s , t h e a r e a of a n a p p r o p r i a t e l y o r i e n t e d m i r r o r m u s t n o t exceed l / l 0 0 of t h i s f i r e a r e a . Once t h e a r e a o f t h e r e f l e c t i n g s u r f a c e exceeds a cer't;ain c r i t i c a l v a l u e , t h e e x c e s s a r e a performs no f w l c t i o n . The c r i t i c a l a r e a i s s u f f i c i e n t l y g r e a t , however, t h a t t h e

s p e c i f i c a t i o n f o r t h e minimum d e t e c t a b l e f i r e area would c e r t a i n l y be l e s s t h a n one hundred t i m e s - t h i s a r e a . Thus, i f t h e plane of t h e r e f l e c t i n g s u r f a c e i s t a k e n t o be normal t o t h e s u n , t h e c r i t i c a l a r e a w i l l be c i r c u l a r w i t h a d i a m e t e r e q u a l t o a p p r o x i m a t e l y a hundredth of i t s d i s t a n c e from t h e r e c e i v e r . Thus, a t a range of a m i l e , t h e cri-trical. d i a m e t e r of t h e " m i r r o r " would be 50 f t .

The c a s e of d i f f u s e r e f l e c t i o n i s even Inore r e a d i l y a n a l y z e d . The r a t i o of t h e " t a r g e t " r a d i a t i o n (from t h e f i r e ) t o t h e d i f f u s e l y r e f l e c t e d s o l a r r a d i a t i o n w i l l be t h e

r e c i p r o c a l of t h e product of the c o n f i g u r a t i o n f a c t o r of t h e sun a t t h e t a r g e t and t h e f a c t o r 100 (mentioned p r e v i o u s l y a s b e i n g t h e r a t i o of t h e e m i s s i o n of t h e

sun

p e r u n i t a r e a

t o t h a t of a s u r f a c e a t 1150°K). The c o n f i g u r a t i o n f a c t o r i s approximately 1/45,000 s o t h a t t h e r a t i o i s approximately

450.

I f t h e s e n s i t i v i t y of t h e d e t e c t o r i s s e t t u d e t e c t a f i r e of minimum s p e c i f i e d p r o p o r t i o n s t h e a r e a of view of t h e s e n s i n g head must n o t exceed 450 t i m e s t h j . s minimum f i r e a r e a .

The above a s s e s s m e n t s i n v o l v e c e r t a i n assumptions which make them r e p r e s e n t bad c o n d i t i o n s , e.g. i n t h e l a s t c a s e , it was assumed t h a t 100 p e r c e n t of t h e s o l a r r a d i a t i o n i n c i d e n t on t h e t a r g e t a r e a would be d i f f u s e l y r e f l e c t e d . N e v e r t h e l e s s , it i s a p p a r e n t t h a t t h e l i m i t a t i o n s of a simple i n f r a - r e d d e t e c t i o n system o p e r a t i n g i n t h e r e g i o n of 2.5 microns a r e t o o s t r i n g e n t t o be a c c e p t a b l e .

To e a s e t h e s e l i m i t a t i o n s it was decided t o make use of t h e f a c t t h a t f i r e s i n h e r e n t l y give r i s e t o f l i c k e r i n g flames while ambient l e v e l s of s o l a r r a d i a t i o n a r e n o t

u s u a l l y s u b j e c t t o such r a p i d v a r i a t i o n . Roeser and McCarny ( 2 )

of t h e National Bureau of Standards have i n v e s t i g a t e d t h e a p p l i c a t i o n of t h i s technique t o t h e d e t e c t i o n of f i r e i n a i r c r a f t engine spaces and have found t h a t t h e p r i n c i p a l components of f l i c k e r i n t h e average s m a l l f i r e Z j e i n t h e range of

3

t o 25 cps. The amplitude o f t h e v a r i a t i o n i n t h e l e v e l of t h e r a d i a t i o n from an average f i r e i s qui-t'e s u b s t a n t i a l whereas t h e corresponding v a r i a t i o n i n t h e r e f l e c t e d s o l a r r a d i a t i o n would be expected t o be almost n e g l i g i b l e . I t would t h e r e f o r e be expected t h a t t h e a r e a which could be monitored a t any one time would be v e r y much g r e a t e r t h a n i s p o s s i b l e w i t h t h e s i m p l e s t i n f r a - r e d system. PROTOTYPE APPARATTJS A Kodak Ektron l e a d s u l p h i d e c e l l w i t h a s e n s i t i v e a r e a 4mm

x

4nm c o n s t i t u t e s t h e s e n s i n g element, t h e window being r e p l a c e d by a Kodak f a r i n f r a - r e d s i l v e r c h l o r i d e f i l t e r w i t h a c u t on a t 1 . 5 microns, This c ~ m b i n a t i o n responds t o t h e band 1.5 t o

3.5

microns and i s t h u s i d e a l f o r r a d i a t i o n from b l a c k body temperatures of about 900°C. The c e l l i s mounted a t t h e f o c u s of a p a r a b o l i c mirror. Using a 5.5-in. diameter 1.75-in. f o c a l l e n g t h m i r r o r g i v e s a n a n g u l a r cover o f about 5 deg; a 1 6 - i n . diameter 6 - i n , f o c a l l e n g t h g i v e s a cover o f about

1 . 5

deg.

The c i r c u i t diagram of t h e a m p l i f i e r and t h e p r e s e n t a t i o n u n i t i s shown i n Figure 2 . The bandwidth of t h e a m p l i f i e r (which i s n o t v e r y c r i t i c a l ) i s of t h e o r d e r of 1 t o 60 cps and t h e maximum v o l t a g e g a i n a t 30 cps i s 1500. P r e s e n t a t i o n i s a u d i b l e , any t r a n s i e n t response of adequate amplitude switching a 1000 cps tone t o a small loud

speaker f o r a period of about 2 s e c . The c i r c u i t marked "scan lock" i s i n c o r p o r a t e d t o l o c k t h e o p t i c a l system on t o a ( s t a t i o n a r y ) t a r g e t when automatic scanning i s i n use,

Scanning c o n s i s t s of r o t a t i n g t h e o p t i c a l system about a v e r t i c a l a x i s a t a r a t e of

1 / 5

rpm by means of a n e l e c t r i c motor,

LABORATORY TRIALS

The f i r s t t r i a l s w i t h t h e a p p a r a t u s were made

before t h e s i l v e r c h l o r i d e f i l t e r had been d e l i v e r e d and i n i t s place a f i l t e r w i t h a cut-on a t 0.8 microns was used.

When covered w i t h t h e s i l v e r c h l o r i d e f i l t e r t h e l e a d sulphide c e l l was only s e n s i t i v e t o about 5 p e r c e n t of t h e energy i n

t h e s o l a r spectrum. Substituting t h e temporary f i l t e r made t h e o p t i c a l system s e n s i t i v e t o a f u r t h e r 73 per c e n t . It was t h e r e f o r e t h o u g h t t h a t when t h e s i l v e r c h l o r i d e f i l t e r was i n s t a l l e d t h e l e v e l of unwanted s i g n a l s would d r o p b y a f a c t o r of abou-t 7.

The o n l y m i r r o r a v a i l a b l e a t t h i s time was t h e s m a l l e r of t h e two.

From a l a b o r a t o r y bench, t h e o p t i c a l system was allowed t o s c a n a n a r e a of g r a s s l a n d a b o u t a q u a r t e r o f a

mile i n d e p t h w i t h a dozen t r e e s s c a t t e r e d a t r a n g e s of between 50 and 200 yd. The a p p a r a t u s was a b l e t o d e t e c t n f i r e i n a 3 - i n . d i a m e t e r t r a y of g a s o l i n e a t a range of a q u a r t e r o f a m i l e , With t h e g a i n c o n t r o l s e t t o a c h i e v e

t h i s l e v e l of s e n s i t i v i t y , s c a n n i n g a t a r a t e of 1 r e v o l u t i o n p e r

5

min d i d n o t g i v e r i s e t o f a l s e a l a r m s on a b r i g h t day e x c e p t when automobiles passed a c r o s s th.e f i e l d of view of t h e o p t i c a l system.

It was a l s o found t h a t t h e performance of t h e a p p a r a t u s was l i t t l e a f f e c t e d by whether o r n o t t h e o p t i c a l system was behind window g l a s s . Some change i n t h e g a i n

s e t t i n g might have been n e c e s s a r y b u t t h e s i g n a l / n o i s e r a t i o ,

i f t h i s term can be a p p l i e d t o t h e r a t i o of t h e t a r g e t t o t h e unwanted s i g n a l s , was s u b s t a n t i a l l y unchanged,

On t w o o c c a s i o n s t e s t s were c a r r i e d o u t i n m i s t when t h e v i s i b i l i t y was a b o u t 100 yd. The a p p a r a t u s was a g a i n a b l e t o d e t e c t a f i r e i n a 3 - i n . d i a m e t e r t r a y of

g a s o l i n e a t a range of a q u a r t e r of a mile. The g a i n c o n t r o l s e t t i n g was n o t markedly d i f f e r e n t from t h a t used on a c l e a r day. No o p p o r t u n i t y a r o s e , however, f o r a t e s t t o be c a r r i e d

o u t through a n a p p r e c i a b l e p a t h l e n g t h of smoke.

The above performance was s o promising t h a t it was t h o u g h t t h a t a simple f i e l d t r i a l would be opportune, d e s p i t e t h e f a c t t h a t t h e s i l v e r c h l o r i d e f i l t e r and t h e l a r g e r m i r r o r had n o t y e t a r r i v e d .

FIELD

TRIALS

Two f i e l d t r i a l s were c a r r i e d o u t a t t h e F o r e s t Range and Experimental S t a t i o n a t Petawawa, through t h e

c o u r t e s y of t h e Department of F o r e s t r y . The complete equip- ment was t a k e n t o t h e t o p of a f i r e o b s e r v a t i o n tower known a s Montgomery Tower and a s m a l l f i r e , a l i t t l e l a r g e r t h a n a t y p i c a l camp f i r e , was lit a t t h e r o a d s i d e , b e s i d e Whichway Lake, a d i s t a n c e of a b o u t 1 1/2 m i l e s . There was a n o p t i c a l p a t h between t h e o b s e r v a t i o n p o i n t and t h e f i r e .

On t h e f i r s t occasir:n t h e _{galra of t h e a m p l i f i e r} was n o t q u i t e a d e q u a t e t o g i v e a clear r e s p o n s e to t h e f i r e . Because t h e I n c i d e n c e of f a l s e r e s p o n s e s , a s s o c i a t e d w i t h s c a n n i n g o r v i b r a t i o n of the t o w e r , w a s low, it w a s t h o u g h t t h a t a g r e a t e r g a i n would p r o v i d e t h e s o l u t i o n T o the problem.

A second s t a g e of t u b e a m p l i f i c a t i o n , i d e n t i c a l . t o t h e f i r s t , w a s p u t i n and t h i s , t o g e t h e r w i t h o t h e r minor m o d i f i c a t i o n s , gave an i n c r e a s e i n g a i n by a f a c t o r o f nearly 20. The l a r g e r m i r r o r w a s now a v a i l a b l e and i n comparison w i t h tlia s m a l l e r one save a f u r t h e r g a i n of a f a c t o r of 8, While t h e c i r c u i t m o d i f i c a t i o n s were b e i n g made t h e s i l v e r c h l o r i d e f i l t e r s wore d e l i v e r e d and t h e s e were i n s t a l l e d i n t h e l e a d s u l p h i d e c e l l s a s s o c i a t e d w i t h b o t h t h e l a r g e and t h e s m a l l m i r r o r s .

The second f i e l d t r i - a 1 was c a r r i e d o u t on a day which was n o t o n l y b r i g h t b u t a l s o v e r y h o t . A s soon a s t h e equipment was s w i t c h e d on it was obvious t h a t t h e i n c i d e n c e of f a l s e r e s p o n s e s was g r e a t e r t h a n on t h e p r e v i o u s o c c a s i o n and it w a s found t h a t t h e f i r e a t Whichway Lake could n o t be d e t e c t e d . F u r t h e r a t t e m p t s a t d e t e c t i o n were made, w i t h o u t s u c c e s s , by s u b s t i t u t i n g a p h o t o - c e l l without; a f i l t e r f o r one w i t h t h e s i l v e r c h l o r i d e f i l t e r and t h e n by s u b s t i t u t i n g t h e s m a l l e r m i r r o r (which had a p h o t o - c e l l , s i l v e r c h l o r i d e f i l t e r combination i n s t a l l e d ) f o r t h e l a r g e r one. The g a i n s e t t i n g of t h e a m p l i f i e r was t u r n e d down t o a v a l u e t h a t wzs no h i g h e r t h a n t h e f u l l g a i n p r i o r t o t h e m o d i f i c a t i o n s . The f a l s e r e s p o n s e s c o n t i n u e d when t h e opti.ca3- system was n o t s c a n n i n g and d i d n o t a p p e a r t o be a s s o c b a t e d w i t h

v i b r a t i o n of t h e t o w e r , which c o u l d be e a s j l y induced b y human movement and which gave r i s e t o f u r t h e r f a l s e r e s p o n s e s ,

p a r t i c u l a r l y when t h e l a r g e r m i r r o r was i n u s e .

The o n l y r e a s o n a b l e e x p l a n a t i o n of t h e above phenomenon i s t h a t s u n l i g h t i n c i d e n t on t h e d e t e c t o r , a f t e r r e f l e c t i o n

from t h e t a r g e t a r e a , was f l i c k e r i n g a s t h e r e s u l t of

r e f r a c t i o n by t h e r m a l a i r c u r r e n t s . Such a phenomenon was a p p a r e n t i n t h e v i s i b l e spectrum on t h i s p a r t i c u l a r day, b u t was one t h a t had n o t been allowed f o r .

The d i s c o u r a g i n g f e a t u r e of t h e r e s u l t was t h a t t h e poor c o n d i t i o n s compared w i t h t h e c o n d i t i o n s of t h e f i r s t t e s t had c o m p l e t e l y masked t h e improvement which s h o u l d have been i n t r o d u c e d by t h e use of t h e new o p t i c a l f i l t e r which would be e x p e c t e d t o c u t down the s i g n a l s a s s o c i a t e d w i t h s o l a r r a d i a t i o n by a f a c t o r of a b o u t

7.

C O N C L U S I O N

-

Although t h e equi~prnent; was i n t e n d e d p.ri..mar.ily f o r u s e on hazy d a y s , a good a l l . - r o u n d perfosn~ance ml~s a l s o

d e s i r e d and t h e r e s u l t s of t h e second f i e l d t r i a l were

c o n s i d e r e d s u f f i c i e n t l y d i s c o u r a g i n g t o warrant; s ~ r s p e n d i n ~ w o r k on a simple f l a m e - f l i c k e r d e v i c e o p e r a t i n g i . n t h e

r e g i o n of 2 . 5 microns.

A t _{t h e time of w r i t i n g t h i s r e p o r t , i.L has n o t} been decided. whether t o devel-op a f l a m e - f l i c k e r dev'ice t c work f u r t h e r o u t i n t h e i n f r a - r e d spectrum. F i g u r e 1 shows

t h a t beyond a 'wavelength of a b o u t 1 0 microns t h e r a t i o o f t h e

e m i s s i o n of t h e sun t o t h a t of a s u r f a c e a t 1 1 5 G U K ( r e p r e s e n t i n g a f l a m e ) f n l . 1 ~ t o

5

a s compared w i t h 100 i n t h e r e g i o n 2 t o

3

microns. I t i s d i f f i c n l t t o e s t i m a t e whether t h i s d e c r e a s e

by a f a c t o r of 20 would. g i v e a marked improvement i n p e r f o r - mance, f i r s t l y because no q u a n t i t a t i v e r e s i l l t s were o b t a i n e d a t t h e fie1.d t r i a l s d e s c r i b e d i n t h i s r e p o r t , and. s e c o n d l y because t h e b e h a v i o u r of t h e t a r e e t a r e a s a s r e g a r d s d i f f u s e r e f l e c t i o n might n o t be t h e same i n t h e d i f f e r e n t s p e c t r s a l =nges.

The t a r g e t r a d i a t i o n l e v e l i n t h e 1 0 micron range would be lower by a f a c t o r of a b o u t 20 and s u i t a b l e d e t e c t o r s would be l e s s s e n s i t i v e and more expensive t h a n a l e a d

s u l p h i d e c e l l . Consequently, t h e c o s t of a n a p p a r a t u s

o p e r a t i n g i n t h i s range would be s u b s t a n t i a l l y g r e a t e r . On t h e o t h e r hand, however, t r a n s m i s s i o n problems i n haze and smoke ( s o f a r n o t d i s c u s s e d ) would be a m e l i o r a t e ? and from t h i s p o i n t of view i+ might b e d e s i r a b l e t o o p e r a t e a t g r e a t e r wsvelengths

.

An i s s u e t o be decided i s whether a f l a m e - f l i c k e r d e v i c e o p e r a - t i n g i n t h e f a r i n f r a - r e d would be competi.tive a s r e g a r d s c o s t , compactness and performance compared t o a h i g h l y r e f i n e d i n f r a - r e d d e t e c t o r h a v i n g a very narrow a n g l e of view and u s i n g two-dimensional s c a n n i n g t o g i v e a p r e s e n t a - t i o n . The l a t t e r d e v i c e would p r e s e n t much more d e t a i l - e d

i n f o r m a t i o n and would n o t g i v e f a l s e r e s p o n s e s a s s o c i a t e d w i t h d i f f u s e s o l a r r e f l e c t i o n . I t would g i v e f a l s e r e s p o n s e s a s s o c i a t e d w i t h s p e c u l a r r e f l e c t i o n , b u t it would immediately become a p p a r e n t t h a t s u c h r e s p o n s e s were f a l s e i f t h e

d e t e c t i n g equipment were moved t o a n o t h e r l o c a t i o n f o r which t h e r e f l e c t i n g s u r f a c e was n o t a p p r o p r i a t e l y o r i e n t e d . Such a t e c h n i q u e would be most a p p r o p r i a t e f o r a i r b o r n e o p e r a t i o n and t h i s t y p e o f a p p a r a t u s would b e w e l l n u i t e d t o a i r b o r n e :lse.

The f l a m e - f l i c k e r technique might n o t be s u i t e d t o a i r b o r n e use because movement of t h e d e t e c t o r a s s o c i a t e d w i t h the v i b r a t i o n o f t h e a i r c r a f t would be l i k e l y t o g i v e r i s e t o f a l s e r e s p o n s e s . The forward movement o f t h e a i r c r a f t

m i g h t also c r e a t e a problem a l t h o u g h e x p e r i e n c e w i t h t h e

scanning of t h e a p p a r a t u s d e s c r i b e d i n t h i s r e p o r t ( o t h e r t h a n on t h e o c c a s i o n of t h e second t r i a l ) suggested t h a t t h i s would n o t be s e r i o u s .

1. McGuire, J. K. Heat transfer by r a d i a t i o n . F i r e Research S p e c i a l Report No- 2. H.M.S.O., 1953, p. 4.

2. Roeser, W. 1;'. and C . S. McCamy. P r i n c i p l e s of f i r e

d e t e c t i o n i n a i r c r a f t engine spaces. W.A.D.C., Tech. Report 54-307- June 1954.