lonaolstroat 10 - DELFT ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^
1 6 JULI 1957
THE COLLEGE OF AERONAUTICS
CRANFIELD
METHOD STUDY OF HOT WORKING CONDITIONS
by
jum, 1956
T H E G O L L E G - E O F A E R O W A Ü T I C S
C E A N g l B L D
Method S'bady of Hot ïïorking Conditions
-by-JoAaC. V/illians
1
a
Introduction and Historical Note
2, Ehysiological Effects of Hot Conditions
3« Psychological Considerations
4e Overall Industrial Effects
3« Heat Stress Indices
5.1 General
5 . 2 E f f e c t of Various C o n d i t i o n s on P4SR 5 «3 B e l d i n g and H^vtch I n d e x
6 . liethod Study Approach 6e1 Ivletabolisr.1 6o2 C o n v e c t i o n
6.3
Radiation
6,31 Insulation
6o32 Surfs.ce Emissivity
6.33 Screens
6 . 4 E v a p o r a t i v e Cooling of t h e Skin 7# Sequence of Heat S t r e s s Study
7*1 Itry "bulb t e m p e r a t u r e 7c2 ¥ e t b u l b temperatui""e 7 . 3 S h o r t c i r c u i t i n g of A i r FIOVT
8• Conclusions
9» Acknowledgements
1 0 . B i b l i o g r a p h y IffiPKanaalstraat 10 - DELFT
2
-1 • Introduction and Historj.cal Note
A convenient dichotomy of the toclmology of Tfork Study in everyday use ^ is
f a^ llethod Study (b) 'Jork lieasurement
This applies to most cuiTent \/ork conditions and techniques although the v/ork sampling tcclrmiques of ratio delay and memomotion do not fill conveniently into either. The usual
chart methods and photographic tecliniques used in method study stem from the practice of the Gilbreths so that sane practitioners equate method study with motion study , Such
a view vTOuld necessarily restrict method study to movement at normal temperatures and leave outside the study of work con-ditions v/hich are the interest of the ergonomist. In
everyday practice no rigid division of the study of industrial vrork situations can be made and an eclectic approach is
required even in the big organisations employing specialists. The term 'ergonomist' is used in the sense that the technician concerned has a wider interest and training than that needed for motion study. Such a use stems back to P.Y/, Taylor whose U3c3 of the term Time Study (as opposed to the modern 'tirae
study') iniplied
(a) the study of vrork and experimentation to develop the best method of performing a job which was then standardised
(b) the division of a work cycle into elements, the timing by stopwatch of these \'/hen
performed by a selected, suitably trained ordhighly motivated \7orker. The addition of allowances for rest, necessary delays etc, and the establishment of a time to be used as a basis for planning and control
and the setting up of a wage incentive scheme.
(c) The analysis of all such established times so that a system of synthetic times can be used for forv/ard planning,
Such a prograi-ime comprises \7hat is now accepted by practitioners to cover T.'ork Study, The genius of the Gilbreths tended to concentrate the attention of later practitioners to the motion study aspect of the first part but Taylor did not make any such a restriction in his experimental approach. His experi-ments on metal cutting and work physiology, started at Hidvale
Steel Coo and continued at Bethlehem Steel Co., v/cre part of /this .,,
this prograixie of what is no\f caiiprehendcd mthin liothod Study, The contributions of Taylor to work physiology were important both in insight and in the fact that they stimu-lated interest and started a tradition vdiich is especially strong in Europe^>5 in centres such as the I'lax Planck Institut fur Arbeitphysiologie at Dortmund,
In industry'' in UoSoA, and Gfce Britain the spread of knowledge of the tenets of motion study, the increased use of powered devices and of electronic control and feed devices (automation) has resulted in the position that, except for isolated cases and for non-repetitive work, there is no industrial work which would be described by physiologists as physically exhausting in normal temperature conditions. This does not imply that physical strain is not present in the form of bad posture (especiallyoaised by bad seating) and from the use of machine controls designed T/ithout attention to human restrictions which require relief by change of posture or, better still, eradication by
better design^. Rather has the restriction on human output, or the conditions in which it is found, shifted from that of phj/'sical effort which can be attacked by Gilbreth motion study into such fields as motivation and morale, inspection processes requiring vigilance, raulti-macliine minding and conveyor belt (machine paced) processes and lastly hot processes in v/hich considerable physical effort may be exerted. In all these fields, Gilbreth techniques have a limited or no application and increased productivity can only be achieved from such vrork by applying psychological, physiological and statistical techniques to them,
Before coming to the main purpose of this paper it v/ill be noted that considerable increases of productivity have been achieved in all the fields noted,
(a) Motivation and morale. The Hav/thome Investigation was an example and later vrork, such as that from
the Tavistock Institute has made claims in this connection. The va"iter does not necessarily
accept such claims at their face value as frequently no attempt is made in such, instances to control
the technical aspects of production, overall production being taken as a morale index - this is considered to be fallacious in many contexts, (b) Inspection processes have been studied since the
vrork of Taylor at Simonds ilanufacturing Co, in 1883, Some notable advances have been made recently by psychologists in Gt, Britain deriving from the pioneer vrork of the Applied Psychology Unit at Cambridge,
(c) llulti machine mdnding processes and conveyoi" belt processes are susceptible to statisticeJ. analysis using queuing theory9 and various statisticians
in U,S,A,, U,S,SoR, and Gt, Britain are working on these problems, Conrad has reordered conveyor belt vrorking''0 using a rational approach to this
type of problem,
This paper is concerned vd.th the method study of hot working environments v/hich are not susceptible to present study methods, The vnriter v/ishos to call attention to the pioneer work in
industry of Prof, G, Crov/den and Etr, T, Bedford in this field which has been the inspiration for deriving a ' system' for
the routine use of vrork study technicians,
2, Physiological Effects of Hot Conditions
Hot vrorking conditions differ from normal temperate conditions in that physiological strain vd.ll result v/hether the vrorker is vrorking or not. The object of method study of hot conditions must be the statement (and measurement in the broadest sense) of those conditions and the redesign of the vrork-place and vrork to reduce the effects on the worker to a minimum,
The conditions v/hich result in decreased
efficiency of the vrorker through heat stress derive from (a) environmental conditions of radiant heat, and air
humidity, temperature and speed
(b) internal heat (metabolic rate) generated by the vrorker especially in exerting physical effort
(c) clothing vrom v/hich interferes v/ith body convection and radiation processes
(d) mental activity of the vrorker
(e) individual physiological and psychological tolerance of the heat stress and degree of acclimatisation,
In order to discuss heat stress it is necessary to examine the effect of increased heat on the body and this v/ill be done for conditions in v/hich pain or burning of the skin does not occur. The human being retains relatively constant temperature conditions vdthin certain internal regions of the body, notably the brain and viscera^ by autcnomic nervofus
control v;hich affects the heart rate and the distribution of the blood flow, A large pta^t of the body, notably the
extremities of the limbs (such as the feet), is not controlled at such constant conditions and is subject to tciaperature fluctuation vd.thin everybody's experience. The effect of increasirg heat in the environment results in increasing
differential heating of the bod^^ tissue (especially the lijnbs) a redistribution of blood flow towards the surface of the body, increasing oxygen consumption from breathing and an
increase in svroating. In physiological laboratory vrork the
effects of heat on the 'body are generally measured by changes
in rectal temperature, skin temperature, pulse rate and svroat loss. The response of human subjects to various test conditions is shovrti in Pigs. 1 and 2 derived from Kuno''''. These figs, and other studies show
(a) the quickness of sweating response to exposure to high temperature depends on the degree of stress
(b) svroating rates differ bet\7een various areas of the body. In general at lov/ heat stress con-ditions svroating occurs mainly on the legs but increase of temperature results in recruitment from all areas but chiefly on the chest and upper areas v/hich are the main sources at high
stress conditions,
(c) the onset of svroating in an area results in decreased skin temperatures,
(d) rectal temperature and heart rate increase vdth exposure
(e) mental activity inhibits sweating and therefore may add to heat stress although authorities differ in view on this
(f) physical effort (increased metabolic rate) adds to heat stress,
The increases of body temperature, svreacting and pulse rate v/hich aare typical for the vrorker in hot conditions compared v/ith temperate, are not 'unhealthy' per se. The body tolerates such increases up to levels which differ betiroen individuals and accommodation takes place within a short period of time in v/hich the capacity to sweat is incroasedj Pig. 33- shov/s the decreases in heart rate, rectal and skin temperatures \;'ith the passage of time of exposure to heat ''^ and Pig, 3b the increase in svroat loss''5, Individual differences in response to the same conditions are largo and reports for tv/o or'Eichnas subjects 14 shov/ this.
Data at end of v/ork in an environment of 96*^ saturated S u b j e c t W I T , HOP D u r a t i o n ( h r s ) 4 1.3 R e c t a l temp*^ 103.5 1 0 2 . 0 H e a r t r a t e 132 171 R e s p , r a t e
36
12 Sv/eat l o s s g r / h r . 1240 2425 C o n d i t i o n T i r e d v/aIking e a s i l y C a n ' t b r e a t h e a l l i nThe physiological response for a x-Jarticular individual v/ill vary v/ith age; Weiner and Tjind^° have investigated age differences for fit mine rescue personnel of different age gpoups but much v/ork has to be done in this area. The work of Eichna ' has shovm that \ander conditions of non-radiant heat the main determinant of tolerable v/ork levels is v/et bulb temperature. Pig.4 shows that only small differences in wet bulb temperature exist between tolerable and impossible v/ork conditions; large variations in dry bulb temperature at these levels of v/et bulb temperatures havin,g small effect.
A question frequently raised by work study technicians is what physiological conditions can be used as a basis for rest allov/ances in time stiidy. The v/riter's view is that individual differences between workers are large and that for workers engaged on hot v/ork both acclimi-tisation and selection of the v/orker has occurred so that the level tolerated by sxjch workers is higher than for other v/orkers. Only rough indications can be given therefore for rest allov/ances and the conditions in vtó-ch they are taken based on statistical average values. Such
allowances should preferably allow the workers physiological state to return to a safe level in a similar manner to tliat first established by P.W.Taylor in his pig-iron leading experiment v/ith 'Schmidt' under
temperate conditions: the v/ork of Muller in the realm of muscular effort is important here as it has been shown that the stress (or rather the physiological effect) varies v/ith the way the rest is split up and that Taylor's practice of compelling rest after each.work cycle results in the lowest physiological stress. Brouha has shavn that hot conditions affect the recovery that is obtained after heavy work Pig.5.
Various figures are available for 'sai"e' limits of sv/eating; a limit discussed later is 1 litre/hr. for fit young men v/orking for 8 hours. This figure results in a higher limit than v/ould be suggested by psycho-logical considerations discussed later and it is suggested that l/4 litre/ hr. shoiild be regarded as a maximimi industrial figure. Por short terra
(not day long) exposure higher values could be tolerated.
At high sweat lejrels, unbalance of the body v/atcr ?.nd salt levels occurs vidthout replacement of water or salt takes place. The intake of water by drinking corresponds closely v/ith sv/eat and urination loss but salt is also needed as it is lost in sv/eating and the eating of salt tablets increases the toleration of hot conditions.
Under conditions of extreme radiant lieat the skin temperature may rise to values of 45°C and above at v/hich Pain nerves are affected and special conditions exist in the breathing passages. It is not intended to discuss these
conditions which are partially covered in references 38 and 39»
3. Psychological. Considerations
It is a useful convention in discussion to use the terms mind and body but, in practice, psychological and physiological performance or behaviour must be regcjrded as
t\/o aspects of the overall beha.viour of the human organism. Psychological behaviour such as the making of judgments of distances, v/eights and velocities or in reaction to stimuli such as v/arning signs on gauges is affected by the physiol-ogical state of the body. Psycholphysiol-ogical behaviour is very important v/here safety, timing and attention is concerned and deterioration of perfomiance often occurs before physiological danger levels,
In considering the effect of heat on psychological behaviour comment is required on the methodology involved.
kn. investigator can ask workers v/hat they consider to be the
effects of heat but such reports are a. notoriously -unreliable indication of performance deterioration. wlien the numbers involved airo large such a method is valid in obtaining^such useful temperature scales as 'effective temperature' by which the feeling of equivalence betv/een satura.ted air at various temperatures and zero air speed is made v/ith unsaturated air at other temperatures and air speeds, However, for most cases in industrial production better indications of the effect of heat stress arc obtained by tests such as those used by Hackv/orthl9, These tests, some results of which are sho-ïm in Pig, 6, shov/ that performance begins to fall off at conditions of 95°P dry bulb/85°P v/et bulb which is much belov/ the physiological level of 1 litre/hr,
sv/eat loss which young fit v/orkers can tolerate, /m
important finding of llackworth is that performance also falls off at temperatures belov/ a certain level when the subjects begin to complain of feeling cold. Reaction times lengthened
as temperatures increased and this is suggestive that accident rates should increase v/ith increased temperature, A multiple reaction time experiment carried out for Eraser and Jackson^O has been found to give useful indications of the deterioration of psychological performance. M l such laboratory tests have face validity but if possible saiie validation is required for practical cases although this may be difficult to obtain,
4, Overall Industrial Effects
A main difficulty in all research v/ork under industrial conditions is to control the largo number of variables (causal factors) that occur. The extension of laboratory v/ork to factory conditions is difficult and any results obtained must be scrutinised carefully to remove bias in interpretation. If data be gathered frcra a large number of sources over a long time period it is often taken
that effects due to various causal factors such as v/ork place design, v/orker skill etc. v/ill be randomised and therefore cancel each other so that the effects of such factors as heat stress (a., assignable cause) can be determined. Such an assumption requires close 33rutiny in all cases,
The v/ork of the I.H.R.B. investigators betv/een the wars is v/orthy of study in the present context of heat stress.. Etr. H,M. Vernon shov/ed^"^ as early as 1919 that production
varied with monthly average temperature and Vernon and later v/orkers such as Dr. T. Bedford and Eric Parmer showed the effect of heat on production and accident rate in a number of trades including coal mining and gleiss manufactvire^^» •^,
Seme of the accident results are shovm in Pig. 7» An altemo-tivo explanation of such curves could, in certain ^ cases, be that there v/as a substantial correlation between temperature and the accident potential of the T,7ork apart fran the effect of heat on the v/orker, but the results shov/n v/ould be expected from the ex^ilanations of laboratory tests,
5. Heat Stress Indices 5.1, General
In deriving a basis for the method study of hot conditions certain indices are required for the strain
(effect) on the worker of hot v/orking conditions. Current tables of C.R, (fatigue) allov/ances used for hot conditions are not helpful in this context for reasons v/hich vn.ll appear from this paper. Extracts from three typical tables are given belov/ (24) from the four given in that reference,
SOURCE
\Ihen work raust be done under trying conditions of heat, cold, dust,, fumes, noise, vibrations, bad light danger etc, a.dd up to 15 ^ . If 1 5 % is found to be insufficient, consider if and hov/ conditions can be improved,
Preezing below 30°P 0-5% Normal 55-70°P 0 High over 85°P 5-10% High humidity conditions raise allov/ances towards maximum circulating air
conditions reduces allow-ances tov/ards minima,
Good ventilation 0 jPoor ventilation 3 .Pumes and/or dust 3-8 IUse of respirator 8-15
D
lijmidity ^^^ N Exc Moving Belov/ 30°P 20 25 - I 3035°P 510 612 -0 5 2 55-75°P 75-100 P 15-40 - 50The terms and the vajucs used in these tables arc not in accord with factors Icnov/n to effect heat stress and it is suggested that
such tables be abaJidoned in favour of a more rational description using certain heat stress indices if a rationalised system based
on scientific concepts be required,
Various measuring instruments for temperature have been used by physiologists working in this field'^*^'' and have been discussed by P,E. Smith in relation to certain indices^o, The most widely used index is probably the 'effective
temperature' scale originally determined by Yaglou (Yagoglou) for persons clothed in pants only and modified by Drs. Vernon and Bedford to take into account radiant heat conditions,Tliio is shov/n in nomograra form in Pig, 8, It lias been used successfully by Dr. Bedford in controlling shipboard conditions in the Royal Navy, A temperature value on the 'effective temperature' scale is that of the temperature of satura.ted air r.t zer-o air velocity
v/hich gives an equivalence of feeling v/ith the measured. • conditions of non-radiant heat. Por conditions of radiant
heat, Dr, Bedford introduced the use of the globe (black bulb) tempera.ture reading which replaced the dry bulb temperature required in the original scale. Effective temperature v/ill continue to be used but a difficulty arises in the present context as degrees at various points of the scale are not equivalent and the scale must still be related to effects on the worker and environmental conditions v/hich it is required to alter in order to reduce heat stress,
An extremely useful index is the Predicted Pour Hour
Sv/eat Rate ( P 4 S R ) developed from a statistical analysis of
laboratory experiments conducted by Mc/irdle and his collea>gues at the National Hospital, London,2? This index is determined f rem a nomogram Pig, 9. There hr,s been verification of this „ by v/ork ct the Tropical Research Unit, Royal Navy in Singapore ,
The calculation of the P4SR index requires the folla/ing quantities to be knov/n:
(a) black bulb (globe) temperat\ire (b) dry bulb temperature
(c^ v/et bulb tempera.ture (d) air speed
(e) metabolic output of the -vrorker (f) clothing condition - values for tv/o
conditions are given viz, that of pants only and overalls over prints,
The ncraogrcm gives a predicted value for the sweat f lev/ v^en the vrorker is immersed in the hot environment for 4 hours continuously and from this the probable increase of rectal temperature can be determined. This nomograjn has been determined empirically and no regression equation is knov/n for it,
Another useful index, recently developed by Belding and Hatch29 depends upon engineering concepts of body heat gain. It stems from the development by Hains and Hatch30 of mathematical equations for
(a) the heat gain of the body from metabolisni, radiant heat and convected heat sources (^rmn) (b) the evaporative capacity of the wetted skin
of the body (Ej^^) ,
The Belding and Hatch Index = T^^ X 100
i'lAX
v/ith the restriction E,,.„ . ^ 2400 BTUs/hr,
This index is confined to clothing conditions of pants only but otherwise requires the same factors to be knov/n a.s v/ith
P4SR, Its main advantage in the writer's viev/ is that its mathematical form aJ.low3 mathematical expressions to be developed as a method study basis. It can be determined
from the namograra shov/n in Pig. 10,
The v/riter hp.s proposed P4SR a.s a rational basis for the allocation of fatigue (C.R.) allov/ances in time study^
and by extension the Belding and Hatch Index can be used in a SDjnilar manner. Such a use requires the determination of objective measurements of the v/ork situation and is more realistic than current fatigue (C.R.) allov/ance tables as important interaction factors of environmental heat and metabolism are taken into account. The main difficulty in
such an application is the determination of a mean basic
sv/eat value throughout the rest and v/ork periods. Probable values of indices that can be taken for an acceptable sv/eat value is 1 Jitre for a period of 4 hours for use v/ith P4SR and frctn heat gain considerations a mean Belding and Hatch Index of 40ï these values are proposed tentatively and require valida.tion in industrial situations as many complica-ting factors arise v/hich are not accounted for in the indices, It is suggested that the main object of v/ork study should be the reduction of stress by method study and in the case of hot vrorking conditions this implies the redesign of the v/orking environment. In some cases even after redesign, heat stress iTill still occur in which ca.se rest from the hot conditions vri.ll be required follov/ing a basis such as that proposed,
5.2, Effect of Various Conditions on P4SR,
The measurable quantities enumerated above v/hich are needed for the determination of PlfSR have been assigned arbitrary values by the writer to derive curves of P4SR on a systematic basis^'' from the nanogram. Extracts frora these are presented in Pig. 11 for conditions v/here radiant heat is present and it is suggested that in everyday practice practitioners could use such arbitrarily determined charts for use in time study and method study rather than derive values from the nomogram. These curves shov/ the importance of the determination of all the variables especially
metabolic ra.te, v/et bulb and black bulb (globe) temperatures v/hich do not normally receive attention by technicians. Eiclrma's results'lS have already been commented upon as
shov/ing the importance of v/et bulb temperature on hea.t stress but the other variables are obviously important.
The use of measured quantities in determining P4SR values is the obvious approajch favoured by an engineer to determine the human stress index value but the black bulb, v/et bulb and dry bulb temperatures are not independent. Consequently in a method study approach to the redesign of
it may be necessary to malce a restatement of the variables used. This ha.s been done in Pig. 12 in v/hich mean radiant vrall temperatiorc and air moisture content have been maintained constant (instea.d of globe temperature and ii/et bulb tempera-ture) and these values are compared with a lattice from Pig. 11 one condition (120°?., 80°P. D.B. and 70°?. ÏÏ.B,) being made common between the tv/o lattice sets,
5.3» Belding and Hatch Index (B.H.I.)
T M s index is a rationalised approach to assessing heat stress ba.sed on mathematical formulae deduced by Haines
and Hatch30 +.o calculate the heat gained and lost by the body
from the various heat sources. It has been assumed that the skin is completely vrotted at a temperature of 95°^ aJ^d that evaporative cooling of the skin occiirs, A further simpli-fying assumption has been made that the gain of heat of the body derives from
(a) metabolism of the body
(b) heat gained from radiating heat sources in the v/orking environment
(c) heat gained or lost by forced convection of the air flov/ing past the body,
üT-ien equilibrium is attained the heat gained from the three sources is balanced by the heat lost. The Belding and Hatch
Index (B,H,I,) is the ratio of heat gain (EOTQ) '^° "the
evaporative cooling capacity (Ry..„) multiplied by 100, This has been reduced to nomogram form. Pig. 9 • ^^^ mathe-matical equations for these quantities are
\ E Q = ^' -^^5.5 X 1,73 X 10-5 (j4 _ ^ ^ ^ ) ^ ^/T { ^ - ^ ^
(BTUs/hr) : : :
Metabolism radiant heat gain convective loss or gain
and
.0)
^,iAX = 10,3 V ^ ( i ^ - V P J (2) (BTUs/hr)
v/here M i s body metabolism i n BTUs/hr
T,-f i s the mean r a d i a n t temperature of environment
(°P absolute)
TCT.,.|.„ is assumed skin temperature P absolute (95°P = 554.6°P absolute)
T is ambient dry bulb temperature ( P a.bsolute) a
VP is vapour pressure of aii-- in mms.Hg
(N.B. h2 is this value at 95°P)
V is air velocity in ft/nun.
The v/riter has compared values of this index vdth P4SR values for assi:imed arbitrary conditions32^ This' comparison is shovvn in Pig, i3 which shov»s the same general trend but that B.H.I. is more sensitive to changes in dry bulb temi:)erature than is P4SR at lower values of wet bulb temperature.
The Haines and Hatch equations involve a simplifi-cation of the body conditions and are intended to be for oïily one clothing condition v/hich is rather atypical of most
working clothes, viz. v/earing only pants. Nevertheless for engineers required to take action v/ithout being able to afford money or time for extensive experimenta^tion. This index
allovTS a relatively simple method study approach to hot working conditions35. This approach is helped by the ma.the-matical form of the index, the partial differentiation i/hich, v/ith respect to independent variables, gives
^/BHI'X I MOO/ „ J , / / B H I V ^ , ,,.46!lSAT / R H ^ \ / „ . .
r F ~ - =2^iv + ( ^ _ ^ 1 0 . 3 V i T ^ i ^ T ö ^ j ^ S ° ^ )
HïöÖ/ s/if - V Ï Ö Ö ) T ^ i SAT 100 j
d V l l O O / _ •^vIAX '.'lAX ( f o r c o n s t a n t T ) ^ a ' These e q u a t i o n s allov/ t h e c a l c u l a t i o n t o b e made f o r t h en e c e s s a r y changes i n T , V, T...^, and R,H, /^ t o change B , H , I , a g i v e n amount when
100 . . , , ^ - 100 _. , . . .
-—=— IS v / r i t t e n a s —-—=;— as a f i r s t a p p r o x i m a t i o n ,
If the enviroruucnt cannot bc changed by redesign then, on a supposition of the v/orker tending to -vvork at a. constant B.H,I. value, the metabolism (pliysical effort tern) must decrease by an amount given by (36)
(-^M) = ZiT^ .107 ( ^
in the case of radiant heat relative to a fixed base etc. Such an approach may be helpful in considering seasonal or diurnal variations in conditions such as those shov/n in the v/ork of Vernon (ref. 21).
6, Method Study Approach
The approach of Taylor and Gilbreth to the study of v/ork was essentially that of redesigning work conditions to reduce the physiological strain on the v/orker and to mininiise unproductive operations and movement. In hot v/orking conditions there is an obvious need for the latter but the physiological stress on the v/orker can only be partly relieved by applying Gilbreth's techniques in hot conditions.
Heat stress arises in the v/orker from a number of factors which have been coraaented on already. Por the
purposes of method study it is convenient to accept the basic approach of Haines, Ha.tch and Belding that the human body acts as on inert mass gaining or losing heat from
(a) metabolism (b) convection (c) radiation
and subject to evaporative cooling of the skin,
In order to reduce physiological heat stress it is necessary to minimise eo.ch one of the sources of heat gain and to maximise the evaporative cooling subject to limitations imposed by chilling. Each one of these factors v/ill now be examined in more detail,
6,1, Metabolism
The body generates intemiil heat in chemical changes necessary for life, IThen muscle auction occiors either in maintaining postxire, in moving the limbs or in doing external v/ork, the glycogen of the muscles is broken dovm into lactic
acid and carbon dioxide, v/hich is relea.sed v.ith the generation of some heat. The muscle potential for further v/ork is
restored by an oxygenated blood supply to the muscle converting the lactic acid back to glycogen v/ith the formation of
further heat (approximately tv/ice that fomed in the broakdovm of glycogen). The heat generated in this process (the meta-bolism) varies v/ith the intensity of the v/ork and the training of the v/orker. Prof, Crüv/den33 shov/cd that the metabolism
(expressed here in terais of ojjygen consumption) depended upon the trair.ing of the -v/orker, Pig. 14, and this must be ronembered in considering the acclimatisation of all nev/ vrorkers. The metabolic rates of different types of work obviously vary but
B. useful indica.tion of expected levels is given by Spitzer3^.
The rate car be detemained by determining oxygen uptake by the lungs but this requires skilled technicians to take measiorements and has limited application for practical purposes in the
v-vriter's view. Muller''7 has made useful contributions in
this field for v/orkers engaged on heavy v/ork and-ftiose interested in the physiological aspects are referred to his papers. In the case of method study the general practice of the Gilbretlis is applicable for the reduction of metabolic rate although physiologists do not necessarily accept all the Gilbreth rules. Movement should be generally reduced'and especially bending
from the v/aist to pick up objects should be avoided by pro-positioning v/ork,
6,2, Convection
Heat is gained or lost by the body when air, hotter or cooler than the body surface, passes over it. This heat transmission is somewhat complicated but Haines and Hatch30 have derived the expression belov/ (for a vrorker clothed only in pant^ in order to estimate this quantity,
Convected heat gain = 2 / V (T -T^^^J (BTUs/hr.) '' ^ a SKIN^
The value of Tcrrz-jivr taken by Belding and Hatch is 95 P» v/hich is ass-umed constant. In practical cases variation of skin temperatures occur over the body and londer hot con-ditions values up to 97°P» occur frequently and such a value can be easily substituted in the equation,
The air speed V is the speed relative to the body and therefore in v/orking conditions involving for instance v/alking (e.g, fettling openheaxth steel furnaces) adjustment will have to be made for v/ind direction and body movement A n accepted time s-tudy pace of 3 m.p.h. = 264 ft/min, which is
seen to be large in the present context.
that normally spoken of as 'tenijerr.ture' in present fatigue
(C.R.) allowance tables. This tempera.ture will often very
from point to point in the v/ork space. It should be obtained v/hen the thermometer is shielded fraa radiation effects and
some relative air novonent to the bulb should be present. In some cases nov/ Icnov/n in industry temperatures are often taken v/ith the bulb iinshielded and to seme extent the tempera.-tures obtained are higher than the real dry bulb temperature.
It is a common misconception that blowing air onto a v/orker is alv/ays beneficial. The cqua.tion above shov/s that the body can gain heat from this practice under certain con-ditions determined by interaction of the convection tcna v/ith the convective cooling tern discussed below.
The main difficulty in using such an expression for the convected heat term is tha.t clothing interferes v/ith con-vection and further experimental work is required to estimate
this effect for practical clothing conditions.
6,3. Ra.diation
In many industrial processes e.g. glass making, tyre moulding and open hearth fumaroe fettling, the main source of heat gained by the vrorker's body caaes from radia.tion effects.
In the developed theory of radiation every body is considered to emit and absorb radiation following the Stefan-Boltzmann lav/. The amount of energy radiated per unit area varies v/ith the temperature and other conditions of the rad-iating siorface. Such energy is emitted at various v/ave-lengths v/hich for industrial cases occur mainly in the
infra-red region (^ .8|J,). Pig. 15 shov/s typical radiant energy
distributions at different temperatures for a 'black body' v/hich shov/s the temperature ha^s to be very high before any radiation is 'seen'.
The 'black body' conception in heat transfer theory
is useful, A black body (not to be confused v/ith colour) is
one tha,t emits the maximum possible ra^diation at a given temperature. It v/ill be seen from Pig. 15 that
(a) an increase in body tempera-hire causes a decrease in the v/ave length (tov/ards the
visible part of the spectrum) at v/hich maximum emission occurs,
(b) Dxi increase in temperature causes increased
radiation at all VTa.velengths
(c) the total energy emission rate is given by the area tinder the curve,
As visible ra^diation extends fraa approximately ,4|J. (violet
end of spectrum) to ,Q[X (red end) it v/ill be appreciated
that considerable radiation occurs in the infra-red region, This radiation causes heat effects but may not othcrv/ise be detrimental to the cyesj exaraination of the heat tolerance
of the eyes is a matter for further examination. Such radiation v/ill be generally absorbed before the retina.
The actual radiant energy emitted from hot surfaces in the industrial enviroraaent is dependent upon the area of the emit'cing surface, its temperature and the emissivity coefficient. The latter is numerica.lly equal to the absorp-tivity coefficient of the surface and is the ratio of the emissive poiror of tlie body to that of a corresponding bla.ck body. The emissivity coefficient of a.ctual surfaces does not remain constant \/ith changes in temperature but the follov/ing seem to be generally true.
(a) highly polished metals have low emissivities (b) the emissivity coefficient of most substances
increases v/ith increaso in their temperatures (c) most non-metals have high ei^oissivities
(d) the emissivity of any surface varies v/idely v/ith the condition of the surface,
Typical emissivity coefficients (fraa Hottel, ref, 42,)are
Surface
Al-uminium, highly polished Oxidised alijminium
Rolled plate brass, natural surface
Brass oxidised by heating at 11loop
Iron and Steelj metallic surface (or very thin oxide layer)
Iron freshly emeried Smooth sheet iron Iron and Steelt oxidised
Rolled steel sêM^^°° Ca.st plate, smooth
at °P, 440, 1070 390, 1110 72 390, 1110 68 1650, 1900 70 73 Emissivity Coefficient .039, .057 .11 , .19 ,06 .61 , .59 .242 .55 , .6 .657 .8
Surface
Tin, bright, tinned iron sheet Linseed Oil layer on aluminium
foil:
Aluminium foil
Aluminium plus 1 layer oil Paints, lacquers, vaornishesl
Snow v/hite enamel varnish on rough iron plate
Black shiny lacquer, sprayed on iron
Aluminium paints and lacquers;
1 0 % Al, 22% lacquer body
on rough or smooth surface 2 6 ^ Al, 2 7 ^ lacquer body on rough or smooth s\irface Alumini-um pa^int after heating
to 620°P. Asbestos board at °P 76 212 212
73
76 212 212 300,60074
Emissivity Coefficient .043, .064 .087 .561 .906 .875 .52 .30 .35 .96The energy radiated behaves in a similar manner to light waves as to directionality and therefore the energy absorbed by any surface subject to radiation v/ill depend upon the incidence of the v/ave s. It follows therefore tha.t radiation can be screened from the vrorkers in a manner v/hich is being increasingly practised in industry.
The workers clothes and skin have high absorption coefficients approximating to 1 and therefore the radiation effect on the standing \/orker is given by considering his clothes and skin as a 'blacdc' body. The heat gain is therefore
= 1,73 X 10"^ (^. - T^^jj^'^ BTUs/hr for an unclothed v/orker,
T-.- is an integrated mean tempera-bure of the vrorking environment" taking into account incidence effects in a
manner given by Hottel^,
In practice a convenient method of determining both TL-, and heat stress factors is with the use of the black bulb (globe) thermometer. This gives an equilibrium temperature
e s t a b l i s h e d be-tvroen r a d i a n t energy irapinging on 'bhe globe and c o n v e c t e d energy f o l l o w i n g t h e e m p i r i c a l e q u a t i o n
where
T^^-^ = T^^ + ,103 X 10^ -/V (l^ - T^)
T.-r = mean radiant temperature in P, absolute
T- = black bulb (globe) temperature in P, absolute T = ambient dry bulb • i i i
a '-' V = air speed in ft,/min,
Such an equation applied to the v/orking environment obviously involves siraplifications as, for instance, pos-ture (crouching compared with standing erect) in a given en-vironmen-fc would involve different radiant energy incidence. The globe
(bla.ck bulb) temperature needs to be determined ai'ter twenty minutes exposure a.s stabilisation requires that tii:ie. This
time requirement means that the globe therraoneter ' can only be used when the radiant energy is constant and of a time
duration v/ith a lainim-um of tv/enty minutes. Per durations belov/ tv/enty minutes, thermopile instr-uraents can be used and readings taken at discarete points to integrate radiant tem-per a-ture spherically.
This equation is not simple to solve directly and nomograms ,g are available for easy solution. The v/riter has given values to allow easy solution. Globe temperature enters into all the heat s-tress indices nov/ used (viz. effective temperature, P4SR and Belding and Hatch Index) so that it has been
con-sidered useful to calculate mathema.tica.lly the required
changes needed in either T,-, T or V to alter Tp a given amount and this has been done in the equations quoted earlier.
The means open to the technician to reduce the body heat gain from radiant heat sources are three
(a) reducing the temperature of the radiating soiorce by suitable insulation
(b) reducing the emissivity coefficient of the radiating source
(c) use of suitable screens betv/een the v/orkers skin and the radiating source.
6,3,1. Insulation, Processes require a certain v/orking temperature v/hich usually cannot be altered. Insulation of the apparatus saves fuel and this results in another practical
saving in tha.t the process can be placed in cCT.i:iission quicker from a cold start than when insulation is not used. insulation can be either of the conductive type using
tjrpically asbestos derivatives, glass fibre etc. or reflection insulation using high purity metal foils (usually aluminium). It is not intended to consider tlie relative merits of these forms in detail but an important practical point to note is that the foil -v/eight will be lower for the saaae s-urface temperatiire.
6.3.2, Surface Emissivit.v, The emissivity coefficient of the apparatus surface must be considered. If surfaces are not suitable for insulation the use of alijminixom paints (see the previous Table) might be considered as these have low emissivity coefficients. The lov7 emissivity coefficient of high purity metallic foils is v/orth consideration v/hen surface finishes are considered.
6.3.3. Screens. The sinrplest form of screen is of v/ood or metal but this absorbs a laorge axaount of heat and is
impracticable for many opera.tions as it impedes the v/orkers. Protective clothing is a portable form of screen v/hich is effective in decreasing tlie radiation effects on the v/orker although it may interfere seriously v/ith convective cooling. This clothing may be of two types
(a) conduction insulation such a.s the well known asbestos suiting
(b) reflection insulation using aluminiuj-i foil. This latter type v/as originally developed in Germany but is increasing in use and can wholly or partially cover the v/orker,
I/[iss Slade37 has reported its use in steel making shomi in
Pig, -16 . It v/ill be appreciated from the previous Table that the heat absorptivity of bright aluminium foil is only some 4 - 5 per cent, the rest being reflected (absorptivity = 1 - reflectivity). In such protective clothing convection cooling is prevented v/ithout special measures axe trikcn and therefore exposure times may still have to be limited,
6,4. Evaporative Cooling of the Skin
The body loses heat to the atmosphere by convection and radiation under some circumstances but the chiel mechanism is the evaporative cooling caused by moisture absorbed into the a-traosphere. This is dependent upon the rcla.tive vapour pressure of the moisture at the skin and the atmospheric vapour pressure. A difficulty arises in practice in forra-ulating a reasonable estimate of this cooling because the
skin temperature varies greatly over the surface at temper-atures belov/ 70 - 80°P dry bulb, being low on the extremities.
At conditions v/here hea.t stress occurs the skin tempera-ture does tend to be the same all over the surface and for cal-„_ culation purposes Haines and Hatch30 and Belding and Hatch have assuraed a value of 95°P» which might be lo\/ in many
instances especially if Mie skin be subjected to direct
radiation.
In addition the evaporation process is subject to interference by clothes especially if these are non porous (or porous but clogged by grease and sv/eai.t,)
Verbal reports from the Institute of Preventive Medicine, Leyden suggest that the Haines and Hatch formulae
accord v/ell v/ith calculations using measured rectaJ tempera-•tures of clothed -vrorkers and it is assiomed that clothes do not affect the overall picture but merely delay body tempera-ture increase (and decrease),
It follows fraa the equation for evaporative cooling capacity (E.,.„) that such cooling will alv/ays take place v/hen the ' air tempera-turc is belov/ 95°P (sa-fcurated) but
that it is obviously desirable to maintain lev/ humidity for maximum effect. In practice no a.ir humidity control is exercised in most v/orkshops so that the air mois-fcure content at the v/ork place varies v/ith that in tlie outside air. The atmospheric conditions at the work place is therefore subject to di-umal and sea.sonal variations v/hich must be taken into account in any redesign of the vrorkplace,
7. Sequence of Hea.t Stress Study
In order to study a v/orker in a hot v/orking environment the follov/ing steps are suggested.
1, i'lake a ratio delay, memomotion or stopv/atch study of the v/ork cycle to detenaine the relative time spent in
each working position in which the hea.t stress index -VTIII be
different,
2, Determine for each v/orking position v/ith different heat stress index (a) bla.ck bulb (globe) temperature: 20 mins, exposure is required and this may be difficult to obtain, Alternative means by thermopile instruments etc, are described by Morpurgo^,
(b) dry bulb tempera-ture taking care to shield the. thermometer from radiation
(c) wet bulb temperature: it is iraportant to maintain a steady airflow over the v/et bulb in order to get a true
reading,
(d) p^ir speed relative to the v/orkers body, talcing into a.ccount his movements,
Kanaalstraat 10 - DELFT
2 2
-(e) an estimate of metabolic ou-tput using S p i t z e r ' s
tables3A-,
30 3. Calculate a time integral of -bhe heat gain , heat gain index29 or predicted 4 hour sweat ratc2o using da.ta from 1 and 2 above,
4» Apply Guhreth motion s-tudy techniques in order to shorten the v/ork cycle and . reduce the metabolic load for the vrork cycle,
5, RecaJ.culate 3 for the shortened vrork cycle v/ith reduced metabolism calculated from 4. By examination of this recalculation decide on the particular vrork positions in which
the chief heat gain occurs,
6, Using the formulae for the heat index given above calculate the required changes in T,-, T , V and R,H. to reduce B.H.I, by a given amount,
7, Decide on which of the factors in 6 to alter to reduce the stress. In some cases Vi^ere T,. varies vri.dely during the cycle it may be advantageous to reduce one or
more of the other three independent variables (T , V and R,H,) as they v/ould generally affect -the whole cycle,
8, Alter the workplace by shielding, air ducting etc, in compliance with the decision of step 7, and. measure the factors given in step 2,
The procedure outlined in the above steps is rational and depends upon the validity of heat stress indices being applied to indus-trial workers and this remains to be shov/n,
It is necessary to comment upon certain aspects of air conditioning in which common faults are often present in v/ork places, in order to better effect lov/er heat stress,
7.1 • Dr.y Bulb Temperature
AssTjming no cooling air conditioning is fitted, the dry bulb tempera-ture at the work place depends primarily upon the atmospheric dry bulb air temperature in a vroll designed system. As a.-tmospheric temperature is seasonal, allov/ance v/ill have to be made for possible high summer values occurring and therefore some appreciation of
meteor-ological data is required,to make suitable forecasts,
Air temperature is often increased greatly before it reaches the vrorker by passage over hot apparatus. This
can be obviated by ducting air to the v/orlqjlace, c;ire being taken to insulate the ducting,
7.2. T.X3t Bulb Temperature
This depends upon both air moisture content and dry bulb temperatiire, For a given moisture content, v/et bulb
tempera-ture increases v/ith dry bulb (1°P increaLse in dry
bulb results in approximately -g-^P increase in v/et bulb). In cases in practice high wet bulb conditions may sometimes be
avoided by ducting air to the v/arkplacc and avoiding cjoy
ventilating air passing over (through) steam leaks or hot appara-fcus. Diurnal and seasonal variation occurs v/ith v/et as well as with dry bulb temperature and again appreciation of meteorological data is required,
7.3.- Shortcircuiting of Air Flow
Although ducting and fans are often installed in workshops, bad sit -.ing of the ducting or fans results in the ventilating air passing fraa air inlet to outlet v/ithout passing through -the work position. The stagnating air at the v/orkplace incrca.ses in tempera-ture to the detriment of vrorking efficiency. In one v/orkshop knov/n to the v/riter,
incaaing air at the duct v/as 56°P and in the v/orkshop generally 80°P although a temperature of 65°P v/ould have been expected if the ducting had boon redesigned to give a proper circulation of air,
8, Conclusions
This paper has attempted to present a rational system of study of hot v/orking conditions for v/ork study technicians. The system depends upon the determination of heat stress indices using mainly measurable quantities at -the workplace in terms of temperature and airspeed. Such a
system of study is an alternative but not a replacement of the conventional experimental physiologists techniques determining various quantities such as rectal tempera-ture, pulse rate, etc. The systera proposed does not require co-operation of v/orkers for its operation but care should be taken in applying such a system to the establishment of rest allov/ances in that -the va.lidity of such indices is not
established in the industrial situation for such a purpose. If the indices are used for a rest allowance basis then a
scaling factor ' may be required. The system discussed should be suitable for method study as in that 'case relative rather than absolute values are required and such values are given in the system.
9, Acknov/ledgements
The author v/ishes to record his thanlcs to numerous workers in this field especially physiologists for their
critical coraiaents. This v/ork derived fraa research sponsored by the Committee on Individual Efficiency in Industry set up
jointly by the Department of Scientific and Industrial Research and the Medical Research Coiuacil and financed from United States economic aid.
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3 0 4 0 6 0 MINI
F I G I g ORDINARY SWEATING WITH RAPID RISE OF ROOM TEMPERATURE.
Fig It). SWEATING WITH HEAT STROKE.
PVk 5 MNS. 1 / ' \ 1 1 / I
nTT
' / 1 1 ƒ 1 / / 1 / i 1 ' f . 1 -VHECTAL TCMf \ . » f t L M T C I * ICW&T C H C s \ ^ * - ^ RATE \ ^i^PALM « « « \ — f \FIG ; a SUPPRESSION OF GENERAL SWEATING BY MENTAL ARITHMETIC AT HIGH TEMPERATURE. MENTAL
ARITHMETIC GIVEN AT M. ,_M2 t M».
FIG a t SWEATWG DUE TO STRENUOUS EXERCISE (W) ROOM T E W 13 5° C ( 7 4 ^ t ^
16 3 0 34
tun o f EXPOSUHt
(o.^ « A R T RATE AND BOOT TEMPERATURE (b^ RATE OF SWEATING
FIG. 4. VARIATION OF DRY t WET BULB CONDITtONS FOR THREE CONDtTIONS OF WORK REPORT
CFROM EICHNA ET AÜ
08-95°^
D B - 9 Ö * F R H - 6 S %
RH-ftCfto
TIME N MINUTES
FIG. 5. HEART RATE BEFORE, DURING, AND AFTER A STANDARD EXERQSE FOR VARIOUS ENVIRONMENTAL CONDITIONS.
3-a > 9 •O M l'2 13 14 I'S 1-6 17 1 8 1-9 2C a-i 2 2 2 3 2 4 -/ ^ / \ / / / / / / / / / / / / 1 1 » — # F R S T HOUR OF TEST O O SeCOKD HOUR OF TEST
\ \ \ \ \ \ \ 1 1 O S . o » 1 0 l l -U ' I - . i-s-I f r 17-l « 1 9 - »o2 1 - as-2 3 2-4 i ; OBY , WET 75?65* EFFEcTivE TEMPERATURE 8 * 7 5 ^ BTS'ET. ROOM TEMPERATURE
FIG. 6. PERFORMANCE ON CLOCK TEST AT VARIOUS TEMPERATURES.
MACKWORTH ( 19 )
AGE OF MEN (YEARS^
FIG7ACCIDENT FREQUENCY RATES OF COAL FACE *(ORK£RS IN RELAHON TO AGE i TEMPERATURE
FIG. 8
CHART SHOWING NORMAL SCALE OF CORRECTED EFFECTIVE (OR EFFECTIVE) TEMPERATURE
FIG. 9
NOMOGRAM FOR PREDICTION O F 4 HOUR SWEAT LOSS (Reproduced by kind p e r m i s s i o n of the Controller, H. M, S. O.
ENTEft I . Af Mit^ccp* »Mi mctAboKim. dr«» koriiM»*! bw; oktAin toUl l,*At lead in tcrmi of *««pw«ti«i iiqwifd fir h««t b«UiK« {Ereq ). Eiiend liofilon4«l CA* le ENTER C. ENTER X. Dr«« herii. Rn* frftn, inUfCApl «f dfe «ad wb Hm-p»r«hirt: obtAHi wApof prtiiura 9rAdi*At b*tw««A M*vr«Hd •liA «I n F «nd tmbÏAAl «tr. Ettend lin« to
ENTER Y. Al inftrctpt »»h «ir «.««d drê« VAHÏCAI GA*; m^M^ mAitinvm «v«por«tïeA from »ot itiA Af tS P lElAAS t- E>*AAd
RAO to
ENTER Z. Mov« to iftttrcopt wüh IwriL bit <r.M C. (K I A M A ••etodi 2400. OAlAf Z at 24001. READ HEAT STRESS I N H X VALUE. HEAT STRESS (INDEX) 120 KM W h t . E .MI t W H d t Z W X ) • ° uM IntrMpt . ( EfM »ini Hilt l l n . 60 / 2 4 0 0 I MAXIMUM EVAPORATIVE CAPACITY I E M I '
5 0 0 1000 1500 2 0 0 0 2 9 0 0 3 0 0 0 ! ^ 1 1
^¥;
V \ Y
— N
AIR S P E E D ^ - ' I F T / M I N l 1 1 t' 1 fe1.
^ \ \ \ \ \IO0 \ i \ ^ \ v , » 0 0J \ ^
\ \ \ , ^ 8 0 SO 100 110 120 130 ORY BULB TEMPERATURE ( ' F l500 1000 1500 2000 2500 5000 HEAT OF VAPORIZATION FROM 100% WETTED SKIN AT 9 5 * F I B T U / M R I
FIG. 10
FLOW CHARTS FOR DETERMINING HEAT STRESS INDEX VALUES
Example: Globe 110, dry bulb 90, wet bulb 75, a i r speed 100, metabolism 600 (light a r m work standing at bench). F o r solution follow broken lines: Heat s t r e s s = 90, (Reprinted by p e r m i s s i o n of the American Society of Heating and Air-Conditioning Engineers)
I O O " F GT
I TEMP J
MO F C T _ A.ACK BULè\ V TEMP J
J
FIG. I I PREDICTED SWEAT LOSS IN 4 HRS FOR MkRIOUS CONDITIONS OF TEMPERATURE t WORK
P 4 S R (LITRES) 3^
^U
3 lt.CAL/^^M gr
¥
k^'
/ / / ~ 7 * I"=*'-'MN.^^<A.c...„
CONSTANT G,T - \X?f CONSTANT w a - 7 C P F(a) \<^RYING D.B. t METABOLIC RATE CONSTANT MEAN RADIANT TEMP CONSTANT MOISTURE CONTENT
( L ) VARYING D.& t METAB RATES WITH CONSTANT G.T t W.B.
FIG, a . P 4 S R CURVES DRAWN FOR TWO DIFFERENT SYSTEMS OF THE VARIABLES COMMON CONDITIONS I20°F GLOBE TEMPERATURE. SO^F DRY BULB.
IOC I I O 130 1 3 0 SO 6 0 SO 6 0 7 0 8 0 6 0 7 0 fiO 6 0 7 0 ao V A < > c u 0 o + T L 1 1 X A 1 7 •
TWO VALUES OF PBY BULB TEMPERATURE TAKEN AT EACH SIGN CONDITION. VALUE ON LEFT IS HIGHEB P. B. VALUE
- L . mU _ L . .a_ . J
AC a o 120 1 6 0 a o o BELDING I HATCH INDEX
F I G . 13.
CORRESPONDENCE VALUES OF P 4 S R S B » H INDEX FOR ARBITRARY C O N D I T I O N S .
PANTS ONLY t 7 5 F T / M I N AIR S P E E D .
\ \ \ . \
V \ i
\
w
\ \ \ \ "^ \ \ —) V POOR P DCGREE OF TRAININGFK». 14. EXCESS OXYGEN CONSUMPTION FOR BARROW WHEELING
WITH VARIOUS WORKERS UNDERGOING TRAINING ( " F R O M C R O W D E N C33j ) ÜO
I
i
0 J 6 0 SO 4 0 2 0 ID _.7
W ' ^ / /l/y^-i
1 \A
V
^ ^ V ^ k^^è>
a
N
WWE LENGTH \. p MICRONS f l MKHON-KT* C M )
FTG15. MONOCHROMATIC INTENSITY OF RAPtATtON FOR A BLACK BODV AT VARIOUS ABSOLUTE TEMPERATURES ( P L A N C K ' S LfHtX
pTEMPgRATURE ON ABSOLUTE CELSIUS SCALE)
KIG. li. ALUMINIUM CLAD WORKER (Ruproduced by pBrJiiiasion o( ( l e o r g c Angus &. Co, Lid.)
