electrical detection of ~races of poisonous gases \ . in the atmosphere

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NOVEMBER
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1946
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.ELECTRICAL DETECTION OF ~RACES OF POISONOUS GASES
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IN THE ATMOSPHERE
by J. ,BOEKE.
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, A catalyst of finely divided platinum which càtalyzes an oxidation re;ction is found to be
hindered in its function by many gases which are also poisonous to' the human body. Based
on this fact, an instrument has been designed which can detect the occurrence of poisonous
gases, for instance' carbon monoxide in mines, garages' or boiler-houses, hydro-cyanic acid
gas in galvanizing shops, etc. The instrument contains a platinum catalyst on a platinum
wire serving as one of the'resistance branches in a Wheatstone
bridge and along'
which a mixture of methylalcohol and atmospheric air is coitducted. The heat developed by
the oxidation of the alcohol keeps the platinum Wire at a temperature.óf 120-150 oe, af
which the bridge is brought into equilibrium. When the catalyst is poisoned the oxidation'
reaction ceases, the Wire c~ols off and, due to its change in 'rp.sistance"the bridge is thrown
out of equilibrium, thereby setting an alarnl arrangement into action. A simple portable
model of the iristrumen:t is described .
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Suffecation and poisoning
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The energy for' th~ vital processes III man: and
substances éalled ferments or enzymes, ,The hàe~oanimal is furnished by "combustion" of the organic
globin combines with carbon monoxide more readily
compounds in the food: an oxidation of thé hy- than with oxygen, so that where carbon monoxide
drogen- and the carbon in these compounds by the
is present the oxygen-blood compound can no
oxygen in the air, For this oxidation it is necessary
longer be formed and the supply of oxygen through
that oxygen 'can be inhaled from the surrounding
thebody stops. In a similar way the ferments in the
air through. the respiratory organs, eventually being
body can'be rendered inactive by'certain gases in,
carried by the blood to the various organs involved
the. air inhaled. Theu function as carriers also'
--, in the oxidation processes.
'ceases; they ar~ "poisoned". Since the ferments áre
~,
If, therefore, the air inhalen by an animal were present only in very small amounts it is under- ,
replaced by a gas containing little or no oxygen,
standabie that ,even only traces of a poisonous,'
for instance only nitrogen or laughing gas (N20),
gas may be sufficient to' block a special ferment
the animal would die for lack of oxygen: it would
function essential in the chain of metabolism of the
, __ suffocate, or some ofits vital, functions would cease;
organism, and therèby cause the entire o;ganismit would become unco~scious.'
to suffer a "death by star\ration". Only 0.03 vol. % '
Elihu 'I'h om s o n showed in 1873, by experiof the well-known prussic acid (hydrogen cyanide)
ments on rthe .Thoînson's family eat, that ill the
in the atinosphere inhaled for a, few moments is
case just described the rerm suffocatio~
must
sufficient to kill an adult. It is less well known that
be used in contrast to the poisoning
which oc- hydrogen sulphide (given off by rotten eggs) is
curs,' for instance, due 'to .the presence of carbon
just as poisonous and in fact, because its action is ' •.~onoxide in the atmosphere. In the case of .suffo~ cumulative, 'even more dangerous. Due to its
cation (at least if life is not already extinct), upon
extremely strong odour, however, we are warned in mçygen being administered, the organism .quickly
time and can escape from the poisonous atmosphere, .
resumes -its normal functions, whereas' in the ca~e , . Unfortunately in many other cases of poisonous
, , 'of the poisoning mentioned the after-effect lasts
gases the nose does not function as an alarm instruI longer and in spite of renewed supply-of oxygen--,-- ment. Our sense of smell very often fails to react to
-death may still sèt in after some time. In the case 'gases which ordinarily do not occur free iIÎ nature
of 'such poisoning as this it is .not the external
but: which are usually' only formed by human
supply of oxygen that ceases, but' rather the
agency: carbon monoxide is quite odourless for
transfer
ofthe oxygen from the lungs to the com- one, hydrogen cyanide for many. Even. with re, pounds to be oxidized. The agents serving for this
spect to hydrogen sulphide, which does often occur
• t~ansfer are the haemoglobin (red pigment] in the
in nature; the nose is not always reliable.. after a
blood =--- which together with oxygen forms a relatively short time, thè sense of smell becomes
"chemical compound, oxyhaemoglobine, that can
so used to it that the concentration could be in-' - ,
-easily be decomposed again and carries it in that'
creased until a fatal dose is unsuspectingly inhaled .
-form to the' organs
and a number of specific - In practice there are many cases where there is a
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PHILIPS
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TECHNICAL
REVIEW
'.VOL'. B, -No.. 11
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', - risk of coming into contact with these or other - and oxygen are absorbed. This phenomenon, which.
, 'poisonous gases. For example, in garages or boilerwas discovered as early as 1823 by, Döhereiner,:
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houses, where carbon _monoxide may be formed ,may be explained as follows. In the adsorbed state
owing to incomplete combustion; in mines where
the molecular structure of the gases 'is' somewhat
carbon monoxide mayalso
occur; in painters'
loosenedbythefieldsofforceoftheplatinumatoms:'
workshops whe;e carbon monoxide is liberated
which means a lowering of the above mentioned
in the drying of paint containing linseed oil; in the
energy threshold,' so that the latter can already ~
. fumigation of ships' holds and of houses with
be exceeded at room température and reaction takes'
'hydrogen cyanide preparations, where traces may
place. The platinum itself does not take part inthe - .
linger after the treatment;
in galvanizing shops
reaction, it only c at.al ys es it. Thi~ may be ex.
where hydrogen cyanide may be developed owing : pressed by saying that the' oxygen, which in a free r.
to incorrect handling of óyanide-containing
baths, > state does not react with f~ee hydrogen, is carried' - .
etc. In such .cases it may often be desirable to have
.over to the hydrogen by the platinum, acting in ;
" at one's disposal an objective alarm ~pparatus
much the same way as the organic carriers, the fer--sufficiently sensitive to react even to traces of ments, enzymes; etc., which may also be considered
invisible, more' or less odourless poiso~ous gases
as catalysts. Sin'èe the platinum does not take part
, and vapours.
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' in th~ reaction and thus is not consumed, and the,
At first, glance this might seem ,impossibl~ of. adsorhed molecules after their mutual reaction
achievement. How can an instrument. distinguish
leave the surface, to ..make room for the adsorption
, whether 'a gas is poisonous to human beings or not?
of new molecules, a small amount of platinum can
-::
-The instrument slioud not, of course, react to noncatalyse the reaction of any desired large amount of
I gas, as is the case with ferments.
poisonous gas~s.
- A solution of this problem is made possible by . Upon platinum black being heated to a:hightemperature its 'catalytic properties 'are lost. This'
_the fact that th~ phenomenon of poisoning in the
can be explained by the assumption that the cata-,
",'. human body showa a far-reaching parallelism with
lysis does not t~,ke place unifórmly 'over the whole
the phenomenon of the poisoning of a "platinum
.. catalyst -. An alarm. arrangement based upon this. platinum surface, but at special "active" spots,
,for instance the edges' and corners of the 'nlinute'
will-be. described here 1). For a good understanding
crystals of platinum on the surface 2» This a!l-,.'.
• v- .of its .working, however, it is necessary first- to say
sumption agrees' entirely with, the explanation of'
something about the functioning. of a e,ata!yst. .
,catalysis ..by _adsorption, since the platinum atoms
, . Catalysts
, at such spots are surrounded by fewer neighbouring
atoms than. in the plane face; and 'therefore' have
'Oxid;tion
by. tra~fer '< in 'àdditiori to _ordinary
, ....
left
over; äs it were, forces of attraction !o attract
o:rldation is also met wjtlt' outside 'the living'
organism.
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foreign molecules (seefig.l).':1t'a
high iemperature " "
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gas which on
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'fz(1 ~. ~~ ,
room temperature, however, no combustion takes
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place, dU:~to the fact that between the two mole- "
" - _' . cules of' hydrogen and oxygen approaching each
Fig. 1. Diagram of the surface of a catalyst with the fields of
attraction shaded (a). Edges and corners of crystals in the surother there is an energy threshold preventing them
face form active spots where foreign molecules, are preferen_from combining at room temperature. If the temp~tially adsorbed (b).
-, táture is, increased by the flame of a match the'
recrystallization
takes place, the many small.".
energy of motion of a .number of molecules is incrystals combining to form a Jew large ones. The:
creased sufficiently to cause them to p.ass over the'
numbe~ of edges and corners is thereby considerably ,
~nergy threshold; the heat of combustion then libe"
reduced, until the' number of active spots is too
rated keeps the .reactiön going.
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small to make any catalytic action perceptihle.
'However, there is yet another, possibility of
Düb er ein er, whoalso knew ofthis phenomenon, '
, bringing about the oxidation of hydrogen (and of
noted that traëes of hydrogen sulphide, hydrogen,
I,
'cómpounds containing hydrogen), namely bymëans
cyanide and carbon 'monoxide also made 'platinum '.
'of a preparation of finely divided platinum" so.
black'
ineffective; the. catalyst
was "poisoned". "
'c~ed platinum black, ,upon whi,ch .hoth hydrogen
. -:»:
'Hydrogen is an- easily -oxidizable
ifI
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'1) Netherlands
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patent no, 54716, applied 'for 23 nee. 1939.
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2) See the' survey; E. K.-Rideal
and H. S. Taylór, Catalysis in Theory and Practice, Mac Millan~ Co. London 1~26;
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NOYEMBER
1946
~ eDETECTION 'OF poisONOUS
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GASES
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'The' explanation is that the poisonous substances
, .. - ,are very' easily' adsorhed on the platinum, their
molecules, occupying the active spots and staying
, there, thus. blocicing the catalytic function. Since
, the adsorption is a !,urfaêe effect, it is also clear,
-
. ,,-
Concentration of poison
(in gram molecules)
for 'a certain:
for a certain
preparation
.platinum
of blood
preparatien
catalase
J,
Poison
that a very small volume of the poison can make the ,Sublimate,' HgCl
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2
. ,.- éatalyst ineffective, sinèe every active spot need
Hydrogen sulphide, H2S ".
~,only be occupied by one or a few poison 'molecules.' Hydrocyanic acid, HCN .
For reactions in which different gases are involved;
Mercuric Bromide, HgBr2
,different catalysts are generally, used, and the
Mercuric Cyanide, Hg(CNh
poisons ' also' seem to he' more or less specific. "Iodine solution, 12-in KI .
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Hydrogen sulphide, a strong 'poison for platinum;
is not poisonous for nickel sulphide catalysts, which
. are u'sed technically in' the preparatien
syn-,
'of,
,
, thétio petrol (Am. .gasoline)." Oxygen, ,a normal
"componerrt in reactions áccelerated by platinum,
, is a strong poison for the, iron catalysts in the synthesis of ammonia, etc. ' ,
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rh.e~e is, however, a remarkably 'pronounced
, parallel between the poisoning 'properties of áll
Hydroxylamine 'hydroèhloride, NHaOH.HCl
1/2000 one
1/300000
1/20000000
'1/2 000 000 -: , ,
1/1 000 000' _
1/1000000
.
1/300000'
1/200000
1/300 ÓOO
1/50 000 000 '
1/50000'
,
'.
1/2~ 000
Phenyl hydrazine,
: CaHs NHNH2 . . . .
Aniline, C6H.:;NH2'. c.' •
Ar
0
serrio tr~oxi~e, ~S2 . 3 ~
,
'Hydrochloric.acid,
HCl
'
1/80000
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1/20000
- -_ -'
, ,"
1140~ ,
non-poisonous
.to 1/2000,
non,_~~,.~..-,,-....
poisonous'
1/100 ~OO :'" -:', ,
1/50000
1/50
.
Car.bop monoxide" CO ...
,
very
, poisonous
, 1/3,0'00
-.
=;===========k=~=~:d====;===
, kinds of substances for platinum and for the organic Principle of the alarm apparatus
. catalysts which-play a part in the body.' Let us
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.: compare, for' 'instance, platinum with catalase,
' -The fundamental principle of the alarm appà"
, an. enzyme occurring in most body liquids such as- , ratus, which is based upon the ab~ve parallelism; ,'" .
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blood and milk. Both substances may function as is the following. An oxidation reaction is caused.
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acc'elerators of the reaction in the decomposition" to take place 011' a platinum catalyst: Heat i~there- J;,. ,
"of hydrogen peroxide' into. water and oxygen. It ,~y liberated. ':Çhe oxygen "for the oxidation is fur~'
:ik
,has neen 'studied how this catalvtic function is ; nished by suction of atmospheric air.r If the latter'
< ...
, ,';. hindered by certain "poisons"; in the table below' 3) 'coptains,a'p'oison
(in: the for~ of gas, vapour' or
'~ the number of. gram molecules of a series of poi- mist)' the catalyst loses its effect, the reaction stops ..': :
'son~is given ~hich is necessary; to redrice to ó~e -and-thusalso the hèan.development.
The chec~~on '.'
half the velocity of the reaction mentioned for a' poisonous substances in the air is thereby reduced
given platin~'
or 'catalase preparation. It may be' , to a check on the heat development, .
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As oxidation reaction we have chosen the oxi-'
substances which are als~ poisonous to 'catalase.
dation of methylalcohol. ,A mixture, 'of methjrl' e,','~,
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Carbon monoxide is,' it is true, not poisonous for alcohol vapour and air flows along a layer of pla-, .' ,.'
catalase, but it-is important that platinum is also tm"u"mblack deposited 'on a platinum wire with an' T:';'
-e-,
sensitive to it, since, as has already been mentioned,
intermediate
layer of aluminium
oxide.' 'I'his :": 'c~rbon "monoxide is poisonous for' haemoglobin. "wire forms' one of the' arms' ~f a Wheat~t'one
'.;:: ':';~
It is impossible to state exactly upon what the bridge (fig. 2). By the heat'of oxidation released by ,
_' parallelism between the poisonousness for 'piati- '"
'-,
<
~> .
num \~nd that for the organic catalysts is based.
Tt may be noted, ho~ever, that they both have the
property of catalysing reactions at low température
. ._: where oxygen is involved. The inorganic' catalysts
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which catalyse the same reactions at higher temperatures, such as certain oxides, are insensitive to
many ofthe poisons listed, probably because ~fthe
~
, ; fact that at higher temperature the poison mole:,' :' " c~e~ themselves begin to react a~d therefore 'do not
. , continue 'to block the active centres.
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• ~3) Borrowed from an article by W. D.>B ancr of t in
book referred to in footnote I).
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the
,
'----:---0'"
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0------"--'
~??70
Fig. 2. Diagram showing the principle of the circuit iof the
alarm apparatus. The platinum wire P bears 'the catalyst
(a layer of platinum black) alongwhich a mixture of methyl- ,
alcohol and atmospheric air is blown. R variable, Rl'8 fixed ,:.
bridge resistance. T headphone. ' ,
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..,------
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PHIqPS
344
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the catalyst the température of the platinum wire
and thus' its resistance is increased. The bridge'
is brought into equilibrium at the tempm'ature
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ultimately attained by the wire after the oxidation
has proceeded for several, moments (120-150 °C).
The presence of a poison will immediately cause
the temp'erature of the catalyst and ,its carrier to
fall, the equilibrium of the bridge is .disturbed and
a buzz is heard in the head phones shown in the
diagram, Of course it is also possible to cause the
disturbance' of the equilibrium of the bridge. 'to
actuate a series of sirens or signalIamps by means
of a relay.
After an alarm has been given the apparatus is
prepared again for detection by "depoisoning" the
catalyst. This is done by raising its temperature
for a short time to about 400°C, which evaporates
the poison or causes it to disappear owing to a
reaction' with the mixture of methyl alcohol and
,
,
air.
Activity arid equilibrium temper~ttire of the è~talyst
VOL. 8, No. 11
TECHNICAL REVIEW
,
In order that the catalyst should be hindered in its fuiIctiob
" . by very smáll amounts of poison, it is desirable that the activity of the catalyst (i.e. the number of active spots) should he
small compared with the amount of the reaction components
passed over it pe):'unit of time. This means th'at all the active
spots available are continually occupied with the reaction..
If this' were not the. case the blocking of á certain number
. of active spots due to occupation by poison molecules would-not
_', necessarily affect the total amount of reaction product formed
- per unit of time. The heat dèveloped per unit of time ,~ould
then also remain unchanged, and with 'certain conditions of
cooling (by radiatio"itand convection) the temperaturevwhich
is determined by thë equilibrium between heat developed and
heat dissipated, would not necessarily undergo any change
either:
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" The activity of the catalyst must therefore' expressly be .
redu~cd until it operates under ,fullload. This is done byheat" ing the catalyst wire for some time at about 700°C. Obviously,
however, o'ne must not go beyond a certain limit, because an
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entirely inactive catalyst can no longer be neutralised by
a poison.
This limitation can be. expressed in another way. As the
activity of the' èatalyst is reduced so the temperature at which
it works must increasè in order that the oxidation reaction may
obtained with a very ,simple and not highly sensitive 'apparatus.
.The fact that we may' eount on a cooling to room tcmperature'
may be explained as follows. Upon poisoning, i.e: reduction
in the number of active spots available, less heat will he produced by the reaction. On the other hand, as already stated, to
maintain the reaction with a smaller number of active spots
a higher ~emperature is necessary. From this it may he seen
that upon a slight poisoning of a fully loaded catalyst no new .
equilibrium can be established; the temperatnre falls rapidly
to room temperature,
The desired equilibrium temperature of l20-150 °Cis realized '
by annealing the catalyst until with the' given glow of the
alcohol-air mixture it adjusts itself to' that temperature. Of
course the velocity of supply of the reaction components must
then remain constant,in order that the equilibrium of the bridge
shall not be disturbed already without any poisoning. The
regulation of the velocity of flow is found not to be very
critical, since the influence of the more rapid supply of the
reaction mixture as the How increases is approximately cornpensated by the deeper cooling then taking place. It is
important, however, to keep the concentration of alcohol in
the mixture very constant.
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When the catalyst is put into action th~ oxidation reaction
does not start spontaneously. The catalyst must first be heated
artificially for a moment above. the equilibrium temperature
to be reached. This is - done in the same w:ay as the
"d~poisoning" pre~iously mentioned. .
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Construction of a. portable instrument'
.
A portable al~rm apparatus constructed according
to. the principles described is' shown in fig. 3. In
addition to great sensitivity, reliability and sim. it was necessary r+: to make
. it .
.plicity of operation,
portable - to' aim at light weight and small energy
consumption. The solution was found in the
following details.
The 'catalyst wire is mounted in an exchangeable
.
tube in the same way as the metal
filament in !in
incandescent lamp.
The air to he tested is supplied by means of a
diaphragm pump. This con~ists of a flat loudepeaker
diaphragm with. a cover in front of it with a blast
opening. The diaphragm is vibrated by a kind of
electric bell. This simple arrangement works as a
pump with a capacity' of abo.ut 7 liters air per
minute.
, Via :iieedlevalve and a capillary methyl alcohol
drip's from a reservoir into a mixing chamber where
keep i_t~~lfgoing. In the extreme case of zeroactivity, the working temperature must simply be equal to the ordinary flame
temperature. at which the oxidation can keep itself going
without a catalyst, From this"it follows that with increasing .the alcohol vapour is carried along by the current
. equilibrium temperature of the catalyst there can be less and
less question of "poisoning": the parallelism
with the senof air passing through. A fine wire in the capillary,
sitivity to poisons of tlie «<nzy~es and ferments which act at' permits à fine regulation of the alcohol supply and a
blood temperature becomes less and less pronounced.
. rapid cleaning of the capillary if it becomes stopped
An equilibrium temperature of 120 to 150
was found to be
up. The mixing chamber is pre-heated by the ex- '
the most suitable. In making'this choice it must also be taken
hausted oxidation gases; thanks to this measure
into account that the working temperature should not be too
low, in order to ensure adequate disturbance of thè bridge
the working temperature of' the catalyst is little
'equilibrium when thè catalyst becomes poisoned and cools
affected by temperature variations of the outside air.
do~n to room temperature, 60 thàt a clear indication can be
In order to obtain
buzzing tone in the he ad-
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..NOVEMllIÛl
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1946 '
DETECTION Olf, POISONOUS G,ASl!:S
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connects the' catalyst wire with another', battery,
'so that' it can he heated for a few seconds _by "
3. Watts to about 400
for "starting" and "depoisoning" ,
'.)' :in: front of the openings .through which the pump .::
draws in air from the outside a filter can, if desired,
be placed to' bl~ck the, entrance for certain gases: .....
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In/this wày it' is possible to demonstrate the pres_',';'ençé ~f certain. gases sel!!ct!vely,:' ~or instance in ""
garages where petrol vapour is alwasy present' and, \
. where the' instrument is not ;equired to react to "
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,that birt only to' possible carbon ,monoxide; in: .this
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case, therefore, a filter must be used which absorbs
petrol vapour but allows carbon monoride to pass' ',':'.:.
throug~:. The, fact that the catalyst is also ~pois-,·::.:.:'
.' oned by petrol vapours which 'are only, slightly
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harmful for the hody is Olie.of the' deviations from "
,I',
" the above-mentioned parallelism.' However, it i;
also possible to tak~ advantage .of this fact in ordei·...
to, detect' petrol or other vapours in' places where,
'.~ . Fig. 3. Cross-section ofa portable model of the alarm instru- .
theIT presence 1vould lead to the fórmution of
.' "~ . ment. _P exchangeáble "lamp" with platinum catalyst, ,M
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. reservoir with methylalcohol which, via the needle valve N . explosive ~mixtures. An analogous possibility ",'of
and the capillary C, drips into the 'mixing chamber IC. B elec,
application is based on the fact' thàt the, catalyst'
tric bell whieh sets the )oudspeaker diapliragm L vibrating
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by means of which the air entering, possibly via ...
..he filter F,
is sensitiv~ to Ftè'on andother voltatile halogenàt~d,
"/
is blown through the mixing chamber. The exhausted air leaves
hydrocarbons, ónly slightly harmful for human' "0. ' • ;:',
the apparatus at U. R variable resistance for balancing the
., , bridge circuit. Zl pocket torch battery for normal operation,
beings ... '
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Z2 battery for starting and depoisoning the catalyst, T connection for the he_!ldphone;
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, 'phone used- as indicator 'the' resistance bridge has'
.~',to be fed with A.C. voltage or intermittent
, voltage. For this purpose the voltage aèross' the
'" 'win~ing of the electric hen' system driving the"
pump is' used. ,
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: The resistances in the arms of the bridge take' up'
.~ ,{ ,êttergy;' Sinc~ the'"energy consumption mU,sthe a"
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minimum; the winding of the bell systèm, in which
Fig. 4, More d~tailed diagram of the cirç~it of th~ apparatus:
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in any case current must flow, is provided with à
.middle tap, and t'Pe two halves are used as fi~ed
, " bridge resistances, Moreover ~1~ith the help" of 'a
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• The two halves of the exciter coil of the bell system also serve
,as the fixed resistances RI' R2 of the bridge circuit. Z battery,
C condenser, L di~p!üagm pump; other letters as in fig. 2.
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. cOllden~er' the winding is brought into electrical
, These fairly expensive gases ani used, for example, .
• . , resopa:nce with the characteristic. .frequenoy of th~
'in refrigerating _Plant,s, and the instrument
deinter~ilptor of the belf syste~; 'see fig. 4. By these
scribed would be very well suited for detecting leaks
"~,me~su~~!3the total, ~onsumption of energy of the
in such plants, It is clear; however, that such appli. "apparatus is reduced to 0.5 Watt, which issupplied .. cations havè' nothing to' do \~ith the original pur. by, a"pockeHÓÎ:ch' battery.
'''pose of the apparatus arid that other '§ipecific
'-, . When the variable resistance
setting the bridge
actions or reactions of the gases 'in question could
in equilibrium i3 turned ~0:rIlfletely hack, 'a switch
probablyalsohe
used for the purpose,
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