Note: Descriptions are shown in the official language in which they were submitted.
6Z5
This invention relates to a fire sensor device includ-
ing at least two ionization chambers and an electrical evaluation
circuit arranged to initiate an alarm condition when the resis-
tance of one at least of the ionization chambers exceeds a pre-
determined threshold level as a result of the entrance of smoke
or combustion aerosols into the ionization chamber.
Arrangements of this kind are known an ionization smoke
detectors or, in the case where several ionization chambers are
arranged at different locations, as ionization smoke~detector
installations. They make use of the fact that smoke or combustion
aerosol particles entering an unsaturated ionization chamber cap-
ture some of the ions formed in the chamber and thus give rise to
a reduction in the ion current flowing between the electrodes. As
compared with other types of smoke detectors they have the advan-
tage that they not only provide information as to visible smoke
but also as to the presence of the very substantially smaller,
invisible combustion aerosols generatsd in the early stages of a
fire.
It has however proved to be a disadvantage of the known -
2Q ionization smoke detectors that the ion current between the elec~
; trodes of the ionization chamber depends not only upon the density
of smoke or aerosols in the chamber, but is also affected by the `
speed with which the air flows within the chamber. This influence
. :
^~ has proved to be particularly disturbing in low-voltage operated
ionization chambers, with particularly low electrical field-
strengths appearing in the ionization chamber, which have in
practice proved particuarly suitable because of their high sensi-
tivity and fast response characteristics. It has been found that,
in accordance with the construction of the ionization chamber
either external movements of air make the sensor more sensitive
with rising velocity of the air, in fact up to an air speed at
which a false alarm is initiated, without smoke being present in
' ^ ' :- . . ~ . .
.
;ZS
the chamber, or that with increasing air velocity the chamber
becomes less and less sensltive until it reacts either b~latedly
or even not at all to smoke or combustion aerosols. ¦ ~-
The reason for this behaviour of ionization smoke
detectors has obviously not been understood and therefore the !
problem of overcoming the disadvantages mentioned by an appro~
priate construction of the ionization chamber could not be
solved. Instead of this it was sought in a number of previously -
known ionization chambers it was sought by means of said wind
shielding devices to ensure that only a slight flow of air
occurred within the ionization chamber, even when a fire caused
the smoke-laden air to flow with considerable velocity. This
was effected, for example, by appropriate construction of the
walls of the chamber, for example as a double shell with off-
set openings, ~hrough appropriate positioning of the air entry
apertures or by deflecting the air flow by means of appropriate
construction of the walls of the ionization chamber. While it -
is true that in this manner the air velocity in the ionization ;~
; chamber could be considerably reduced, there resulted at the same I `~
time the disadvantage that the smoke-laden air could penetrate
only slowly and with difficulty into the chamber. In many cases 1;~
the alarm response could thus be delayed to an undesirable extent. 1 ~
With smouldering fires especially, for which no substantial move- ;
ment of air occurs, such draft-screened ionization smoke detectors
` respond very late or even not at all. `~
It is however required of fire alarm installations that
:~ ;
they shall detect and signal at as early a time as possible all ~
, :.: .
the different kinds of fire that occur in practice, more particu- ;~
larly both open fires with strong air circulation and also
3~0 smouldering fires producing smoke or combustion aerosols without
substantial movement of air. Since this has not been possible ;~
with the required ~enerality by the use of the known ionization
- 3 -
1~6~Z5
smoke detectors, in practice ionization smoke de-tectors were
often combined with optical scattered-light or extinction smoke
detectors, which are known not to be affected by air flows.
The known disadvantages of optical smoke detectors must then be
taken into consideration, that is, the iarge current required by
the light source, their instability resulting from dirt accumula-
tion and aging and the alterations of sensitivity and liability
to false alarms, as well as the defective compatibility between
ionization smoke detectors and optical detectors, which makes
necessary a mutual adaptation of the evaluation circuits with
abandonment of the advantages of the individual constructions.
The object of the invention is to overcome the above-
mentioned disadvantages and to provide a fire sensor device which
makes possible the development of an alarm signal at an early
instant for all the kinds of fire occurring in practice, which
hence responds rapidly and reliably, regardless of the kind of
fire, without giving false alarms.
According to the present invention there is provided a
fire sensing arra~gement comprising at least two ionization cham-
:, ~ . .
bers and an electrical evaluation circuit arranged to yield analarm signal when the resistance of one at least of the ionization
chambers exceeds a predetermined threshold level as a result of
the entranceof smoke into the chamber, one of said ionization
chambers being a smoke-sensitive and air-accessible ionization
chamber of a first kind including means for ionizing the air which
are so constructed and arranged that ions of both polarities are
formed in the whole of the space between the electroaes of the -
chamber, and being provided with means for retarding the entry of -
air flowing into the chamber, and another of said ionization ;~
chambers of a second kind being arranged for substantially
unhindered entrance of air and including ionization means so -
constructed and arranged that a part only of the space between -~
.
6462t;
the electrodes so that the ion current flowing in the chamber
comprises ions of only one polarity, the evaluation circuit being
so arranyed that an alarm signal is initiated upon a predeter~
mined increase in the resistance of either or both of the two
chambers
Advantages of a smoke detector in accordance with the ;.
invention are that the evaluation circuit employed is simple, `
accessible and not liable to defects and has a low power demand.
The invention will now be further described with 1
reference to the accompanying drawing, in which:
Figure 1 is a schematic representation of a first kind
of ionization chamber; - ~:~
Figure 2 is a schematic representation of a second kind
of ionization chamber; :~
Figure 3 is a graph illustrating the different character~
istics of the two kinds of ionization chamber in moving air;
. Figure 4 is a schematic circuit diagram of a fire sensor `~
in accordance with the invention; : ~`
.. . .. .
Figure 4a shows details of the construction of the fire ~.
. 20 sensor used in the arrangement of Figure 4;
Figures 5 to 9 are circuit diagrams of five different `~:~
embodiments of ionization smoke detectors in accordance with the ; ~.
invention.
Figure 1 shows sahematically an ionization chamber of a
first kind, specifically what is known as a bipolar ionization ..
chamber, in which an electrical potential is applied between
opposed electrodes 1, 2 so that electrode 1 is held positive with
, . . .
respect to electrode 2`.~ The air in the space between the elec- z~.
: trodes 1, 2 is relatively uniformly ionized by a radioactive
30 source 3. Everywhere within this space ions of both polarities
are thus generated and there flows everywhere a bipolar ionic
current, the positive and negative ions moving in opposite direc- r
- 5 -
~0~;~a625
tions. Upon the penetration of smoke or combustion aerosols into
the space between the electrodes the ions are captured by the
particles in known manner and the ionic current diminishes. The
same effect is however produced by flowing air, since as a result
of the movement of air through the ionization chamber some of the
ions are withdrawn from the chamber and no longer reach the elec-
trodes. A circulation of air thus likewise gives rise to a reduc-
tion of the ion current and in extreme cases a false alarm may be
given without a fire being present. In order to prevent this,
ionization fire alarms with bipolar ionization chambers must be
provided with screening means for preventing air flows and to
reduce the velocity of air entering the ionization chamber, for i;
which various solutions are known. As a result, the giving of an ¦-
alarm may in some cases with slow air circulation be undesirably
delayed or even prevented.
Figure 2 illustrates another embodiment of ionization
chamber, in which the radioactive source 3 is screened so that the
i air becomes ionized in only a small part of the space between the
electrodes 1 and 2. Only in this region do ions of both polari-
ties appear. Owing to the voltage applied between the electrodes
only electrodes of one polarity are drawn out of this region 4
into the remaining major part 5 of the ionization chamber. Thus
a unipolar ion current flows-in this region ~. This has the
result that a space charge is formed in the region 5. The charac- ~ -
teristics of such a predominantly unipolar chamber are thus
altered in a definite manner as compared for example to a bipolar
chamber, It has been found that a predominantly unipolar ioniza-
tion chamber has a higher sensitivity for heavy particles, e.g.
smoke or combustion aerosols, even at low voltages or currents. ¦~
; 30 In this connection it has already been observed and described
1 ~
that the sensitivity to wind of a unipolar ionization is less
than that of a bipolar ionization chamber.
- 6 -
.. ~
`' :' ' . , , . '`: `
~06~2~
It is true that in practice unipolar ionization chambers
have been employed for smoke detection, but obviously the differ-
ent responses of unipolar and bipolar transistors to flowing air
were not recognised. The reason for this probably was that even
unipolar ionization chambers must always have a portion of the
chamber in which the ions are formed and which must therefore have
bipolar characteristics. In previously known embodiments of ioni-
zation chambers this bipolar portion of the ionization chamber
apparently had so great an extent that the different and to a `~
`~ 10 certain degree exactly opposed characteristics of the two types
of chamber were not recognized.
The different behaviour of the ion current in a bipolar
and in a unipolar ionization chamber in accordance with the velo-
city of transverse flow of the air, which has become apparent ;
through the present invention, is contrasted in Figure 3. It is
known that the ion current of a bipolar ionization chamber `
. , ~
i diminishes progressively with increasing air velocity, while in a
unipolar ionization chamber it first rises, so long in fact, as
the unipolar region is maintained between the electrodes. For high
air velocities, because of turbulence, the unipolar region is ~` ;
destroyed and the chamber assumes the same characteristics as
those of a bipolar ionization chamber, that is, the ion current ~ -
, . .: ~ .
falls with furtherincrease in air velocity. ~ ;~
This behavicur has the result that an ionization fire " ~ ~
alarm with a predominantly unipolar ionization chamber shows an ~ -
increase of sensitivity upon the appearance of air currents up to ~
a certain limiting air velocity. Wind-shielding means are there- ~`
fore superfluous and a unipolar ionization fire-alarm may comprise
a substantially open chamber to which the air has unhindered
access, although for purely constructional reasons mounting j
elements for the components or, for preventing the entrance of
foreign bodies, e.g. insects, a thin-wire grid, may be arranged
.~ .
_ 7 _
,,' '.
~i4~Z5
in front of the ionization chamber, which however only slightly
hinder the entrance of air. Thus under normal conditions there
is no risk of a false alarm initiation, such as occurs with
bipolar ionization chambers.
The invention now makes use of the formerly not clearly
recognized different characteristics of the two types of chamber,
the two types of chamber being combined in a single detector
arrangement but in such a manner that the disadvantages are
.,
eliminated, while the advantages are retained.
Figure 4 shows generally an embodiment of fire-alarm
installation in accordance with the invention. Here an alarm
unit C is connected by way of common leads Ll and L2 with an
ionization fire sensor Bl with a bipolar ionization chamber and
with a second ionization fire sensor Ul with a predominantly
unipolar ionization chamber, both so arranged that they supervise
a first space Dl to be protected. The two fire sensors may be of
.. :, . .
known construction and circuit arrangement. To the bipolar ioni-
~ation chamber there are fitted known means for retarding the
entry of air, while the unipolax ionization chamber allows the
entry of air substantially uninterrupted. The leads Ll L2 may
be led through further spaces, such as D2, which likewise contain ;
a bipolar ionization fire sensor B2 and a unipolar fire sensor U2.
In addition, other types of fire sensor such as F may be provided `~
in some at least of the spaces to be protected. These may for
example be flame sensors or optical smoke sensors, so that the
efficiency of the installation is still further improved. ^`~
Figure 4a shows by way of example the construction of a
unipolar and bipolar ionization smoke sensor, such as may be ;~
employed in a fire alarm installation as described above with
-3Q reerence to Figure 4.
A unipolar fire sensor Ul includes an ionization chamber
60, which is separated from the atmosphere by a grid 70 that ¦~
; ~ .:
~ -- 8 ~
.,.' ' 1' ,.
''~ . , . , 1:
:.:, : . . , , .: , . . ..
1~6~625
serves as an electrode. The air thus has substantially unhindered
access to this ionization chamber 60. ~n the centre of the cham-
ber there is situated a second,rod-shaped electrode 80, and
radioactive sources such as 90 are arranged on an annular support
plate so that their radiation zone does not include the central
electrode 80, but only the air in an annular zone 110 in the
viclnity of the grid electrode 70 is ionized, so that the ioniza-
tion chamber possesses a unipolar characteristic. In the lower ~ ~
portion of the fire sensor there is fitted a further ionization - ```
chamber 111, which in contrast to the ionization chamber 60 is
operated in the saturation range so that its ion current remains -
constant and is not affected by smoke. In order to ensure that
the reference chamber 111 is completely insensitive to smoke, its
casing 112 is made practically impermeable by air, except for
. . .
pressure-equalizing capillaries. Within the chamber 111 are
~- situated a radioactive source 113 and a counter-electrode 114.
The reference chamber 111 and the control electrode 80 of the
unipolar ionization cha~ber 60 are connected together and to the
control electrode 115 of an electronic switching device 116, `~
,~ ,
which may as shown be a cold-cathode tube or in known manner may
be in the form of a semiconductor switch. In the illustrated
embodiment the two other electrodes 117, 118 of the switching
device are respectively connected with the two other electrodes 70
,, ~ , , A
and 114 of the two ionization chambers as well as with base con-
tact plns 119 and 120. `~
The bipolar ionization smoke sensor Bl includes an
ionization chamber 121 which is closed at its upper end by a grid- ~-
'~ form electrode 122. The side wall 123 is impermeable to air.
Over the grid electrode 122 there is fitted a wind-shielding ~;
plate 124, which prevents the direct entrance of air into the `
chamber 121 and has the effect that in flowing air can enter the
chamber through the aperture 125 only after being deflected.
;' '
,: ~
~L~6462~i -
Instead of this, however, ~ther known wind-shielding means could
be employed. In the ionization chamber 121, a radioactive source
127, having adjustable screening to permit its intensity to be
regulated, is fitted upon a pot-shaped counter-electrode 126, so
that the air in practically the whole interior of the chamber is
ionized, to that a bipolar ion current flows everywhere. Further-
more there is again provided a reference chamber 129, enclosed by
a casing 128 and therefore insensitive to smoke, which contains a
further radioactive source 130 and a counter electrode 131. Again
`10 the electrode 126 and 128 which is common to the two ionization
chambers is connected to the trigger electrode of an electronic
switching device 132, the other two electrodes of which are again
connected with the free electrodes 122 and 131 respectively of the
two ionization chambers and also with base connecting pins 133 and
134.
One connecting pin 120 and 134 respectively of each of ;
the two ionization smoke detectors Ul and Bl is connected with the
lead Ll and the other two connecting pins 119 and 113 with the
~ other lead L2, each by way of an appropriate base or connecting `
.~ ' :i,.
socket. By way of the two leads ~1 and L2 the two sensors are
~ connected in parallel with one another to an alarm unit C, which
; in the simplest case includes a voltage source L and a current-
; sensitive switching device SW, e.g. a relay, through the normally~
open contacts N of which an alarm signal device AS is connected
to a voltage source B'. ;
It is remarked that the two sensors Ul and Bl may be ~ -
arranged either immediately adjacent one another or on a common
base unit that ensures their correct connection to the leads
going to the alarm unit and ensures that an increase of the
resistance of either of the two ionization chambers 60 and 121
will give rise to the initiation of an alarm signal.
However, the invention is not limited to the use of the
'' -- 10 --
3l~11646Z5
kinds of ionization smoke detectors described above with reference
to Figure 4a, but a fire sensor in accordance with the inven-
tion may also be e~uipped with other sensors. It is merely
assumed that there are present in the same zone to be protected `~
at least a respective substantially open ionization smoke sensor
with a predominantly unipolar characteristic and another, wind-
shielded sensor with a-bipolar ionization chamber, the two sen-
sors being connected in a kind of OR circuit.
By the combination of a screened bipolar and an open
~`~10 unipolar ionization smoke sensor it is already arranged that the
fire-alarm installation reacts more rapidly than known installa
tions. For an open fire with development of smoke and detectable ~-
air circulation the smoke enters relatively rapidly into the
bipolar ionization chamber, so that this sensor gives rise to an
. .: - .
increased live current and thus initates an alarm signal, even in
the initial stages of a fire, when it is true the density of `
; aerosols is high, but the density of smoke is still small. On
the other hand, smouldering fires with small circulation of air ~ ~`
are detected even in the initial stages by the open unipolar
2~ ionization chamber. In order also to detect extreme types of
fire still other types of fire sensor may be connected in parallel ~ `~
e.g. flame sensors, which respond particularly rapidly to li~uids
burning without the development of smoke, gas sensors that signal `
the carbon monoxide or halogenides formed in a fire, or other fire
sensors.
For the sake of completeness it should be said that other
.. .. . .
means, known in themselves, may be used for the generation of ~~;
unipolar ion currents, e.g. placing a radioactive layer with a
short radiation range on an electrode, the electrode separation
3a being greater than the radiation range. The amount by which the
unipolar region extends into the ionization chamber can be influ-
, enced by the choice of the radiation range and so also the degree `
~646ZS
of dependence of the ion current upon wind velocity. It ~ould
also be possible to match the unipolar and the bipolar regions ;~
to one another so that the ionization chamber would up to a
certain limit be independent of flowing air.
In the fire alarm installation descri~ed above two
different types of ionization smoke sensor are necessary at each ^;~
measuring position. In order to avoid the increased expense thus
resulting it has proved to be advantageous to combine a bipolar
- and a unipolar ionization chamber into a single ionization smoke
sensor, so that a compact unit results, the evaluation for giving
an alarm being effected by means of an OR circuit.
In the embodiment of this kind represented in Figure 5,
an ionization smoke sensor Ml is connected by way of voltage-
carrying leads L1.L2 to a central signalling position C. In
parallel therewith there may be connected further, similar sensors
such as~2 at other locations.
The ionization smoke sensor Ml includes two smoke-
sensitive ionization chambers U and B. Both chambers contain a -
respective central electrode, which carries a radioactive prepara- ;;
` 20 tion and in both chambers the housing or casing serves as a
counter electrode. In the ionization chamber U this housing is
~ . . , ~ ,
made as permeable to air as possible, e.g. as a grid of thin wire,
so that the external air has ~ubstantially unhindered access to
the interior of thè chamber, ~hile forelgn bodies, e.g. insects,
are kept out. The other ionization chamber B is, on the contrary,
provided with a housing substantially impermeable to air, through `~
which the air can enter into the interior of the chamber only after
being retarded and deflectedO The two ionization chambers differ
additionally with respect to the kind of ionization occurring in
the interior of the chambers, i.e. between the electrodes, in a
characteristic manner. In the ionization chamber B a radioactive
source is arranged at an appropriate position, e.g. on one elec-
i
- 12 -
:. : . : .. ,. . :
~6~2ci ~ -
trode or on the wall of the chamber, the radiation range being
chosen so that the air within practically the whole of the chamber
is ionized. Ions of both polarities are thus generated in the ;
whole of the space between the electrodes and in all regions a
bipolar ionic current flows between the electrodes~ In the
other ionization chamber U, on the other hand, the radiation -~
range of the radioactive source is chosen and the source is so
- positioned and/or screened so that ionization occurs within only
a small part of the interior of the chamber. Ions of both polar~
; 10 ities are generated only within this ionization region. In the `
remainder of the ionization chamber, ions of only one polarity
are then present and a unipolar ion current flows. Instead of - -
applying to one ~f the electrodes a radiation source with a `~
radiation range of substantially smaller extent than the electrode - :~
separation, e.g. a tritium-containing source, as is shown in Fig-
ure 5, there may alternatively be used an isotope such as is
usually employed in ionization smoke detectors, e.g. Americum 241,
the range of which is reduced by appropriate sheilding. The
source may however be arranged at one side and shielded so that
?o only a portion of the space between the electrodes is irradiated
. so as to ionize the air in that portion. -
By the combination of the two kinds of chamber in one
ionization smoke detector together with differently effective
.,~ .... .
h screening against flowing air it is possible here also to combine
the characteristics of the two chambers in such a manner that both
smouldering fires with only very slight thermal movement of air `
., ~
and al~o open fires with strong air movements can be reliabl~
~` and rapidly reported, while false alarms are prevented. For this
~ purpose the two ionization chambers U and B are connected in
~ 30 series with respective resistors Ri and R2 between the voltaye-
carrying leads Ll and L2. Th~ junction points between the ioni~a-
tion chambers and the respective resistors are connected with the
,
- 13
. - .
: ,,
: - : :
~ L~646Z,5
:';'
inputs ofrespec-tive threshold swltches Sl and S2, of which the
outputs are connected with the lead Ll. If the ion current in
one of the two chambers falls below a predetermined threshold
level, then the corresponding threshold switch Sl or S2 causes an
increased live current to be drawn over the leads from the central
position C and there initiates an alarm signal in known manner.
In the case of a smouldering fire the smoke now enters
relatively rapidly into the unipolar ionization chamber U open to
:
the external air and when the smoke density is sufficient it
initiates an alarm by way of the threshold switch Sl in the manner
described. At first the bipolar chamber B remains relatively
unaffected, since because of the small movement of air the smoke
can enter its interior only with difficulty. In the case of an
open fire, which almost always proceeds with strong air movements
the smoke penetrates relatively rapidly into both of the ioniza-
tion chambers U and B. Because of the strong air movement, how-
: .
ever, the chamber U is less sensitive, while because of the wind-
shielding means the sensitivity of the chamber B is retained or
even improved. In any case, upon reaching a definite smoke den~
sity, that is, upon a definite change in resistance of this chamber `-
B, an alarm signal is given by way of the corresponding threshold ;~
switch S2. Thus here also it is arranged that both with a
- smouldering fire and also with an open fire an alarm is given ~ `
more rapidly than in the case of known ionization smoke detectors,
~hile however the disadvantages of both types of lonlzation smoke
detectors are avolded. Thus the Iiability to the giving of false
alarms is in no way higher than in previously known detectors.
In the embodiment represented in Figure 6 there is pro-
-" vided instead of the two series resistors Rl and R2 a reference
, 30 ionization chamber R that is substantially closed or unresponsive
.. , , :
to smoke. This reference ionization chamber may for example be
operated in the saturated condition and put in connection with
`'~; . ~:
. . , .:.
., , . . ,, , . : .
~``~ ;:
1(~6~6Z~i
the external air only through very small apertures, so that while
pressure equalization for compensating variations of atmosphere
pressure is possible, yet the entrance of smoke particles is at
least made very much more difficult.
Each individual ionization path in this reference cham~
ber R is now connected in series either with the unipolar ioniza- .
tion chamber U or with the ~ipolar chamber B, between the leads
Ll and L2. The junction points of these two series circuits are . ::.:
.
. connected with the control electrodes of respective field-effect .
transistors Tl and T2 ~ that have like main electrodes of each .
connected together and to line L , while their other main elec~
-~
trodes are connected respectively to the junction points formed .~. :
i .
:.;. by pairs of resistors Rl, R3 and R2 I R4 connected in series
l across the lines Ll, L2. The field-effect transistors Tl, T2 act ~
.l as threshold setters, that is as soon as a variation of ion current . ::
~ flowing in one of the ionization chambers U, B causes the potential
.l applied to the gate of the respective field-effect transistor Tl l~ -
l or T2 to exceed the threshold level set by the voltage dividers l`
. Rl-R4, the transistor becomes conductive. The output of the two ,.
20 transistors Tl and T2 are each connected with one input of an OR
gate, at the output of which a signal thus appears as soon as one
of the two ionization chambers U and B shows a definite reduction :.
of current or incxease in resistance as a result of the entrance .:
of smoke into the interior of the chamber. The output of the OR
gate is connected to a switching device S, e.g. a thyristor or an .
electronic switch, which initiates an alarm signal by way of one
. of the leads Ll and L2, or over a separate signalling lead if pre- ~ :~
.~ .
-. ferred, as soon as one of the two ionization chambers U and B ~ ;
. . . .,
signal the presence of smoke.
. 30 In the embodiment represented in Figure 7 three ioniza- . ~
: tion chambers U, B and R are combined into a mechanical unit. ..
:
The upper portion U is separated from the external air only by a ~
:
` - 15 -
.
~. .
1~ti46ZS
thin-wire grld 150, so that the air has substantially rree access
to this region. A radiation source 161 carried on a central elec-
trode 160 ionizes air within a small part only of the space U, so ~ ~
that a unipolar ion current flows within a large part of this ~ ;
space. The lower part of the sensor is shielded against the
external air by a cylindrical outer wall 170. Within this is
situated a further cylinder 180 which together with an insulating
member 190 forms a substantially closed reference chamber R. The
annular space B between the two cylinders 170, 180 forms the bi-
polar ionization chamber. The external air can enter this chamber
B only after being retarded by a detour through the chamber U and ` ~
the direct passage of flowing air through this chamber B is impos- ;~ -
sible. The air everywhere within the space B is subjected to
ionizing radiation from a source 181 carried by electrode 180,
which source serves also to ionize the air within reference cham-
ber R. In chamber R a counter electrode 200 is carried by insulat-
ing member 190 which is itself secured to a support and connecting
. :, .. : .
~ member 160 that carries electrode 160. The whole assembly is
,,, . ~ . ..
mounted on an insulating base 210 through which pass leads 162,
:, -.:
181 and 201 that respectively provide electrical connections to ~`
electrodes 160, 180 and 200. The electrodes of the three --
ionization chambers are in this embodiment connected to an evalua- ` ~
tion circuit as described in connection with Figure 6, but which ;
, - is shown only incompletely.
..
Figure 8 shows another embodiment of combined ioniza- ` ;~
~! ' tion chamber with the same evaluation circuit. Here the substan-
tially closed or smoke-sensitive reference chamber ~ is formed
by a space within the lower part of the device, surrounded by an r~
.; .: '
enclosure 210 formed of insulating material. Chamber R encloses
an electrode 220 carrying a radioactive source 221 and is traversed
~ ,.
by lead3 222, 223 carrying respective electrodes 224, 225. Leads `;
222, 223 pass through the top of enclosure 210 and supports respec-
- 16 -
Z5
tive electrodes 230, 2~0 each carrying a radioactive source 231,
241. Source 231 is chosen to have a restricted ionization range
and electrode 230 is disposed within a substantially open unipolar
ionization chamber U partly enclosed by thin wir~ mesh 250 while : .
source 241 has an ionization range extending throughout the bipolar
ionization chamber B, into which the external air has only .
restricted access through flow-deflecting means 261 provided in ~ -
an otherwise impermeable housing 260. Thus one half of the upper .
part of the device may form a unipolar ionization chamber and the
10 other halfmay form.a bipolar chamberO However, in order to obtain
a uniform sensitivity of the ionization smoke sensor, regardless
t of the direction in which the external air flows, that is, of the
position of the seat of the fire with respect to the sensor, it
is advantageous also to divide the unipolar and bipolar ionization ~:. .
chambers and to arrange the four ionization chambers thus result- ~;ing in opposite quadrants as shown by ~1~ U2 and Bl, B2 in diagram
: 300. Obviously further division into more than four sectors may .;
be effected. Alternatively the alltogether more heavily screened ~1
bipolar ionization.chamber B may be placed centrally and concen-
~0 trically surrounded by the open unipolar ionization chamber U as
.: . . "
shown at 600. ~ith this arrangement also sensitivity approximately
independent of the direction of air flow is guaranteed.
. In order to avoid high-impedance evaluation circuits and .: - :
; the problems of lnsulation and stability associated therewith, it .`
. may be found suitable, instead of making use of analog evaluation
of the voltage drop across the ionization chamber, to employ the
chamber resistance for controlling the frequency of an oscillator
or pulse source and to make use of the change in frequency to
yield an alarm signal. Such a pulse or a.-c. signal evaluation may
... .
30 be arranged to be considerably less liable to disturbances and more reliable than an analog evaluation using a high resistance
voltage divider. Figure 9 shows the circuit diagram of such an
- 17 -
,' ~ .
1~4~25
embodiment. In a unipolar lonization chamber ~ a radiation source ~`
Ql of restricted ionization range is carried on an electrode El.
A housing Hl, which is made substantially air-permeable, e.g. in
the form of a grid, forms the counter-electrode. Adjacent to the
ionization chamber U is arranged a second chamber B, in which is
enclosed a second electrode E2 carrying a radiation source Q2' the ~; ~
ionization range of which is so chosen that the air within prac- ~'
tically the whole of the chamber is ionized, and ions of both
polarities are present everywhere within the chamber. The chamber
housing H2 again serves as the counter-electrode. Thus while for ~,
the most part a unipolar ion current flows withln the ionization '" '
; chamber U, in the chamber B the ion current has a bipolar char- - -
' acter. In contrast to the open nature of the housing H1, the ,,
,i housing H2 again forms a screen against flowing air. This may be ~ ,
arranged by providing only a few apertures A, behind which are ,' ~'
arranged flow-deflecting and shielding means.
The two housing portions Hl and H2 are electrically
connected together and to the earthed negative supply 'lead Ll.
The electrodes El and E2 of the two ionization chambers are each ,~
` 20 connected to the gate electrode of a respective field-effect tran- ,
sistor Tl and T2, of'which the source electrodes are connected to ,
: . ~
', one another and with'the positive supply lead Ll. The drain
electrodes of the two field-effect transistors T1 and T2 are con- ' ,~
nected with respective series-connected resistors 6 and 6' placed
~ithin an insulating housing H3, the junction between the resis-
, tors being connected to the housings H1 and H2 and so with the
; negative supply lead. The-drain resistors 6 and 6' of the two ':
fi'eld effect transistors Tl and T may aiternatively be formed by ',
~ , 2 - ,,
,~ a potentiometer of which the adjustable tapping is held at the `
negative supply potential. ~In addition the drain electrodes of
'~ each of the two field-effect transistors is cross-coupled with !
the gate electrodes of theother transistor by way of a capacitor
7, 8. ~ '
.' 1."
- 18~
62~ii
The circuit arrangement comprisiny the field-effect ~ ~.
transistors Tl and T~ with cross feedback, in which the ioniza-
tion chambers U and B form the gate resistances, therefore oper-
ates similarly to a freely oscillating astable multivibrator.
Because of the high values of the indivldual resistances, the
pseudo-stable periods are however of the order of magnitude of
one second, that is, there appears at the output point 9 a fre-
i- quency that will be between 0.1 and 10 Hz, in accordance with the -
embodiment. The two ionization chambers U and s, of which the
resistance changes in the presence of smoke, thus act as frequency- -
~determining elements for this extremely slowly oscillating oscil-
`; lator and in fact in such a manner that the pulse or oscillation
~- frequency dimenishes when the resistance of one of the two ioniza- -
tion chambers U or-B rises. Both ionization chambers thus act in
i the same direction, so that no OR circuit is necessary. In addi-
tion the circuit has the advantage that no refexence resistances ;-~
are necessary for the ionization chambers U and B, so that the
problems of stability and insulation associated with the use of '-
such resistors for the most part do not appear. The field-effect
transistors Tl and T2 and the capacitors 7 and 8 can advantageously
be combined into an integrated circuit, which may be fitted in the ;~
base housing H3 together with the reactors 6 and 6'.
To the output of the multi~ibrator there is connected
~` by way of a coupling capacitor 10 a frequency-detector circuit D.
The pulse-shaped alternating voltage appearing at the output point
9 is fed by way of an RC circuit consisting of the capacitor 10
and a resis~or 14 and by way of a diode 15 to a storage capacitor
16, which is discharged with a constant time-constant by way of a
parallel resistor 17. The state of charge of the capacitor 16 is
` 30 thus dependent upon the pulse repetition rate, or frequency. If
.~ :' ,
-~ the frequency falls, as a result ofthe entrance of smoke into one
of the two ionization chambers U or B, then the charge on capa-
citor 16 falls. The potential appearing on capacitor 16 now
'~. ' '
' - 19_
~al 64G25
controls the base of a transistor 18, in the collector path of
which is arranged the actuating coil 19 of a relay. In the normal
condition, that is, as long as no smoke is present, the charge on
capacitor 16 is sufficient to keep the transistor 18 conductive,
the relay coil 19 energized and the normally-closed contact 20 of
the relay in its open condition, as shown, so that an indicator ~
device 21 in series with the contact 20 does not yield any signal. ~ -
i , - ,
If however, the charge on the capacitor falls as a result of the `
effect of smoke, then the relay 19 releases, its contact 20 closes
and the indicator device 21 signals an alarm.
To supervise the circuit it may be advantageous to use
as the relay 19 a two-step relay with a further contact 22 which
is actuated only at a higher threshold level. With such a circuit,
if the charge on the capacitor rises still more, so that the
current through the transistor 1~ and relay winding 19 increases
further, then this contact 22 also closes and through the indica~
tor device 23 connected in series with it, signals a fault condi-
tion. The alarm contact 20 of the relay 19 may also be made self-
holding and instead of an electro-magnetic relay 19 a known elec-
tronic circuit performing the same function may be employed.
An ionization smoke sensor of this kind thus has in
addition to the advantages of the other embodiments, that it
responds reliably to the different types of fire occurring in
practice, the further advantage that high-resistance reference
elements are unnecessary and an a.-c. or digital evaluation may
. ., ~i -
be employed instead of an analog evaluation. It is therefore --~
~- particularly reliable and not subject to disturbance. ~ -
i ~' ~'''
::. .,
- 20 -
. 1~
.. . ..
