Note: Descriptions are shown in the official language in which they were submitted.
Z010105
, ~
A method of operating an ionization smoke alarm and ionization smoke
al arm
The invention is directed to a method of operating an ionization smoke
alarm according to the introductory portion of patent claim 1.
It is known to detect the increasing aerosol content (smoke~ in the air5 by means of an open ionization chamber. A radioactive member generates a
ion current in the ionization chamber which current is decreased by the
so-called small ion agglomeration effect if smoke aerosols are present.
Conventional ionization smoke alarms give the alarm through the alarm
line if a predetermined threshold for the ion current or a potential
10 generated thereby (at the measuring electrode) is exceeded or fallen
short. In rec~nt times more and more so-called analog alarms are used
(German Publication Letter 22 57 931, German Disclosure Letter 29 46
507, EP O 070 449~. According to these alarms a corresponding signal for
the evaluation means is generated in response to the analog value of the
- 2 - 2~ L05
respective measuring chamber current.
I~ormally, a fire alarm unit consists of a plurality of fire alarms which
are connected in groups with a fire alarm central office by means of
5 current supply lines and signal lines. The evaluation of the analog
signals makes necessary an associated definite identification signal for
each alarm as well as for its respective measuring value. An output of
analog signals in short time intervalls is necessary in order to
recognize a fire immediately. 5ince a great number of fire alarms is
10 normally connected to a common cable, an agglomeration of signal
results. A high-grade alarm identification word each consisting of a
signal sequence and an identification data word containing the
associated analog value as well as a high-grade cable network are
absolutely necessary for a safe communication to the central office
which is often far away from the alarms (EP O 121 048 or also EP O 070
449). Also in the central office relative high expenses are necessary
for the data process1ng of the plurality of signal sequences (EP O 067
339) .
20 These expenses are made i n order to recognl ze as early as possi bl e
modifications of the measuring chamber current which are not caused by
fire and to avoid false alarm (German Publication Letter 22 57 931 or
German Disclosure Letter 29 46 507).
25 Apart from climatic influences, as for instance temperature, pressure
etc., as well as aging effects, especially of the radioactive member,
the correct operation of such smoke alarms is influenced by
contamination which naturally considerably vary dependent on which
atmosphere the alarm is exposed to. One distinguishes substantially two
30 detrimental effects which are based upon different contamination. If the
- ~ 3 ~ 20~Q105
contamination at the insulation of the structure supporting the
measuring electrode predominates, a reduction of the responsiveness or
even a non-response results on account of leaking currents. In order to
- detect this condition in time solutions have been already proposed
(German Patent Letter 20 29 794, EP 0 033 888, German Disclosure Letter
30 04 753 or German Patent Letter 20 04 584 ) .
- However, if a contamination of the radioactive member predominates, for
instance on account of dirt depositions, a reduction of the measuring
chamber current results on account of a reduction of the movement energy
or of the ionization capability of the radioactive radiation; the
ionization smoke alarm becomes more sensitive with respect to smoke. If
the continuing contamination of the radioactive member is not
recognized, a false alarm results if corresponding precautions are not
taken.
Several solutions have been already suggested for detecting this highly
critical condition of an alarm very early. So, for instance, with
conventionally operating threshold alarms one or a plurality of
additional pre-alarm thresholds are provided which give alarm already at
relatively small chamber current decrease (Swiss Patent Letter 629 90S
or Swiss Patent Letter 574 532). In order to check the function of the
ionization smoke alarms from the central office or to determine the
actual responsiveness or, more precisely stated, to determine the
voltage difference which has to be overcome for giving alarm at the
measuring electrode, it has been already proposed to continuously or
stepwisely increase the voltage at the outer electrode of the measuring
chamber either (German Publication Letter 20 19 791, German Patent
Letter 202 764 or German Patent Letter 20 S0 719). Furthermore, it is
30 already suggested in German Disclosure Letter 21 21 382 to evaluate only
_ 4 201~105
such modifications of the medsuring chamber current which extend about
longer periods for a discrimination whether smoke or for instance dirt
is the reason for a chamber current variation. Here, Yery slow
- variations of the current are attributed to the influence of dirt.
5 Furthermore, in the last-cited publication also the installation of a
radiation detector is proposed with which the radioactivity is directly
measured in order to be able to immediately identify variations of the
ionization capacity. In the same publication the installation of
assisting electrodes is described either in order to be able to better
10 recognize or compensate an increase of the insulation leaking current.
It is also known from EP 0 121 048 to provide each ionization smoke
alarm with so-called noise levels. Here, additional thresholds are
formed below the alarm threshold, and additionally a superimposed
long-time drifting is taken into account. A comparable method has also
become known with analog alarms (EP 0 070 449).
Furthermore, it has become known from EP 0 067 339 to use modificationsof the static current of the measuring chamber caused by varying
20 environmental conditions as criterion whether the alarm is at all in a
correct oper ati on al condi t i on .
However, all the known methods do not show any way which, in a
sufficiently safe manner, allows a discrimination whether dirt
25 depositions on the radioactive member or floating smoke aerosols are the
reason for a reduction of the measuring chamber current. The reaction of
an alarm at so-called pre-alarm thresholds makes necessary an
eximination whether a fire develops which has to be carried out directly
by a person, i. e. an extenslve alarm organization is produced with a
30 responsible user. Indeed, in most of the cases a contamination 1s the
20101~
5 23327-126
reason for the triggering of the pre-alarm threshold, however,
the danger is present that the attentlveness is reduced thereby or
that at least a great loss of confidence is caused. Fire alarms
suppressing an alarm at a relatively slow variation of the
measuring chamber current bring along the danger that they detect
slowly smouldering fires very late or that they do not detect them
at all. A considerable short-time contamination or for instance a
dewing of the radioactive radiators cannot be discriminated by
means of this method from a current variation in the measuring
chamber caused by a fast smoke increase.
On principle, also the known analog systems have these
above-cited deficiencies. Also with comparable high technical
efforts only few of the actually existing defects which are
slmulated by contamination can be detected. With the most known
solutions concerning analog alarms either contamination or aging
i~ imputed if the measuring chamber current values change very
slowly or a rather worthless evaluation of the variations of the
measuring chamber current o~curring during normal operation is
carried out.
It is the ob~ect of the invention to provide a method of
operating a ionization smoke alarm with which lt can be recognized
in a safe manner whether the modification of the measuring chamber
current is caused by the entering of smoke aerosols or by
contamination or other deteriorations of the radioactive source.
According to a broad aspect of the invention there is
provided a system for detecting smoke, said system comprising:
a first device, said first device comprising:
a chamber open to ambient atmosphere, said chamber including:
,,. ~
. .
` ~ 201~1~5
5a 23327-126
a radloactive source within said chamber for lonization of a
selected portion of said atmosphere within said chamber; and
a first pair of electrodes ~paced apart;
a supply circuit connected to said first pair of electrodes
suited for connection to an electrical energization source for
providing at least temporarily a first voltage across said first
pair of electrodes to establish a first fleld strength
therebetween, said supply circuit having a supply circuit
parameter value for a supply cirauit parameter having a
characteristic variation based on environmental factor variation
within said chamber;
an initiation circuit for measuring said supply circuit
parameter value with said first field strength present and
generating an initiation signal if said supply circuit parameter
value reaches a selected first reference value; and
means for determining a characteristi~ of environmental
factor variation within said chamber, including:
means for providing a second field strength occurring at
least temporarily within said chamber and
determination circuit means, for producing a determination
signal indicative of environmental factor variation within said
chamber based on a comparison of (a) a determination circuit
parameter value of a determination circuit parameter responding to
said second field strength to exhibit a characteristic variation
ba~ed on environmental factor variation within said chamber with
~b) a selected second reference value.
According to another broad aspect of the invention there
is provided a method for detecting smoke occurring in a chamber
` ~ 2~
5b 23327-126
open to ambient atmosphere, sald chamber containing a radioactive
source for ionization o~ a selected portion of said atmosphere
within said chamber and at least two electrodes spaced apart, said
electrodes having a supply circuit connected thereto, said supply
circuit having a supply circuit parameter value of a supply
circuit parameter having a characteristic variation based on
environmental factor variation within said chamber with said first
field strength present, said supply circuit having a determination
circuit means provided therewith, said determination circuit means
having a determination circuit parameter value of a determination
circuit parameter having a characteristic variation based on
environmental factor variation within said chamber with a second
field strength present, said method comprising:
applying voltage across said electrodes to form first and
second field strengths at least temporarily therebetween said
supply circuit;
measuring said supply circuit parameter value with said f irst
iield strength present and said determination circuit parameter
value with said second field strength present between said
electrodes; and
producing an initiation signal if said supply circuit
parameter value in the presence of said first field strength
reaches a first selected value and producing a determination
signal i~ said determination circuit parameter value reaches a
second selected value with said second reference value.
According to the inventive method one uses the
perception that the measuring chamber current, upon a change of
the field strength, will get
- 6 - 20~6~105
different values dependent on the fact whether a current reduction has
been caused for instance by contamination and thus part coverage of the
radioactive member or by the entering of smoke aerosols. Independent of
the degree of contamination upon change of the voltage reduction by the
5 increase or the decrease of the applied supply voltage a measuring
chamber will have another behaviour than if floating smoke aerosols
would be present in the measuring chamber. That is, according to the
agglomeration law of Schweitler (German Publication Letter 12 53 277)
the relative change of the ion concentration is a function of the
10 duration time of the ions in a respective volume element. However, the
ion duration time depends on the electric field strength. With other
words, with increasing field strength in the ionization chamber the
relative change of the measuring chamber current will become more and
more less at the same smoke density. At the same smoke density, with
15 smaller field strengths (for instance in the order of a few V/cm) a
percentally greater reduction of the measuring current is the result
compared with the reduction at higher field strengths. The reason for
this is the agglomeration capability with regard to aerosols which
become s smal 1 er wi th i ncreasi ng f i el d strength .
On account of the above-cited perception a plurality of embodiments of
the inventive method is possible. The inventive method can be applied
not only to an arrangement consisting of one or two ionization chambers
but also to a system working with thresholds or analog values. A
25 relative simple embodiment of the invention can work in the following
manner .
With a non-saturated ionization chamber working in a field strength
range of a few Volt/cm which is favourable for the agglomeration process
30 of ions to smoke aerosols, a defined change of the field strength is
7 2~
carried out upon reaching a predetermined change of the measuring
chamber current. lf smoke aerosols are the reason for the occurence of
the field strength change, a col,~spo"ding new (modified) chamber
- current will occur in accordance with the agglomeration law. However, if
5 for instance the field strength has become considerably higher, it is no
more optimal for the agglomeration of ions, and a correspondingly
smaller value for the chamber current will occur. However, if the
deposition of dirt or a humidity film on the radioactive member is the
reason for the chamber current variation, in the case of the field
lû strength increase a considerably greater modification of the ionization
current will result if the other conditions are the same. On account of
the evaluation of the chamber current values occuring at the different
field strengths a discrimination is possible whether a fire alarm has to
be given or whether for instance only a cleaning of the corresponding
1s fire alarm is necessary. Accordingly, by the invention a false alarm on
account of contaminated or dewed radioactive radiators can be avoided.
Furthermore, the inventive method allows a field strength change in
defined time intervalls in order to be able to already determine a
20 slight contamination and, if necessary, to cause a corresponding
correction of the responsiveness to smoke. Here, the use of the method
is possible not only in ionization smoke alarms working in an analogous
manner but also in such working as threshold alarms. So, the field
strength change-over and the evaluation can be also carried out not
25 before reaching one or a plurality of different changes of the chamber
current. Dependent on the degree of the determined contamination either
a correction of the alarm threshold at slight contamination or a service
request from a certain contamination degree on can be induced or failure
of the fire alarm can be signalized at high contamination. By means of
30 the inventive method even different smoke densities can be recogn~zed in
20~105
-- 8 --
order to induce corresponding pre-alarms and alarms. However, the
detection of different smoke densities is also known in the prior art.
If one emanates from a field strength favourable for smoke
5 agglomeration, upon an increase of the field strength in the presence of
smoke, as described above, a relatively smaller change of the ion
chamber current will result than if dirt depositions on the radioactive
member were the reason for reaching the original chamber current change.
~owever, if one carries out a decrease of the field strength under
lO identical starting conditions, smoke will result in a greater chamber
current change than a dirt deposition on the measuring chamber radiator.
For carrying out the inventive method it is necessary that the
characteristic curve in the measuring chamber (chamber current in
15 relation to the chamber voltage) is at least known point by point. In
order to determine the change of the potential at at least one further
field strength one can for instance refer to a potential value which a
measuring chamber has in its new condition. For example, the reference
values can be directly derived by measuring the new ionization chamber
20 or from its data.
~3
rf the characteristic curve runs favourably sometimes measuring of the
potential at only one second field strength is sufficient to make a
statement whether the measured potent1al change has the reason in the
25 presence of smoke aerosols or dirt depositions on the radioactive
radiator. Preferably, a measurement of the potentials at the measuring
electrode is done for at least one field strength above and at least one
field strength below the first field strength (operation field strength)
in order to be able to carry out a safe evaluation. As already
30 mentioned, the checking of an ionization smoke alarm with regard to
... .. .. ..
9 2~Q:l~S
contamindtion can be started for instance if a chamber current reduction
and thus a potential increase has occured. Alternatively, the
examination can be carried out according to a fixed time schedule which,
above all, is advantageous if, as in systems working in an analogous
5 manner, the evaluation of the data is not carried out in the individual
smoke dlarms but in a central office.
According to the invention a possibility of carrying out a measurement
at another field strength consists in associating the check circuit with
lO a switch-over device which changes the field strength in the measuring
chamber by applying different supply voltages. Alternatively, it can be
provided that in the measuring chamber, by a specific structure of the
same, at least two different field strength ranges are always formed in
the same. For this, an embodiment of the invention provides that the
15 measuring chamber contains at least two pairs of electrodes which are
connected to different voltages and that the measuring electrodes of the
two pairs are connected to the check circuits. Alternatively, the
measuring chamber can have at least two separated measuring electrodes
connected to the check curcuit as well as a common counter electrode.
20 The counter electrode includes two electrode portions associated with
the measuring electrodes, sa1d portions being differently spaced with
respect to the associated measuring electrodes. If a predetermined
voltage difference with regard to the normal condition is reached with
the chamber range working in the smaller field strength range or with
25 the measuring electrode assoc~ated therewith, also in the range working
with the higher voltage a voltage difference corresponding to the field
strength can be observed if smoke aerosols have an effect. However, if a
dirt deposltion on the radioactive member is the reason for the
potential change in the one chamber range, in the other range a voltage
30 change w;ll occur in a correspondingly significant manner. with the
- 10- ZC~10~;
last-cited construction poss~ble deviations are primarily dependent on
the design of the transition ranges of the measuring chamber, especially
on the field strength acting there.
In the following the invention is described in more detail in connection
- with drawings.
Figure 1 shows a current-voltage-diagramm of an ionization smoke
alarm for different conditions.
Figure 2 shows a similar diagramm as figure 1 with additional
characteri sti c curves .
Figure 3 shows a similar diagramm as figures 1 and 2, however, with
the use of an ohmic resistance as reference for the
measuring chamber.
Figure 4 shows a sectional view of an ionization chamber arrangement
according to the invention with different field strength
ranges.
Figure 5 shows another ionlzation chamber arrangement with different
field strength ranges.
5 Figure 6 shows a block diagramm for the operation of an ionization
smoke alarm according to the invention.
Figure 7 shows in a detailed manner the functional flow for the
control and evaluation logics of the block diagramm
according to figure 6.
Before the details shown in the drawings are commented on, it is to be
pointed out that each of the descr~bed features per se or connection
with features of the specification can be of inventive importance.
5 Figure l shows characteristic curves of a chamber of an ionization smoke
alarm according to which an ionization measuring chamber which is freely
accessible for the ambient air and a closed ionization reference chamber
are connected in series. Each of the chambers includes a radioactiYe
member. The chamber voltage UK is shown on the abscissa while the
lo chamber current IK is shown on the ordinate. The characteristic curves
with solid lines show the course of the characteristic curves of the
measur1ng chamber in its new condition MK (new) and in the presence of
smoke NK (smoke) of predetermined constant density. The dash-dotted
characteristic curves RK show the course of the characteristic curves of
the reference chamber. The dotted characteristic curve MK (contaminated)
shows the course of a characteristic curve at a significant
contamination of the radioactive member in the measuring chamber.
On the assumption of a normal voltage UN applied across both chambers a
20 voltage potential according to the po~nt of intersection C results at
the common measuring electrode. If during operation a potential
displacement is observed at the measuring electrode, for ~nstance for
the voltage difference X, a point of intersection D is reached. Now, for
the provision of another field strength the chamber voltage is varied
25 according to the invention, for instance by going down to the voltage
Upl. In the new condition of the measuring chamber the working point A
would result at the measuring electrode. However, if smoke has been the
reason for the potential change X the lowered characteristic curve MK
(smoke) becomes valid. Accordingly, the potential B will result at the
.. . .. ... . . ...
20~0~
- 12 -
measuring electrode upon reduced chamber voltage. The potential
difference between A and B is al. HoweYer, if dirt on the radioactive
member has been the reason for the potential change X, the measuring
chamber characteristic curve MK (contaminated) becomes valid and a point
5 of intersection K resu1ts, i. e. only the potential difference bl is
- reached.
However, ~f after occurence of the voltage difference X it is switched
to a higher chamber voltage Upz, in the new condition the potential L
lO would result at the measuring electrode. However, if smoke is in the
chamber the point of intersection M results, i. e. the potential
difference a2. Now, this can be evaluated for the smoke detection.
However, if a contaminated radiator has been the reason for the
potential change X at the nominai voltage the po1nt of intersection N
15 would result at the higher check voltage. This high potential difference
b2 would not be suited for a safe detection of dirt. The very high
potential differences result from the fact that the characteristic
curves are substantially located in the saturation range at the higher
chamber voltage.
However, one can already discriminate whether smoke or dirt has been the
reason for the potential reduction at nominal voltage upon reduction of
the chamber voltage to smaller evaluatable potential differences with
regard to the nominal voltage. With the selected courses of the
25 characteristic curves and points of intersection not only higher
potential differences but also sign~ficant differences with regard to
the reason of the chamber current reduction or potential change result
with the increase of the chamber voltage. Furthermore, one recognizes
that the ratio of the potential differences al:bl is larger than l at
30 low chamber voltage. Compared with this, the ratio of the potential
.. _ . . .. . . .
-- 13 --
differences a2:b2 iS smaller than 1 at a higher check voltage than
normal voltage. If one emanates from a medium normal chamber voltage, a
deposition of dirt has a smaller effect than smoke at a smaller check
voltage. However, at a higher check voltage dirt has a significantly
5 higher effect than smoke in the measuring chamber. As already mentioned,
- high potential differences result upon saturation conditions ~n the
chambers, espec~ally at the shown check voltage Up2, which enable an
exact evaluation of the respective smoke density or a clear
discrimination whether dirt is present on the radiator.
The diagram according to figure 2 is substantially the same as figure 1.
However, it shows a mere detailed evaluation possibility according to
the inventive method. The solid lines MK (new) and MK (smoke) as well as
the dotted line MK (contaminated) correspond to the lines according to
15 figure 1. An additional characteristic curve characterizes the measuring
chamber at MK (little smoke) at predetermined identical smoke density
during the measurement. An additional characteristic curve MK (little
dirt) characterizes the measuring chamber at smaller dirt depositions on
the radioactive member. The course of the reference chamber
20 characteristic curves is fdentical with the same according to figure 1.
If not much smoke is in the measuring chamber the potential difference y
results at the measuring electrode. This can be the reason for switching
over to a higher check voltage Up2. If smoke has been the reason for the
25 potential change y, the measuring electrode voltage will displace from
the point of intersection L to the polnt of intersection P which causes :=
a potential change d at the measuring electrode. However, if dirt has
been the reason the measuring chamber characteristic curve gets the
cited course MK (little dirt). Emanating from the point of intersection
30 at UN which is reached after the occurence of the potential difference
~ 2~10~
- 14
y, the potent1al of the measuring electrode at the check voltage Up2 is
shifted to the point of intersection R. Now, the potential difference
from the point L to the point R reaches the higher value f through the
effect of dirt instead of the difference d through the effect of smoke.
5 The potential difference d can serve as pre-alarm for little smoke, and
upon occurence of the potential difference f the same can be evaluated
as hint for a necessary cleaning of the ionization alarm.
After having finished the evaluation the chamber voltage can be switched
lO back to is nominal value UN. However, if during the operation the
potential difference at the measuring electrode becomes higher and, for
instance, reaches the value x, it is again switched over to the higher
check voltage Up2. As already described in connection with figure l,
now, upon the presence of smoke, the potential difference a2 will result
15 for an alarm evaluation or the potential difference b2 will result for
the deposit10n of dirt. b2 points to a considerable contamlnation of the
alarm and, at a very high degree of contamination, can be eYaluated as
hint for a smoke alarm which i5 no more completely functionable.
20 The diagram according to figure 3 is based upon an arrangement of
chambers according to which the ionization reference chamber is replaced
by an ohmic resistance. The resistance line passing the point UN
1ntersects th~ new measuring chamber line in point U. If a potential
difference z is achieved by a change of the chamber current a switching
25 to the low chamber voltage Upl results. Now, the point of intersection P
with the characteristing curve MK ~smoke) results through the influence
of smoke. The potential difference ml is reached. Upon the influence of
dirt the characteristic curve of the measuring chamber gets the course
MK (contaminated) which is shown by a dashed line. The resistance line
30 at Upl intersects the dashed characteristic curve of the measuring
, .. .. ... . . ...... .. . . . .
0
chamber in point Q. Now, the potential difference gets the value rl.
Upon switching to a higher check voltage Up2 the point of intersection T
and the potential difference m2 result upon the influence of smoke.
However, upon the influence of dirt the measuring electrode potential is
5 displaced to the point of intersection S, and the potential difference
r2 iS measured. The differences determined according to the arrangement
of figure 2 are smaller than the potential differences received
according to figure l, however, also in this case the ratio ml:rl
exceeds l (check voltage Upl. At the check voltage Up2 the corresponding
lO ratio m2:r2 iS smaller than l. Accordingly, an unambiguous evaluation
whether dirt or smoke caused the chamber current change can be carried
out .
In order to make a very detailed and safe discrimination with regard to
15 the smoke density and the degree of contamination it can make sense to
additionally switch over to a reference chamber (characteristic curve
RK). Now, with identical chamber conditions the points of intersection
L, M and N and thus the potential differences a2 and b2 would be suited
for a very exact evaluation.
Furthermore, a defined course of the characteristic curve can also be
adjusted by means of a resistance combination and poss~bly a reference
chamber in order to get potential differences with which primarily
either the influence of smoke or the influence of dirt can be evaluated
25 in a prefered manner.
As already mentioned, the evaluation whether smoke is present in the
alarm or whether a contamination 1s present can be done within the alarm
itself or at a central location. If the evaluation ~s made at a central
30 location it can be advantageous to also carry out a change of the
2Q~ 05
-- 16 --
electric -ield strength from a central location, for example by changing
the supply voltage from line to line. However, if one selects an
embodiment according to which the ionization chambers and the circuit
are disposed within a common housing, it is useful to carry out the
5 check process with each alarm dependent on its respective measuring
- chamber condition. In order to be able to carry out a check automaticly
only at a specific smoke alarm working at the same alarm line, i. e.
voltage supply line, and to leave the other alarms in the condition of
supervision, changing over the voltage or varying the field strength is
10 practically carried out only in the alarm which has to be checked. It is
a matter of course that the necessary change-over possibilities and the
necessary evaluation and signal components are contained in the
electronic circuit of the alarm.
15 The above-described method has the advantage that it can be carried out
with conventional ionization chambers. However, if it is necessary to
give alarm of a fire developing very rapidly in a very short time the
arrangement which will be described In the following is preferred.
20 In figure 4 an ionization chamber arrangement 10 is shown which consists
of a measuring chamber 11 and of a reference chamber 12. The reference
chamber 12 has a reference chamber electrode 13 and the measuring
chamber 11 includes a outer measuring chamber electrode 14. Both
chambers 11, 12 have a common measuring electrode 15 as well as an inner
25 measuring electrode 16 which are separated from one another by a
suitable insulation 17. Radioactive radiators are located on both sides
of the inner measuring electrode. The arrows in the chambers 11 and 12
are to show the range of action of the radioactive radiators. ~he
electrodes 13, 15 and 16 are formed plainly. However, the outer
30 electrode 14 is formed stepwisely in a cup-like manner with a central
..... .. .. ...
- 17-
portion 18 and a portion 19 annularly extending around the central
portion. These portions are connected through a substantially axial
annular wall portion 20. Thereby the central measuring electrode 16
largely cooperates with the central portion 18 of the outer electrode
5 14, and the outer measuring electrode 15 substantially cooperates with
the outer annular portion 19 of the outer electrode 14. Accordingly, in
the measurlng chamber 11 two ranges of different field strengths are
present if one does not take into conslderation the transition ranges of
the field strength. For example, a supply voltage of 12V is applied to
10 the outer electrode 14 and the reference chamber electrode 13. As
mentioned, the field strength in the central range is smaller than in
the outer range since the outer electrode 14 or the portion 19 has a
smaller distance to the outer measuring electrode 15 than the central
portion 18 to the inner measuring electrode 16. If, according to a
15 chamber arrangement of figure 4, the depositon of dirt on the
radioactive radiator of the measuring chamber 11 is the reason for a
change at the inner measurlng electrode 16 which is operated in the
range of the smaller field strength, a deviating potential will occur at
the outer measuring electrode 15 which works in the range of the higher
20 field strength. If one compares this with figure 1 and if the potential
at the inner electrode 16 would have been displaced from the operation
point C to point D, than at the outer electrode 15 the potential L is
displaced to point N. In this example which serves for the clarification
of the method the balance currents flowing on account of the potential
25 difference between the measuring electrodes have not been taken into
account. However, if smoke is the reason for the potential decrease
changed values occur at the electrodes 15, 16 since the agglomeration of
ions at smoke aerosols is better in the range of a smaller field
strength than in the ranges of higher field strength. The conditions
30 shown in figure 1 to 3 can be used in a corresponding manner.
.. . . ... .... .. _ . . _ ... . ... .. .. .
- 18- 2~
Such a chamber arrangement has the advantage that time de~ays after
changing over to one or a p1urality of different field strengths on
account of the respective transient effects can be avoided.
5 The chamber arrangement 15 shown in figure 5, in its essential parts, is
the same as that of figure 4. A measuring chamber 26 and a reference
chamber 27 are radially spaced from one another by an outer measuring
electrode 28 and an inner measuring electrode 29 which are separated by
an insulation 3û. The inner measuring electrode 29 has on both sides a
lû radioactive radiator, respectively. The arrows show the range of action
of the radiation. The reference chamber 27 includes a reference chamber
electrode 31 and the measuring chamber 26 has an outer electrode which
is ~ormed by an inner part electrode 32 and an outer part electrode 33
which are insulated from one another by an annular insulation 34. The
15 inner part electrode 32 is also plain as the measuring electrodes 28, 29
and the reference chamber electrode 31. A part of the outer part
electrode 33 is also plain. This part is joined by a cylindrical portion
by which the chamber 26 is terminated. Now, another voltage is applied
to the central part electrode 32 than to the outer part electrode 33
20 whereby two ranges of different field strength result in the measuring
chamber 26 (again the transition ranges not taken into account). The
central measuring electrode 29 is substantially assoclated with the
central part electrode 32 while the annular outer measuring electrode 28
is associated with the annular part electrode 33.
Applied to the example of figure 1, the supply voltage can be UN and the
other can be Up2. Also in the range operating with the higher voltage
Upl a voltage difference corresponding to the field strength can be
observed if a predetermined voltage difference with regard to the normal
.. . , . .. . . _ . ....
~a~
- 19 -
condition is reached in the chamber range or the associated measuring
electrode operating in the smaller field strength range upon the
influence of smoke aerosols. However, if the deposition of dirt on the
radioactive member is the reason for the potential change in the one
5 chamber range, in the other range a voltage change will occur in a
- correspondingly significant manner.
In the figures 4 and 5 it was presupposed that the inner and outer
measuring electrodes are at the same electrical potential in their new
condition at the normal operating voltage. This can be achieved by a
corresponding geometrica1 dimensioning of the measuring chamber ranges
operated with different field strengths, for example by the selection of
mating measuring electrodes surfaces, chamber volumes as well as also by
the number of ion pairs in the two measuring chamber part ranges formed
15 by the radioactive radiation. If different potentials occur at the two
measuring electrodes during the operation by the influence of smoke or
dirt a corresponding change of the electrical field results. Thereby the
presence of balance currents is favoured, especially in the range around
the electrical insulation between part measurlng electrodes. These
20 balance currents result in a reduction of the potential differences and
have to be taken into account when the measuring thresholds are fixed.
In figure 6 a conventional ionization chamber arrangement 40 is shown.
The chamber arrangement consists of a measuring chamber 41 and a
25 reference chamber 42 connected in series therewith, the common inner
electrode or measuring electrode 43 bearing a radioactive radiator at
both sides. By means of a switch 44 the chamber arrangement 40 normally
is applied to the normal operation voltage UN (block 45) or to a check
voltage Up (block 46a). A comparator 47 is connected to the measuring
30 electrode 43 by means of an electronic circuit 46 which preferably
20~Q~O~ii
- 20 -
contains a field effect transistor. Four threshold stages are provided
within the comparator 47, namely alarm threshold 48, dirt threshold 49,
pre-alarm threshold SO and test threshold 51. Control and evaluation
logics 52 are connected to the output terminal of the comparator 47. One
5 output terminal thereof is connected to a pre-alarm signal stage 53 for
smoke, one is connected to a contamination signal stage 54 and one to an
alarm signal stage 55.
The shown circuit functions in the following manner. During the normal
lo operation voltage UN only low field strengths of a few V/cm are
effective for the ion transport in the chambers 41 and 42. The potential
occuring at the measuring electrode 43 is supplied to the comparator 47.
If the potential reaches the test threshold 51, for instance potential O
in figure 2, a corresponding signal is fed to the control and evaluation
15 logics. A switch 44 is operated by the same and is switched oYer to a
higher check voltage Up2 (46a). If a potential R occurs during the check
time at the higher voltage or the higher electric field strength the
comparator responds with its dirt threshold value, and a contami-
nation signal is produced in stage 54 by means of the control and
20 avaluation logics. However, if this potential is not reached but the
potential P in place of that the pre-alarm threshold 50 is reached by
means of the comparator 47 and a pre-alarm signal is output by means of
the control and evaluation logics 52. This signal means that a small
smoke density is present. The control and evaluation logics of the alarm
25 40 is left in this condition in order to immediately give alarm (alarm
signal stage 54) at a further smoke increase after having reached the
alarm threshold 48. However, if within a predetermined time the alarm
threshold is not reached or the potential P is again fallen short
(towards the normal value L), the alarm is again switched back to its
30 normal supervising condition with the supply voltage UN. However, if the
..... .
- 21 - 2~
test threshold potential 0 should be again reached, a new test cycle is
produced .
The function of the control and evaluation logics 52 is shown in figure
5 7 in a more detailed manner. If the test threshold 51 is reached (figure
- 6~ a storage 60 is set and a control signal is applied to the switch for
the voltage switch-over (line 61). In order to introduce a further
evaluation of the measuring electrode potentials not before the
transient state caused by the voltage switch-over, a delay element Tvl
starts to work. This element is connected to the contamination threshold
43 by means of the line 62. If the signal (potential R in figure 2)
responding to the contamination is present after termination of the
delay time also a signal from the storage 60 corresponding to the higher
voltage Up2 is present at the gate G1 as second AND-condition. A signal
15 is sent to the output terminal 64 signalizing contamination, and a
contamination signal (stage 5~; see also figure 6) is produced. If the
threshold (contamination; potential R in figure 2) should not have been
reached after expiration of the delay time a gate G2 receives an inverse
signal. Furthermore, a signal from the storage 60 characterizing the
20 higher operation voltage is present at the gate G2 either. A gate G2
triggers a delay element TV2 the time constant of which is larger than
that of the delay element TVl After expiration of the time from T 2 the
observation time is started by a timer Tv3. If the alarm threshold at
the higher check voltage is reached within the observation time the
25 conditions of a gate G3 are fulfilled. A signal is fed to the alarm
output terminal 65 and thus the alarm signal is given (stage 55; see
also figure 6). However, if the alarm threshold is not reached during
the observation time but the potential P representing a small smoke
density is present the conditions for a gate G4 are fulfilled, and a
30 signal is fed to the pre-alarm output terminal 63 and a pre-alarm signal
. . . . . . .. . . ....
- 22 - 2~
is produced ~stage 53; see also figure 6). If then during the
observation time a further potential displacement caused by smoke
increase should not occur, a signal is fed to a timing element My by the
delay stage Tv3. This timing element bridges the transient stage which
5 occurs due to the resetting into the supervising condition at low supply
voltage. Simultaneously the storage 60 is reset. Again, the alarm
functions under normal conditions. However, if the test threshold 51 is
again reached a new check cycle follows. It is self-evident that
e~uivalent threshold values which are stepped in a finer manner can be
used with an extended check.
It is not necessary for carrying out the method that complete control,
evaluation and signal electronics as described-above are individually
associated with each ionization fire alarm. At least a part of the said
15 electronics can be located in the supervision central office so that it
can be connected to the respective alarm which has to be checked either
in a predetermined order or after having reached predetermined chamber
current changes for the evaluation according to the method.
