Ionizing smoke sensor

The present invention relates to an improvement of an ionizing smoke sensor having one radiation source and a two chambered structure consisting of an internal ionization chamber housing an internal electrode having a radiation source and defined by an intermediate electrode, and an external chamber where smoke can flow in, defined by an external electrode and irradiated from the radiation source through an opening in the intermediate electrode. Hitherto, in an ionizing smoke sensor having one radiation source and a two chambered structure, there has been a problem in that the giving of misinformation and noinformation occur when a little amount of thread, moisture or dust invades or a current of air flows an internal ionization chamber through an opening and an external ionization chamber. In addition, the electric field in the internal ionization chamber loses its homogeneity because of the opening, hence V-I characteristics preferable for the internal ionization chamber can not be obtained. Furthermore, stable V-I characteristics can not be obtained since hardly any smoke can hardly flow into the internal ionization chamber because of the mesh structure. In the ionizing smoke sensor of the present invention, the above described problems are solved by forming the opening part in the intermediate electrode with a mesh structure having a rate of porosity more than or equal to 50%.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ionizing smoke sensor having one 
radiation source and a two chambered structure comprising of an internal 
ionization chamber housing an internal electrode having a radiation source 
and defined by an intermediate electrode, and an external chamber where 
smoke can flow in, defined by an external electrode and irradiated from 
the radiation source through an opening in the intermediate electrode. 
2. Description of the Related Art 
Hitherto, an ionizing smoke sensor having one radiation source and a two 
chambered structure, for example, the sensor shown in FIG. 8, has been 
known. 
In FIG. 8, an internal ionization chamber 1 (internal chamber) and an 
external ionization chamber 2 (external chamber) are formed in a body 
cover 20 and an outer cover 30. 
The internal ionization chamber 1 is defined by an insulating member 3 and 
an intermediate electrode 4 and so constructed that smoke can hardly flow 
in. An internal electrode 6 having a radiation source 5 which generates 
pairs of ions is housed in this ionization chamber 1. 
The external ionization chamber 2 is defined by an external electrode 7 
which covers the outside of the internal ionization chamber 1 and the 
intermediate electrode 4. A smoke inflow entrance is installed in the 
external electrode 7 so that smoke can flow in when a fire occurs. 
As shown in FIG. 9, an opening 8 is mounted on a part of the intermediate 
electrode 4 dividing the internal ionization chamber 1 from the external 
ionization chamber 2 facing the radiation source 5. The radiation 
irradiated from the radiation source 5 can irradiate the external 
ionization chamber 2 through this opening 8 and ion pairs can be generated 
in the external ionization chamber 2. 
The operation of this ionizing smoke sensor will be explained as follows. 
When voltage is applied between the external electrode 7 and the 
intermediate 4 and between the intermediate electrode 4 and the internal 
electrode 6, a faint electric current flows due to the movement of the ion 
pairs generated by the radiation source 5 from the minus pole to the plus 
pole due to an electric field generated in each of the ionization chambers 
and V-I characteristics which generate a specified voltage between the 
electrodes are obtained. 
When the smoke flows into the external ionization chamber 2, the ion 
electric current flowing between the intermediate electrode 4 and the 
external electrode 7 decreases and the voltage between the electrodes 
rises because the mobile speed of the pairs of ions decreases because of 
the attachment of ion pairs to smoke particles. Then the smoke sensor 
detects that the smoke concentration has reached a specified value based 
on the change in the V-I characteristics of the external ionization 
chamber 2 and sends a fire detecting signal. 
However, in this conventional ionizing smoke sensor, since the relatively 
large opening 8 through which the radiation passes is installed in the 
intermediate electrode 4 to generate the specified ion pairs in the 
external ionization chamber 2, a problem occurs, namely, a little amount 
of thread, moisture, dust and so on which flows into the external 
ionization chamber 2 invade the internal ionization chamber 1 through the 
opening 8 and cause misinformation to be given, or a current of air which 
flows into the internal ionization chamber 1 from the external ionization 
chamber 2 through the opening 8 carries away the ion pairs, fluctuates the 
reference voltage and causes sensitivity change to be given. 
In addition, since the part where the opening 8 is formed does not have the 
same function as the intermediate electrode, problems occur, namely, the 
electric field is hardly formed in the part of the internal ionization 
chamber 1 facing the opening 8 and hence a sensor having the internal 
ionization chamber 1 with good V-I characteristics can not be obtained. 
Conventional ionizing smoke sensors are described in U.S. Pat. Nos. 
4,361,763 and 3,935,492. 
Only a bar or a centerpiece is mounted on the opening hole of the 
intermediate electrode in the smoke sensor of U.S. Pat. No. 4,361,763, and 
this intermediate electrode is essentially the same as the intermediate 
electrode shown in FIG. 9 and has problems similar to those of the 
intermediate electrode shown in FIG. 9. 
Although U.S. Pat. No. 3,935,492 discloses a structure having a meshed 
intermediate electrode, since the whole body of the intermediate electrode 
is meshed, there are problems in that it is easily affected by a current 
of air and the electric field is easily fluctuated by ion pairs being 
carried away. 
SUMMARY OF THE INVENTION 
The present invention has been devised to solve these above described 
conventional problems. The purpose of the present invention is to provide 
an ionizing smoke sensor which can prevent the disordering of ion pairs 
caused by the invasion of a little amount of thread, moisture, dust and so 
on into the internal ionization chamber and the inflow of a current of air 
and which can satisfactorily improve the V-I characteristics of the 
internal ionization chamber. 
To accomplish the above purpose, there is provided in the present invention 
an ionizing smoke sensor comprising an internal ionization chamber housing 
an internal electrode having a radiation source and defined by an 
intermediate electrode, and an external ionization chamber where the smoke 
can flow in from the outside formed at the outside of the internal 
ionization chamber and defined by an external electrode, the internal and 
external chambers being formed in the cover, and radiation being 
irradiated from the radiation source to the external ionization chamber 
through an opening part installed in a part of the intermediate electrode 
facing the radiation source, the opening part of the intermediate 
electrode being made of a mesh structure with a rate of porosity more than 
or equal to 50%. 
In addition, according to the preferred embodiments of the present 
invention, the mesh structure is formed by attaching the meshed opening 
plate to the opening part or by stamping the center part facing the 
radiation source of the intermediate electrode to form a meshed opening 
part. 
In the ionizing smoke sensor of the present invention having this 
structure, the invention of a little amount of thread, moisture, dust and 
so into the internal ionization chamber through the opening part which 
causes misinformation to be given can be satisfactorily prevented, since 
the opening part of the intermediate electrode has the mesh structure. 
In addition, since the opening part is defined as the mesh structure with a 
rate of porosity more than or equal to 50%, the external ionization 
chamber can be almost similarly irradiated with radiation to the 
irradiation through the opening. 
Furthermore, the electric field can be effectively generated at the opening 
part of the internal ionization chamber by making the opening part with 
the mesh structure, and as a result, the V-I characteristics of the 
internal ionization chamber can be improved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be explained as 
follows, together with reference to the figures. 
As shown in FIGS. 1 and 2, an ionizing smoke sensor of the present 
invention has a body cover 20 mounted on the side of ceiling surface and 
an outer cover 30 removably mounted on the body cover 20 and having smoke 
inflow entrances surrounding the outer cover 30. An internal ionization 
chamber 1 and an external ionization chamber 2 are formed in this body 
cover 20 and the outer cover 30. 
The internal ionization chamber 1 is defined by an insulating member 3 and 
an intermediate electrode 4, and an internal electrode 6 whose lower end 
is caulkingly fastened to a radiation source 5 is housed in this internal 
ionization chamber 1. The internal electrode 6 is supported by the 
insulating member 3 and is electrically connected to a circuit board 9 
positioned at the upper part of the sensor. 
An external electrode 7 having a surrounding smoke inflow entrance 7a where 
smoke can flow in from outside is installed in a position covering the 
outer side of the internal ionization chamber 1 defined by the insulating 
member 3 and the intermediate electrode 4, and the external ionization 
chamber 2 is defined by this external electrode 7. Needless to say, both 
the intermediate electrode 4 and external electrode 7 are supported by and 
fastened to the insulating member 3. 
An opening 8 is mounted on the part of the intermediate electrode 4 facing 
the radiation source 5 of the internal electrode 6 housed in the internal 
ionization chamber 1, and a meshed opening plate 10 having many mesh holes 
is fastened to the lower side of the opening 8 by, for example, spot 
welding and so on. 
As the plan view of the intermediate electrode 4 seen from the lower side 
shows in FIG. 3, the opening 8 is given a mesh structure by installing the 
meshed opening plate 10 having many hexagonal mesh holes 10a in the lower 
side of the opening 8. In addition, smoke passing holes 4a are 
peripherally placed on the intermediate electrode 4 at specified pitch 
intervals. 4b denotes a mounting hole for the intermediate electrode 4 on 
the insulating member 3. 
The size of the mesh is determined so that the mesh holes 10a of the meshed 
opening plate 10 has a rate of porosity more than or equal to 50% per unit 
area; so that the incident dose of the radiation irradiated from the 
radiation source 5 to the external ionization chamber 2 will not be 
spoiled; to prevent the invasion of a little amount of thread, moisture 
and dust to the internal ionization chamber 1; to properly suppress the 
inflow of the current of air and to effectively generate the electric 
field in the internal ionization chamber 1 facing the opening 8. In this 
embodiment, the mesh holes 10a is determined to have a 65% rate of 
porosity. 
Needless to say, since the meshed opening plate 10 also has the same 
function as the intermediate electrode 4, the same metal plate as is used 
in the intermediate electrode 4 is used in the meshed opening plate 10. 
The operation of the embodiment shown in FIG. 1 will be explained. 
First, many smoke inflow holes 7a are installed in the perimeter of the 
external electrode 7 defining the external ionization chamber 2 to make it 
possible for smoke to inflow from outside and gauze is usually installed 
on the outside of the smoke inflow holes 7a to prevent an invasion of 
insects. However, there is the possibility that a little amount of thread, 
moisture and dust may invade the external ionization chamber 2 through the 
gauze on the mesh. 
However, a little amount of thread and so on invading the external 
ionization chamber 2 can not pass the mesh holes 10a of the meshed opening 
plate 10 because the meshed opening plate 10 is fastened in the opening 8 
of the intermediate electrode 4, thus an invasion by a little amount of 
thread and so on of the internal ionization chamber 1, which could change 
the V-I characteristics of the internal ionization chamber 1 and cause a 
misoperation can be reliably prevented. 
On the other hand, as is obvious from FIG. 2, because the meshes of the 
meshed opening plate 10 are uniformly dispersed throughout the whole of 
the opening 8 and the rate of porosity of the meshed opening plate 10 is 
more than or equal to 50%, the incident dose of the radiation sent from 
the radiation source 5 mounted on the internal electrode 6 housed in the 
internal ionization chamber 1 to the external ionization chamber 2 is 
almostly unchanged compared to when the meshed opening plate 10 is not 
mounted in the opening 8, and a sufficient amount of ion pairs can be 
generated in the external ionization chamber 2 by the irradiation of the 
radiation. 
In addition, the opening having the above mesh structure which acts as the 
electrode is formed on a part of the internal electrode 4 by installing 
the meshed opening plate 10 in the opening 8. As a result, the electric 
field can be effectively generated in the region facing the internal 
electrode 6 of the internal ionization chamber 1 and the opening 8, so 
that the distribution of the electric field in the internal ionization 
chamber 1 between the internal electrode 6 and the intermediate electrode 
4 becomes almost homogeneous and the ion electric current can be increased 
by increasing the total amount of the moving ion pairs. Therefore, the V-I 
characteristics of the internal ionization chamber 1 can be improved. 
FIG. 4 is a diagram showing the principle of the ionizing smoke sensor of 
the present invention. In FIG. 4, Vi and Vo denote the voltage between the 
internal electrode 6 and the intermediate electrode 4, and the voltage 
between the intermediate electrode 4 and the external electrode 7, 
respectively. I, Vi' and Vo' denote the electric current and the voltages 
after changing of Vo and Vi, respectively. 
The V-I characteristics of the conventional sensor shown in FIG. 8 is as 
shown in FIG. 5. When smoke flows in the external ionization chamber 2 at 
the time of a fire and so on, the ion current is decreased by the 
interference of smoke particles and the V-I characteristics of the 
external electrode change as shown by the broken line. At this time, the 
voltage changes by V and this change is judged to be a fire when this 
change V exceeds the specified level. 
In the conventional sensor, since the opening 8 of the intermediate 
electrode 4 does not have the same function as the intermediate electrode, 
the ion electric current flowing into the internal ionization chamber 1 is 
reduced at the portion where the opening 8 is formed, and as a result, the 
gradient of the characteristic curve of the internal electrode 6 is 
enlarged as shown in FIG. 5. Therefore, the voltage change V accompanied 
by the change in the V-I characteristics of the external electrode 7 is 
reduced and stable detection of smoke can not take place. 
In contrast, in the sensor of this embodiment of the present invention, the 
distribution of the electric field in the internal ionization chamber 1 is 
made homogeneous by the meshed opening plate 10, as described above, and 
the value of the ion electric current in the internal ionization chamber 1 
can be saturated, as shown in FIG. 6, because the amount of the moving ion 
pairs increases. As a result, the V-I characteristics of the internal 
electrode 6 become favorable, with no gradient. Therefore, the voltage 
change V accompanied by the change in the V-I characteristics of the 
external electrode 7 becomes greater than that of the conventional type 
and stable detection of smoke can take place. 
Furthermore, the amount of smoke flowing into the internal ionization 
chamber 1 of the present invention is greater than the amount of smoke 
flowing into the internal ionization chamber 1 of the conventional smoke 
sensor due to the installation of the meshed opening plate 10 in the 
opening 8 even if the smoke flows into the external ionization chamber 2, 
and since the ion electric current in the internal ionization chamber 1 is 
saturated sufficiently, even if the current of air carried away the ion 
pairs, the problems concerning the fluctuation of the reference voltage is 
minimized. Therefore, the stability of the internal ionization chamber 1 
which acts as the standard chamber at the time of the inflowing of the 
smoke can be improved and the giving of misinformation and noinformation 
can be prevented. 
FIG. 7 explains another embodiment of the intermediate electrode 4 of the 
present invention. In this embodiment, a meshed opening part 12 is formed 
on the center of the intermediate electrode 4 facing the radiation source 
5 by stamping and so on. 
By forming the meshed opening part 12 at the center of the intermediate 
electrode 4, its structure can be simplified and the cost is less than 
that of the embodiment shown in FIG. 1, in which the meshed opening plate 
10 is installed as a separate member. 
Although the mesh holes of the meshed opening plate 10 or the meshed 
opening part 12 are defined as the hexagonal holes in the above described 
embodiment, the form of the mesh holes can be circular, rectangular and so 
on when only a rate of porosity of more than or equal to 50% is 
maintained. Furthermore, gauze having a rate of porosity more than or 
equal to 50% can be installed.