AU616844B2 - Ion type smoke sensor - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1952 COMPLETE SPECIFICATION Form

(ORIGINAL)

FOR OFFICE USE 616844 Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: a 4 TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: o* 04 HOCHIKI KABUSHIKI KAISHA 10-43, KAMIOSAKI 2-CHOME

SHINAGAWA-KU

TOKYO

JAPAN

GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

o Actual Inv(ctor: Address for Service: a 0 a Complete Specification for the invention entitled: ION TYPE SMOKE SENSOR.

The following statement is a full description of this invention including the best method of performing it known to me:-

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ION TYPE SMOKE SENSOR BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ion type smoke sensor which uses a two-chamber and one-radiation-source method and senses a fire by detecting a change of ion currents caused by a change in the concentration of smoke by 9> 9 means of a radiation source.

C U 02. Description of the Related Art o A conventional ion type smoke sensor, which adopts such 60 9 two-chamber and one-radiation-source method, is composed of an internal electrode with a radiation source, an 15 intermediate electrode with a radiation transmitting hole a od o and an external electrode into which smoke is capable of S*o* flovinitg from the outside, as disclosed in U.S. Pat. No.

S,234,877. Formed between the internal electrode and the intermediate electrode in such ion type smcke sensor is an 20 internal chamber as a reference chamber where the voltage between the electrodes is maintained substantially constant without being influenced by an inflow of smoke. Formed between the intermediate electrode and the external electrode is an external chamber where the voltage between the electrodes changes in accordance with the concentration

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0 0 0 0 0 0 o 0 a o 00 0 e o S 0 o 0 go 0 of smoke which flows in from the outside. When the smoke flows in, the ion type smoke sensor senses the smoke by detecting the change in the voltage between the electrodes in the external chamber by means of an element, such as a field effect transistor (FET), having high input impedance.

It is known that the range of the change of the voltage between the electrodes in the above external chamber changes in accordance with the ratio of the distance between the electrodes in each of the internal and external chambers.

10 Then, conventionally, in order to set each optimal interelectrode distance in the ion type smoke sensor having such internal arrangement, the ratio of each interelectrode distance in the internal and external chambers is determined so that the change of the output voltage of the FET may be 15 at maximum when the smoke flows in the external chamber.

Concretely, since the interelectrode distance H between the internal electrode and the external electrode is determined at a fixed value by the size of the sensor, the position of the intermediate electrode within the interelectrode distance H, that is, the distance Hin between the internal electrode and the intermediate electrode and the distance Hout H-Hin) between the external electrode and the intermediate electrode, are set so that the change of the output voltage of the FET may be at maximum when the smoke flows in.

ao 0 o B00 O 0 00 0 0 00 0 0 0 0 In recent years, attempts to make the ion type smoke sensor small and thin have been made. In other words, though the distance between the internal electrode and the external electrode is normally set within the range of 30mm in the conventional ion type smoke sensor, the necessity of further shortening the distance between both -the above electrodes arises from the demand for making the sensor small and thin. In this case, it is also necessary, 0 in the same manner as in the conventional method, to determine the interelectrode distances so that the change of o 0 oo the output of the FET may be at maximum when the smoke flows in.

However, experiments for trial manufacturing by the inventor of the present invention and others reveal that an 15 ion type smok.e sensor which is made small and thin, that is, in which the interelectrode distance is shorter than 16mm, ois more remarkably influenced by the change in atmospheric pressure than before being made small and thin. Therefore, in the conventional method of setting the interelectrode 00 distance a new problem arises, that is, 'the output change 0 0 caused by the change in the atmospheric pressure is too heavy -to expect a stable performance of the sensor. For example, when the sensor is made small and thin by setting the interelectrode distance based on the conventional ratio of the interelectrode distance, the output of the FET i urchanges more than 20% if the atmospheric pressure is lowered from that of sea level to that of, for example, 3500m above sea level. Therefore no sensor has been developed in which the interelectrode distance is shorter than 16mm.

SUMMARY OF THE INVENTION 0*06 ao t *o I *0 *0 60 1c 060 0 0 0I 0000 0 00 00 0 00* 0, With such conventional problems in riew, an object of the present invention is to provide an ion type smoke sensor in which the interelectrode distance is set so as to meet the requirements of both the sensitivity for detecting smoke and the output change caused by the atmospheric pressure.

An ion type smoke sensor of the present invention, which achieves the above results, comprises an internal 15 electrode provided with a radiation source, an intermediate electrode provided with a radiation transmitting hole and an external electrode into which smoke flows from the outside, possesses an internal chamber between the internal electrode and the intermediate electrode, possesses an external 20 chamber between the intermediate electrode and the external electrode, and detects a change in the interelectrode voltage caused by the smoke which flows into the external chamber. In the ion type smoke sensor, the distance H between the internal electrode and the external electrode is shorter than 16mm and the ratio Hin/Hout of the distance Hin 0.6 0 0 00 0 between the internal electrode and the intermediate electrode in -the internal chamber and the distance Hout between the intermediate electrode and the external electrode in the external. chamber iLs set within the range of 0.3 to 0.6.

In such an arrangement, if the change in the atmospheric pressure is permitted to some extent, tile characteristic of the sensor, which places emphasis on the detection sensitivity-, can be obtained by determining the interelectrode distance so -that the ratio Hin/Hout of the 4interelectrode distances is near 0.3. On -the other hand, in order to minimize the influence of the change in the atmospheric pressure, when the atmospheric pressure is lowered to, for example, 'the pressure at 3500m above sea 15level, the rate of the output change is limited to around a 404a few percent while maintaining the detection sensitivity within the range which meets the performance of the sensor by setting the interelectrode distances so the ratio Hin/Hout of the interelectrode distances is approximately 0.6. As a result, when -the sensor is made small and -thin, its optimal performance can be obtained. in consideration of both the detection sensitivity and the influence of the atmospheric pressure. Particularly, it is possible to solve a problem of an erroneous alarm issuance which is caused because the small and thin sensor is subject to be influenced by a change in atmospheric pressure.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a sectional view of an ion type smoke sensor according to an embodimeint oF the present invention; Fig.2 is an exploded view of the sensor shown in Fig.l; 8 8 Fig.3 is a circuit diagram showing a basic circuit 10 arrangement of each electrode and output circuit in the ion o 0 o type sensor shown in Fig.i; 0 0 at o° Fig.4 is a characteristic view showing a detection characteristic of the ion type smoke sensor shown in Fig.l; and 8 8' 15 Fig.5 is a characteristic view showing an output change So£ AV of a FET between a normal condition and when smoke flows S in, a rate of output change of the FET relative to change in o. o atmospheric pressure, and an output value of the FET, in accordance with the change in the ratio Hin/Hout of the i 0 8 20 interelectrode distances in the ion type smoke sensor.

o 0 g4 8t 80 8a DESCRIPTION OF THE PREFERRED EMBODIMENT Fig.l is a sectional view of an ion type smoke sensor as an embodiment of the present invention.

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0*901 0 oL) n 00 0 00* 0 In Fig.1, 1 denotes a sensor body and 2 denotes a sensor base. The sensor base 2 is fixed to a ceiling and so on, while the sensor body 1 is detachably mounted.

The sensor body 1 comprises a body cover 3 mounted on the side cf the sensor base 2 and an outer cover 4 mounted from below the body cover 3.

The outer cover 4 is shaped in a cup whose upper portion is open and a plurality of smoke influent openings are arranged around its peripheral side surface which inclines upward. Each of the smoke influent opening 5 is almost rectangularly formed and is mounted at regular intervals.

A cylindrical partition wall 6 vertically stands inside the inclined peripheral surface, which has the smoke 15 influent openings 5, of the outer cover 4 and is unitedly formed in the outer cover 4. Around the peripheral surface of the partition wall 6, a plurality of rectangular smoke influent openings are also arranged at regular intervals.

An insect net 8 is mounted inside the partition wall 6 which is formed in the outer cover 4. The insect net 8 is formed so that its height is almost the same as that of the partition wall 6 and in close contact with the inside of the peripheral surface of the partition wall 6.

Furthermore, an external electrode 9, which is cupshaped in the same manner as the outer cover 4, is 0000 00 0 o 000 0 0t 0 000 00S o u o ono 0 0 positioned inside of the insect net 8 in the outer cover 4.

The external electrode 9 has a peripheral surface which is inclined upward and a plurality of almost rectangular smoke influent openings 10 arranged at regular intervals around the inclined peripheral surface.

On the other hand, an insulating block 11 is incorporated into the sensor b)ody 1. An internal electrode 12, which is provided with a radiation source, is mounted at the center of the insulating block 11 and an intermediate ,10 electrode 13, which is provided with an opening, is arranged 0 0 besides the internal electrode 12. Furthermore, as shown in ,the figure, an internal chamber A is formed between the internal electrode 12 and the intermediate electrode 13, and an external chamber B is formed between the intermediate 15 electrode 13 and the external electrode 9 outside the 0 internal chamber A.

On the rear side of the insulating block 11, formed is a circuit store space 15 which has a shield case 14 at its upper portion. A printed circuit board 16, which packages a sensor circuit, is arranged in close contact with the rear side of the insulating block 11. Furthermore, in a FET Sstore portion 19 mounted on the rear side of the insulating block 11, a FET 1.7 and an electrode lead 18 of the intermediate electrode 13 are electrically insulated and sealed in the state they are potted by pouring heat-melting i i resin, for example, hot melt of synthetic resin in order to certainly prevent electrostatic breaking which is caused when an assembly and so on is touched with the hand. In this embodiment, the FET 17 is a junction type.

Fig.2 is an exploded view of the sensor body 1 shown in Fig.1.

As shown in Fig.2, the body cover 3 has an internal opening 3a at its lower portion and contact pins 20 are mounted at two points of the internal opening 3a in order to electrically connect the printed circuit board 16.

S Engageable terminals 21 are fixed to the rear side of the contact pins 20 in order to engageably connect the sensor base 2.

The shield case 14 is incorporated into the internal 15 opening 3a of the body cover 3 and subsequently the printed i circuit board 16, which packages the sensor circuit, is Soo. incorporated.

Furthermore, the insulating block 11 is incorporated next to the printed circuit board 16. The internal 20 electrode 12, which is composed of an electrode member 12a, a radiation source 12b and an electrode cover 12c, is mounted at the center of the insulating block 11. The doughnut-shaped intermediate electrode 13 is arranged outside the internal electrode 12 and further outside is mounted the external electrode 9 which is provided with the smoke inlet opening .0 around it.

The external electrode 9 is mounted by putting contact metal fittings 22 into slits 23 of the insulating block 11.

The leading edges of the contact metal fittings 22 further penetrate the printed circuit board 16 and are in contact with contact portions 24 of tlie shield case 14. The contact metal fittings 22 are soldered Lo the ground portion of the printed circuit board 16 at a portion where the contact metal fittings 22 penetrate the printed circuit board 16.

0 Therefore, the contact metal fittings 22 have a function for mounting the external electrode 9 on the side of the t00 insulating block 11 and a function for making an electrical contact in order to shield the store portion of the printed 15 circuit board 16 in conjunction with the shield case 14.

0000 t 4, Subsequently to the external electrode 9, mounted is the outer cover 4 which compuises a bottom plate 25 and smoke inlet openings 5 around it. The insect net 8 is arranged inside the outer cover 4.

Fig.3 is a circuit diagram showing a basic circuit arrangement of each electrode and output circuit in the ion type sensor shown in Figs.1 and 2.

As shown in Fig.3, a drain D, which detects a change of the interelectrode voltage when the smoke flow into the external chamber B from the outside, is connected to both the internal electrode 12 and the positive side of an direct current power source 32. Furthermore, a gate G is connected to the intermediate electrode 13 and a source S is connected to the external electrode 9 and the negative side of the direct current power sou t )rc 32 through a load resistance R33.

Fig.4 is a characteristic view showing a detection ,t characteristic of the ion type smoke sensor shown in Fig.1.

An operation principle of the ion type smoke sensor of the 10 two-chamber and one-radiation-source type shown in Fig.1 S will now be explained with r.eferencre to Fig.4 as follows: 0• The inside of the internal chamber A and the external chamber B is ionized by radiation rays from the radiation source 12b mounted in the internal electrode 12. At this o° 15 time, the current-voltage characteristic between the o internal electrode 12 and l.he intermediate electrode 13 in the internal chamber A is as shown as a curve 26. The current-voltage characteristic between the intermediate electrode 13 and the external electrode 9 in the external chamber B is as shown as a curve 27. When the smoke enters, J the ion current is interfered and therefore the currentvoltage characteristic in the external chamber B changes as shown as a curve 28.

A cross point of the characteristic curves 26 in the internal chamber A and 27 in the external chamber 27, when -11no smoke exists, shows an electric potential of the intermediate electrode 13 when no smoke exists. Then, when smoke flows in and the characteristic curve 28 of the external chamber B changes as shown in Fig.4, the voltage Vo between the intermediate electrode 13 and the external electrode 9 in the external chamber B is increased, while the voltage Vi between the internal electrode ]12 and the intermediate electrode 13 in the internal chamber A is reduced. Therefore, the voltage change of the intermediate electrode 13 with smoke and without smoke is detected as AV by the FET 17 and a source output voltage Vout is obtained.

By detecting that the change AV of the source output voltage Vout exceeds a reference voltage level, a fire is detected.

In the ion type smoke sensor of the present invention, 15 which has the above mentioned arrangement, the distance between the internal electrode 12 and the external electrode 0 9 is set shorter than 16mm in order to make the sensor 0 o0 9 smaller and thinner than the conventional sensor. In this case, if the interelectrode distance is determined by the 04 9 a °20 conventional ratio of the interelectrode distance, the same as the conventional sensor, the influence of the change in the atmospheric pressure, noises and so on cannot be ignored and it is impossible to obtain a sensor capable of stably sensing smoke.

-12- In the present invention, the above-mentioned problems are solved by setting the above distance shorter than 16mm, which has been considered as a minimum up to the present, and setting the ratio as mentioned below.

Concretely, in the ion type smoke sensor of the present embodiment, the above distance is set 12mm, which is shorter than 16mm.

is a characteristic view of an embodiment according to the present invention which is used to determine the distance between the internal, intermediate and external electrodes 12, 13, 9 in the direction of their Sheight.

Then, it is assumed that the distance between the internal electrode 12 and the external electrode 9 in the 15 direction of their height is H, the distance between the internal electrode 12 and the intermediate electrode 13 in the direction of their height is Hin, and further the distance between the intermediate electrode 13 and the external electrode 9 in the direction of their height is t 1 20 Hout.

As shown in Fig.5, a horizontal axis denotes the ratio (Hin/Hout) of the interelectrode distance Hin in the internal chamber A and the interelectrode distance Hout in the external chamber B shown in Fig.l. Furthermore, it denotes the interelectrode distance Hin in the internal chamber A in -13relation to the ratio when the above interelectrode distance H is 12mm.

On the other hand, a right vertical axis denotes the output voltage Vout of the FET and the change rate of the output voltage Vout caused by a change in atmospheric pressure of the FET 17 and a :light vertical axis denotes the change AV of output voltage Vout of the FET 17 when the srnoke having the concentration of light reduction per meter) flows in.

First, as the change AV of the output voltage, when the smoke flows in, relative to the ratio Hin/Hout of the V interelectrode distances, a characteristic shown as the curve 29 is obtained. The characteristic curve 29 increases as the ratio Hin/Hout increases, and AV is at its peak when I. 15 the ratio Hin/Hout is approximately 0.4. After that, the characteristic curve 29 decreases.

A, Next, a curve 30 is obtained as the change rate of the output voltage Vout of the FET 17 if the ratio of the interelectrode listances is changed when the atmospheric pressure around the sensor is lowered from the standard pressure of sea level to the pressure of 3500m above sea level. According to the characteristic curve 30, the smaller the ratio of the interelectrode distances is, the larger the rate of the change caused by the pressure change is. When the ratio of the interelectrode distances reaches -14near 0.75, the rate of the change of the output voltage Vout of the FET 17 in accordance with the change in atmospheric pressure from the standard pressure of sea level to the pressure of 3500m above sea level is 0%.

Furthermore, shown in Fig. 5, the output voltage of the FET 17, when no smoke flows in, in accordance with the change in the ratio Hin/Hout of the interelectrode distanc:es is shown as a characteristic curve 31. On the ft characteristic curve 31, the smaller the ratio Hin/Hout of i the interelectrode distances is, the lower the output voltage Vout of the FET 17 is. Furthermore, the larger the t0 ratio Hin/Hout of the interelectrode distances is, the higher the output voltage Vout of the FET 17 is. In other words, as shown in Fig.5, as the ratio Hin/Hout of the interelectrode S- 15 distances changes from 0.3 to 0.75, the output voltage Vout St 1 of the FET 17 increases from approximately 5.3V to 7V.

Then, when the FET 17 is completely in a saturation state, the maxi.Lmum value of the source voltage Vs is approximately 8V in the power source voltage Vc of Therefore, the lower the output voltage Vout ol the FET 17, S' when no smoke enters, that is, the further lower than 8V the J output voltage Vout of the FET 17 is, the more change of the output voltage Vout can be permitted until the output saturation value is reached. In other words, the detection range for the smoke concentration can be wide. Since, when the ratio of the interelectrode distances is small, the output voltage Vout of the FET is low, the change of the FET 17 to the saturation voltage value 8V is large and the range of the smoke detection concentration can be wide.

How to find the change rate of the output voltage Vout of the FET 17 relative to the change in the atmospheric pressure in reference to the characteristic curve 31 shown in Fig.5 will be explained. For example, in the case the ratio of the interelectrode distances is 0.6, the change Sr° 10 rate given in the characteristic curve 30 is 4% and the output voltage Vout of the FET 17 is 9V in the characteristic curve 31. Therefore, the change value of the output voltage of the FET 17 is 9V x 4% 0.36V. In other words, when the o Q o standard atmospheric pressure is lowered to the atmospheric pressure of 3500m above sea level, the output voltage Vout changes to 9V-0.36V 8.64V in the case it is 9V under the standard pressure.

In the present invention, in order to stabilize a smoke sensing operation, each value of the output change AV of the FET when smoke flows in, a change in the change rate of the I* t' output Vout of the FET 17 relative to the atmospheric a't' pressure and the change of the source output voltage Vout of the FET 17, which are respectively shown as the characteristic curves 29, 30 and 31, is set so that the ratio Hin/Hout of the interelectrode distances within the -16- Zkrange of 0.3-0.6, that is, tne value is within a range defined by vertical broken lines in For example, in the case of an ion type smoke sensor in which H 12mm, if the ratio Hin/Hout of the interelectrode distances is set a value within 0.3-0.6, for example, 0.4, the interelectrode distance Hin in the internal chamber A is 3.4mm and the interelectrode distance Hout in the external chamber B is 8.6mm in Fig.l.

Next, the reason why the minimum value of the ratio 10 Hin/Hout of the interelectrode distances is set 0.3 and the maximum value 0.6, as shown in Fig.5, will be explained.

First, as for selecting the ratio Hin/Hout of the ,interelectrode distances within the range of 0.3 to 0.6 shown in Fig.5, there seems to be several cases in which the following three demands are regarded as important. Then, Sdifferent values within the range of 0.3-0.6 are selected for each case as the ratio Hin/Hout of the interelectrode distances. The three demands are as follows: 1. to maximize detection sensitivity 2. to minimize an output change of the FET 17 relative to a change in atmospheric pressure S3. to maximize an extra output range of the FET 17 In is, the order to maximize the detection sensitivity, that output change AV of the FET 17 when the smoke flows -17- 1 Urr. I-1 I" c.

in so as to meet the first demand, the ratio Hin/Hcout of the interelectrode distances is set a value within the range of 0.3-0.6 on the characteristic curve 29 shown in Fig.5, for example, 0.4 so that AV may be a peak value.

In order to minimize the output change of the FET 17 relative to the change in the atmospheric pressure so as to meet the second demand, since the characteristic curve says which is the minimum, as the change rate relative to the atmospheric pressure in the case of the ratio of the 10 distances between the electrodes of 0.6, 0.6 is selected as the ratio of the interelectrode distances. In the case of the ratio of 0.6, it is assured that the output change AV, which is given on the characteristic curve 29, of the FET 17 S *o is 0.58V, nearly the same as the case of the ratio of 0.3 and there is no problem in the detection sensitivity.

Furthermore, the output voltage Vout, which is given on the characteristic line 31, of the FET 17 is 6.2V and has an t l t extra range of approximately 1.8V to the saturation value 8V and therefore the smoke concentration can be detected without causing any practical problem.

SOn the other hand, in order to maximize the extra Soutput range of the FET 17 so as to meet the third demand, since the minimum output voltage Vout of the FET 17 is 0.3 on the characteristic curve 31, a value near 0.3 is selected as the ratio of the interelectrode distances. In the case of -18i the ratio of 0.3, the detection sensitivity given on the characteristic curve 29, that is, the change AV of the output voltage of the FET 17 is 0.6. Though the value is lower than the peak value AV of 0.7, it can assure a sufficient voltage change when the smoke flows in. However, in the case of the ratio of 0.3, as shown on the characteristic curve 30, the change rate of the output voltage AV of the FET 17 relative to the change in the atmospheric pressure is 18%, which is the maximum.

10 Therefore, if the ratio near 0.3 is selected, the sensor should not be mounted in a place which is subject to be Z influenced by the change in the atmospheric pressure.

The sensor in which the ratio of the interelectrode S sI distances is less than the minimum value 0.3 cannot be used because, as shown on the characteristic curve 29, the output o tvoltage change AV of the FET 17 when the smoke flows in, that is, the detection sensitivity is too low and as shown on the characteristic curve 30, the influence of the change o 6 in the atmospheric pressure on the output voltage of the FET 17 is too heavy. On the contrary, the sensor in which the ratio of the interelectrode distances is more than the maximum value 0.6 is favorable in point of the chang. in the atmospheric pressure because the change rate of the output voltage of the FET 17 relative to the change in the atmospheric pressure further decreases on the characteristic -19i- *I 1. I curve 30. However, as for other two points, that is, the detection sensitivity shown on the characteristic curve 29 and the extra output range of the FET shown on the characteristic curve 31, if the ratio exceeds 0.6, a practical problem arises and that is out of the scope of the present invention.

As described above, in the ion type smoke sensor according to the present invention, the ratio Hin/Hout of the interelectrode distances depends on which demand of the 10 above three demands takes priority. Then, by selecting an t S optimal value from the range of 0.3-0.6 as the ratio Hin/Hout 0 of the interelectrode distances and determining the 4 interelectrode distances Hin and Hout, the optimal sensor to meet the demand can be obtained without lowering the performance of the sensor.

The distance H between the internal electrode 12 and o o the external electrode 9 is determined at a fixed value by the size of the sensor. Therefore, the intermediate electrode 13 may be mounted adjustably in the direction of its height and may be movable to a proper position within the range in which the ratio Hin/Hout of the interelectrode distances is 0.3-0.6 shown in Fig.5, as necessary. Thus, by moving the position of the intermediate electrode 13 for adjustment, ion type smoke sensors which are respectively intended for use according to each of the above-mentioned hi.- three prior demands can be embodied in a single electrode arrangement.

Needless to say, other electrodes, for example, the internal electrode may be movable for adjustment in order to change the ratio of the interelectrode distances.

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Claims (1)

1. An ion type smoke sensor comprising an internal electrode provided with a radiation source, an intermediate electrode provided with a radiation transmitting hole, and an external electrode into which smoke can flow from the outside, possessing an internal chamber between said internal electrode and said intermediate electrode and an external chamber between said intermediate electrode and said external electrode and detecting a change in an 4" 10 interelectrode voltage caused by said smoke which flows into 0 i* said external chamber, wherein the distance H between said internal electrode and said external electrode is set 16mm or less and the ratio Hin/Hout of the distance Hin between said internal electrode and said intermediate electrode in said internal I chamber and the distance Hout between said intermediate electrode and said external electrode in said external t chamber is set within a range of 0.3-0.6. DATED THIS 17TH 'DAY OF MAY 1990 i HOCHIKI KABUSHIKI KAISHA S By its Patent Attorneys: GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia -22-