Gas Alarm Device

Abstract

A detecting element for inflammable gases which includes a pair of electrodes formed of a relatively nonoxidizable material which are supported in spaced relationship by an intervening member having parallel grooves for receiving the coils and to which the coils are fixed at least in part and a metal oxide semiconductor covering said electrodes and intervening member.

Description

This invention relates to a device for the detection of gases and generating an alarm in response thereto. More specifically the device is particularly useful for the detection of inflammable gases as well as the detection of carbon monoxide which may be produced at the time of a fire.

It has been found that a particular form of metal oxide semiconductor when heated exhibits a variation in electrical resistance when in contact with an inflammable gas. For example a gas detecting element may be arranged with two electrodes in the form of helical coils wound in spaced relationship on a helical bobbin of a heatproof insulating material. The two coils and the surface of the bobbin are then coated with a metal oxide conductor. With this structure, however, the characteristics of the finished product may not be uniform because of the displacement of the wires which occurs during assembly. In some cases the wires may contact one another or may slide off the bobbin. To overcome these difficulties extreme care must be exercised during assembly. In order to avoid displacement of the wires a glass like bonding material may be used to fix the wires to the surface of the bobbin, but the bonding agent tends to adhere to the wires covering them wholly or in part with the result that a good electrical connection between the wires and the metal oxide conductor cannot be obtained.

One object of the invention resides in the provision of an improved gas detecting element structure which will afford uniform characteristics and at the same time simplify manufacture. The improved detecting element in accordance with the invention includes two helically wound coil electrodes formed of a metal which is relatively nonoxidizable at a high temperature, and a block of heatproof electrical insulating inorganic material having concave grooves on two side faces with the radius of the grooves conforming with the peripheral surfaces of the electrodes. An inorganic high melting point bonding agent is used to fix the electrodes to the concave grooves of the block. The structure is then embedded in a metal oxide semiconductor to form in effect a unitary structure. In this way the electrodes are permanently fixed to the insulating block by a bonding agent, even though the bonding agent may cover part of the electrodes, a major portion of the electrodes are in good electrical contact with the semiconductor. Furthermore, the spacing between the electrodes is maintained constant and accordingly, the characteristics of the finished products are uniform.

With the invention as described above, the coiled electrodes are permanently fixed to the insulating block during assembly and accordingly cannot move one relative to the other during the manufacturing process with the result that the manufacture is greatly facilitated. Furthermore, the bonding agent for bonding the coiled electrodes to the insulating block and baking of the semiconductor can be performed very simply by passing an electric current through the electrodes of the assembled structure so that complicated heating furnaces generally required are not necessary.

A still further object of the invention resides in the provision of an improved mount for the detecting element. As previously mentioned, the detecting element does not have to be heated in a furnace and accordingly, does not require the utilization of a heatproof stem. With this invention an inexpensive synthetic resin stem can be employed and further a normal organic bonding agent can be utilized for sealing the envelope and the stem. This results in a still further reduction in manufacturing costs. In a preferred embodiment of a gas alarm circuit for use with the detecting element in accordance with this invention, heater current is supplied from a tap on the autotransformer to one of the coiled electrodes in order to heat the electrode to a proper operating temperature which is generally lower than the treating temperature at the time of manufacture. Normal operating temperatures are within the range of 50.degree. C. to 300.degree. C. The primary voltage of the transformer is applied between the heated electrode and the unheated electrode through a buzzer or other suitable device for generating an alarm. If an inflammable gas is not present in material quantities in the air, the current traversing the semiconductor of the element is insufficient to cause operation of the buzzer. When the quantity of inflammable gas exceeds a predetermined limit, the current flowing through the semiconductor increases and causes the buzzer to operate. The buzzer may preferably be of a structure wherein the spacing between the magnetic pole piece and the vibrating element is adjustable in order to fix the minimum threshold current for operation of the buzzer. Because of some irregularities of the detection characteristics of the detecting element and because of variations in the secondary voltage of the autotransformer, some variation in sensitivity of the assembled circuit will be experienced. However, by adjusting the buzzer as mentioned above the alarm circuits can be arranged to have uniform resultant sensitivity.

A preferred embodiment of a container for the gas alarm device utilizing a detecting element in accordance with the invention includes a main body for containing the alarm circuit and a skirtlike resonator extending from the side face of the main body. An opening is formed in the body for the admission of air to the detecting element housed within the body. By utilizing the skirtlike resonator, a relatively weak sound produced by the buzzer which is directly driven by the detecting element can be greatly enhanced. Furthermore, the resonator can be used as a container for the power cord for supplying power to the alarm circuit.

The above and other objects of the invention will become more apparent from the following description and accompanying drawings forming part of this application.

In the drawings:

FIG. 1 is a perspective view of a detecting element in accordance with the invention;

FIG. 2 is a cross-sectional view of the structure shown in FIG. 1;

FIG. 3 is a side elevational view in partial section of a detector mount in accordance with the invention;

FIG. 4 is a circuit diagram of an alarm system utilizing the invention;

FIG. 5 is a plan view of the base plate of the alarm structure showing the positioning of the various components;

FIG. 6 is a side elevational view of an adjustable buzzer utilized with the invention;

FIG. 7 is a plan view of a housing for the structure shown in FIG. 5;

FIG. 8 is a perspective view of the housing shown in FIG. 7;

FIG. 9 is a bottom view of the housing with the structure of FIG. 5 removed; and

FIG. 10 is a cross-sectional view taken along the line 10--10 of FIG. 7.

The gas detecting element in accordance with this invention and illustrated in FIGS. 1 and 2 includes a pair of coils 1 and 2 made of a metal wire such as platinum, palladium, or platinum-iridium alloy which does not oxidize readily at high temperatures. These coils are contained within cylindrical concave grooves 4 and 5 on opposite faces of the block 3, the latter being made of a heatproof electrically insulating inorganic oxide such as Al.sub.2 O.sub.3, SiO.sub.2, or BeO. These coils are fixed in part to the faces of the grooves 4 and 5 by an inorganic high melting point bonding material 6 and 7 of a glassy substance consisting of a solid solution of Na.sub.2 O and one or more elements selected from the group consisting of K.sub.2 O, CaF.sub.2, Al.sub.2 O.sub.3, Ba.sub. 2 O.sub.3, and SiO.sub.2. In so doing the coils are maintained in precise parallel relationship. It will also be observed that the coils 1 and 2 are embedded in the bonding agents 6 and 7 only in the portions contained within the respective grooves so that the major portions of the coils remain exposed. The block 3 and the coils 1 and 2 are then embedded in a metal oxide semiconductor material 8 which is sensitive to inflammable gas at a high temperature and the resultant assembly is in the form of a blocklike structure. The ends of the coils 1 and 2 extend from opposite faces of the block 8 and are denoted by the numerals 9, 10, 11 and 12.

The metal oxide semiconductor material 8 may be SnO.sub.2, ZnO, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, TiO.sub.2, or Fe.sub.2 O.sub.3. When the semiconductor material heated to a temperature of 50.degree. C. to 300.degree. C. contacts an inflammable gas, the gas removes oxygen ions from the semiconductor material and thus oxidizes the semiconductor material. Thus the balance between the cations and the anions within the semiconductor material is upset and the resistance of the material will therefore be changed.

The detecting element is preferably formed in the following matter. The lead wires 9, 10, 11 and 12 of the coils 1 and 2 are fixedly connected to four posts 13, 14, 15 and 16 which are secured to and extend through a stem 17 of plastic or other good insulating material. This procedure positions the coils in a predetermined spaced parallel relationship. The insulating block 3 having grooves 4 and 5 coated with the high melting point bonding agents 6 and 7 is then placed between the coils 1 and 2 with a portion of a periphery of the coils being seated within the grooves 4 and 5. A current is then passed through the supports 13 and 14 and the supports 15 and 16 to heat the coils and in turn melt the bonding agents so that the coils 1 and 2 will become fixed within the grooves of the block 3. The coils 1 and 2 and the block 3 are then coated by a nonsintered material consisting of a gas sensitive oxide. A current is then passed through the coils to sinter the gas sensitive oxide and thus complete the fabrication of the element. As the heater current of the element during normal use is materially below the current required for heat treatment during manufacture, neither the bonding agent nor the oxide material will be affected during actual use.

FIG. 3 illustrates a method particularly useful for supporting the gas sensitive element. As pointed out above, the stem 17 is formed of a suitable plastic and the supports 13, 14, 15, and 16 extend through the stem. The portions of the supports protruding from the lower face of the stem are utilized for making external connections. A cylindrical container 18 is formed of a metal mesh and the upper end is closed by a metal cap 19. The stem includes an upper portion 20 of reduced diameter and a lower portion 21 of an enlarged diameter. An annular groove 22 is formed in the lower portion 21 and the metal mesh container 18 extends over the stem portion 20 and is suitably bonded in the annular groove 22.

Referring now to FIG. 4, which illustrates a circuit embodying the gas detecting element described above, the numeral 23 denotes a pair of power source terminals to which the power cord 24 as shown in FIG. 5 is attached. Alternating current energy of the order of 100 volts to 200 volts is applied to the cord for operation of the circuit. One of the terminals 23 is connected through a fuse 25 to the terminal 35 of an autotransformer 26 while the other terminal 23 is connected to the other terminal 31 of the autotransformer. A pilot light 27 in the form of a neon discharge tube is connected in series with the resistor 28 and in parallel with the terminals 31 and 35 of the autotransformer. One of the coils 30 of the detecting element generally denoted by the numeral 29 is connected between the terminals 31 and 32 of the autotransformer, the latter terminal being a tap to provide the desired low voltage to the coil 30. The ends of the second coil 33 of the detecting element 29 are connected one to the other and to the terminal 35 of the autotransformer through the buzzer or other suitable alarm 34. If the voltage applied to the autotransformer is relatively high as for example approximately 200 volts, the autotransformer may be provided with a second tap for attachment of the buzzer 34 in order to apply the proper voltage to the buzzer. The components of the circuit of FIG. 4 are assembled on a printed circuit base plate 36 as illustrated in FIG. 5, and the wiring of FIG. 4 is printed on the back surface of the base plate. Holes 37 and 38 are used for mounting the base plate in the gas detector.

In the circuit as described above, when the terminals 23 are connected to a source of alternating current power, the neon tube 27 will be illuminated and at the same time a low voltage will be supplied to the coil 30 of the detecting element 29 in order to heat the element to a temperature of 50.degree. C. to 300.degree. C. Although at this time a relatively high alternating current voltage is also applied through the buzzer 34 to the coils 30 and 33, and if the air surrounding the detector 29 does not contain an inflammable gas, the current flowing between the coils 30 and 33 will be too small to actuate the buzzer. When an inflammable gas does contact the detector 29, it will cause a relatively large current to flow between the coils 30 and 33 which in turn causes the buzzer to operate.

The buzzer, as will be explained, is adjustable to compensate for slightly varying characteristics of detecting elements 29 and slight variations of the voltage produced by the autotransformer. In this way, sensitivity of the device can be adjusted.

FIG. 6 illustrates a buzzer 34 which comprises an iron pole piece 41 having a magnetizing coil 39 mounted thereon. The pole piece 41 is secured at one end to the yoke 40 and is positioned at one end of the yoke. The other end of the yoke 40 carries a diaphragm 42 made of thin iron which is secured to the yoke by screws 43 and 44. The free end of the diaphragm 42 extends over the pole piece 40 and forms a narrow gap between the end of the pole piece. An adjusting screw 45 extends through the diaphragm at a point near the supported end and the lower edge of the screw 45 abuts the bottom of the yoke 40. With this arrangement, rotation of the adjusting screw will modify the spacing between the diaphragm 42 and the pole piece 41 and thereby modify the sensitivity of the buzzer to the magnetizing current applied to the coil 39. In this way, adjustment of the sensitivity of the buzzer will compensate for irregularities in the characteristics of the detecting element 29 as well as the variations in the secondary voltage of the autotransformer. Thus substantially the same sensitivity can be obtained for all alarm devices. Moreover, the adjustment is relatively simple and is accomplished by means of a single screw 45.

FIGS. 7 through 10 show the case or housing for the alarm circuit as described above. The case includes a main body portion 46 which is provided with a bottom closure 47. The top surface of the body 46 has an opening carrying a red transparent material 48. A peripheral flange 49 extends outwardly from a point approximately midway of the side walls of the main body 46 and the flange carries a downwardly and outwardly extending skirt 50 which skirt terminates generally in the same plane as the bottom cover 47 as will be observed more clearly in FIG. 10. An elongated slot 51 is formed in one corner of the body 46 and at a point adjoining the top side of the flange 49. Within the main body 46 a detecting chamber 53 is formed by a partition wall 52 and this chamber is positioned at the corner of the body 46 containing the gap 51. The bottom end of the partition wall 52 terminates below the bottom edge of the main body 46. The corner of the main body opposite the corner enclosed by the partition 52 has a shoulder disposed in substantially the same plane as the bottom edge of the partition 52, and the enclosed corner is also provided with a similar shoulder. The shoulders are threaded to receive screws 59 and 60. In addition, the bottom edge of the main body 46 is also provided with a slot 56. It is preferable that the main body 46, the flange 49 and skirt 50 are formed integrally of a strong resistant material such as acrylobutadiene-styrol resin (ABS resin). The cover 47 is formed of similar material and is provided with a ridge adjoining the peripheral edge portion of its inner surface. It also includes a pressure sensitive bonding agent 58 on the back surface and at opposing corners which include holes through which the screws 59 and 60 will extend.

The structure shown in FIG. 5 is then inserted in the main body 46 as shown in FIG. 10 with the detector 29 entering the compartment formed by the partition wall 52. The cover 47 is then placed in overlying relationship and the assembly is secured in place by screws 59 and 60 as shown in FIG. 9. The neon tube 27 is positioned adjoining the indicating window 48 and the circuit base plate 36 is supported by the partition wall 52 as well as the shoulders 54 and 55. The ridges 57 of the cover hold the base plate in position and the cord 24 extends outwardly through the cutout 56.

While only one embodiment of the invention has been illustrated and described, it is apparent that alterations, modifications and changes may be made without departing from the true scope and spirit thereof as defined by the appended claims.

Claims

What is claimed is:

1. An inflammable gas detector comprising a pair of coiled electrodes formed of a metal that is relatively stable at high temperatures, a body of heatproof electrically insulating material having a pair of opposing edges thereon, means securing a portion of the periphery of each electrode to one of said opposing edges and a metal oxide semiconductor enclosing said body and electrodes with the ends of said electrodes extending therefrom, the resistance of said semiconductor when at a high temperature changing in value in the presence of an inflammable gas.

2. An inflammable gas detector according to claim 1 wherein said opposing edges of said body having cylindrical concave grooves and portions of the peripheries of said electrodes are secured in said grooves by a high melting point bonding agent.

3. An inflammable gas detector according to claim 3 including a stem of insulating material and supports carried by said stem, and wherein the ends of said electrodes are secured to said support.

4. An inflammable gas detector according to claim 3 including a metal mesh surrounding at least part of said element and secured to said stem.

5. An inflammable gas detector according to claim 1 including a transformer having at least one winding with terminals at the ends thereof and a voltage tap, means connecting one electrode to one terminal and the tap to energize said electrode and means including a buzzer connecting the other electrode to the other terminal of said winding.

6. An inflammable gas detector according to claim 5 wherein said buzzer comprises a pole piece, a coil surrounding said pole piece, a yoke of magnetic material magnetically coupled to one end of said pole piece, a diaphragm secured at one end to said yoke and extending in spaced overlying relationship to the other end of said pole piece and means engaging said diaphragm to modify the spacing between it and said pole piece.

7. An inflammable gas detector according to claim 5 including a housing enclosing said element, transformer and buzzer, said housing including an opening for admission of air to said metal oxide semiconductor enclosing said electrodes and a skirtlike wall surrounding said housing and in spaced relationship thereto, said wall forming with said housing a resonant chamber for increasing the level of sound produced by said buzzer.