US 3719160 A
An audio alarm mechanism warns a diver using a self-contained underwater breathing apparatus (scuba) that the air tank pressure has dropped to reserve level. The mechanism includes an oscillator piston having one end which cooperates with an annular seat to gate air flow between the scuba first and second stage regulators. When the air tank pressure is sufficiently high, a stem spaces the piston away from the seat to allow unimpeded air flow to the mouthpiece and to prevent oscillation of the piston. When the tank pressure drops, the stem retracts, permitting oscillation of the piston at an audio frequency. Oscillation is sustained by feedback of the outlet pressure via a passageway to an actuating chamber at the other end of the piston. The resultant acoustic vibrations are transmitted both through the air supply hose and through the water to warn the diver to begin his ascent.
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Description (OCR text may contain errors)
United States Patent [191 Christianson 5] March 6, 1973 1 AUDIO RESERVE ALARM 3,244,196 4/1966 Replogle ..137 557 MECHANISM FOR SELECONTAINED 3,508,542 4/1970 Browner ..128/142.2
BREATHING APPARATUS Primary Examiner-Louis J. Capozi  Inventor: Raymond A. Christianson, In- Anomey 1:1am & Flam glewood, Calif.  Assignee: Under Sea Industries, Inc., Comp-  ABSTRACT I011, Calif. An audioalarrn mechanism warns a diver using a self-  Filed: March 1, 1971 contained underwater breathing apparatus (scuba) App]. No.: 122,594
US. Cl. ..116/70, 73/419, 128/1422,
that the air tank pressure has dropped to reserve level. The mechanism includes an oscillator piston having one end which cooperates with an annular seat to gate air flow between the scuba first and second stage regulators. When the air tank pressure is sufficiently high, a stem spaces the piston away from the seat to allow unimpeded air flow to the mouthpiece and to prevent oscillation of the piston. When the tank pressure drops, the stem retracts, permitting oscillation of the piston at an audio frequency. Oscillation is sustained by feedback of the outlet pressure via a passageway to an actuating chamber at the other end of the piston. The resultant acoustic vibrations are transmitted both through the air supply hose and through the water to warn the diver to begin his ascent.
30 Claims, 7 Drawing Figures PATENTEB 61915 3,719,160
SHEET 30F 3 TO MOUTHPIECE 7 g T FROM REGULATOR H 22" FROM REGULATOR-H FROM TANK 14 IN VENTO/Q RAVMO/VD A? CH2 /5 r/fi/vsa/v 5 TO MOUTHPIECE -17 y/4..
A 7 To/eA/Ew- AUDIO RESERVE ALARM MECHANISM FOR SELF-CONTAINED BREATHING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio alarm for warning the user of a self-contained breathing apparatus that the pressure in the air supply tank has dropped to reserve level.
2. Description of the Prior Art Exhaustion of the air supply without adequate warning is an ever present danger in the use of a self-contained underwater breathing apparatus. Various devices have been suggested to provide such warning. For example, a widely used reserve valve mechanism utilizes a restriction in the air flow path that can be removed by operating a lever. The pressure drop caused by the restriction is inconsequential while tank pressure is high, thus adequate pressure is delivered to the unit. However, when tank pressure is low, the pressure drop becomes critical and the diver notes that extra effort is required to draw sufficient breathable gases. When this condition is noted, the diver operates the lever to remove the restriction. Thereupon, sufficient pressure is available for effortless breathing and the diver begins his ascent.
More recently, alarms which provide a sonic warning of low tank air pressure have become popular. With such sonic alarms, the diver is not required to operate a lever, nor does the diver encounter any restriction in air flow. Once triggered, the alarm operates repeatedly whenever the diver inhales, or continually, whereby the diver is constantly aware of his peril.
One such sonic reserve alarm, shown in U.S. Pat. No. 2,854,001 to Humblet, incorporates a whistle located in the conduit between the first and second regulator stages. In other devices, described in U.S. Pat. No. 3,149,752 and No. 3,144,171 to Cousteau-Gagnan, sonic alarms are mounted on the gas tank itself. While this arrangement is functionally satisfactory, a problem arises in that the diver often exchanges his empty tank for a full one, thus each tank must be equipped with an alarm mechanism.
Structures shown in U.S. Pat. No. 3,056,378 to Simmonds and No. 3,244,196 to Replogle achieve good sound signals by metal striking metal, and avoid the disadvantages of a tank-mounted device by locating the alarm mechanism between the first and second regulator stages. Both the Simmonds and Replogle structures are quite complicated. Simmonds utilizes a substantial number of parts to provide a pneumatic relaxation oscillator which commences operation when tank pressure is reduced. Pressure slowly builds up in a capacitor chamber until explosive outflow causes a striker to engage a rigid object. The outlet recloses and the cycle repeats. The slow charging path is controlled by a snap switch positioned in accordance with the tank pressure.
The structure of the Replogle patent operates a noise maker on a more steady state basis as compared with Simmonds. Air expands into a larger diameter chamber upon opening of a spring-pressed closure. A striker coupled to the closure causes repeated clanging each time the diver inhales. Replogles noise-maker path is shunted via a pressure-sensing valve to inhibit the sonic alarm so long as the tank pressure is above a critical value.
Neither Simmonds nor Replogle recognizes that the oscillator principle can be used in a vastly simpler manner to cause sonic vibrations quite independently of mechanical parts striking each other. Accordingly, the primary object of this invention is to provide an extremely simple audio reserve alarm system of high reliability. A companion object is to provide an audio alarm mechanism in which maintenance and manufacturing costs are minimized, all without sacrificing the essential desirable characteristics of an audio reserve alarm. These include repeated operation at least at every inhalation by the diver, and utilization ofa selfcontained available power source, namely the pressurized breathing gas being supplied to the diver.
SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an audio reserve alarm mechanism for use with a self-contained underwater breathing apparatus. The mechanism is provided with a pneumatic oscillator so arranged as to produce an audio alarm not caused by impingement of metal upon metal, but instead generated by oscillation of a piston or diaphragm at audio frequency. The acoustic alarm signal is transmitted to the diver both via the breathing apparatus hose and through the water itself. Thus, the diver not only hears the audio alarm, but also tactilely senses the signal. Thus, even if the divers hearing is impaired for any reason, he nevertheless will be warned that the air supply is near exhaustion.
In a preferred embodiment, the inventive alarm mechanism comprises an oscillator piston disposed for reciprocation in a chamber within a housing. The piston divides the chamber into an inlet region and an actuating region. Air from the breathing apparatus first stage regulator is supplied to the chamber inlet region, and thence flows between the piston and an annular seat to an outlet passageway leading to the breathing apparatus second stage regulator. A feedback path, which may comprise an opening through the piston or a channel through the housing, communicates between the outlet passageway and the chamber actuating region behind the piston. A spring biases the piston toward the annular seat.
An inhibit mechanism spaces the piston away from the seat, permitting unimpeded air flow between the first and second regulator stages while the pressure in the air supply tank is sufficiently high. When the pressure in the air supply tank drops to a preselected reserve level, the inhibit mechanism retracts, and the piston is free to oscillate. Upon inhalation by the diver, the reduced pressure in the outlet passageway is communicated via the feedback path to the actuating chamber. The resultant pressure differential between ends of the piston initiates oscillation of the piston to generate the audio alarm.
BRIEF DESCRIPTION OF THE DRAWINGS Detailed description of the invention will be made with reference to the accompanying drawings wherein like numerals designate like parts in the several figures. These drawings, unless described as diagrammatic or unless otherwise indicated, are to scale.
FIG. 1 is a perspective view of a self-contained underwater breathing apparatus incorporating the inventive audio alarm mechanism.
FIG. 2 is an end view, partly cut away and in section, of the housing containing the first stage regulator and audio alarm components of the apparatus of FIG. 1, shown attached to a conventional air tank or cylinder.
FIG. 3 is a transverse sectional view of the first stage regulator and audio alarm mechanism, of FIG. 2, as seen generally along the line 3-3 thereof.
FIG. 4 is a fragmentary transverse sectional view of the audio alarm mechanism of FIG. 2, as seen generally along the line 4-4 thereof.
FIG. 5 is a simplified diagrammatic view of an alternative embodiment of the inventive audio alarm mechanism.
FIG. 6 is a diagrammatic view of yet another audio alarm mechanism incorporating a diaphragm as the oscillating member.
FIG. 7 is a transverse sectional view of an alarm actuating valve useful with the mechanism of FIG. 6, as seen generally along the line 7-7 thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description is of the best presently contemplated modes of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention since the scope of the invention best is defined by the appended claims.
Structural and operational characteristics attributed to forms of the invention first described also shall be attributed to forms later described, unless such characteristics obviously are inapplicable or unless specific exception is made.
Referring now to the drawings and particularly to FIGS. 1 and 2 thereof, there is shown a self-contained underwater breathing apparatus (scuba) 10 of the type typically used for aquatic diving. Apparatus 10 comprises a first stage regulator, generally designated 11, included in a housing 12 having a yoke 13 adapted for attachment to a tank or cylinder 14 in which air is stored under pressure. Air at reduced pressure is supplied from the regulator 11 via a hose 15 to a second stage demand regulator 16 incorporated in this instance in a mouthpiece 17. The apparatus 10 further includes an audio alarm mechanism 18, also situated within housing 12, which provides an audible warning to the diver when the air pressure in tank 14 drops below a reserve level. This audio alarm 18 advises the diver to begin his ascent while a sufficient reserve of air still is available.
As best seen in FIGS. 2, 3 and 4, high pressure air from the tank 14 is supplied via a channel 21 to the first stage regulator 11, which in turn provides at an outlet channel 22 air at a constant small pressure (e.g., I40 p.s.i.) over and above that of the ambient medium. This low pressure air is supplied to the mouthpiece 17 via a path including an annular chamber 23, a space 24 between one end 25a of an oscillator piston 25 and an annular seat 26, and an outlet passageway 27 to which hose 15 is connected.
So long as the pressure in the tank 14 is above reserve level, the piston 25 is held away from the annular seat 26 by a stem 29 (FIG. 4), thereby maintaining the space 24 open. To accomplish this, the rear end flange 28 of a stem 29 is subjected to air at tank pressure supplied via the channel 21 and a branch channel 31. When the tank 14 pressure is above reserve level, i.e., above about 350 p.s.i., the air pressure exerted on the stern via flange 28 overcomes the counterforce of a bias spring 32, thereby forcing the stem 32 and the piston 25 to the position shown in solid lines in FIG. 4.
When the air pressure in the tank 14 drops to reserve level, the force of the bias spring 32 retracts the stem 32 to the position shown in phantom in FIG. 4. Such stem retraction permits the piston 25 to oscillate, generating an audio alarm which warns the diver that only a limited reserve of air remains in the tank 14.
Oscillation of the piston 25 is initiated when the diver inhales, thereby reducing the pressure in the outlet passageway 27 to below that in the annular chamber 23. This reduced pressure is communicated via a small diameter opening 33 through the piston 25 to the actuating region 34a of a chamber 34 containing the piston 25. The relatively higher pressure in annular chamber 23 is exerted against the piston end 250, urging the piston 25 away from the seat 26. The resultant air flow from the chamber 23 through the space 24 causes the pressure in the passageway 27 to increase. This increased pressure is fed back via the opening 33 to the actuating chamber 34a, equalizing the pressure on the piston ends 25a and 25b, and permitting the piston 25 to be urged back toward the seat 26 by a spring 35. Once more, air flow through the space 24 is impeded, and continued inhalation again reduces the pressure in the passageway 27, the feedback opening 33 and the actuating chamber 34a, therebyinitiating the next oscillation cycle. The piston 25 continues to oscillate, generating the audio alarm, during each diver inhalation period.
By appropriate selection of the mass of the oscillator piston 25 and the diameter-to-length ratio of the feedback opening 33, the piston 25 will oscillate at a relatively low audio frequency, producing an audio alarm signal which is communicated via the air in the hose 15 to the diver. Moreover, the oscillation of the piston 25 is coupled, partly via the spring 35, to the housing 12, which housing in turn radiates acoustic energy into the water. The resultant pressure pulses transmitted through the water and via hose l5 and mouthpiece 17 can be both heard and felt by the diver. Thus, the diver will be warned of the low tank air pressure level even though his hearing may be impaired.
Referring to FIGS. 3 and 4, note that metal-to-metal contact is prevented by providing the oscillator piston end 25a with an O-ring 37 seated in an annular dovetail groove 38 aligned with annular seat 26. The oscillator piston 25 also is provided with a peripheral O-ring 39 situated relatively closer to the end 25a than to the other piston end 25b, thereby permitting a slight tilting movement of the piston 25. This arrangement makes the unit very sensitive, so that the piston 25 readily will go into oscillation.
For simplicity of construction, the communicating- 35. An O-ring 43 prevents leakage to or from chamber- 34. Further, as best illustrated in FIG. 4, the piston 25 may comprise a two-piece assembly, including a generally cylindrical case 25c and an interior plug 25d threadingly connected to the case. A vent hole 44 communicates with the bottom of the groove 38 to prevent air from becoming trapped behind the O-ring 37. The passageway 27 may be provided with a conventional threaded fitting 45 for connection of the hose 15.
The first stage regulator 11 (FIG. 3) includes a generally cylindrical piston 52 having a knife edge 53 cooperating with a hard plastic seat 54 to form a valve for high pressure air supplied from the tank 14 via the channel 21 and a chamber 55. Air passing the knife edge 53 is communicated via the interior longitudinal opening 52a through the piston 52 to a chamber 56a and thence to the outlet channel 22. The valve separation between the edge 53 and the seat 54, and hence the outlet pressure, is determined by the force exerted on the flanged piston end 52b by a bias spring 57 and by the ambient water pressure vented to the spring enclosing chamber 56b via one or more holes 58 through the body 12. The spring 57 preferably is selected so that the air pressure at the outlet channel 22 is on the order of 140 p.s.i.
Again for ease of construction, the inlet chamber 55 may be formed by boring inwardly from body' end 12a, the opening being capped by an appropriate threaded plug 61 counterbored to receive the valve member 54, and sealed with an O-ring 62. The chamber 56a, 56b, in which the flanged piston end 52b reciprocates may be bored inward from the body end 12b, and may be capped with a threaded plug 63. A threaded bore 64 extending through plug 63 provides an outlet directly from the first stage regulator 11, bypassing audio alarm mechanism 18. Typically, this outlet may be used for connection to pneumatic tools (not shown) or to another second stage regulator (not shown) for use as a backup regulator in the event that the primary second stage regulator malfunctions, or for use by another diver in an emergency. Alternatively, the outlet may be closed offwith a plug 65 as shown in FIG. 3.
An alternative embodiment 70 of the audio alarm mechanism is shown schematically in FIG. 5. In this embodiment, the various elements designated by primed numbers correspond respectively to the unprimed, but like-numbered components of the above described audio alarm mechanism 18.
Note in FIG. 5 that the oscillator piston 25 is not provided with a feedback opening of the type designated 33 in FIG. 3. Rather, a feedback passageway 71 is provided within the housing 12' between the outlet passageway 27' and the piston actuating chamber 34a. The dimensions of passageway 71 in part determine the oscillation frequency of the piston 25 Of course, such alternative feedback passageway arrangement could be incorporated in the audio alarm mechanism 18 by providing a feedback opening (not shown) within housing 12.
Still referring to FIG. 5, the stem 29 is coaxial with the piston 25' and extends concentrically through a portion 72 of the L-shaped passageway 27'. Thus, when the pressure in tank 14 is sufficiently high, the air pressure against the stem end 28 will overcome the force of the bias spring 32' and cause stem 29' to maintain piston 25' away from its seat. This will permit unimpeded air flow from the output of first stage regulator 11 to the mouthpiece 17. When the air pressure in the tank 14 is sufficiently low, the spring 32' causes retraction of the stem 29', and the piston 25' goes into oscillation at an audio frequency. Again, acoustic vibrations are communicated to the diver both through the water and via hose 15.
In FIG. 6 there is shown an embodiment of the invention utilizing a diaphragm 81 rather than an oscillator piston. When the pressure in tank 14 is sufflciently high, air from the first stage regulator 11 flows via an alarm bypass line 82, an open valve 83, a line 84 and a resonating chamber 85 to hose 15". When the pressure in the tank 14 drops to reserve level, the valve 83 closes, and the air from regulator 11 flows to the annular inlet chamber 23".
As the diver inhales, the resultant reduced pressure in the resonating chamber 85 is communicated back via a feedback path 71" to the actuating chamber 34a behind the diaphragm 81. As a result of the pressure differential between chambers 23" and 34a", diaphragm 81 flexes away from the annular seat 26", permitting air flow from inlet channel 22" past the space between the diaphragm 81 and the seat 26" into the resonating chamber 85. The resultant increased pressure is fed back through path 71" to chamber 34a", causing diaphragm 81 to flex back toward seat 26". Accordingly, diaphragm 81 begins to oscillate ata relatively high audio frequency. Again, the acoustic vibrations are communicated through hose 15 and via the body 12" and the water to the diver, who then can begin his ascent while air still remains in tank 14.
A typical embodiment of valve 83 is shown in FIG. 7, and comprises a piston 87 urged by sufficient tank air pressure to the position shown. In this position, air from the inlet line 82 can flow through the annular space 88 surrounding the reduced diameter region 87a of the piston 87 to the line 84. When the pressure in tank 14 drops to a sufficiently low level, the force of a spring 89 overcomes the air pressure exerted via channel 21" on the piston end 87b. As a result, the piston 87 moves to the right (as seen in FIG. 7), and the piston region 870 is interposed between the lines 82 and 84, blocking air flow therethrough. As a result, the air flow from regulator 11 is diverted to alarm mechanism 80 to initiate oscillation of diaphragm 81, causing production of .the audio alarm warning.
Intending to claim all novel, useful and unobvious features shown or described, I make the following claims:
1. An audio alarm mechanism for use with a self-contained breathing apparatus of the type including a supply tank of air, a first stage regulator receiving air at tank pressure and providing air at a regulated pressure, and a conduit for conducting air to a second stage demand regulator, said mechanism comprising:
a. an oscillator member adapted for motion between a flow communicating position in which air from said first stage regulator flows past one end of said oscillator member to said conduit, and a flow impeding position in which air flow from said first stage regulator to said conduit is impeded,
b. first means for subjecting said oscillator member one end to air at said regulated pressure,
c. feedback means for subjecting the other end of said oscillator member to the demand air pressure in said conduit, and
d. bias means for urging said oscillator member toward said flow impeding position, a differential between said demand pressure and said regulated pressure initiating oscillation of said member at audio frequency to generate said audio alarm.
2. An audio alarm mechanism according to claim 1 further comprising:
alarm inhibit means for maintaining said oscillator member in said flow communicating position when the air pressure in said supply tank is above a preselected reserve level, and forpermitting oscillation of said member when the pressure in saidsupply tank drops below a reserve level.
3. An audio alarm mechanism according to claim 1 wherein said oscillator member comprises a piston.
4. An audio alarm mechanism according to claim 3 wherein said member is disposed in a chamber, and wherein said feedback means comprises a feedback opening communicating between said conduit and the actuating region of said chamber defined by said member other end.
5. An audio alarm mechanism according to claim 4 wherein the mass of said piston, the force of said bias means and the dimensions of said feedback opening cooperate to cause oscillation of said piston at a relatively low audio frequency.
6. An audio alarm mechanism according to claim 1 and enclosed in a housing containing said chamber, said member comprising a piston disposed for reciprocation in said chamber, and wherein said bias means comprises a spring urging said piston toward one end of said chamber.
7. An audio alarm mechanismaccording to claim 6 wherein said feedback means comprises an opening through said piston.
8. An audio alarm mechanism according to claim 6 wherein said first means for subjecting comprises an annular chamber communicating with said first stage regulator output and opening into said piston-containing chamber one end.
9. An audio alarm mechanism according to claim 8 further comprising an annular seat at said piston-containing chamber one end, and an outlet passageway central of said seat and communicating between said chamber one end and said conduit, said piston being situated against said seat in said flow impeding position and being spaced from said seat in said flow communicating position.
10. An audio alarm mechanism according to claim 3 wherein said member comprises a diaphragm mounted across a chamber and flexurally biased into contact with a seat at one end of said chamber, and wherein said feedback means comprises a resonating cavity communicating between said chamber and said conduit, and a feedback passageway between said resonating cavity and the actuating region of said chamber adjacent to other end of said diaphragm.
11. An audio alarm mechanism according to claim 10 wherein said diaphragm oscillates between a How impeding position in which said diaphragm one end is in contact with said seat, and a flow communicating position spaced from said seat to permit an air flow from said first stage regulator through the space between said diaphragm and said seat to said resonating cavity and thence to said conduit.
12. In a self'contained underwater breathing apparatus of the type including an air tank supplying a first stage regulator, and a conduit connected to a second stage demand regulator, an audio reserve alarm mechanism comprising:
a. a housing including an interior chamber,
b. an oscillator piston disposed for reciprocation in said chamber,
c. an annular seat at one end of said chamber, I
d. an air inlet chamber surrounding said seat and communicating between said first stage regulator and said chamber,
. an outlet passageway central of said seat and communicating between said chamber and said conduit,
f. a feedback passageway communicating between said outlet passageway and the other end of said chamber, and g a bias spring urging said piston toward said annular seat, h. the pressure differential between ends of said piston resultant during demand by said second stage regulator initiating oscillation of said piston to produce an audio alarm.
13. An alarm mechanism as defined in further comprising:
inhibit means for maintaining said piston spaced from said seat to prevent oscillation of said piston when the air pressure in said tank is above a preselected value and for permitting oscillation of said piston to produce said audio alarm'when the air pressure in said tank drops below said preselected value.
14. An alarm mechanism as defined in claim 13 wherein said inhibit means comprises a stem situated within said housing and adapted to i move between an extended position maintaining said piston spaced from said seat and a retracted position permitting oscillation of said piston,
a spring biasing said stem toward said retracted position, and
means for subjecting said stem to the force of air at tank pressure, said force overcoming said spring bias to urge said stem to said extended position only when said tank pressure is above said preselected level.
15. An alarm mechanism as defined in claim 13 wherein the mass of said piston and the dimensions of said feedback passageway are selected so that said position oscillates at a relatively low audio frequency, the resultant acoustic vibrations being transmitted both via said conduit and through the water to provide both aural and tactile warning that air tank reserve pressure has been reached.
16. An alarm mechanism as defined in claim 12 wherein said feedback passageway comprises an opening through said piston.
17. An alarm mechanism asdefined in claim 12 wherein said feedback passageway comprises a channel through said housing.
18. An alarm mechanism as defined in claim 12 wherein said first stage regulator also is situated in said housing.
19. An alarm mechanism as defined in claim 12 further comprising an O-ring situated in a groove at one end of said piston facing said seat.
claim 12 20. An alarm mechanism as defined in claim 19 wherein said piston is adapted for limited tilting motion within said chamber.
21. An audio alarm for indicating to a diver when his scuba air tank pressure has dropped to reserve level, comprising:
a. a housing having an interior chamber,
b. an oscillator piston reciprocally disposed in said chamber and dividing said chamber into an air inlet region at one end and an actuating chamber 10 region at the other end, one end of said piston and the corresponding one chamber end cooperating to gate air from the scuba first stage regulator to an outlet passageway leading to the scuba second stage regulator,
c. an air feedback path communicating between said outlet passageway and said actuating chamber region,
d. means for biasing said piston toward said one chamber end, and
e. means for inhibiting reciprocation of said piston until said air tank pressure drops to reserve level,
f. whereby diver inhalation causes reduced pressure in said actuating chamber, thereby causing said piston to move toward said actuating chamber region permitting air to be gated from said first stage regulator to said outlet passageway, said biasing means then returning said piston back toward said one chamber end, resulting in oscillation of said piston and generation of acoustic vibrations warning the diver of said reserve level condition.
22. An audio alarm according to claim 21 wherein 23. An audio alarm according to claim 22 further comprises an O-ring mounted on said piston one end and cooperating with an annular seat at said one chamber end to gate said air.
24. An audio alarm according to claim 23 wherein said O-ring is disposed in an annular, dovetailed groove vented to said actuating chamber.
25. An audio alarm according to claim 23 further comprising an O-ring around the periphery of said piston and situated relatively nearer to said piston one end than to the other piston end, thereby permitting limited tilting motion of said piston for sensitive initia-' tion of oscillation.
26. An audio alarm according to claim 21 wherein said means for inhibiting maintains said piston spaced from said one chamber end, permitting unimpeded air flow from said first stage regulator to said outlet passageway, while said tank pressure is above reserve level.
27. An audio alarm mechanism according to claim 1 wherein said oscillator member comprises a diaphragm.
28. An audio alarm mechanism according to claim 27 wherein said member is disposed in a chamber, and wherein said feedback means comprises a feedback opening communicating between said conduit and the actuating region of said chamber defined by said member other end.
29. An audio alarm mechanism according to claim 6 wherein said feedback means comprises a passageway through said housing from said conduit to the region of said chamber adjacent said member other end.
30. An audio alarm according to claim 21 wherein said feedback path comprises a feedback passageway through said housing.