Abstract:
There is disclosed a self-contained breathing apparatus of the positive pressure type wherein there is provided a pneumatic servo mechanism responsive to a pressure sensor to drive a breathing storage chamber by expansion of an oxygen containing gas from a storage tank therefor.

Description:
BACKGROUND OF THE INVENTION (1) Field of the Invention 
     This invention relates to a self-contained breathing apparatus, and more particularly to a closed-circuit positive pressure self-contained breathing apparatus for temporary use by a wearer in a noxious environment, such as is worn by a firefighter. (2) Description of the Prior Art 
     Self-contained breathing apparatus (SCBA&#39;s) are worn by industrial workers, and in particular firefighters, to provide a safe, respirable breathing condition while the user works in a hostile environment. Currently, breathing performance and service life rating of such apparatus are based upon user consumption at the rate of 40 liters per minute, wherein inhalation and exhalation reach peaks instantaneous flow rates of about 115 liters per minute. For firefighting duty, the National Fire Prevention Administration (NFPA) has defined new performance standards (for open circuit SCBA&#39;s) wherein peak instantaneous breathing rates exceed 200 liters per minute. 
     SCBA&#39;s are classified as open-circuit (where the user&#39;s exhalation is dumped from the system) or closed-circuit (where exhalation is returned to the system for subsequent reuse after carbon dioxide is removed and oxygen is added). 
     Although closed-circuit, self-contained breathing apparatus have existed longer than open-circuit types, there are some inherent disadvantages of closed-circuit systems which offset the substantial weight and size advantage they offer for extended duration. One disadvantage is the sluggish response of the system to the user&#39;s breathing requirements, particularly at high metabolic work rates. A second disadvantage is the design difficulty encountered in trying to create a constant positive pressure in the facepiece (positive pressure substantially increases the degree of respiratory protection to the wearer). 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide an improved self-contained closed-circuit positive pressure breathing apparatus of extended rating which provides an amplified response to the user&#39;s breathing effort, resulting in very low breathing resistance. 
     Another object of the present invention is to provide an improved closed-circuit self-contained breathing apparatus which is capable of maintaining a positive pressure in the facepiece, even during high inhalation flow rates, but without having substantial resistance to high exhalation flows. 
     Still another object of the present invention is to provide an improved closed-circuit positive pressure self-contained breathing apparatus with breathing performance which meets or exceeds that of an open-circuit apparatus. 
     Yet another object of the present invention is to provide an improved closed-circuit self-contained positive pressure breathing apparatus. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention are achieved in a self-contained breathing apparatus of the positive pressure type wherein there is provided a pneumatic servo mechanism responsive to a pressure sensor to drive a breathing storage chamber by expansion of an oxygen containing gas from a storage tank therefor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further objects and advantages of the present invention will become apparent from the following detailed description thereof when taken with the accompanying drawings wherein like numerals designate like parts throughout, and wherein: 
     FIG. 1 is a schematic flow diagram of the positive pressure self-contained breathing apparatus of the present invention; and 
     FIG. 2 is an enlarged partial cross-sectional plan view of the pressure regulating assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is schematically illustrated a self-contained breathing apparatus of the present invention, generally indicated as 10, mounted in a supporting carrier assembly (not shown) including a helmet (H) and comprised of a compressed breathing gas supply tank 12; a pneumatic servovalve assembly, generally indicated as 14; an inhale/exhale delivery conduit 16; a canister assembly 18 and a breathing storage assembly 20. The breathing gas supply tank 12 (containing either air, oxygen, oxygen enriched air, or another mixture of oxygen in combination with other breathable gases) is of a size to provide about 2 to 5 liters of air to the system, depending on the user&#39;s metabolic work rate and is connected by a conduit 22 under the control of a pressure reducer valve 24 via a conduit 26 to the pneumatic servovalve assembly 14. 
     The inhale/exhale delivery conduit 16 is connected by a conduit 28 under the control of one-way valve 30 to the chemical canister assembly 18 for the removal of carbon dioxide (and possibly chemical addition of oxygen) therein as known to one skilled in the art. The canister assembly 18 is connected by a conduit 32 to the breathing storage assembly 20 defining a variable volume gaseous storage chamber 34 in fluid communication by a conduit 36 under the control of one-way valve 38 with the inhale/exhale delivery conduit 16. A conduit 40 is provided between the pneumatic servovalve assembly 14 and the inhale/exhale delivery conduit 16 to sense the breathing circuit pressure therein. 
     The canister assembly 18 contains a chemical bed (or beds) of one or more well known carbon dioxide-adsorbing material, and possibly one or more oxygen producing chemicals. Various carbon dioxide-adsorbing materials are well known and readily available at relatively inexpensive prices. The material producing oxygen could be one of several known chemicals which react with moisture and carbon dioxide to generate oxygen, such as potassium superoxide, lithium superperborate or the like. 
     The breathing storage assembly 20 is comprised of an upper wall 42 and a lower wall 44 connected by flexible bellowed-shaped side wall 46 and reinforced to withstand significant positive and negative internal pressure. Through the upper wall 42, referring particularly to FIG. 2, there is slidably positioned a rod member 48 vertically disposed for reciprocal movement and mounted by a nut 50 to the lower wall 44 of the breathing storage assembly 20. The rod member 48 is provided with a piston 52 disposed in a cylinder member 54 mounted to an upper surface portion of the upper wall 42 of the breathing storage chamber 34 and defining an upper chamber 56 and a lower chamber 58 therein. 
     The cylinder member 54 is preferably mounted to the upper wall 42 of the breathing storage assembly 20 to ensure positive displacement of the rod member 48 mounted to the lower wall member 44 with respect to the movement of the rod member 48 and associated piston 52 disposed within the cylinder member 54 as more fully hereinafter described. The rod member 48 is provided with appropriate sealing assemblies, such as ring members 60 disposed in upper and lower walls 62 and 64 of the cylinder member 54. The piston 52 is provided with a ring seal 66 to prevent fluid flow between the upper and lower chamber 56 and 58 of the cylinder member 54. 
     The pneumatic servovalve assembly 14 includes a cylindrically-shaped pressure sensor housing 70 defining a chamber 72 and having a diaphragm member, generally indicated as 74, including a disc-shaped body portion 76 and a spindle member 78 positioned within the chamber 72 and defining an upper chamber 80 and a lower chamber 82 with a donut-shaped flexible membrane member 84 attached between the body portion 76 and the pressure sensor housing 70. The upper portion of the pressure sensor housing 70 is provided with channels 86 to provide fluid communication between the atmosphere and the upper chamber 72 of the pressure sensor housing 70. A channel 88 is provided in the lower portion of the pressure sensor housing 70 to provide fluid communication between the lower chamber 82 and the conduit 40 as more fully hereinafter described. 
     The spindle member 78 is provided with an upper valve portion 92 and a lower valve portion 94 for positioning within valve seating chambers 96 and 98, respectively, formed in the upper and lower wall portions of the pressure sensor housing 70. Diaphragm seals 100 and 102 are mounted to the upper and lower portions of the spindle member 78 and to proximate portions of the pressure sensor housing 70 to isolate the chambers 80 and 82 from the valve seating chambers 96 and 98. The upper valve seating chamber 96 is in fluid communication via an orifice 104 with the chamber 56 of the member 54 by a conduit member 105 and with the conduit 36 via a conduit 106. The lower valve seating chamber 98 is in fluid communication via an orifice 108 with the chamber 58 of the cylinder member 54 via a conduit member 110 and with the conduit 36 via a conduit 112. 
     A positive pressure of approximately 20 mm. of water in the breathing conduit 16 is transmitted to the chamber 82 via the conduit 40 so that the spindle member 78 is lifted to a neutral position with equal flow restriction through orifices 104 and 108. An intermediate conduit 114 including reducing orifices 116 and 118 is provided for fluid communication between the conduit member 105 and the conduit member 110 and the conduit 26 to provide fluid communication of the compressed air from the pressure reducer 24 to the pneumatic servovalve assembly 14. 
     A coil spring 118 is provided in the upper chamber of the pressure sensor housing 70 to bias the valve portion 94 of the spindle member 78 against the needle valve chamber 98 in order to create a positive pressure in the breathing circuit. A positive pressure of approximately 20 mm. of water in the breathing conduit 16 is transmitted to the chamber 82 via the conduit 40 so that the spindle member 78 is lifted to a neutral position with equal flow restriction through orifices 104 and 108. 
     In operation, the spindle member 78 acts a rod stem at each end by restricting flow through either the upper orifice 104 or the lower orifice 108 thereby creating an imbalance in the pneumatic network comprised by the pneumatic resistance of the orifices 104, 108, 116 and 118. Thus, the chambers 56 and 58 of the cylinder member 54 function to control the movement of the piston 52 (and thus the movement of the breathing storage chamber 20) as a result of the pressure imbalance between the chambers 56 and 58. 
     During inhalation, the pressure on the breathing circuit in conduit 16 drops slightly below the 20 mm. static pressure, causing the pressure in the lower chamber 58 of the cylinder 54 to increase thereby applying a net upward force against the piston 52 of the rod member 48 to cause the lower wall 44 to rise and compress the breathing storage chamber 34 with sufficient force to overcome what ever resistance is present between the storage chamber 34 and the helmet (H). As the storage chamber 34 responds, pressure in the breathing circuit increases towards the 20 mm. static pressure thereby providing feed-back to the pneumatic servovalve assembly 14 thereby returning to a neutral position the spindle member 78 of the pressure sensor housing 70. 
     Similarly, exhalation causes an increase in breathing circuit pressure in conduit 16 which causes an increase in pressure in the upper chamber 56 of the cylinder member 54 causing the breathing storage chamber 34 to expand to assist overcoming resistance in the exhalation circuitry. 
     In accordance with the present invention, use is made of the energy in the pressurized breathing gas tank 12 to drive a servo assembly in response to the user&#39;s demand, energy otherwise unused, and without requiring substantially more gas than would be required to replenish oxygen or flush the rebreather system. Following its use in pneumatically amplifying the breathing response and driving the breathing storage chamber, the breathing gas is vented into the breathing circuit conduit 36 via conduits 106 and 112. In summary, the user&#39;s breathing creates a pressure change in the breathing conduit 16 which is transmitted through the conduit 40 to the lower chamber 82 of the pneumatic servovalve assembly 14 causing the spindle member 78 to respond up or down to restrict flow through either of the restrictive orifices 104 or 108, respectively. 
     While the invention has been described in connection with an exemplary embodiment thereof, it will be understood that many modifications will be apparent to those of ordinary skill in the art; and that this application is intended to cover any adaptations of variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equivalents thereof.