Surgery – Respiratory method or device – Means for supplying respiratory gas under positive pressure
Reexamination Certificate
2000-05-15
2004-03-30
Lewis, Aaron J. (Department: 3761)
Surgery
Respiratory method or device
Means for supplying respiratory gas under positive pressure
C128S201250, C128S201270, C128S201280, C128S203140, C128S204260, C128S205110, C340S508000
Reexamination Certificate
active
06712071
ABSTRACT:
The present invention relates to self-contained breathing apparatus such as may be used for underwater diving or in other hostile environments in which a user may need a supply of breathable gas. Such uses include fire fighting where the atmosphere may be heavily polluted with combustion products and noxious gases, other industrial environments where the atmosphere may be polluted or otherwise unbreathable, or at high altitude where the atmosphere itself is too thin or effectively non-existent.
Although applicable to a wide range of other uses the present invention will be described hereinafter with particular reference to its application to underwater breathing apparatus for diving applications. It will be understood, however, that this description is provided without prejudice to the generality of the invention or its range of applications.
It is well known to provide divers with self-contained underwater breathing apparatus in order to prolong the time for which they can remain below the surface of the water. The most widely used self-contained breathing apparatus comprises a rigid container within which is housed a supply of compressed air which is allowed out of the container via a high pressure or first stage regulator and directed through a flexible hose to a mouthpiece containing a demand valve including a second stage regulator which acts automatically to open and close as the diver inhales and exhales. Such systems are known as open-circuit breathing apparatus because exhaled gas is allowed to pass directly out into the marine environment so that a stream of bubbles is emitted upon each exhalation. If the compressed gas breathed from the gas container is air a large proportion of the exhaled gas will constitute nitrogen which is present in air in an approximate ratio of 4:1 with oxygen as is well known. In other words 80% of the air which is breathed by the diver, and therefore 80% of the content of the compressed air container, or air bottle, comprises little more than a vehicle for the oxygen some of which is converted to carbon dioxide during its residence in the lung. Thus 80% of the breathed gas is not really needed by the body except to dilute the oxygen. It is not possible to breath pure oxygen below 10 m since at higher pressures oxygen is toxic.
Proposals have in the past been made for so-called closed circuit or “re-breather” apparatus in which the carbon dioxide content of exhaled air is removed from the exhaled air outside the body, fresh oxygen is introduced to replace that consumed, and the thus-reconditioned air returns to the diver for re-breathing. In this way it is necessary for the diver only to carry two or three lungfuls of nitrogen sufficient to circulate around the closed circuit. Such a system is described, for example, in U.S. Pat. No. 4,964,404 to William C Stone and in U.S. Pat. No. 3,555,098 to John W Kanwisher and Walter A Starck II. These Patentees were not the first to devise closed circuit re-breather apparatus, however, it being known that so-called “frogmen” used re-breather apparatus during World War 2 in order to avoid the tell-tale bubbles rising to the surface upon exhalation in an open-circuit system such as the traditional compressed air bottle arrangement described above. U.S. Pat. No. 4,964,404 describes an improved such mixed gas breathing apparatus in which a container for exhaled gas (the so-called counterlung) is formed in two parts, a first part communicating with a hose leading from a mouthpiece to a carbon dioxide removal filter, and a second part in the line between the carbon dioxide removal filter and the mouthpiece. The carbon dioxide removal filter in the system described in U.S. Pat. No. 3,556,098 includes a chamber housing oxygen partial pressure sensors used to detect the oxygen content in the exhaled gas and to reinstate the oxygen balance by introducing oxygen through a valve controlled indirectly by the sensors. The oxygen sensor system is described as comprising three sensors with the average value of the sensor signals being taken to produce the control signal. Three sensors are used on the grounds that the appropriate introduction of the right amount of oxygen is so critical, in these circumstances, that it is not possible safely to rely on the signal from a single sensor or even two sensors because any failure of a sensor may not be detected or recognised sufficiently quickly to prevent inadequate oxygenation of the circulating gas, or excess oxygenation depending on the nature of the failure. The argument presented for utilising three sensors is that by taking the average of three sensors the departure from the correct value introduced by a single faulty sensor is minimised. The effect of a faulty sensor on the average value is limited by electronically “clipping” the values to predetermined maximum and minimum values. The three sensors are monitored so that should one start to produce a signal which differs materially from that produced by the other two an alarm is indicated and the dive can be aborted. This strategy is based on the fact that the probability of two sensors being faulty is low, and the probability of two sensors being faulty at the same time is lower and can be reduced even further by taking remedial action immediately a faulty sensor is detected.
However, although this makes concessions to absolute safety by using an alarm signal upon departure of one sensor beyond a predetermined threshold from the other two, this results in the need for the diver to make a judgement on whether the other two sensors are performing properly and risks disruption to the diving activity unless the remaining two sensors are so clearly providing the correct control signal that the diver can come to the conclusion that he can safely ignore the third. The safety strategy adopted by W Stone in U.S. Pat. No. 4,964,404 is further reinforced by the provision of two entirely separate closed circuit re-breather systems each having front and back counterlungs and each being adapted to utilise components of the other system in the event of failure. Such 100% redundancy is necessitated by the chosen strategy in the management of the sensor signals and results in considerable additional equipment expense and bulk.
The present invention seeks to provide self-contained breathing apparatus of the closed-circuit re-breather type in which an improved strategy for management of the oxygen sensors is adopted which, whilst recognising the possibility of failure of an oxygen sensor, monitors the operation in a more practical manner and avoids the necessity for the duplication of all the components without loss of safety. Indeed, safety of the diver remains of paramount importance and numerous features of the apparatus formed in accordance with the present invention are directed at minimising the risk to the diver whilst nevertheless avoiding the need unnecessarily to resort to open circuit emergency breathing due to minor malfunctioning of equipment.
According to a first aspect of the present invention, there is provided self-contained breathing apparatus of the type having a container for receiving exhaled gas, means for removing carbon dioxide from the exhaled gas, sensor means for detecting the oxygen content of the exhaled gas and means for injecting oxygen into the exhaled gas to reinstate the oxygen content so as to lie within a desired range for re-breathing, in which the signals from the oxygen sensor means are delivered to two independent signal processing circuits which are interconnected in a primary and secondary relationship with the primary signal processing circuit acting in use to control the operation of a solenoid valve for injection of oxygen into the exhaled gas and the secondary signal processing circuit acting in use to display information concerning the sensor output signals to provide confirmation of the satisfactory operation of the master signal processing circuit.
Preferably the said signal processing circuits are interconnected with a signal line and the secondary signal processing circuit is able c
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
Lewis Aaron J.
Parker Martin John
Weiss Joseph
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