1. Field of the Invention
This invention relates to the field of underwater breathing apparatus (UBA), more specifically to self-contained, closed-circuit underwater breathing apparatus, which operates without need of air or breathing gas from an outside supply remote from the diver, and wherein carbon dioxide gas (CO.sub.2) generated by a diver is constantly removed and oxygen (O.sub.2) needed for metabolism is constantly supplied.
2. Description of the Prior Art
A closed-circuit UBA is a form of self-contained underwater breathing apparatus (SCUBA) in which a diver's breathing gas is recycled through a closed loop, adding oxygen and removing carbon dioxide gases as needed. The carbon dioxide gas is typically removed by chemical absorption. Elements of UBA typically comprise an oxygen supply bottle, a canister containing CO.sub.2 absorbent, a breathing bag or flexible volume element, connecting hoses, a mouthpiece for the diver, and a diluent gas or inert gas bottle. Diluent gases such as nitrogen, inert gases such as helium, or helium/nitrogen mixtures are often used. Some UBA versions monitor O.sub.2 electronically and add oxygen when inspired O.sub.2 concentrations drop below desired levels. The only mechanical adjustments that can be made by a diver involve the degree of filling of the breathing bags. As ambient pressure changes, for example as the diver goes deeper or rises, the volume of gas within the breathing bag(s) contracts or expands respectively, and diluent gas may be added or dumped, either manually or automatically. Some UBA have a single breathing bag; others have one for the inhalation side and another for the exhalation side of the recirculating loop that handles the gas movement through the rig.
The closed-circuit breathing apparatus and its basic construction and principles of operation have been known for some time (U.S. Pat. No. 3,837,337 to LaViolette, 1974). Improvements to such equipment are always being made for the diving community, which includes military, commercial underwater construction and salvage, and sport divers. For example, improvements in the control of air or breathing gas flow within the apparatus are discussed in U.S. Pat. No. 4,440,166 to Winkler, et al, (1984), particularly with respect to emergency mechanical control system in the event that the electronics fails.
The majority of patents in the art center around the two critical performance factors for UBA, namely CO.sub.2 absorption and O.sub.2 control. Almost none deal with improving or lessening the difficulty of breathing at depth, particularly during arduous exercise or heavy work. Those that do, deal only with the fluid mechanical flow resistance and not other sources of impedance or mechanical resistance to breathing arising from the elements in the UBA including tubing or hoses, canister, etc. One exception to this is the prior invention (now U.S. Pat. No. 5,315,988 to Clarke, et al., issued May 31, 1994) by three of the inventors common to the present invention. In general, breathing resistance in a UBA can be significant for the diver; it can reduce his effectiveness or the duration of work capability, and may, more seriously, contribute to loss of consciousness.
The mechanical resistance to breathing on a UBA is complex, because the breathing is sinusoidal or periodic in nature and not a steady flow. In this kind of flow situation dynamic analyses must be employed, such as those common to the art and discussed in detail by R. Peslin and J. J. Fredberg, "Oscillation mechanics of the respiratory system", chap. 11 in Handbook of Physiology; Vol. III, The Respiratory System, A. P. Fishman (ed.), 1987, American Physiol. Soc., Bethesda, Md., and H. D. Van Liew, "The electrical-respiratory analogy when gas density is high", Undersea Biomedical Research, vol. 14, no. 2, (1987) pp. 149-160. In a periodic flow, allowance must be made for additional resistances to the motion of the breathing gas. These resistances are termed elastic and inertial, and they cause increased energy loss, because the diver must overcome them, as well as the resistance due to flow, to keep breathing. Inertial resistance or inertance arises from accelerations and decelerations in the gas flow or displaced water due to the periodic nature of the flow. Elastic resistance or elastance arises from pressure changes due to flow entering a closed volume or pressure changes due to volume changes (in submersed breathing bags for example). Because of the oscillatory or periodic nature of the flow, complex algebra must be used to describe the overall resistance to flow, which is termed the impedance. Therefore, for a linear model; EQU Z=R-(jE/.omega.)+j.omega.I Eqn. 1
where Z is the impedance in units of pressure/flow rate, R is the resistance due purely to flow (the flow resistance) in the same units, E is the elastance in units of pressure/volume, I is the inertance in units of pressure/flow acceleration, and .omega. is the radian frequency in units of reciprocal time. Eqn. 1 applies to a series arrangement of R, E and I with linear resistance, elastance and inertance, which is fundamentally typical of UBA, but other more complex models may also be used. Impedance is composed of a real part, namely the flow resistance, and an imaginary part, which is a combination of the inertance and the elastance. The magnitude of Z can be computed by; EQU .vertline.Z.vertline.=.sqroot.[R.sup.2 +(.omega.I-E/.omega.).sup.2 ]Eqn. 2
so that all three components of impedance contribute to the pressure required to drive the flow in the system.
At the natural or resonant frequency, the inertial and elastic terms in Eqn. 2 cancel, leaving only the flow resistance contributing to the impedance. Thus impedance is at a minimal value when the system is oscillating at the natural frequency, the condition of which is given below; EQU .omega..sub.n =.sqroot.(E/I) Eqn. 3
The foregoing are terms of the art necessary to understand the present invention, but they do not constitute the invention.
Impedance in the UBA adds to the positive and negative respiratory pressures that a diver must generate to breathe. Impedance is generated by the elastance, inertance and resistance in the UBA. Resistance in UBA arises from breathing hoses, valves, changes in flow diameter, the canister and other similar obstructions in the flow path. Resistance is the fluid mechanical cost of moving a volume of fluid at a given rate. The ratio of pressure difference required to cause a given flow rate to the flow rate is termed the flow resistance.
Elastance is the reciprocal of compliance in the system and is derived in UBA primarily from changes in volume of the breathing bag when immersed. If these changes in volume lead to a vertical expansion or contraction of the bag, pressure is altered by hydrostatic forces, wherein the pressure change (.increment.P) is given by; EQU .increment.P=.rho.g.increment.h Eqn. 4
where .increment.h is the vertical displacement and p is the density of the ambient fluid, usually water or sea water, and g is the acceleration of gravity. The shape of the bag and its orientation in the water have an effect on the elastance. Reference is made to D. D. Joye, J. R. Clarke, N. A. Carlson and E. T. Flynn, "Formulation of Elastic Loading Parameters for Studies of Closed-Circuit Underwater Breathing Systems", NMRI Technical Report 89--89, Bethesda, Md. This reference is available from the National Technical Information Service (NTIS). Elastance is inversely proportional to the cross-sectional area that is perpendicular to the vertical direction. In general, as the breathing bag changes volume, the hydrostatic component of pressure from the top to the bottom of the bag is the elastic pressure. There are other contributions to elastance in a UBA, for example the volume of internal hoses and containers in the breathing loop, that have an additional, but much smaller, effect.
Inertance arises from the acceleration of mass in a system. The larger the mass the higher the inertance. Accelerated masses comprise breathing gases, water displaced by the breathing bag and various UBA components. Inertance (I) can be calculated from the formula: EQU I=m/A.sup.2 Eqn. 5
where m is mass and A is the cross-sectional area through which mass is moved, or the sectional area which moves with the mass. For a fluid moving in a tube or conduit and filling the cross-sectional area, this reduces to .rho.p L/A, where L is the tube length and .rho. is the density of the fluid.
The force that moves the flow of breathing gas is respiratory pressure. Although the respiratory pressure imposed by UBA elastance can be relatively high at the low frequencies commonly encountered in a diver's breathing pattern, which is typically in the range 5-60 breaths/minute, particularly 10-40 bpm, there have been no efforts to either statically or dynamically reduce UBA elastic impedance to make it easier for the diver to breathe.
Some efforts have been made in the design of breathing machines (not UBA) that simulate human breathing or can be adjusted to generate other breathing conditions, and/or to provide adjustable impedance. Reference is made to M. Younes, D. Bilan, D. Jung and H. Kroker, "An apparatus for altering the mechanical load of the respiratory system", J. Applied Physiology, vol. 62, no. 6 (1987) pp. 2491-2499, wherein adjustment to elastance by changing gas volume in the machine is shown. Inertance is not adjusted, and oscillatory behavior is damped, not fostered.
In U.S. Pat. No. 5,315,988 to Clarke, et al., issued May 31, 1994, a reactive, closed-circuit underwater breathing apparatus is described in which tuning is accomplished continually by electronic sensing, computing and control action. In general, the tuning principles used in this invention are the same as those described therein, i.e. adjusting inertance of the UBA by changing the amount of water moved by displacement or volume changes of the breathing bag during inhalation/exhalation. Because Clarke, et al. is the work of common inventors and the present invention is a variation of the application of the same principles, the disclosure of the present invention hereby incorporates by reference Clarke, et al. to this specification, specifically from col. 1, line 1 to col. 4, line 10 and substantial portions from col. 6, line 22 to col. 8, line 28.