Rebreather system with depth dependent flow control and optimal PO.sub.2 d e

• US 5,924,418 A
• Filed: 07/18/1997
• Issued: 07/20/1999
• Est. Priority Date: 07/18/1997
• Status: Expired due to Fees

• Current Assignee: HO Underwater Acquisition LLC
• Original Assignee: John E. Lewis
• Inventors: Lewis, John E.
• Primary Examiner(s): Asher, Kimberly L.
• Application Number:
  US08/897,092
• Time in Patent Office: 732 Days
• Field of Search: 128/201.27, 128/201.28, 128/204.21, 128/204.26, 128/205.11, 128/205.24, 128/205.22, 128/204.29, 128/205.28
• US Class Current: 128/204.22
• CPC Class Codes: B63C 11/24 in closed circulation

Abstract

A method and apparatus for a self contained underwater breathing apparatus in which a breathing gas is supplied to a flow loop from two separate gas sources each having a different oxygen fraction, and each controlled by separate mass flow controllers having variable flow rate. The mass controller flow rates are adaptively adjustable to deliver gas at variable flow rates which depend solely on a function of depth. An algorithm determines these specific flow rates from each of the tanks at particular depths, such that the gas flow from an oxygen rich gas source decreases as a function of depth, while the gas flow from a diluent gas source increases as a function of depth, so as to maintain the oxygen partial pressure in the flow loop within a specific pre-deternined range. The algorithm allows calculation of an optimum oxygen partial pressure, for a particular dive, which allows construction of a dive profile which maximizes bottom time while taking into account no-decompression time at depth, tank capacity limited time, and single-dive and daily pulmonary oxygen toxicity limits.

Claims

1. In a semi-closed circuit rebreather system of the type comprising an oxygen rich gas source and a diluent gas source, configured to provide a breathing gas mix to a flow loop including a counterlung, a method for adaptively controlling flow rates of said oxygen rich and said diluent gasses so as to provide the partial pressure of oxygen of the breathing gas mixture within a specified range comprising;

providing a first, oxygen rich gas source having a first oxygen fraction, F_O.sbsb.2 ;

providing a second diluent gas source having a second oxygen fraction, F_AIR ;

providing first and second flow control means, coupled respectively to said first oxygen rich gas source and said second diluent gas source, the flow control means individually adjustable for controlling gas flow from their respective gas sources to the counterlung flow loop; and

adaptively adjusting said first and second flow control means so as to vary flow rates from the oxygen rich gas source and the diluent gas source in a manner solely dependent on ambient pressure expressed as a function of diving depth.

2. The method according to claim 1 further comprising the steps of:

choosing a first, maximum, oxygen partial pressure, P_O.sbsb.2^MAX, the maximum oxygen partial pressure value defining a parametric limit;

choosing a second, minimum oxygen partial pressure, P_O.sbsb.2^MIN, the minimum oxygen partial pressure defining a parametric limit;

defining a first, minimum, oxygen consumption rate, O₂^MIN ;

the minimum oxygen consumption rate defining a parametric limit;

defining a second, maximum, oxygen consumption value, O₂^MAX, the maximum oxygen consumption rate defining a parametric limit;

to thereby define a parametric boundary space governing the adaptive adjustment of oxygen rich and diluent gas source flow rates; and

adaptively adjusting said flow control means within the parametric boundary space and in accordance with ambient pressure.

3. The method according to claim 2, wherein the flow rate of the oxygen rich gas source is adaptively adjusted as a function of depth in accordance with an algorithm defined as:

space="preserve" listing-type="equation">V.sub.O.sbsb.2 =(CE-B)/(AE--BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)-P_O.sbsb.2^MAX V_FL (DR/33)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)-P_O.sbsb.2^MIN V_FL (DR/33)

and where P_AMB is the depth dependent ambient pressure.

4. The method according to claim 3, wherein the flow rate of the diluent gas source is adaptively adjusted as a function of depth in accordance with an algorithm defined as:

space="preserve" listing-type="equation">V.sub.AIR=( AF-CD)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)-P_O.sbsb.2^MAX V_FL (DR/33)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)-P_O.sbsb.2 MINV_FL (DR/33)

and where P_AMB is the depth defined ambient pressure.

5. The method according to claim 4, wherein the oxygen rich gas source comprises pure oxygen, such that the first oxygen fraction, F_O.sbsb.2, is equal to 1.0.

6. The method according to claim 5, wherein the diluent gas source comprises compressed air, such that the second oxygen fraction, F_AIR, is equal to 0.21.

7. The method according to claim 6, wherein the minimum oxygen partial pressure value is defined as that sufficient to avoid the onset of hypoxia, and wherein the maximum oxygen partial pressure is defined as that required to avoid the onset of CNS oxygen toxicity, the minimum and maximum oxygen partial pressure values comprising 0.21 and 1.60 atmospheres, respectively.

8. The method according to claim 7, wherein the minimum and maximum oxygen consumption rates are chosen from a range of from about 0.5 to about 3.0 standard liters per minute.

9. The method according to claim 1, further comprising the steps of:

providing an oxygen sensor in the rebreather flow loop;

providing a signal processing circuit, coupled to the oxygen sensor and configured to perform oxygen consumption rate calculations in operative response to signals received from the oxygen sensor;

calculating and recording a diver'"'"'s oxygen consumption rate, as measured by the oxygen sensor, the signal processing circuit thereby defining a maximum and minimum oxygen consumption rate for a diver under actual conditions;

defining an oxygen consumption parametric range in accordance with the calculated maximum and minimum consumption rates; and

adaptively adjusting said first and second flow control means in accordance with the defined oxygen consumption parametric range, to thereby increase the rebreather'"'"'s gas utilization efficiency by substantially reducing both oxygen rich and diluent gas flow rates, thereby substantially extending dive time.

10. The method according to claim 9, further comprising the steps of:

choosing a first, maximum, oxygen partial pressure, P_O.sbsb.2^MAX, the maximum oxygen partial pressure value defining a parametric limit;

choosing a second, minimum oxygen partial pressure, P_O.sbsb.2^MIN, the minimum oxygen partial pressure defining a parametric limit, to thereby define a parametric boundary space governing the adaptive adjustment of oxygen rich and diluent gas source flow rates; and

adaptively adjusting said flow control means within the parametric boundary space and in accordance with the calculated oxygen consumption parametric range and ambient pressure.

11. The method according to claim 10, wherein the flow rate of the oxygen rich gas source is adaptively adjusted as a function of depth in accordance with an algorithm defined as:

space="preserve" listing-type="equation">V.sub.O.sbsb.2 =(CE-BF)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)-P_O.sbsb.2^MAX V_FL (DR/33)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)-P_O.sbsb.2^MIN V_FL (DR/33)

and where P_AMB is the depth dependent ambient pressure.

12. The method according to claim 11, wherein the flow rate of the diluent gas source is adaptively adjusted as a function of depth in accordance with an algorithm defined as:

space="preserve" listing-type="equation">V.sub.AIR =(AF-CD)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)-P_O.sbsb.2^MAX V_FL (DR/33)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)-P_O.sbsb.2^MIN V_FL (DR/33)

and where P_AMB is the depth defined ambient pressure.

13. The method according to claim 12, wherein the oxygen rich gas source comprises pure oxygen, such that the first oxygen fraction, F_O.sbsb.2, is equal to 1.0.

14. The method according to claim 13, wherein the diluent gas source comprises compressed air, such that the second oxygen fraction, F_AIR, is equal to 0.21.

15. The method according to claim 14, wherein the minimum oxygen partial pressure value is defined as that sufficient to avoid the onset of hypoxia, and wherein the maximum oxygen partial pressure is defined as that required to avoid the onset of CNS oxygen toxicity, the minimum and maximum oxygen partial pressure values comprising 0.21 and 1.60 atmospheres, respectively.

16. A semi-closed circuit rebreather apparatus of the type comprising a flow-loop including a counterlung and a carbon dioxide scrubber canister, the rebreather further comprising:

a first gas supply, comprising an oxygen rich compressed gas, the oxygen rich gas further having a pre-determined oxygen fraction, F_O.sbsb.2 ;

a second gas supply, comprising a diluent gas, the diluent gas having a pre-determined oxygen fraction, F_AIR, less than the oxygen fraction, F_O.sbsb.2, of the oxygen rich gas;

first and second pressure regulators, respectively coupled between the first and second gas supplies and the flow loop of the rebreather;

a first flow controller for controlling the flow rate of oxygen rich gas to the counterlung, coupled between the first pressure regulator of the first gas supply and the rebreather flow loop, the first flow controller having a variable flow rate and adaptively adjusting the oxygen rich gas flow rate to the counterlung in a manner solely dependent on a function of depth; and

a second flow controller for delivering diluent gas to the counterlung, coupled between the second pressure regulator of the diluent gas supply and the rebreather flow loop, the second flow controller having a variable flow rate and adaptively adjusting the diluent gas flow rate to the counterlung in a manner solely dependent on a function of depth, so as to substantially extend dive time.

17. The semi-closed circuit rebreather according to claim 16, wherein the first and second flow controllers are configured to adaptively adjust the oxygen rich and diluent gas flow rates to the counterlung in accordance with a substantially linear dependence to a depth defined ambient pressure.

18. The semi-closed circuit rebreather according to claim 17, wherein the first and second pressure regulators provide intermediate pressures in a manner varying substantially linearly with depth, the flow controllers comprising sonic orifices configured to deliver oxygen rich and diluent gasses to the counterlung in operative response to ambient pressure at flow rates varying substantially linearly with depth defined ambient pressure.

19. The semi-closed circuit rebreather according to claim 18, wherein the flow rate of the oxygen rich gas source is adaptively adjusted as a function of depth in accordance with an algorithm defined as:

space="preserve" listing-type="equation">V.sub.O.sbsb.2 =(CE-BF)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)

and where P_AMB is the depth dependent ambient pressure.

20. The semi-closed circuit rebreather according to claim 19, wherein the flow rate of the diluent gas source is adaptively adjusted as a function of depth in accordance with an algorithm defined as:

space="preserve" listing-type="equation">V.sub.AIR =(AF-CD)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)

and where P_AMB is the depth dependent ambient pressure.

21. The semi-closed circuit rebreather according to claim 20, further comprising means for reducing the flow rate of the oxygen rich gas during descent so as to control the increase of oxygen partial pressure.

22. The semi-closed circuit rebreather according to claim 21, wherein the means for reducing the flow rate of the oxygen rich gas during descent comprises a rigid volume interposed between the oxygen gas source and the counterlung, the rigid volume operative to allow oxygen rich gas to flow to the counterlung only when a descent rate is less than a critical rate.

23. The semi-closed circuit rebreather according to claim 22, wherein the means for reducing the flow rate of the oxygen rich gas during descent further comprises:

a pressure transducer;

an electronically controlled valve coupled to the oxygen rich gas source; and

signal processing circuitry for calculating a descent rate, the signal processing circuitry further providing control signals to the electronically controlled valve to thereby adjust the oxygen rich gas flow rate to the counterlung in accord with the descent rate.

24. The semi-closed circuit rebreather according to claim 16, further comprising a pressure transducer and a digital signal processing circuit, configured to receive data from the pressure transducer, the signal processor being firmware programmable to perform calculations on data input by a user consisting of minimum and maximum values of oxygen partial pressure, minimum and maximum values of oxygen consumption, the oxygen fraction, F_O.sbsb.2, of the oxygen rich gas, the oxygen fraction, F_AIR, of the diluent gas, and the depth as provided by the pressure transducer, the processor functionally connected to the first and second flow controllers for adaptively adjusting the oxygen rich and diluent gas flow rates therethrough so as to maintain an oxygen partial pressure within the rebreather'"'"'s counterlung within a pre-determined maximum and minimum value, solely as a function of depth.

25. The semi-closed circuit rebreather according to claim 24, wherein the first and second flow controllers are electronically controlled mass flow controllers, calibrated to restrict or enhance oxygen rich or diluent gas flow rates therethrough in accordance with control signals provided by the digital signal processing circuit.

26. The semi-closed circuit rebreather according to claim 25, wherein the signal processing circuit calculates a depth dependent flow rate for the oxygen rich gas in accordance with a user defined parametric boundary space consisting of maximum and minimum oxygen partial pressure, P_O.sbsb.2^MAX, and P_O.sbsb.2^MIN, maximum and minimum oxygen consumption rates, O₂^MAX, and O₂^MIN, the oxygen fractions of the oxygen rich and diluent gas sources, F_O.sbsb.2 and F_AIR, the oxygen rich gas flow rate determined according to:

space="preserve" listing-type="equation">V.sub.O.sbsb.2 =(CE-BF)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)-P_O.sbsb.2^MAX V_FL (DR/33)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)-P_O.sbsb.2^MIN V_FL (DR/33)

and where P_AMB is the depth dependent ambient pressure.

27. A semi-closed circuit rebreather according to claim 26, wherein the signal processing circuit calculates a depth dependent flow rate for the diluent rich gas in accordance with a user defined parametric boundary space consisting of maximum and minimum oxygen partial pressure, P_O.sbsb.2^MAX, and P_O.sbsb.2^MIN, maximum and minimum oxygen consumption rates, O₂^MAX, and O₂^MIN, the oxygen fractions of the oxygen rich and diluent gas sources, F_O.sbsb.2 and F_AIR, the diluent gas flow rate determined according to:

space="preserve" listing-type="equation">V.sub.AIR =(AF-CD)/(AE-BD)

where

A=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MAX)

B=(F_AIR P_AMB -P_O.sbsb.2^MAX)

C=O₂^MIN (P_AMB -P_O.sbsb.2^MAX)-P_O.sbsb.2^MAX V_FL (DR/33)

D=(F_O.sbsb.2 P_AMB -P_O.sbsb.2^MIN)

E=(F_AIR P_AMB -P_O.sbsb.2^MIN)

F=O₂^MAX (P_AMB -P_O.sbsb.2^MIN)-P_O.sbsb.2^MIN V_FL (DR/33)

and where P_AMB is the depth dependent ambient pressure.

28. A semi-closed circuit rebreather according to claim 27, wherein the minimum oxygen partial pressure is about 0.21 atmospheres, the maximum oxygen partial pressure is about 1.6 atmospheres, the minimum oxygen consumption rate is about 0.5 SLM and the maximum oxygen consumption rate is about 3.0 SLM.

29. A semi-closed circuit rebreather according to claim 28, wherein the oxygen rich gas source comprises pure oxygen having an oxygen fraction, F_O.sbsb.2, of 1.0, and wherein the diluent gas is compressed air, having an oxygen fraction, F_AIR, of about 0.21.

30. The semi-closed circuit rebreather according to claim 24, further comprising;

an oxygen sensor disposed in the rebreather flow loop; and

a signal processing circuit, coupled to the oxygen sensor and configured to perform oxygen consumption rate calculations in operative response to signals received from the oxygen sensor;

the signal processing circuit calculating and recording a diver'"'"'s oxygen consumption rate, as measured by the oxygen sensor, to thereby define a maximum and minimum oxygen consumption rate for a diver under actual conditions;

the signal processing circuit further adaptively adjusting said first and second flow controllers in accordance with the defined oxygen consumption parametric range and as a function of depth.