What is a Rebreather Four types of rebreathers CCR Oxygen SCR CCR electronic CCR manual (KISS)

1. Chapter 1: Why a CCR • What is a Rebreather • Four types of rebreathers • CCR Oxygen • SCR • CCR electronic • CCR manual (KISS) • Design common to all CCR rebreathers • Gas supply and control • Breathing Loop/ over pressure relief • Counterlungs • Scrubber • Advantages / disadvantages of CCR

2. Chapter 1: Why a CCR What is a Rebreather? • Human Respiration: • On inhalation we inhale 21% O2 • On exhalation we exhale 16% O2 and about 3-4% CO2 • A rebreather has a breathing loop (basically and extension of our lungs) that takes this exhaled 16% O2 and adds more O2, and removes the CO2 (through a chemical reaction)

3. Chapter 1: Why a CCR CCR Oxygen • Simplest design • Gas supply cylinder contains pure oxygen • Breathing loop and scrubber • As diver uses oxygen loop volume decreases and oxygen is added, or oxygen is constantly added at a rate similar to diver metabolic rate (like KISS) • Draw back MOD 20 fsw (6 msw)

4. Chapter 1: Why a CCR SCR? • Gas supply cylinder(s) contains NITROX • Breathing loop, scrubber, over pressure relief valve • Two types: • Passive addition (RMV-Keyed): injects nitrox into loop proportionally to diver's RMV this approach assumes RMV is proportional to metabolic needs (Halcyon) • Active addition (Mass-Flow controlled): gas is injected into loop at a constant mass (i.e. constant # molecules, not volume) (azimuth, dolphin)

5. Chapter 1: Why a CCR SCR • Both systems waste gas • Mass controlled wastes more gas than RMV-Keyed • Unlike the CCR both of these SCR’s do not maintain a • constant PO2. • They have a constant FO2 being supplied to the loop (from • the nitrox cylinder), but the loop FO2 will vary with work • load • Increased workload will result in lower FO2 • Decreased workload will result in increased FO2

6. Chapter 1: Why a CCR CCR electronic control • Gas supply cylinders: one with O2, another with “diluent” • (air, heliox, trimix) • Breathing loop and scrubber • Solenoid and PO2 sensor, add O2 when PO2 drops below • “set-point” • Maintains a relatively constant PO2 • Disadvantage: electronics and water • Advantage: less??!! chance of Hypoxia (vs. KISS) ALWAYS • KNOW PO2 If electronics fail, diver may have false sense of • security in electronic control • Gas efficiency

7. Chapter 1: Why a CCR KISS Manual CCR • Gas supply cylinders: one with O2, another with “diluent” • (air, heliox, trimix) • Breathing loop and scrubber • No solenoid nor electronic controls • Constant flow of O2 into loop (at rate slightly less than • diver metabolic rate)

8. Chapter 1: Why a CCR KISS Manual CCR • The rate is controlled by the orifice size and IP of the • oxygen regulator first stage • Manual O2addition valve • Maintains constant PO2 if controlled correctly by diver • ALWAYS KNOW PO2 • PO2 sensors • Gas efficiency (minimal wastage)

9. Chapter 1: Why a CCR Common Design: Gas Supply • All CCR have two gas supply • cylinders (at least) • Pure oxygen (usually on the right) • Diluent of either Air, Heliox, or Trimix (usually on the left)

10. Chapter 1: Why a CCR Common Design: Gas Supply • STRONG CAUTION: against using high FO2 in diluent • cylinder, this can lead to high PO2 • These two cylinders and scrubber, allow for “on the fly • blending” ensuring the best mix for the depth (constant • PO2) • 13 ft3 (1.8 L) cylinders or larger, O2clean for Oxygen • cylinder and possibly for diluent cylinder • Current VIP and hydro’s • Oxygen regulator has been modified so it Can NOT adjust • to increasing pressure • Over pressure relief valve recommended on regulators

11. Chapter 1: Why a CCR Common Design: Breathing Loop • DSV, inhale and exhale hoses, inhale / exhale • counterlungs • One-way valves on DSV (prevents mixing of exhaled • and inhale gases) • “Scrubber”

12. Chapter 1: Why a CCR Common Design: Breathing Loop • Over pressure relief valve on loop to allow expanding gas to vent on ascent • The loop volume remains constant regardless of depth • As you descend the ADV is activated by diver inhalation effort and adds diluent maintaining a constant volume • On ascent excessive gas is vented through over pressure relief valve, or diver nose

13. Chapter 1: Why a CCR Common Design: Breathing Loop • The loop PO2 should remain constant • The loop is fed a constant mass of oxygen at close to the diver's actual metabolic rate. (KISS CCR manual) • Or a solenoid automatically adds O2when a drop below the set-point is detected. (CCR Electronic) • Additional oxygen can be added manually to increase the PO2 (manual and electronic) • O2sensor (usually on the inhalation side) lets diver know PO2 • Protect sensor from water, heat, O2, and CO2 • If stored in inert gas, allow at least 12 hours in atmospheric • gas before use

14. Chapter 1: Why a CCR Common Design: counterlungs • Back mounted: negative pressure, easy to exhale, resistance on inhale, best position 45 degree • Front mounted: easy inhale, resistance on exhale • Over shoulder: physiologically equal regardless of diver position, more cumbersome, and less protected • Inhale / exhale lungs (verse. one counterlung): increase the time the gas has in contact with the scrubber (dwell time) resulting in increased CO2 removal, also helps keep moisture in the exhale lung, and not in the scrubber

15. Chapter 1: Why a CCR Common Design: The Scrubber • Every rebreather utilizes a scrubber to remove CO2 • Two scrubber designs mainly used: • Axial (KISS, Inspiration): gas flows in a single direction • Resists channeling, • Increased breathing resistance • Radial (Azimuth, Cis-Lunar, Prism): gas enters the center of the scrubber and flows radially outward • Greater chance of channeling • Decreased breathing resistance

16. Chapter 1: Why a CCR Common Design: The Scrubber • Scrubber efficiency is a result of the scrubber design, • granule size, and temperature • Decreased temperature results in decreased efficiency • Increased granule size results in decreased surface • area and decreased CO2 removal capabilities • But the increased intestinal space between the granules results in decreased WOB • Increase canister size results in increased gas • exposure to absorbent resulting in increased CO2 removal • Gas flow will take the path of least resistance this can lead • to “channeling” in a poorly packed canister

17. Chapter 1: Why a CCR Common Design: The Scrubber • The purpose of the scrubber is to remove excess Carbon • Dioxide (CO2) from the breathing “loop” • The scrubber material (sorb) is not a filter, it causes a • chemical reaction • The reaction is affected by temperature, usage, diver • metabolism, dive depth, water and scrubber material used • As temperature decreases the scrubber will not last as long (in • cold water the scrubber must be pre-heated 30 F or less) • This can be done by pre-breathing the system in a warm • environment, and continuing breathing • The scrubber has a recommended amount of time it should be • used, never try to push the limits!

18. Chapter 1: Why a CCR Common Design: The Scrubber • A certain amount of moisture is required to start the • chemical reaction, too much or to little moisture will cause • the scrubber to fail • There is a multitude of scrubber material available • Sodalime (NaOH + Ca(OH)2, lithium hydroxide (LiOH), barium hydroxide (BaOH) • Use only dive rated materials (medical scrubber material • do not have enough moisture)

19. Chapter 1: Why a CCR Common Design: The Scrubber • Scrubber material comes in different sizes • 4-8 grade granules (2.5-5mm) are larger than the 8-12 grade (1-2.5mm), they have a smaller surface area, and decreased breathing resistance • 4-8 will not last as long (3 hrs vs. 5 hrs for the 8-12), but will have less breathing resistance as compared to the 8-12 grade • Scrubber material removes CO2 proportionally to the granule surface area, and gas contact time with the granules • Harder is better as it produces less dust • Hardness is expressed in a percent, 100% being the hardest

20. Chapter 1: Why a CCR Common Design: The Scrubber • KISS CCR it is recommended to use 4-8 grade sorb • Expected time is three (3 hr) hours in moderate water • temperature 40-60 F • Expect the scrubber to last longer in warmer waters and to • last not as long in colder waters • It is highly recommended to start any deep dives with a • fresh scrubber (150 fsw or 45 msw) • It is best not to start any dive with a scrubber expected life • less than 60 minutes. • Before every dive the scrubber needs to be breathed on the • surface to initiate the chemical reaction (time depends on • temperature)

21. Chapter 1: Why a CCR Common Design: The Scrubber • The Reaction: • CO2 + H2O = H2CO3 (carbonic acid) • 2. H2CO3 + 2NaOH = Na2CO3 + 2H2O + Heat • H2CO3 + Ca(OH)2 = CaCO3 + 2H2O + Heat • 3. Na2CO3+Ca(OH)2 =CaCO3 + 2NaOH + Heat • Exothermic reaction results in heat production • If too much liquid is allowed into the scrubber then the NaOH and Ca(OH)2 will dissolve creating a caustic cocktail • The more dust and longer liquid contact time = more caustic the cocktail

22. Chapter 1: Why a CCR Common Design: The Scrubber • Scrubber material specifications: • Sofnolime: is the cheapest material, and safest to handle (sodalime) • Ca(OH)2 calcium hydroxide • NaOH sodium hydroxide • Sodasorb HP: is similar to sodalime • It is harder 92-95% • It also contains KOH potassium hydroxide • Lithium Hydroxide (LiOH) • Will work in very cold conditions • Is more effective than sodalime at scrubbing CO2 • Is extremely expensive • Is extremely corrosive • Floods are very caustic

23. Chapter 1: Why a CCR Common Design: The Scrubber • As the scrubber material is utilized it swells and increases in size, this results in decreased interstitial space in the scrubber and increased breathing resistance • Gas will then take the path of least resistance, so in a flood situation: • The gas will totally by pass the scrubber and exit through the overpressure relief valve • The ADV will then add gas on diver inhalation

24. Chapter 1: Why a CCR Common Design: The Scrubber • It is essential to protect the scrubber material form extreme temperatures • Cold will freeze the moisture out of the granules resulting in less moisture available to start the reaction • Heat will evaporate the moisture out of the granules again less moisture to start the reaction • Protect the granules from crushing (hard container) • Protect form air exposure (sealed container)

25. Chapter 1: Why a CCR Advantages Of CCR • Near silence: UW photographers dream • Decompression efficiency: constant PO2 means decreased • inert gas uptake • Warmth: CO2 scrubbing produces heat • Hydration: moisture from exhalation not lost to environment, • moisture from reaction • Warmth and hydration further help with decompression • efficiency • Constant “on the fly” blending (ideal mix at all depths during • the dive, not just at MOD) • Decreased gas expense (i.e. less Helium)

26. Chapter 1: Why a CCR Advantages Of CCR • A greater range of depth capabilities • Less physical volume of equipment • to pack for a weekend of diving • Less bulky than OC technical gear

27. Chapter 1: Why a CCR Disadvantages Of CCR • Expensive • Greater chance of death if the diver is complacent • More time consuming to prepare and maintain • Bulkier than recreational OC gear