Table of Contents

1. The Problem Being Solved
2. Background: Hydrogen Diving History
3. The Key Constraints: ICD and Combustion
4. The Proposed Architecture
5. Gas Management Strategy
6. Feasibility Analysis: ICD and Scrubber Rate
7. Representative Dive Profiles
8. Engineering Challenges
9. Prior Art
10. Open Questions
11. References

1. The Problem Being Solved

Deep trimix diving beyond ~150 m runs into two hard physiological walls:

• HPNS (High Pressure Nervous Syndrome): caused by helium at high partial pressure. Manifests as tremors, cognitive impairment, nausea. Onset is depth- and rate-dependent; significant above ~150 msw.
• Narcosis: nitrogen (and to a lesser extent helium) have narcotic effects that compound with depth.

Hydrogen has long been known to address both: it produces minimal HPNS, has a slight narcotic effect that actually buffers against HPNS, and has the lowest molecular weight of any gas, giving excellent density characteristics at depth.

The obstacle is combustion risk: H₂ in air is flammable above ~4% by volume, and explosive above ~18%. This caps the O₂ fraction in any H₂-containing breathing mix at ~4% (by volume). At depth, 4% O₂ gives adequate ppO₂. As the diver ascends, that same 4% fraction becomes hypoxic — at the surface, 4% O₂ = 0.041 bar ppO₂, well below the 0.18 bar hypoxia floor.

The conventional solution is a hard gas switch from the H₂ mix to heliox or trimix on ascent. COMEX’s experience (Hydra V, 1983) showed that this switch, done abruptly, reliably causes decompression sickness via isobaric counterdiffusion (ICD).

This proposal describes a rebreather architecture that eliminates the hard switch by using a catalytic H₂ scrubber to continuously and gradually replace H₂ with He in the breathing loop, paced to match tissue off-gassing rate.

2. Background: Hydrogen Diving History

Key Milestones

Why Hydrogen?

Gas density at 150m (16 bar):
  Air (79% N2):     ~19 g/L  [extreme breathing resistance]
  Heliox 16/84:     ~5.6 g/L [GUE limit ~6.2 g/L]
  Hydreliox 4/10/86: ~3.8 g/L [well below density limit]
  Hydrox 4/96:       ~2.2 g/L [exceptional]

At 150 m, a heliox-based trimix approaches or exceeds the GUE/WKPP gas density limit of 6.2 g/L. Hydrogen, being the lightest gas, dramatically reduces work of breathing at any depth.

Diffusivity Hierarchy

H₂ diffuses through tissue faster than any other diving gas (aqueous diffusivity data [4]):

Diffusivity in aqueous tissue (cm²/s):
  N₂:  2.3 × 10⁻⁵  (baseline)
  He:  3.8 × 10⁻⁵  (1.65× N₂)
  H₂:  5.8 × 10⁻⁵  (2.5× N₂, 1.53× He)

This means H₂ loads and unloads tissues ~3.7× faster than N₂ and ~1.53× faster than He. For decompression, the fast loading is a minor concern; the fast unloading is a significant advantage — H₂ off-gasses quickly. The danger is ICD on the switch away from H₂.

3. The Key Constraints: ICD and Combustion

Isobaric Counterdiffusion

ICD occurs when a diver switches from one inert gas to another at constant depth (isobaric). The faster-diffusing outgoing gas leaves tissues faster than the slower-diffusing incoming gas replaces it, causing a transient net supersaturation — bubbles can form with no ambient pressure change [3,4].

For H₂ diving, the critical switch is H₂ → He on ascent:

H₂ (fast, leaving) → He (slightly slower, entering)
Net effect: brief window where total tissue tension > ambient pressure

Because H₂ is only 1.53× faster than He (not 2.65× as with He→N₂), the H₂→He ICD gradient is smaller in magnitude than He→N₂ ICD for technical trimix divers [5]. However, for saturation divers (fully loaded tissues), even a small gradient matters. The Hydra V incident was a saturation dive [2]; bounce dives have much lower total tissue H₂ loading.

How long to reach saturation on 4%O₂/96%H₂? Using ZHL-16C half-times scaled by 1/√2 for H₂: the fastest compartment (t½ = 1.1 min) saturates in ~7 minutes; the slowest (t½ = 216 min) takes ~16 hours to reach 95% and ~24 hours to reach 99%. For practical purposes, a diver is “saturated” in H₂ after roughly 24 hours at depth — similar to helium saturation diving schedules. A 30-minute bounce dive loads the slowest compartments to only ~10%, which is why bounce dive ICD risk is so much lower than saturation.

The Combustion Constraint

H₂ lower flammability limit (LFL): ~4% by volume in air
H₂ lower explosive limit (LEL):   ~18% by volume

Therefore: O₂ fraction in loop must be < ~4% while H₂ is present

This creates a ppO₂ window problem on ascent:

The H₂ must be removed from the loop before the diver reaches ~40 m, or the O₂ fraction must be increased — which is only safe once H₂ is below 4% by volume.

The Classic Solution (and its failure)

COMEX’s first approach: switch the diver to heliox at ~250 msw when ascending from saturation. Hydra V result: DCS from ICD [2].

COMEX’s corrected approach (Hydra VIII): install a palladium catalyst chamber in the habitat atmosphere. H₂ + ½O₂ → H₂O, continuously, over 18 days of decompression, reducing H₂ partial pressure below the ICD-dangerous threshold before the switch [1].

The insight this proposal builds on: catalytic removal is a solved problem. It just needs to be miniaturised and integrated into a rebreather.

4. The Proposed Architecture

System Overview

The Catalytic Hydrogen Rebreather (CHR) is a closed-circuit rebreather with an additional catalytic H₂ oxidation circuit that:

1. Removes H₂ from the breathing loop continuously during ascent
2. Injects He to maintain total loop pressure as H₂ is removed
3. Allows the O₂ controller to gradually increase O₂ fraction as H₂ fraction falls
4. Paces all of the above to stay within ICD-safe tissue loading gradients

Architecture Diagram

flowchart TD
    subgraph Cylinders["Gas Cylinders"]
        D["Diluent\n4% O₂ / 96% H₂\n(cheap, fills at surface)"]
        T["Travel diluent\nAir or trimix\n(surface → 35 m)"]
        O2["O₂ Cylinder\n(metabolic + combustion\nbackfill)"]
        He["He Cylinder\n(wash-in during ascent)"]
        O2s["Scrubber O₂\n(dedicated feed\nfor catalyst)"]
    end

    subgraph Loop["Breathing Loop"]
        INH["Inhale\ncounterlung"]
        EXH["Exhale\ncounterlung"]
        CO2S["CO₂ Scrubber\n(sodalime canister)"]
        CTRL["Controller\n(ppO₂ / ppH₂\ntissue model)"]
    end

    subgraph H2S["H₂ Scrubber Circuit (Pd/Ag membrane, isolatable)"]
        PUMP["Loop pump\n(small, variable rate)"]
        DRYER["Desiccant pre-stage\n(protects membrane)"]
        MEM["Pd/Ag membrane\nH₂ only crosses →\n(loop gas never contacts O₂)"]
        SWEEP["Sweep side\nO₂ feed + H₂O trap\n(isolated from loop)"]
    end

    subgraph Sensors["Sensors"]
        PPO2["ppO₂ sensors ×3"]
        PPH2["ppH₂ sensor\n(thermal conductivity\nor electrochemical)"]
        TEMP["Temperature\n(catalyst bed)"]
    end

    D -->|"diluent add valve\n(depth > 35m)"| Loop
    T -->|"travel diluent valve\n(depth ≤ 35m)"| Loop
    O2 -->|"O₂ solenoid"| Loop
    He -->|"He add valve\n(controller-metered)"| Loop
    O2s -->|"O₂ to sweep side only\n(never enters loop)"| SWEEP

    EXH --> CO2S --> INH
    EXH -.->|"gas sample"| PUMP
    PUMP --> DRYER --> MEM
    MEM -.->|"H₂-depleted loop gas\n(He-richer) returns"| INH
    MEM -->|"H₂ crosses membrane →"| SWEEP

    CTRL -->|"set scrub rate"| PUMP
    CTRL -->|"set He add rate"| He
    CTRL -->|"O₂ injection"| O2

    PPO2 --> CTRL
    PPH2 --> CTRL
    TEMP --> CTRL

    style H2S fill:#fff3cd,stroke:#856404
    style Cylinders fill:#d1ecf1,stroke:#0c5460
    style Sensors fill:#d4edda,stroke:#155724

Key Design Principles

1. The H₂ scrubber circuit is isolatable. At depth (bottom phase), it is closed. Scrubbing only begins on ascent, when the controller determines it is safe to start reducing H₂ fraction relative to tissue loading.

2. He injection is controller-metered. As H₂ is removed, He is injected to maintain total pressure in the loop. The controller tracks the ratio continuously.

3. The scrubber O₂ circuit is isolated from the breathing loop. Two architectures are possible — and the choice has major safety implications:

   • Option A — Direct contact catalyst bed (COMEX habitat approach): Breathing gas passes over a Pd catalyst; O₂ is injected into the same gas stream just before the bed, reacts with H₂, and the scrubbed gas returns to the loop. O₂ is metered stoichiometrically so it is fully consumed before returning. The controller must guarantee no O₂ breakthrough — a safety-critical continuous metering requirement.

   • Option B — Pd/Ag membrane reactor (preferred): A palladium-silver alloy membrane is selectively permeable to H₂ only — H₂ dissolves into the Pd lattice on the loop side and diffuses to the sweep side; nothing else crosses. The sweep side carries a small O₂ flow that oxidises the H₂ to water. The breathing loop never contacts the scrubber O₂. Safety analysis reduces to: “does the membrane fail mechanically?” rather than “does O₂ breakthrough?” At 150 m, the driving force is ~15 bar ppH₂ differential — actually better than many industrial membrane applications. This is the strongly preferred architecture.

   The membrane approach also addresses the batched-pulse question: because transport through the membrane is rate-limited by H₂ partial pressure and membrane area (not by a valve or pump), the scrubbing rate is inherently continuous and self-limiting — no “gulp” of gas can transfer instantaneously. He injection remains controller-metered to maintain loop pressure as H₂ is removed.

4. The controller runs a tissue model. The critical novel element: the controller estimates current tissue H₂ loading using a Haldanean multi-compartment model (similar to a dive computer), and paces the scrubber rate to avoid the breathing-gas ppH₂ dropping faster than tissues can follow. This is the ICD constraint.

5. O₂ fraction is gated on H₂ fraction. The controller will not allow O₂ fraction to rise above the combustion threshold until loop H₂ is confirmed below 4%.

5. Gas Management Strategy

Phase Diagram

Depth (m)   Phase              H₂ loop    O₂ loop   He loop
─────────────────────────────────────────────────────────────
0 → 150     Descent            96%        4%         0%
            (scrubber closed)
150         Bottom             96%        4%         0%
            (scrubber closed)
150 → 50    Fast ascent        96→0%      4%         0→96%
            (scrubber OPEN,
             He injecting,
             O₂ held fixed)
~50m        H₂ < 4% confirmed  0%         4%→rising  96%→falling
            (O₂ control freed)
50 → 0      Standard deco      0%         variable   ~96%
            (normal CCR mode)

Gas Switch Transition Detail

xychart-beta
    title "Loop gas fractions during ascent 150m to 50m (30 min BT, 15 m/min)"
    x-axis "Depth (m)" [150, 140, 130, 120, 110, 100, 90, 80, 70, 60, 50]
    y-axis "Loop fraction (%)" 0 --> 100
    line "H2 fraction" [96, 86, 74, 60, 46, 32, 20, 10, 4, 0, 0]
    line "He fraction" [0, 10, 22, 36, 50, 64, 76, 86, 92, 96, 96]
    line "O2 fraction (x10 for visibility)" [4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4]

Three traces: H₂ fraction (declining from 96% to 0%), He fraction (rising from 0% to 96% as He wash-in replaces H₂), and O₂ fraction (held constant at 4% throughout). He + H₂ + O₂ = 100% at all depths. Note: profile is schematic — actual H₂/He transition is non-linear (scrubber rate is in fraction/min, ascent rate is in m/min, so the depth-vs-fraction curve depends on the ratio of these).

ppO₂ Safety Window

The rebreather controller must maintain ppO₂ between the hypoxia floor and the CNS toxicity ceiling (1.6 bar working / 1.4 bar conservative). The absolute physiological floor is 0.18 bar, but most CCR practitioners use 0.3–0.4 bar as a working floor due to exertion, cold stress, and CO₂ loading effects at depth. This proposal uses 0.18 bar as a conservative lower bound to maximise the window for H₂ scrubbing; operational protocols would likely set a higher floor, which would require H₂ to be cleared slightly deeper (before ~60 m rather than ~40 m):

At 150m: ppO₂(4% O₂) = 0.64 bar  ← well within window
At 100m: ppO₂(4% O₂) = 0.44 bar  ← acceptable  
At  50m: ppO₂(4% O₂) = 0.24 bar  ← above absolute floor; below CCR working floor
At  40m: ppO₂(4% O₂) = 0.20 bar  ← barely above absolute floor
At  30m: ppO₂(4% O₂) = 0.16 bar  ← HYPOXIC

At 0.3 bar working floor: H₂ must be < 4% before ~75m.
At 0.18 bar absolute floor: H₂ must be < 4% before ~40m.

6. Feasibility Analysis: ICD and Scrubber Rate

Method

A Haldanean multi-compartment (ZHL-16C [8], 16 compartments) simulation was written in Python to model:

• Tissue H₂ loading during descent and bottom phase
• Loop H₂ fraction declining as scrubber runs at constant rate
• He fraction rising correspondingly
• Supersaturation (total tissue tension − ambient pressure) at each depth step
• ICD metric: min[(tissue_H₂ − loop_H₂) − (loop_He − tissue_He)] across all compartments

H₂ tissue half-times were derived from ZHL-16C He values scaled by 1/√2 (Graham’s Law: diffusivity ∝ 1/√M, so D_H₂/D_He = √(M_He/M_H₂) = √(4/2) = √2).

⚠️ Critical caveat on the ICD-safety conclusions below: The Graham’s Law scaling assumes tissue diffusivity ratios match aqueous diffusivity ratios. This is not established. The 5.8/3.8 (H₂/He) ratio comes from measurements in water [4]; the composite lipid/aqueous/membrane tissue that Bühlmann compartments [8] represent could have substantially different H₂:He ratios. The safety conclusion — that scrubbing reduces supersaturation — depends entirely on H₂ clearing tissues faster than He loads them (ratio > 1). If the real tissue half-time ratio is closer to 1:1, the conclusion weakens or reverses. All ICD-related results in this section should be read as “consistent with these assumed coefficients,” not as validated findings.

Additionally, the ICD metric defined below has no published threshold value — it is a relative comparison only. A negative delta (scrubber case better than no-scrubber) does not mean “safe”; it means “less bad than the baseline.” No published H₂-specific ICD threshold exists to determine what value is dangerous.

Tissue Saturation at End of Bottom Phase

Tissue H₂ loading as a percentage of full saturation at bottom pressure:

Key observation: fast compartments (≤5 min H₂ half-time) are fully saturated after even a 10-minute bottom time. The slow compartments are still only 10–20% saturated for a 30-minute bounce dive — this is why bounce diving is far safer than saturation for H₂ ICD risk.

Required Scrubber Rate to Clear H₂ by 50m

Transit time from bottom to 50 m at 15 m/min:

• 150 m → 50 m: 6.7 minutes
• 100 m → 50 m: 3.3 minutes

Note: FIRE = H₂ still above 4% at 50m, O₂ increase unsafe.

Important distinction: The table above answers only one question — “can the scrubber clear the loop in time?” It does not answer whether a given scrubber rate is ICD-safe for the tissue loading at that bottom time. Those are separate questions. The loop clearance rate is independent of bottom time; the ICD safety of that rate is not — longer bottom times load slow tissue compartments more heavily, making any given scrubber rate incrementally less safe from an ICD perspective. The ICD analysis in the next section uses a fixed bottom time of 20 min; see the Danger Zone section for bottom-time sensitivity.

Does the Scrubber Add ICD Risk?

This is the critical question. Results for the worst-case scenario (150 m, 20 min BT):

Under the assumed half-time model, the scrubber consistently reduces supersaturation relative to the no-scrubber baseline. The mechanism: if H₂ clears tissues ~1.41× faster than He loads them, then dropping loop ppH₂ causes a faster fall in tissue H₂ tension than the rise in tissue He tension, so net total tension falls.

However — this conclusion is contingent on the 1/√2 half-time scaling being approximately correct (see the caveat in Method above). If the true H₂:He tissue half-time ratio is closer to 1:1 rather than 1:1.41, the scrubber’s delta-SS advantage shrinks toward zero and eventually reverses. The model cannot distinguish between these scenarios without empirical H₂ tissue half-time data.

This is the opposite of the Hydra V scenario because:

• Hydra V: saturation dive (tissues 100% loaded in all compartments, slow and fast)
• Bounce dive: slow compartments are only 10–45% loaded; fast compartments off-gas H₂ within minutes regardless

graph LR
    A["H₂ in breathing loop drops\n(scrubber running)"] 
    B["H₂ diffuses OUT of tissues\n(fast, t½ ~1-40 min for fast comps)"]
    C["He diffuses INTO tissues\n(slightly slower than H₂)"]
    D["Net tissue tension\nfalls faster than ambient"]
    E["Supersaturation DECREASES\n relative to no-scrubber case"]
    
    A --> B
    A --> C
    B --> D
    C --> D
    D --> E
    
    style E fill:#d4edda,stroke:#155724,color:#000000

The ICD Danger Zone: What to Avoid

The scrubber-is-safe conclusion holds for bounce dives. The scenario to avoid is:

• Starting the scrubber before beginning ascent (at depth, fully loaded) — all tissue compartments are near equilibrium, so any breathing-gas ppH₂ drop creates a large ICD gradient simultaneously across all compartments, with nowhere for the gas to go.
• Scrubbing faster than tissue H₂ half-times allow (especially slow compartments) — fast compartments clear H₂ quickly, but a scrubber running at, say, 1.0 frac/min would drop loop ppH₂ to near zero before the 60–300 min compartments have had a chance to off-gas, creating sustained supersaturation in those tissues.
• Long bottom times at depth (>2–3 hours) — this progressively loads even the slowest compartments toward equilibrium, converging toward the saturation diver scenario where any scrubber rate becomes dangerous.

graph TD
    subgraph Safe["✅ Safe operating region"]
        A["Short BT + any scrub rate\ne.g. 10min@100m, sr=0.20\nΔSS = −4.6 bar (safer than no scrubber)"]
        B["Long BT + slow scrub rate\ne.g. 30min@150m, sr=0.15\nΔSS = −0.59 bar"]
    end
    subgraph Caution["⚠️ Caution / avoid"]
        C["Long BT + fast scrub rate\nFull saturation dive + fast scrub\n= Hydra V scenario"]
        D["Any BT + no scrub\nH₂ still 96% at 50m — cannot raise O₂"]
    end
    style Safe fill:#d4edda,stroke:#155724,color:#000000
    style Caution fill:#f8d7da,stroke:#721c24,color:#000000

The key insight (under the assumed model): for bounce dives, more scrubbing appears better — the scrubber’s delta-SS is negative, meaning it reduces supersaturation relative to the baseline. The danger zone is saturation or extended dives where slow tissue compartments are fully loaded and the 1.41× diffusivity advantage of H₂ over He may not be sufficient.

7. Representative Dive Profiles

Profile A: 100 m, 30-Minute Bottom Time

Descent switch ICD safety: Switching from air to 4%O₂/96%H₂ at 35 m is physiologically safe from an ICD perspective. Tissues are still near surface N₂ loading (~0.80 bar total tension), while ambient is already 4.6 bar — a −4.2 bar margin. Although H₂ is faster-diffusing than N₂ and loads fast compartments rapidly, the total tissue tension at any point during the continued descent remains far below ambient (which is still rising at 20 m/min). Even if the fastest compartment transiently loads H₂ at its maximum rate while still holding surface N₂, the combined tension cannot approach the ambient pressure. Supersaturation on descent is physically impossible with this margin.

Gas requirements (approximate):

• Diluent (4/96): ~3–5 bar of a 3L cylinder for diluent adds
• O₂ (metabolic): standard CCR consumption ~0.5–0.8 L/min = ~50–90L for the dive
• He (wash-in): sufficient to replace ~96% of loop volume as H₂ is removed — loop volume ~10L at STP equivalent, so ~960L He. Note: this covers only the gas composition replacement; additional He is needed to maintain loop pressure as ambient falls during ascent (at 15 m/min over 3–7 min this is a small additional volume, ~50–100L, but should be included in cylinder sizing).
• Scrubber O₂ (dedicated): stoichiometric against H₂ removed — modest, ~50–100L at STP

Profile B: 150 m, 30-Minute Bottom Time

Note on deco obligation: The 150 m deco times above are rough estimates from standard Bühlmann models with trimix-equivalent parameters. The actual H₂ deco would need validated tables — decodaitengu’s H₂ coefficients are experimental. The deco obligation from 150 m/30 min is substantial (multiple hours) regardless of gas mix.

Deco Gas Strategy After H₂ Phase

Once H₂ is below 4%, the loop transitions to standard CCR operation:

50 m → 21 m:   He diluent, ppO₂ 1.2–1.4 bar  (accelerated deco)
21 m → 6 m:    O₂-enriched (50–80%), ppO₂ 1.4 bar
6 m → surface: O₂ breaks, ppO₂ 1.6 bar max

This is identical to standard deep trimix CCR procedure once the H₂ phase is complete.

8. Engineering Challenges

8.1 Controller Complexity

The CHR controller must simultaneously manage more variables than any existing CCR:

The tissue model running in the controller is the key novel element. The controller needs to solve: “given current tissue H₂ loading, how fast can I reduce loop ppH₂ without creating a dangerous ICD gradient?”

This is equivalent to the ICD-constrained scrubber rate calculation in Section 6, running in real time.

8.2 Catalytic H₂ Removal: Membrane vs Direct Contact

The reaction in both cases: H₂ + ½O₂ → H₂O (Δ = −242 kJ/mol)

Membrane architecture preferred for diver safety: the breathing gas and the scrubber O₂ are separated by physics, not by a control system.

8.3 H₂ Sensor

A reliable, fast-response, in-loop H₂ sensor is essential. Options:

• Thermal conductivity sensor: responds to H₂ and He (both low thermal conductivity relative to N₂/O₂). Critical calibration challenge: TC output is ambiguous mid-transition — during the H₂→He scrubbing phase, both gases are present in varying proportions, and both shift the TC reading in the same direction. A sensor calibrated for pure H₂/N₂ mixtures will give incorrect readings in a H₂/He/O₂ mixture. Requires a two-gas compensation algorithm or a second independent sensor to disambiguate.
• Electrochemical H₂ sensor: highly specific to H₂, low power, good sensitivity at low concentrations. Less suitable at high H₂ fractions (overranging); may need attenuation or a second range sensor to cover 0–96%.
• Acoustic gas analyser: measures gas density acoustically; can distinguish H₂/He/N₂ mixtures by fitting a density model. Most suitable for the mid-transition phase but more complex and power-hungry.

Existing CCR O₂ sensors (galvanic cells) are unaffected by H₂.

8.4 H₂ Sensor Failure Modes

If the H₂ sensor fails high (reads more H₂ than present), the controller will continue scrubbing when H₂ is already gone — wastes He, but safe.

If the H₂ sensor fails low (reads less H₂ than present), the controller may allow O₂ fraction to rise while H₂ is still present — combustion risk. This is the critical failure mode and requires either redundant sensors or a conservative fallback (hold O₂ at 4% until two sensors agree H₂ < 4%).

8.5 Water Management

Each mole of H₂ removed produces 1 mole of H₂O (18 g). Scrubbing the loop from 96% → 0% H₂ in a 10L loop at 16 bar involves:

~15.4 bar H₂ removed × 10L = 154 bar·L = ~6.3 mol H₂ → ~6.3 mol H₂O → ~113 mL water

113 mL in ~7 minutes is the full capacity of a typical CCR condensate trap. This is not “manageable” without careful design — it is a genuine engineering constraint. Two compounding risks:

1. Desiccant saturation: The desiccant pre-stage protecting the membrane also competes with moisture from the CO₂ scrubber canister, which generates its own condensate throughout the dive. A desiccant stage sized for the H₂ scrubbing event alone, without accounting for CO₂ scrubber moisture over a 2–3 hour dive, will arrive at the H₂ scrubbing phase partially saturated.
2. Membrane flooding: If the desiccant fails to intercept liquid water, the Pd/Ag membrane can be flooded, catastrophically reducing scrubbing rate mid-ascent — exactly when it is needed most.

Design requirement: The desiccant pre-stage must be sized for the total moisture budget of the dive (CO₂ scrubber moisture + H₂ scrubbing water), with significant margin. A separate, isolated condensate trap on the sweep side is also needed for the 113 mL of H₂O product.

8.6 Controller Failure and Manual Bailout

The controller failure scenario during the H₂ scrubbing transition (e.g., at H₂ ~50%, He ~46%, O₂ 4%) is the most safety-critical failure mode in the system and is not addressed by any existing CCR bailout protocol.

The problem: A mid-transition loop is in a state no standard OC bailout cylinder is designed for:

• Loop is ~50% H₂ — not breathable on open circuit (hypoxic at any useful depth)
• Cannot increase O₂ — H₂ still above combustion limit
• Cannot switch to heliox — abrupt H₂→He gas change is the Hydra V scenario
• Cannot ascend on current loop — controller is managing the ICD-safe scrub rate; manual operation may be faster or slower in unpredictable ways

Candidate manual bailout procedure (preliminary):

1. Isolate the H₂ scrubber circuit — close the scrubber loop valve. This freezes the current loop composition and stops any further H₂ removal.
2. Ascend at normal rate — the current loop composition, whatever it is, will maintain ppO₂ at 4% through the ascent. If H₂ is ~50%, the diver can continue ascending on the existing mix without ICD risk (total tissue tension analysis is the same as the full-H₂ case).
3. At 35 m, switch to travel diluent — now breathe air travel gas (which was used on descent). This dilutes the loop H₂ progressively with N₂/O₂, and importantly, raises ppO₂.
4. Do not attempt O₂ injection while H₂ is present — until the loop has been sufficiently diluted with travel gas or the diver has surfaced and the loop has been vented.

Open issue: This procedure has not been modelled and may produce an ICD event at step 3 depending on tissue loading state. A partial-He loop transitioning to air (N₂) replaces a faster gas with a slower one — the classical bad-direction switch [3,5]. Further analysis required.

Design implication: The controller should log its last known loop composition and tissue state to a non-volatile store every 10–30 seconds, so that on failure, the diver or support team can reconstruct the state and determine the safest manual path.

9. Prior Art

No published work describes a fully integrated wearable rebreather with onboard catalytic H₂ scrubbing. The closest:

The apparent novelty of this architecture lies in combining:

1. Miniaturised catalytic H₂ oxidation in the breathing loop
2. Real-time tissue H₂ model in the controller
3. ICD-rate-limited scrubber pace algorithm
4. Coordinated He injection and O₂ gate

The ICD-rate-limiting constraint — pacing scrubber removal to tissue H₂ half-times rather than physical scrubber capacity — does not appear to be explicitly articulated in the published literature, even though it follows directly from the Hydra V lesson.

10. Open Questions

Physiological

• What is the actual ICD-safe H₂→He gradient threshold for bounce dives? (No published data)
• Do H₂ ZHL-16C coefficients scaled from He provide accurate decompression tables? (Unvalidated)
• Is the N₂ trace (for HPNS buffering) worth its ICD complexity on the He wash-in phase?
• What is the minimum ppO₂ that can be safely maintained during the H₂ phase? (affects scrubber design if the controller needs more margin)

Engineering

• Palladium catalyst bed sizing: what volume of catalyst is required for the needed scrubbing rates (~0.10–0.20 H₂ fraction/min in a 10L loop)?
• Loop pump sizing: what flow rate through the catalyst bed is needed?
• Catalyst lifetime characterisation in diving gas environments (contaminants, pressure, temperature)
• Thermal characterisation of the exothermic reaction in a sealed CCR loop at depth
• H₂ sensor validation at high H₂ fractions and high pressure

Regulatory / Safety

• How do existing CCR certification frameworks apply to a novel gas species in the loop?
• What failure-mode analysis is required for the H₂ sensor fault scenarios?
• What is the minimum viable redundancy for the catalytic circuit?
• What is the safest manual bailout procedure from a partially-scrubbed loop? (See Section 8.6 for preliminary analysis — the step 3 He→N₂ switch may itself be an ICD risk; this needs to be modelled before any practical dive)

11. References

Appendix: Simulation Code

The Python simulation used in Section 6 is available at:

decotengu/tools/h2_scrubber_icd.py

It models a Haldanean 16-compartment ascent with configurable scrubber rate and produces the supersaturation and ICD metric tables in this document.

Key parameters used:

• H₂ half-times: ZHL-16C [8] He values × 1/√2 (Graham’s Law)
• Descent: 20 m/min
• Fast ascent: 15 m/min (150 m or 100 m → 50 m)
• dt: 0.5 min
• Surface pressure: 1.013 bar
• N₂ pre-loading: 0.79 × 1.013 bar (surface air equilibrium)

This document was produced as part of the decodaitengu decompression modelling project. All physiological conclusions are theoretical and unvalidated. Do not use for actual dive planning.