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Hydreliox
Hydreliox
Hydreliox
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Hydreliox is an exotic breathing gas mixture of hydrogen, helium, and oxygen.[1][2] For the Hydra VIII (Hydra 8) mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.[3]

It is used primarily for research and scientific deep diving, usually below 130 metres (430 ft). Below this depth, extended breathing of heliox gas mixtures may cause high pressure nervous syndrome (HPNS).[4] Two gas mixtures exist that attempt to combat this problem: trimix and hydreliox. Like trimix, hydreliox contains helium and oxygen and a third gas to counteract HPNS. The third gas in trimix is nitrogen and the third gas in hydreliox is hydrogen. Because hydrogen is the lightest gas, it is easier to breathe than nitrogen under high pressure. To avoid the risk of explosion, as a rule of thumb hydrogen is only considered for use in breathing mixtures if the proportion of oxygen in the mixture is less than 5%. However, the pressure during the dive must be such that the partial pressure of 5% oxygen is sufficient to sustain the diver. (The flammability of the mixture also depends to some degree on the pressure)[5]

The diving depth record for off-shore (saturation) diving was achieved in 1988 by a team of professional divers (Th. Arnold, S. Icart, J.G. Marcel Auda, R. Peilho, P. Raude, L. Schneider) of the Comex S.A., industrial deep-sea diving company performing pipe line connection exercises at a depth of 534 m (1,752 ft) of seawater (msw/fsw) in the Mediterranean Sea as part of the Hydra VIII (Hydra 8) programme.[6][7]

Hydreliox has been tested in 1992 to a simulated depth of 701 metres (2,300 ft) by COMEX S.A. diver Théo Mavrostomos in an on-shore hyperbaric chamber as part of the Hydra X (Hydra 10) programme.[8] The Hydra X team Théo Mavrostomos belonged to spent 3 days at the simulated 675 metres (2,215 ft) depth. After the rest of this team were held incapacitated at 675 m depth, Mavrostomos took a short 2-hour excursion at the simulated 701 metres (2,300 ft) depth, and took 43 days to complete the record experimental dive.[9][10][11] Although breathing hydreliox improves the symptoms seen in HPNS, tests have shown that hydrogen narcosis becomes a factor at depths of 500 metres (1,600 ft).[2][12][13]

Hydrogen can reduce the effects of high pressure nervous syndrome (HPNS), reduce work of breathing, and may be a partial alternative to helium. Use of hydrogen can make it possible to dive deeper, descend faster, and stay at depth longer. Lower work of breathing allows higher levels of exertion by the diver.[14]

Hydrogen is limited to below 4% in normoxic environments or oxygen content is limited to below 6% in high-hydrogen environments.[14]

See also

[edit]
  • Argox – Gas mixture occasionally used by scuba divers for dry-suit inflation
  • Heliox – Breathing gas mixed from helium and oxygen
  • Hydrox – Breathing gas mixture experimentally used for very deep diving
  • Nitrox – Breathing gas, mixture of nitrogen and oxygen
  • Trimix – Breathing gas consisting of oxygen, helium and nitrogen

References

[edit]
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Hydreliox

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Hydreliox is a ternary mixture consisting of (H₂), (He), and oxygen (O₂), designed for ultra-deep to mitigate (HPNS) and alleviate the increased respiratory workload caused by high gas density at extreme depths. Developed in the late , hydreliox addresses limitations of traditional mixtures, which become ineffective beyond approximately 200 meters due to HPNS symptoms such as tremors, , and . The inclusion of , with its properties and low molecular weight, allows divers to operate safely at depths exceeding 500 meters while maintaining work capacity and safety. The development of hydreliox stems from the HYDRA research program conducted by the French diving company COMEX between 1968 and 1992, which systematically tested hydrogen-enriched mixtures to push the boundaries of human deep-sea operations. Early experiments, such as HYDRA III in 1983, explored hydrogen-oxygen () blends at shallower depths of 75–91 meters to assess feasibility and safety. This progressed to hydreliox in HYDRA V (), a 36-day saturation dive to 450 meters in a hyperbaric chamber, where divers breathed a mixture with approximately 54% (25 bar partial pressure), 45% (20.6 bar), and 1% oxygen (0.4 bar), demonstrating no HPNS symptoms and improved breathing comfort without . Subsequent milestones include HYDRA VIII in 1988, an offshore operation to 530 meters near , , where four COMEX divers and two divers performed six days of simulated pipeline connection tasks using hydreliox, achieving full operational efficiency. The program culminated in HYDRA X (1992), an onshore record dive to 701 meters of (msw) equivalent, confirming hydreliox's viability for industrial applications at extreme pressures up to 71 atmospheres. Experimental dives with hydreliox continued into 1996 (HYDRA XII at 210 m), but commercial adoption has remained limited due to handling complexities, risks associated with , and the increasing use of remotely operated vehicles for deep-sea tasks. As of 2023, COMEX's expertise in hydreliox informs broader applications in energy sectors such as transportation and storage.

Composition and Properties

Chemical Makeup

Hydreliox is a ternary mixture composed of (H₂), (He), and oxygen (O₂), designed for extreme-depth . The composition of hydreliox varies depending on the target depth and pressure to optimize safety and performance. In the COMEX Hydra VIII experiment at approximately 53 atmospheres (530 meters equivalent), the mixture consisted of 0.8% O₂, 49% H₂, and 50.2% He. At shallower depths, such as around 300 meters, the oxygen fraction increases to 1-2% to maintain appropriate partial pressures while minimizing risks. For instance, in the 2024 HYDRA 12 operation at 210 meters, hydreliox mixtures were used with higher oxygen fractions suitable for mid-depth , confirming the adaptive composition strategy. Hydrogen is incorporated to lower the overall density of the gas mixture relative to heliox, thereby reducing respiratory workload, and to counteract high-pressure nervous syndrome (HPNS) via its mild narcotic effects at elevated partial pressures. Helium serves as the primary inert diluent due to its low narcotic potential and high diffusivity, enabling operations at depths where nitrogen or other gases would induce severe narcosis. Oxygen is included solely for metabolic respiration, with its concentration strictly controlled. The of oxygen (PPO₂) in hydreliox is typically maintained at 0.4-0.5 to support adequate oxygenation while avoiding toxicity; for instance, in Hydra VIII, the 0.8% O₂ at 50 yielded a PPO₂ of 0.4 .

Physical Characteristics

Hydreliox mixtures exhibit significantly lower than heliox due to the incorporation of , which has a molecular weight of approximately 2 g/mol compared to 's 4 g/mol. In the COMEX Hydra V experimental dive to 450 meters, the hydreliox mixture (with fractional concentrations of 0.87% oxygen, 44.78% , and 54.35% at pressure) achieved a of 5.5 g/L (dry, 37°C), representing a 26% reduction relative to an equivalent mixture and over 50% lower than trimix. This reduced alleviates breathing resistance at extreme depths, where gas compression would otherwise increase ventilatory workload. At (STP, 0°C and 1 ), hydreliox range from 0.1 to 0.2 g/L, calculated as the mole-fraction-weighted average of component (hydrogen: 0.0899 g/L, : 0.1785 g/L, oxygen: 1.429 g/L). The thermal conductivity and specific heat capacity of hydreliox surpass those of air and typical heliox mixtures, attributed to the inherent properties of its components. Hydrogen possesses a thermal conductivity of 0.186 W/m·K and specific heat capacity of 14.3 kJ/kg·K at 25°C, while helium contributes 0.152 W/m·K and 5.23 kJ/kg·K; mixtures thus yield values higher than air (0.026 W/m·K, 1.006 kJ/kg·K) or heliox (approximately 0.13-0.15 W/m·K). These elevated properties increase heat loss from the diver's respiratory tract to the surrounding environment, heightening the risk of hypothermia during cold deep-water operations and necessitating gas preheating systems to maintain thermal balance, though they may facilitate improved heat transfer in certain heated setups. Empirical studies on hydrogen-helium binaries confirm that thermal conductivity remains high across compositions up to 150°C, with minimal pressure dependence at diving-relevant conditions. Solubility of hydreliox components in and tissues is notably low, minimizing narcotic effects and loading under hyperbaric conditions. Hydrogen and exhibit Ostwald solubility coefficients of 0.005 and 0.0085 ml gas/ml /atm at 37°C, respectively, far lower than (0.012) or oxygen (0.024), which reduces the risk of at depths beyond 300 meters. Hydrogen demonstrates a higher rate than helium in tissues, saturating and desaturating approximately 3.7 times faster than nitrogen (versus helium's 2.7 times), enabling more rapid equilibration during compression and decompression phases. This combination of low and enhanced diffusivity for supports efficient gas management in protocols. Viscosity and flow characteristics of hydreliox are advantageous for gas delivery systems, with the mixture's dynamic lower than that of owing to hydrogen's value of 8.8 μPa·s at 25°C (versus helium's 19.8 μPa·s and air's 18.5 μPa·s). Binary hydrogen-helium mixtures show viscosities averaging 0.6% error from predictive models across compositions, resulting in reduced frictional losses in tubing, valves, and regulators. In and open-circuit setups, this translates to lower pressure differentials and improved at high delivery rates, enhancing system reliability and diver comfort during prolonged exposures. Experimental evaluations confirm up to 50% lower resistance with hydrogen-enriched mixtures compared to at equivalent partial pressures.

History and Development

Early Research

Early theoretical research on as a diving gas emerged in the 1960s, driven by U.S. concerns over scarcity and expense, as well as the limitations of mixtures in mitigating high-pressure neurological syndrome (HPNS) symptoms like tremors and convulsions at extreme depths. scientists explored as a substitute due to its lower , which reduces resistance, and its mild properties to counteract pressure-induced neurological effects. Initial laboratory investigations in the and used animal models to assess hydrogen's and under hyperbaric conditions. Studies on such as monkeys and dogs exposed to hydrogen-oxygen mixtures showed reduced incidence of tremors compared to at high pressures, indicating hydrogen's potential to suppress HPNS manifestations. These experiments confirmed hydrogen's lower molecular weight improved ventilatory function, with limited toxicity observed at pressures up to around 70 atmospheres in some cases. Theoretical advancements in the solidified hydreliox's conceptual foundation through publications examining gas and narcosis thresholds. Peter B. Bennett's work detailed HPNS as "helium tremors" and proposed hydrogen's effects could raise narcosis thresholds, allowing safer operations at depths where failed. Concurrently, a U.S. report by James H. Dougherty advocated hydrogen-helium-oxygen blends to optimize (reducing by up to 40% versus at 200 feet) while maintaining oxygen partial pressures below explosive limits. These papers emphasized hydreliox's potential for depths exceeding 500 meters, paving the way for subsequent human trials.

Key Experimental Trials

The Hydra program, initiated by the French diving company COMEX in collaboration with the in 1968, represented a series of experiments that progressed from hydrogen-oxygen () mixtures to hydreliox, validating it as a for extreme depths. Early trials, such as Hydra III in 1983, explored blends at 75–91 meters to assess feasibility and safety. These efforts addressed limitations of mixtures by focusing on mitigating high-pressure neurological syndrome (HPNS) through hydrogen addition, building on prior animal and chamber studies to emphasize manned saturation dives for practical performance. One of the earliest key hydreliox trials was Hydra V in , a simulated saturation dive to 450 meters in COMEX's hyperbaric facility. Six divers, divided into two teams, resided at pressure for up to 36 days, breathing a hydreliox consisting of approximately 54% , 45% , and 1% oxygen at depth (partial pressures: 25 bar H₂, 20.6 bar He, 0.4 bar O₂). No symptoms of HPNS were observed, and divers reported improved breathing comfort due to the lower density of the gas compared to . Physiological, psychomotor, and work performance tests—both in dry chambers and wet pots—demonstrated normal cognitive function and manual dexterity, with successful execution of tasks such as equipment handling and monitoring. This trial marked the first prolonged exposure to hydreliox, confirming its feasibility for depths beyond meters without neurological impairment. Building on this success, Hydra VIII in 1988 advanced to an open-sea operation off , where four COMEX divers and two personnel conducted six days of work at 530 meters while breathing hydreliox. The mixture enabled excursions to 534 meters, with divers performing maintenance tasks on an offshore platform, including tool use and structural inspections. Communication remained clear via voice systems, with no reported delays or distortions typical of HPNS in dives. Performance evaluations showed enhanced efficiency and reduced fatigue, underscoring hydreliox's potential for commercial applications. The trial achieved a for depth at the time and demonstrated safe gas switching protocols between hydreliox and during transfers. The program culminated in Hydra X (also referred to as Hydra 10) in 1992, a chamber-based reaching 701 meters—the deepest manned hyperbaric exposure to date. Diver Théo Mavrostomos and team members endured three days at 675 meters on hydreliox, with the full team progressing to the target depth. No HPNS manifestations occurred, and divers completed complex psychometric tests and operational simulations effectively. This milestone established hydreliox's efficacy up to 700 meters, informing safety parameters for very deep interventions, though economic factors limited immediate adoption. Following these efforts, hydreliox trials became sparse due to regulatory and cost barriers, with shifting toward simulations and biochemical decompression aids in the . As of 2025, interest has revived in exploratory dives, but no large-scale manned validations comparable to the Hydra series have occurred, with focus remaining on refining protocols for potential future use.

Applications in Diving

Use in Saturation Diving

Saturation diving involves divers living in a pressurized habitat at equivalent depth for days or weeks, allowing extended work periods at great depths without repeated decompression, typically using breathing gas mixtures to avoid nitrogen narcosis and manage other high-pressure effects. Hydreliox plays a critical role in saturation diving beyond approximately 300 meters seawater (msw), where heliox mixtures become impractical due to excessive gas density increasing the work of breathing and exacerbating high-pressure nervous syndrome (HPNS). By incorporating hydrogen, hydreliox lowers overall gas density— for instance, achieving about 4.56 g/L at 250 msw compared to 7.63 g/L for certain trimix variants—while hydrogen's mild narcotic properties help mitigate HPNS symptoms like tremors and vertigo, enabling safer and more efficient operations at ultra-deep levels up to 500 msw or more. In saturation protocols, divers often switch from to hydreliox during compression or at depth to maintain low of oxygen (PPO₂) while optimizing and HPNS control; for example, in the Hydra 10 experiment, divers transitioned from between 10–200 msw to hydreliox from 200–701 msw, limiting hydrogen to 2 MPa, before reverting to below 280 msw for decompression. This switching strategy allows initial compression with established systems while leveraging hydreliox's advantages at greater depths, ensuring PPO₂ remains in the safe range of 0.4–1.6 bar to prevent . Equipment adaptations for hydreliox in emphasize hydrogen compatibility to mitigate flammability and leakage risks, including specialized units, non-sparking compressors, and storage systems designed for handling, such as those used in Comex hyperbaric facilities with temperature-controlled gas delivery at 30–33°C. These modifications, validated in controlled environments, support reliable supply via umbilicals and chambers, preventing ignition in oxygen-enriched atmospheres. Notable case studies include the Comex Hydra VIII offshore saturation dive in 1988 off , where divers worked at 520–530 msw for several days using hydreliox, demonstrating improved performance over for subsea tasks relevant to oil production interventions. Similarly, the 1992 Hydra 10 onshore simulation involved three divers saturating to 701 msw and performing eight working excursions (e.g., 660–145 msw) on hydreliox, simulating commercial inspections and confirming operational feasibility up to 500 msw in scientific and potential industrial contexts.

Role in Experimental Dives

Hydreliox has been instrumental in pushing the boundaries of human diving depth through experimental programs, particularly in record-setting operations aimed at evaluating its efficacy against (HPNS). In 1988, the French diving company COMEX conducted the Hydra VIII mission, achieving the deepest saturation to date at 534 meters (1,752 feet) in the off . Divers breathed a hydreliox mixture consisting of approximately 49% , 50.2% , and 0.8% oxygen, enabling six days of operational work at around 520 meters while maintaining functional performance under pressures exceeding 50 atmospheres. This demonstrated hydrogen's ability to mitigate HPNS symptoms, such as tremors, , and motor impairment, through its properties that counteract helium-induced neurological effects, allowing faster compression rates and deeper excursions than traditional mixtures. Key research outcomes from Hydra VIII and related COMEX trials provided critical data on cognitive and physiological performance at extreme pressures. Assessments revealed that divers on exhibited reduced anxiety symptoms and preserved mental acuity during prolonged exposure at 50+ atmospheres, with hydrogen's antagonistic effect on HPNS enabling better task execution compared to controls in prior experiments. These findings, including evaluations of work capacity and saturation fatigue, have informed the refinement of mixtures for future deep-sea interventions, highlighting hydreliox's potential to extend operational limits beyond 500 meters. Despite these advancements, hydreliox remains confined to experimental and professional contexts due to its operational complexity, including stringent handling protocols to prevent ignition and the requirement for advanced hyperbaric systems not feasible for recreational or standard . In the 2020s, ongoing experiments have explored hydreliox variants in controlled settings, such as the first reported deep dive to 230 meters using a hydrogen-enriched mixture in 2023, which further validated HPNS control and gas density management for potential applications in ultra-deep operations and high-pressure simulations.

Physiological Effects

Mitigation of High-Pressure Issues

Hydreliox mitigates (HPNS), which manifests as tremors, myoclonic jerks, and cognitive impairments in deep dives starting around 300 meters, primarily through the antagonistic effects of on pressure-induced neurological disruptions. 's mild potency counters the hyperexcitability caused by high-pressure exposure to , stabilizing function and reducing symptom severity. In the Hydra V experimental dive to 450 meters in , six divers breathed a hydreliox mixture (approximately 54% , 45% , and 1% oxygen by fraction) with no observed HPNS symptoms, including absence of tremors or , unlike comparable exposures where such effects are prominent. This reduction in HPNS is linked to hydrogen's lower compared to , which alters gas and influences conduction dynamics under compression, though the precise mechanism involves reversal of narcotic-like interactions at synaptic levels. Trials such as Hydra V reported complete elimination of motor disturbances, enabling enhanced diver performance and respiratory comfort due to a 26% lower gas than . A 2023 rebreather to 230 meters using a similar helihydrox mixture also confirmed HPNS control, with symptoms resolving upon hydrogen introduction and no adverse neurological effects noted. Hydreliox avoids narcosis by substituting non-narcotic and for , preventing anesthetic impairment until depths beyond 500 meters, where minimal may emerge without compromising operational capacity. In Hydra V, no narcosis was detected at 450 meters despite a partial pressure of 25 bar, supporting its use for extended deep operations. Oxygen toxicity, a risk from elevated partial pressures at depth, is managed through precise control of low oxygen s in hydreliox, typically 0.5-2% to maintain s of 0.4-1 bar, sufficient for while below toxic thresholds. During the Hydra V dive, oxygen was limited to a 1% yielding 0.40 bar , ensuring safety over prolonged exposure without incidents of .

Impact on Human Physiology

Hydreliox's low gas density substantially reduces respiratory workload at extreme depths, enhancing ventilation efficiency by minimizing and effort required for breathing. This benefit arises from 's molecular weight, which is lower than helium's, allowing for smoother gas flow even under high ambient pressures. In the HYDRA 10 chamber dive to 701 meters sea water (msw), divers reported no dyspnea or significant nasal breathing difficulties despite slight ventilatory impairment below 650 msw, with full recovery observed as partial pressure decreased during decompression. The mixture's high thermal conductivity promotes rapid heat dissipation through expired gases, accelerating body cooling and elevating risk during prolonged exposure, particularly in cold environments. Divers must employ heated undergarments, drysuits, or warmed to counteract this effect and maintain balance. During the HYDRA 10 experiment, chamber gas temperatures were controlled at 30–33°C with approximately 50% relative to ensure neutrality throughout the hydreliox phase. Cardiovascular adaptations to hydreliox exposure are generally minimal, with stable physiological responses observed under hyperbaric conditions. Electrocardiogram monitoring in the HYDRA 10 dive revealed normal tracings at maximum depth, including no significant alterations in P-R, QRS, or Q-T intervals and a stable QRS axis. An initial marked occurred upon compression but adapted progressively, while heart rates remained consistent during physical exertion at depths from 640 to 701 msw. Hydrogen's potential for rapid tissue diffusion did not result in detectable disruptions to blood gas profiles or overall cardiac function in this trial. Prolonged exposure to hydreliox in controlled human trials has shown no evidence of , even over extended periods. The HYDRA 10 dive involved 9 days below 600 msw, 5 days below 650 msw, and a total saturation duration of 42 days, with divers exhibiting full physiological recovery post-decompression and no lingering adverse effects attributed to the gas mixture. Earlier experimental dives, such as those simulating 700 msw for up to 20 days of compression, similarly reported no long-term physiological impairments.

Safety Considerations

Explosion and Ignition Risks

Hydreliox, a mixture primarily composed of , , and a small amount of oxygen, presents significant explosion and ignition risks due to 's inherent flammability. has a wide flammability range of 4% to 75% by volume in air, making it susceptible to ignition across a broad spectrum of concentrations. Additionally, its minimum ignition energy is exceptionally low at 0.017 mJ, far below that of other common fuels, allowing even minor energy sources to initiate . In the context of diving, these properties amplify hazards within hyperbaric environments, where elevated pressures alter limits and increase the potential for rapid flame propagation. Potential ignition sources include sparks from mechanical equipment, electrical faults, or static discharge, particularly during gas handling or in chambers where mixtures may inadvertently approach flammable thresholds. For instance, in hyperbaric chambers, binary hydrogen-oxygen mixtures have an upper limit around 71.3% at 24 bar, necessitating careful composition to stay above this threshold for safety in related systems. Binary hydrogen-oxygen mixtures, relevant to hydrox precursors, become above 4% oxygen at normoxic pressures, a risk that persists under compression. Mitigation strategies focus on minimizing oxygen exposure and eliminating ignition opportunities. Oxygen content in hydreliox is rigorously controlled below 1%—often as low as 0.8% in experimental dives—to remain outside flammable limits, achieved through precise gas blending and continuous scrubbing to remove excess oxygen. Equipment employs non-sparking materials, such as or tools, and grounding protocols to prevent static buildup, while real-time monitoring systems track gas compositions and alert for deviations. These measures, informed by standards for atmospheres such as those from the International Marine Contractors Association (IMCA) for operations, ensure operations occur in a "non-explosive" regime by maintaining concentrations above the upper . No major accidents involving hydreliox explosions have occurred during human dives, reflecting effective safety protocols in controlled trials like COMEX's Hydra X onshore record dive to 701 meters.

Decompression Protocols

Decompression protocols for hydreliox mixtures address the unique inert gas dynamics of , which exhibits higher in tissues compared to (ratios of 1.9 in and 2.9 in oil) but faster washout rates, necessitating careful staging to mitigate (DCS) risk. Unlike , 's off-gassing occurs more rapidly in faster tissue compartments, with time constants around 0.5 hours, leading to half-times of approximately 20-30 minutes, while slower compartments require adjustments about 1.5 times longer than 's equivalents. This dual behavior demands extended saturation periods and controlled pressure reductions to prevent bubble formation from uneven desaturation. Protocols typically involve prolonged bottom times at depth to achieve equilibrium, followed by linear ascents with multiple stops, transitioning from hydreliox to once is reduced below critical thresholds (e.g., to 250 msw or P_H2 < 1.2 MPa) to leverage helium's slower kinetics for shallower stages. In the Hydra VIII trial, six divers underwent 18 days of decompression from 500 msw saturation, including catalytic removal of to 250 msw before standard procedures, resulting in no DCS incidents and full recovery. Similarly, the Hydra X trial featured 24 days of decompression from 675 msw, with hydreliox used down to 280 msw and no circulating bubbles detected via Doppler monitoring. These approaches prioritize conservative staging, often 30% longer than equivalent profiles, to account for 's potency in DCS formation, which is up to 35% higher than helium in animal models. Decompression modeling for hydreliox adapts neo-Haldane frameworks, such as modified Bühlmann algorithms (e.g., ZH-L16 variants), incorporating hydrogen-specific parameters for and half-times to simulate multi-compartment tissue loading. These models treat as an intermediate between and , with slower compartment half-times at 0.75 times nitrogen's and 1.5 times helium's, enabling predictive schedules for saturation exposures exceeding 30 days total. Validation from trials like Hydra VIII confirms efficacy, with Doppler showing minimal post-movement bubbles during hydrogen elimination phases.

Comparisons with Other Gases

Differences from Heliox

Hydreliox differs from primarily in its composition, incorporating alongside and oxygen, for example in a 2024 experimental dive using approximately 3% oxygen, 59% , and 38% , whereas consists solely of and oxygen without . This addition of further lowers the overall gas compared to , as has a molecular weight half that of , thereby reducing the at extreme depths. In terms of performance, is generally limited to operational depths around 300 meters due to the onset of (HPNS), which manifests as tremors and other neurological symptoms starting from approximately 160 meters. extends practical diving depths significantly, with successful saturation dives reaching 534 meters in open water during the COMEX Hydra 8 operation and simulated dives to 701 meters in hyperbaric chambers, primarily through hydrogen's role in ameliorating HPNS symptoms. Despite these advantages, hydreliox presents notable drawbacks relative to , including increased complexity in gas handling and preparation due to hydrogen's flammability, which introduces and ignition risks not present in . Additionally, is more cost-effective and safer overall, as it avoids hydrogen-related hazards while still providing effective narcosis resistance for moderate deep dives. Usage thresholds reflect these differences: is typically employed for dives between 100 and 300 meters in commercial and technical applications, while hydreliox is reserved for experimental or ultra-deep operations beyond 300 meters where HPNS mitigation is critical.

Relation to

is a binary mixture consisting of (H₂) and oxygen (O₂), developed for experimental applications to reduce gas density and compared to traditional mixtures. Human trials with were conducted by COMEX in the 1980s, reaching simulated depths of up to 300 meters in hyperbaric chambers, where divers experienced manageable physiological responses during exposure periods. However, its use was constrained by the onset of (HPNS), manifesting as tremors and cognitive impairments, which limited practical deployment beyond this depth. Hydreliox emerged as an evolutionary advancement of , incorporating (He) into the H₂-O₂ formulation to form a ternary (H₂-He-O₂), thereby enabling safer and deeper operations. This addition of dilutes the hydrogen concentration, reducing the overall proportion of H₂ needed while maintaining low gas for improved respiratory at extreme pressures. COMEX's Hydra V experiment in demonstrated this hybrid approach, with divers saturating at 450 meters using a of approximately 54% H₂, 45% He, and 1% O₂, marking a transition from pure testing. Subsequent trials, such as Hydra VIII in 1988, pushed simulated depths to 534 meters, highlighting hydreliox's role in overcoming hydrox's depth barriers. Both and hydreliox leverage hydrogen's low molecular weight to minimize breathing resistance and attenuate HPNS symptoms—hydrogen's mild properties counteract the neurological effects more effectively than alone. Key similarities include their use of H₂ to achieve densities lower than equivalents, facilitating better physical performance during saturation. Divergent traits arise in stability and safety: , with its higher H₂ content (often exceeding 90%), poses elevated explosion risks due to the flammable nature of concentrated H₂-O₂ blends, whereas hydreliox's dilution lowers the explosive potential by keeping H₂ below critical flammability thresholds (around 4-74% in oxygen-enriched environments). This makes hydreliox more viable for prolonged exposures. Despite successes, was largely phased out for operational depths greater than 400 meters owing to persistent HPNS challenges, incomplete decompression data, and heightened fire hazards in pure form. Hydreliox addressed these by hybridizing with , providing a balanced solution for ultra-deep diving up to 701 meters in simulated COMEX trials by 1992, though it remains experimental due to logistical complexities. This progression underscores hydreliox as a refined tailored for industrial and research applications beyond hydrox's constraints.

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