Annals of the Academy of Romanian Scientists

Series on Engineering Sciences

ISSN 2066-6950

Volume 18, Number 1/2026

5

DIVING USING HYDROGEN-BASED

RESPIRATORY GAS MIXTURES

Mircea DEGERATU ¹, Sergiu IONIȚA²

Rezumat. În cazul scufundărilor la adâncimi de peste 200 m, există doi factori care

limitează eficacitatea scafandrului utilizând amestecuri pe bază de heliu și anume:

Sindromul Nervos al Înaltelor Presiuni (SNIP) datorat efectelor neurologice ale presiunii

și limitările ventilatorii datorate densității unor astfel de amestecuri (pierderi de energie

crescute prin sistemul respirator). Pentru a contracara în mod eficace efectele celor doi

factori limitatori, specialiștii în hiperbar au considerat ca necesară utilizarea hidrogenului

ca o componentă a amestecurilor pe bază de hidrogen, așa cum sunt amestecurile binare

HIDROX (hidrogen - oxigen) și ternare hidreliox (hidrogen – heliu – oxigen).

Abstract. When diving to depths of over 200 m, there are two factors that limit the

effectiveness of divers using helium-based mixtures, namely: High Pressure Nervous

Syndrome (HPNS) due to the neurological effects of pressure and ventilatory limitations

due to the density of such mixtures (increased energy loss through the respiratory system).

To effectively counteract the effects of these two limiting factors, hyperbaric specialists

considered necessary to use hydrogen as a component of hydrogen-based mixtures, such as

the binary HYDROX (hydrogen-oxygen) and ternary HYDRELIOX (hydrogen-helium-

oxygen) mixtures.

Keywords: commercial diving, saturation diving, breathing mixture, hydrogen

DOI 10.56082/annalsarscieng.2026.1.5

1. BRIEF HISTORYAND GENERAL INFORMATION

The idea initiated by Lavoisier to use hydrogen as a low-density breathing

gas dates back to the 18th century. After the well-known tests carried out by

engineer Arne Zetterström between 1943 and 1945 as part of a Swedish Royal Navy

program, with the exception of some animal experiments in the laboratory, no

substantial program was undertaken until 1967. In 1968, as part of the HYDRA I

¹Prof., PhD, Eng., Dept. of Hydraulics and Environmental Protection, Technical University of Civil

Engineering Bucharest, Academy of Romanian Scientists (mircea.degeratu@yahoo.com )

²Professional diver, certified in the U.S.A. at Commercial Diving Institute of New York, was

employed by some prestigious submarine intervention companies specialized in underwater

interventions at offshore oil installations in the Gulf of Mexico. (sergiu.ionita@rdslink.ro ).

6

Mircea Degeratu, Sergiu Ioniță

program, COMEX attempted to dive one of its elite divers, René Veyrunes, to a

depth of 250 m using hydrogen. However, the diving equipment was insufficient to

protect him from the cold, and he had to return to the bell. It was not until 1982 that

the research program on the use of hydrogen in deep-sea diving was relaunched,

taking advantage of the evolution of materials and technologies for deep-sea diving

developed on oil rigs.

The aim of the studies is to define the industrial methods necessary for the

practical use of hydrogen-based mixtures and to study the limits of their use in

humans.

The use of hydrogen in diving has been approached with diver safety as the

main technical objective. The definition of the means, methods, and procedures

must be based on precise knowledge of the following elements:

- the flammability limits of hydrox (hydrogen-oxygen) and hydreliox

(hydrogen-helium-oxygen) mixtures under hyperbaric conditions;

- the compatibility of hydrogen with equipment materials and the

breathability of hydrogen (research into pollutants);

- adaptation and compliance of all gas circuits, oxygen regeneration and

automatic refill systems;

- development of a system to remove hydrogen during decompression by

catalytic dehydrogenation.

In practice, with a significant safety margin, a maximum oxygen

concentration of 3% has been set for all hydrogen-based mixtures. With this safety

limit, all cases of hydrogen use in normoxic gas mixtures become impossible in the

0-60 m depth range. During the compression phase of a saturation dive, hydrogen

is not injected into the hyperbaric chamber until 200 m and is then progressively

eliminated during the decompression phase to reach a concentration of 3% at a

depth of 300 m. This removal of hydrogen at the chosen decompression rate is

achieved by catalytic dehydrogenation in a temperature-controlled reactor,

combining hydrogen with oxygen to produce water.

These technological advances have enabled COMEX to successfully

complete its HYDRA research program.

2. HYDRA-COMEX SYSTEM DIVING PROGRAM

In 1983, as part of the Hydra 3 program, sixteen divers, led by Henri

Delauze, President of COMEX, carried out a series of dives between 70 and 91 m

in the sea off Marseille. In the same year, as part of the HYDRA IV program, six

divers breathed hydrogen for the first time in a hyperbaric chamber at a pressure of

31 bar (absolute), equivalent to a depth of 300 m. Two years later, in 1985, six

divers achieved the first saturation with hydreliox (hydrogen-helium-oxygen)

mixture at a depth of 450 m (HYDRA V). Thus, the first saturation dive, with

Diving using hydrogen-based respiratory gas mixtures

7

humans in a hyperbaric environment and using the hydrox (hydrogen-oxygen)

breathing mixture, was carried out in 1985 through the HYDRA V experiment at a

depth of 450 m. The experiments continued in 1986 with the HYDRA VI dive to a

depth of 520 m, in 1987 with the HYDRA VII dive to a depth of 260 m, and in 1988

with the HYDRA VIII dive with 6 people for 6 days at a depth of 530 m, a dive

carried out in real conditions at sea. These experiments carried out by COMEX in

France paved the way for a new method of working underwater on subsea

construction sites.

Therefore, in order to demonstrate the industrial feasibility of hydrogen

diving, the continuation of the research program for humans (divers) required the

demonstration, in real conditions at sea, of the human capacity to work at a depth

never before reached: over 500 m. To achieve this ambitious project, two training

and selection dives were designed and carried out in the COMEX hyperbaric

chambers in Marseille: HYDRA VI (1986) at 520 m with hydreliox (hydrogen-

helium-oxygen) and HYDRA VII (1987) at 260 m with hydrox (hydrogen-oxygen).

Confirmation under real conditions (HYDRA VIII) was carried out in the

Mediterranean in February-March 1988, departing from the ship Orelia. This

experiment demonstrated the remarkable effectiveness of the divers at depths of

520 and 534 m, similar to that commonly observed on a construction site at a depth

of 200-250 m with HELIOX (helium-oxygen) mixture.

This was followed by the HYDRA IX experiment in 1989, which clarified

the range of using hydrox (hydrogen-oxygen) mixture and its effects on humans

during long-term exposure.

Ten years after the launch of the HYDRA program, the results were

compiled and used to push the boundaries of human intervention. This was achieved

through test dives as part of the HYDRA X program, which took place at the end

of 1992 in the hyperbaric chambers of the COMEX Hyperbaric Testing Center in

Marseille (fig. 1). The team consisted of three professional divers: Serge Icart, Théo

Mavrostomos, and Régis Peilho. After thirteen days of compression, one of them,

Théo Mavrostomos, reached a record depth of 701 m (71,1 bar (abs.), or 7,11 MPa

(abs.)) (fig. 2) and performed a three-hour working demonstration in the water

hyperbaric hamber (fig. 3).

6

Mircea Degeratu, Sergiu Ioniță

Fig. 1. Hyperbaric hambers belonging to the system for simulation diving with

hydrogen-based mixtures at C.E.H. COMEX in Marseille.

Fig. 2. Diver Theo Mavrostomos

at a record depth of 701 m as part of the HYDRA X program

Diving using hydrogen-based respiratory gas mixtures

9

Fig. 3. Demonstration by Theo Mavrostomos in the water pressure

chamber (HYDRA X experiment – 701 m)

Recently, a new method of using hydrogen has been developed in order to

limit the cost of adapting systems. This method, called "Helium IN / Hydrogen

OUT", consists of saturating divers with the classic HELIOX (helium-oxygen)

mixture in the living hyperbaric chamber and in the diving bell, and supplying the

diver, who is in the water, with hydreliox (hydrogen-helium-oxygen) through a

specially adapted circuit. Two test dives, HYDRA XI and HYDRA XII (1994 and

1996), were carried out using this technique at depths of 350 m and 210 m

respectively. HYDRA XI enabled procedures to be established and HYDRA XII

demonstrated the advantages of this method at sea: greater ease of breathing for

divers during strenuous efforts and greater efficiency at work.

Therefore, in 1998 and 1999, through the research program "HYDRA

LUDION" (I and II) research program, COMEX studied the possibility of using

hydrogen to achieve rapid pressure variations between the saturation depth of the

diver (living level) and that of the underwater site (working level). Compared to the

helium ludions currently in use, hydrogen has been shown to allow for twice as

large pressure variations.

Hydrogen-based breathing mixtures can be used successfully at depths of

150 to 650 m.

Through the HYDRA program (Table 1), 17 years of intensive research on

the use of hydrogen in deep diving has shown that psycho-sensory and behavioral

manifestations, namely hydrogen narcosis, only occur at a partial pressure of

hydrogen of 2 MPa (abs.) or 20 bar (abs.). Below this limit, the hydreliox

6

Mircea Degeratu, Sergiu Ioniță

(hydrogen-helium-oxygen) breathing mixture, by limiting the effects of SNIP and

increasing breathing comfort, significantly improves the effectiveness and working

capacity of divers.

Table 1

Experimental saturation dives with hydrogen-based breathing mixtures

based on hydrogen, carried out by C.E.H. COMEX

Partial

pressure

of

Breathing

mixture

pressure

p_am

Exposure

time to

hydrogen

mixture

[days]

Type of

breathing

mixture

used

Diving

depth

[m]

Experimental

dive

Nr. of

divers

hydrogen

[bar (abs.)]

bar (abs.)

HYDRA V

(1985)

450

hydreliox (H₂-He-

O₂)

46

25

4

18

3

HYDRA VI

(1986)

HYDRA VII

(1987)

HYDRA VIII

(1988)

hydreliox (H₂-He-

O₂)

hydrox

(H₂-O₂)

hydreliox (H₂-He-

O₂)

520

260

53

2

24

25

2

15

5

8

4

6

520/534

53/54.4

18

HYDRA IX

(1989)

300

225/200

hydrox

(H₂-O₂)

31

23.5/21

30

19

30

4

3

HYDRA X

(1992)

hydreliox (H₂-He-

O₂)

675/701

68.5/71.1

2

29

3

HELIOX

(He-O₂)

hydreliox (H₂-He-

O₂)

HYDRA XI

(1994)

350/335/

365

36/34.5/37.5

-

4

HELIOX

(He-O₂)

hydreliox (H₂-He-

O₂)

HELIOX

(He-O₂)

hydreliox (H₂-He-

O₂)

HYDRA XII

(1996)

200/210

21/22

-

4

2

HYDRA

LUDION I

(1998)

200/260/

300

21/27/31

HELIOX

(He-O₂)

HYDROGEN-

HELIUM

MIXTURE

(H₂-He-O₂)

HYDRA

LUDION II

(1999)

200/290

100/160/

21/30

11/17

-

5

Diving using hydrogen-based respiratory gas mixtures

11

3. PRESENTATION OF THE HYDRA VII PROGRAM

For example, Figure 4 shows the saturation dive profile, with hydrox

(hydrogen-oxygen) breathing mixture, at a depth of 260 m, of the HYDRA VII

experiment. Figure 5 shows a general diagram of the diving assembly used for this

experiment.

In the HYDRA VII experiment, the divers were compressed in 4 hours with

HELIOX mixture at a depth of 180 m, after which the HELIOX (helium-oxygen)

mixture was replaced with the hydrox (hydrogen-oxygen) mixture until the hydrogen

concentration in the new mixture in the diving assembly reached 90%. During this

period, the divers experienced a slight euphoria for 15 to 30 minutes, comparable

to the narcosis experienced in compressed air dives at a depth of 40 to 50 m. This

slight narcosis disappeared fairly quickly, allowing pressurization to continue at the

pressure corresponding to a depth of 260 m. Before decompression, dehydrogenation

was performed for 40 hours at 260 m, at a rate of 0.6 bar (absolute) of hydrogen per

hour. The final decompression was performed with a HELIOX (helium-oxygen)

mixture.

Fig. 4. Profile of the saturation dive with hydrox breathing mixture

performed as part of HYDRA VII (COMEX) experiment.

6

Mircea Degeratu, Sergiu Ioniță

Fig. 5. General diagram of diving assembly used for the HYDRA VII experiment

(COMEX).

The experimental dives using hydrogen carried out by COMEX at the

Hyperbaric Testing Center in Marseille continued with the HYDRA IX experiment

in 1989, which made it possible to specify the field of use of the hydrox (hydrogen-

oxygen) mixture and its effects on humans during long-term exposure.

The results obtained in the use of hydrogen as an oxygen diluent for

saturation diving at great depths were positive. Technical problems related to the

injection of oxygen into hyperbaric chambers to replenish that consumed by divers,

as well as problems related to sealing, were solved, and it was concluded that

hydrogen could be a new solution as an oxygen diluent for life under pressure.

During dives conducted as part of the French HYDRA program, using

hydrogen-based breathing mixtures at partial hydrogen pressures of 1.9...3 MPa

(abs.) (Table 1), certain effects of the hydrogen-based mixture on human body were

observed, effects that can be grouped under the name of hydrogen narcosis. These

Diving using hydrogen-based respiratory gas mixtures

13

effects could be studied in relation to clinical observations made during dives at

equivalent depths using a HELIOX (helium-oxygen) mixture.

It was found that hydrogen acts on the human body as follows:

– causes controlled psycho-sensory and behavioral manifestations

(perceptual disturbances, sleep changes, psychological and intellectual changes, for

2,4 MPa (abs.)) or uncontrolled manifestations (anxiety, depression, confusion and

disorientation, for 2,4...3 MPa (absolute pressure)) in divers;

– mitigates the effects caused by high-pressure nervous syndrome (SNIP)

(almost complete disappearance of tremors), high-pressure joint syndrome (SAIP)

(significant reduction in joint pain and discomfort), and high-pressure respiratory

syndrome (SRIP) (easier ventilation with less intense respiratory muscle effort).

Tests carried out by COMEX as part of the HYDRA diving program have

shown that, for a hydrogen-based mixture with a partial hydrogen pressure in the

range of 2 MPa (absolute), the disorders caused by "hydrogen narcosis" are well

controlled and compensated for by divers. It can also be concluded that, through the

use of hydrogen-based breathing mixtures, depths between 150 m and 650 m can

be considered accessible to a wide range of divers. The hydreliox (hydrogen-

helium-oxygen) breathing mixture, by limiting the effects of SNIP and increasing

respiratory comfort, significantly improves the efficiency and working capacity of

divers at underwater hydrocarbon production facilities. Thus, oil companies are now

assured of human technical capacity in areas with depths exceeding the possibilities

of conventional helium diving (depths of over 200 m).

The advantages of using hydrogen are also related to its abundance compared

to helium. However, hydrogen cannot completely replace helium, especially during

decompression, when the oxygen concentration increases as the pressure decreases,

up to 24% in the last 10 m toward atmospheric pressure.

Hydrogen has the disadvantage of being an explosive gas when mixed with

air in proportions that include the presence of 5,3% oxygen. When mixed with more

than 4% oxygen, hydrogen spontaneously becomes explosive. To avoid the risk of

chemical combination, the volume concentration of oxygen in the hydrox

(hydrogen-oxygen) breathing mixture must be less than 4% (< 0,04). Between 2,5%

and 0,6% (= 0,025...0,006), and therefore harmless in combination with hydrogen,

this oxygen concentration allows the use of the hydrox (hydrogen-oxygen) mixture

for diving at depths between 70 and 700 m.

4. ROMANIAN HYDROGEN DIVING PROGRAM HYDRODIVE

Hyperbaric engineering specialists from the Hyperbaric Laboratory at the

Constanța Diving Center belonging to the Romanian Navy, in collaboration with

specialists in the thermohydraulics of compressible fluids applied to human

underwater penetration from the Department of Hydraulics and Environmental

Protection at the Technical University of Civil Engineering Bucharest, will develop

6

Mircea Degeratu, Sergiu Ioniță

a research program called HYDRODIVE on using hydrogen as a component of

hydrogen-based mixtures, such as the binary hydrox (hydrogen-oxygen) and

ternary hydreliox (hydrogen-helium-oxygen) mixtures.

The HYDRODIVE program will be designed to include the following main

stages:

- The theoretical preparation phase of the program, tracking the physical

characteristics of the components of hydrox binary mixtures (hydrogen-

oxygen) and ternary hydreliox (hydrogen-helium-oxygen) mixtures and the

limits of volume fractions and partial pressures of gaseous components to

avoid ignition of the mixture and to reduce the narcotic effects of hydrogen-

based mixtures.

- The phase of performing dives with a open (wet) diving bell or a open bell

pressurized with HELIOX (helium-oxygen), divers breathing from the

umbilical, for a short time, binary hydrox mixture (hydrogen - oxygen), at a

depth where both the danger of ignition of the mixture and the occurrence

of narcosis at an unbearable level will be avoided.

- Calculation and preparation phase for a diving technology using hydrogen-

based mixtures, i.e., binary hydrox (hydrogen-oxygen) and ternary

hydreliox (hydrogen-helium-oxygen) mixtures in a hyperbaric system, with

the preparation of hyperbaric chambers and related installations.

- Phase of performing a system dive with professional divers from the

Hyperbaric Laboratory, using hydrogen-based mixtures at depths of up to

300 m.

R E F E R E N C E S

[1]

[2]

J.E. Cayford, Underwater Work (Lorella Maritime Press Centreville, Maryland, 1982).

A. Constantin, Transportul gazelor prin sistemul respirator uman şi mijloacele de protecţia

respiraţiei, în procese hiperbare (PhD Thesis, "Ovidius" University Constanţa, 1998).

M. Degeratu, A. Petru, V. Beiu, Computer – aided Simulation of Theoretical Processes in

Binary and Ternary Mixtures of Hyperbaric Systems Used in Deep Diving (Biochem.

Methods, Columbus, Ohio, U.S.A., 1986) Vol. 107, p. 346.

[3]

[4]

[5]

M. Degeratu, A. Petru, Şt. Georgescu, S. Ioniţă, Tehnologii hiperbare pentru scufundări

unitare şi în saturaţie (Matrix ROM, Bucureşti, 2003).

Mircea Degeratu, Sergiu Ioniță, Respiratory Gases used in Professional Diving (Annals of

the Academy of Romanian Scientists, Series on Engineering Sciences, ISSN 2066-8570

Volume 16, Number 1/2024, DOI 10.56082/annalsarscieng.2024.1.100).

[6]

[7]

Mircea Degeratu, Sergiu Ioniță, Processes and Installations for the Preparation of Breathing

Gas Mixtures (Annals of the Academy of Romanian Scientists, Series on Engineering

Sciences, ISSN 2066-8570 Volume 16, Number 1/2024).

Mircea Degeratu, Sergiu Ioniță, Blending Respiratory Gas Mixtures (Annals of the Academy

of Romanian Scientists, Series on Engineering Sciences, ISSN 2066-8570 Volume 16,

Number 2/2024).

Diving using hydrogen-based respiratory gas mixtures

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[8]

[9]

Mircea Degeratu, Sergiu Ioniță, System dives with gas mixtures based on helium (Annals of

the Academy of Romanian Scientists, Series on Engineering Sciences, ISSN 2066-8570

Volume 16, Number 1/2024).

B. Gardette, HYDRA IV and HYDRA V, human deep hydrogen dives 1983-1985. In hydrogen

as a diving gas (33-rd Under sea and Hyperbare Medical Society Workshop. BRAUER R.

W., 1987).

[10] C.J. Lambertsen, Effects of Excesive Pressure of Oxygen, Nitrogen, Helium, Carbon Dioxide

and Carbon Monoxide. (V.B. Mountcastle, Missouri, 1980) Vol. 2.

[11] G. Poulet, R. Barincou, La plongée (Denöel, Paris, 1988).

[12] * * *, U.S. Navy Diving Manual (U. S. Government Printing Office, Washington, 1975).

[13] * * *, Linde Gasekatalog (Linde AG, Werksgruppe Technische Gase, München).