Diving using mixed gas

Mixed gases are breathing gases other than air or EANx used by divers to dive to greater depths.

Commonly used mixes include:

• helium and oxygen (HELIOX)
• helium
• nitrogen
• oxygen (TRIMIX).

Mixed gases are breathed by divers in both open circuit SCUBA and rebreather equipment. Generally using mixed gases reduces the diver's exposure to the toxic and narcotic effects of elevated partial pressures of nitrogen and oxygen while managing their exposure to decompression illness and preventing hypoxia. However careful planning, gas analysis, training and proper equipment is required for this type of diving to be undertaken safely.

Rebreathers are underwater breathing devices using a breathing circuit rather than an open system. The breathing quality of the air is maintained using chemical absorption to remove carbon dioxide while oxygen and diluent (air, EANx or other inert gases) are added. These systems use in line sensors and computers to monitor the breathing gas and alert the diver if required. The benefits of rebreathers include quietness, the ability to vary the breathing gas mix during the dive and efficient gas usage. The risks include the system flooding and failures in the use of the various components of the breathing circuit resulting in toxic or hypoxic breathing gas.

Diving using mixed gas with SCUBA

The person conducting the business or undertaking should ensure:

• mixed gas diving does not take place unless a dive supervisor is present at the dive site
• only a certificated mixed gas scuba diver undertakes recreational scuba diving using mixed gases unless the diver is undertaking training for the purpose of certification as a mixed gas scuba diver in accordance with the Recreational Diving, Recreational Technical Diving and Snorkelling Code of Practice 2018 (PDF, 0.61 MB)
• oxygen partial pressure exposure times are not exceeded
• maximum depth of a dive does not exceed the depth where:
  • oxygen in the mixture being breathed at any time exceeds a partial pressure of 1.4 bar while diving and 1.6 bar during decompression
  • nitrogen in the mixture being breathed at any time exceeds a partial pressure of 4.0 bar while diving
• minimum depth does not exceed the depth where oxygen in the mixture currently being breathed is less than a partial pressure of 0.16 bar
• before a breathing mixture is used, the diver conducts a gas analysis to verify the oxygen content. The results should be recorded in the mixed gas dive safety log and on the cylinder.

Divers should be advised about:

• the dive plan and gas change over depths
• the sequence and role of each diver
• gas turn around pressures and analysing gas mixtures
• maximum and minimum depths for each breathing gas and run times
• omitted decompression procedures
• analysing their gas mixture
• emergency procedures including:
  • loss of breathing gas procedures
  • buddy separation procedures
  • loss of ascent path procedures
  • the location of and contact procedures for the nearest recompression facilities
• checking the position and correct operation of their own equipment and their buddy's equipment
• performing for themselves and their buddy an in-water check (leak test) and an in-water regulator location and correct operation check.

Find out more about mixed gas  qualifications, equipment for mixed gas diving and the blending, testing, storage and use of mixed gas.

Diving using mixed gas rebreathers

The person conducting the business or undertaking should ensure:

• mixed gas  rebreather diving does not take place unless a dive supervisor is present at the dive site
• only a certificated mixed gas rebreather diver undertakes recreational diving using a mixed gas rebreather unless the diver is undertaking training for the purpose of certification as an mixed gas rebreather diver in accordance with the Recreational Diving, Recreational Technical Diving and Snorkelling Code of Practice 2018 (PDF, 0.61 MB)
• oxygen partial pressure exposure times are not exceeded
• maximum dive depth does not exceed the depth where:
  • oxygen in the mixture being breathed at any time exceeds a partial pressure of 1.4 bar while diving and 1.6 bar whilst on a mandatory decompression stop
  • nitrogen in the mixture being breathed at any time exceeds a partial pressure of 4.0 bar while diving
• before a breathing mixture is used, the diver conducts a gas analysis to verify the O₂ content. The results should be recorded in the mixed gas rebreather dive safety log and on the cylinder
• rebreathers should not be used for introductory experiences or resort dives in open water for non-certificated diving.

Divers should be advised about:

• the dive plan
• omitted decompression procedures
• checking the position and correct operation of their own equipment and that of their buddy
• dive objectives
• maximum and minimum depths for the breathing gas
• loss of breathing gas procedures
• buddy separation procedures
• safety requirements
• emergency procedures, including the location of and contact procedures for the nearest recompression facilities
• manufacturer's pre-dive checks, emergency procedures and advice on carbon dioxide scrubbers.