This chapter describes what is meant by the term ‘life-support’ in the context of hyperbaric-diving and-examines the reasons for and requirements of secondary life-support systems, sometimes called bail-out systems. Equipment and some of the operational aspects, including procedures, training and ancillary items are discussed.

An opinion on the level of secondary life-support required in shallow, medium and extreme depths is presented to provoke discussion and help identify potential solutions for some of the remaining problem areas. Primary life-support systems are only discussed where background information is relevant.


In hyperbaric diving operations, life-support is the term given to equipment that supports the function of biological processes in an otherwise lethal environment, that is:

  • respiration;

  • maintenance of body temperature;

  • control of environmental pressure.

In saturation diving, facilities must also be provided for:

  • nutrition;

  • excretion,

  • sleep.

Failure of the system to provide any of these facilities will, after an initial ‘buffer’ period, when the human body depletes its built-in storage capacity, or its physiological tolerance to change is exceeded, lead to severe distress and ultimately death.

In commercial diving operations, exposure to a non-life-supporting environment, the in-water work period, is normally less than 8 hours. It is, therefore, only those requirements with a ‘buffer’ or tolerance period shorter than 8 hours that are normally provided for by the life-support system, that is breathing gas, maintenance of body temperature and control of environmental pressure.

In the case of in-water incidents, a period of 10–15 minutes is usually considered adequate to allow a diver to return to a more permanent ‘dry’ life-supporting environment (the surface, a submarine or a diving bell) if the primary in-water system fails. It is, therefore, only those requirements with a ‘buffer’ period shorter than 10–15 minutes that are provided for by secondary life-support systems. In shallow water, this is usually breathing gas and a method to control environmental pressure. In deeper water, as pressure increases and ambient temperature decreases, the ‘buffer’ periods are reduced due to a complex interaction of physical and physiological factors. A point is reached where the secondary system must also maintain body temperature. As depth increases, it also becomes more difficult to design and build secondary life-support systems with an endurance of 10–15 minutes.

An indication of the length of the ‘buffer’ periods before unacceptable distress sets in can be gained from the following examples. The times are necessarily approximate.(Situations are available in full paper)


For the purposes of this discussion, I am going to define an incident as an event, or series of events, that remove the primary life-support facilities from a diver in the water, forcing him to become autonomous.

The title ‘In-water Secondary Life-support Systems: removing the emergency from incidents’ serves to imply that secondary systems should be regarded as complete escape systems, rather than a simple ‘bail-out bottle’ that supplies breathing gas. I would like to labour this point by describing an incident.

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