Diver emergency heating report


Version history

May 2024 Rev. 0.1 – Preface and bookmarking updated; no change to content

March 2017 – Following a review, it was decided that the report should continue to be made available to IMCA members. In order to ensure continued access, the report was republished as IMCA D059 with only minor editorial changes

September 1984 – Initial publication of report as AODC 026 – Diver emergency heating



The need for emergency heating for divers trapped in a diving bell underwater has always been apparent, however specific incidents highlighted the need for positive action. The details of these incidents and accidents have varied but in all of them, the divers involved have suffered degrees of hypothermia when the conventional external heating systems failed.

These accidents prompted extensive research and development into means of providing emergency heating for divers and the formal requirements for such heating systems were included by AODC, in February 1980, in the document ‘Emergency Diving Bell Recovery – Guidance Notes/Code of Practice’. This states ‘The equipment should aim to provide sufficient heat for a period of up to 24 hours in an oxygen/helium environment, with a bell in water at 5°C’. This requirement was further enforced in the UK by the ‘Diving Operations at Work Regulations’ in July 1981 which required diving bells to be provided with ‘a means by which the lives of trapped persons can be sustained for at least 24 hours or, where that is not practicable, sustained for as long as is practicable’.

The research and development which was carried out in 1979 and 1980 was mainly focused on providing active emergency heating, i.e. devices which generated heat in order to keep the ambient internal atmosphere of the diving bell at a suitable temperature or keep the divers themselves warm. The overall insulation of the diving bell was considered and, while helpful in reducing heat loss and thus reducing the required heat input, could not in itself resolve the problem as it would not be practicable to insulate the diving bell to provide sufficient heat conservation for a 24-hour period.

A number of active heating devices were developed to the prototype stage and at least one device was marketed. It became apparent however, when many of the prototypes were tested, that they did not perform as efficiently as had been hoped and that in the main they were bulky and heavy, thus imposing other penalties on the diving bell.

While the development of active heating devices was under way parallel developments were in hand to insulate the trapped divers individually, to minimise their personal heat loss without heating the ambient atmosphere inside the bell. It soon became apparent that this could be done relatively simply and further that this insulation, coupled with a small heat exchanger to conserve the expired gas heat loss and topped up with the heat generated by the exothermic reaction of the chemicals used to remove CO2 from the divers’ exhaled breath, should be sufficient to maintain the divers in thermal balance. This method became known as the passive system.

Three passive systems were developed and showed various advantages over active systems, not the least of which was that they were immediately available to provide protection to divers in a lost bell situation.

As a result, all diving contractors purchased passive systems and almost overnight the developers of active systems were left with very little prospective market – accordingly, development stopped.

The effectiveness of passive systems has now been comprehensively tested and there is reasonable evidence to suppose that, if correctly designed and used, these systems will considerably increase survival times in a lost bell. In one incident in 1981 when a diving bell in the North Sea was trapped for ten hours, a form of passive system was used, and the divers’ survival was undoubtedly assisted by the equipment.

It is apparent however that active heat systems are still desirable, in order to increase the chances of survival during a prolonged entrapment and also to provide ‘belt and braces’ security against malfunction or misuse of the passive systems. It must be accepted however that passive systems offer considerable technical attractions and will always be used as the primary safety protection.


  • Introduction and aims
  • Acknowledgements
  • The requirements for diver emergency heating
  • Passive systems
  • Active systems
  • Future developments
  • Discussion
  • Conclusions
  • Companies mentioned in the report
Reference(s): IMCA D059
Published:May 2024
Grade:IMCA Informative Guidance

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