Guidance on prevention and mitigation of lost bell emergencies


Version history

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

May 2023 – Initial publication



A closed diving bell is any compression chamber which is capable of being manned and is used or designed for use under the surface of water in supporting human life at a pressure greater than atmospheric pressure during normal operation.

A potential risk when using closed diving bells is that the diving bell and its divers cannot be recovered from depth to the surface, either because the main lift wire, secondary lift wire(s) and main umbilical are severed, or because the bell is trapped. Such an incident is referred to as a ‘lost bell’ if supplies of breathing gas or heating from the surface are lost.
It is recognised that a lost bell diving incident is a rare occurrence, but one which is life-threatening for the divers. Previous lost bell incidents have indeed proved fatal.

There have been no documented lost bell incidents in recent years and very few recorded ‘near misses’. However, there is no room for complacency as any lost bell diving incident has the potential to cause multiple fatalities. The most pressing threats to the lives of divers in a lost bell are severe hypothermia and the risk of carbon dioxide poisoning. It should also be noted that bell diving in warm areas of the world may introduce problems with hyperthermia, which also need addressing.

All closed diving bells need to be capable of sustaining the lives of trapped divers for at least 24 hours.


  • Introduction
  • Aims
  • Application
  • The Prevention of Lost Bell Diving Incidents
  • The Mitigation of Loss Bell Diving Incidents
  • References
  • Appendix 1 – Bell Mounted Acoustic Transponder Specification
  • Appendix 2 – Diver Operated Interrogator/Receiver
  • Appendix 3 – Bell Emergency Communication
  • Appendix 4 – Bell Ballast Release Systems
Reference(s): IMCA D077
Published:May 2024
Grade:IMCA Recommended Practice

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