Key areas of concern are where an octopus set-up, in which two second-stage demand valves operate off a single first-stage regulator, is based on a less expensive, lower-performance first stage; and where the second stages feature either inefficiently old or inappropriately paired units.

The HSE commissioned the research following difficulties experienced by some divers using second stages as octopus rigs in emergencies. Dual hyperbaric breathing simulators to test a number of octopus configurations, at varying ventilation rates, depths and air supply pressures.

The research found that the performance of a first-stage regulator is a vital factor when determining the performance of a complete system, and that reduced breathing performance was experienced when using low cost/performance first-stage regulators compared to high cost/performance models, says the HSE.

In addition, tests showed that the poorer performing demand valve of any octopus pair will experience a greater loss of performance with increasing depth and ventilation rate when compared to the better performance valve.

It was recommended that a high-performance first stage should be acquired if an octopus rig is to be based on it. Any two demand valves set up as octopus partners should be of similar performance. Older valves, or ones where performance may have degraded, should not be used.

Although octopus rigs are used by most divers, the HSE stressed that they are best avoided altogether when considering emergency arrangements.

HSE recommends the use of a completely independent secondary gas supply system, for example a pony cylinder set-up, said Chris Sherman, Chief Inspector of Diving. That way, if there is a problem with the octopus system or if a buddy pair become separated, divers have a much increased chance of survival in the event of running out of air.

An alternative is the twin-cylinder set-up, which works well for the wearer, though not of course for another diver if separation occurs.

The HSE report, Breathing Performance of Octopus Demand Diving Regulator Systems, can be viewed here . Printed copies are available from HSE Books, tel. 01787 881165.

  • DIVER magazines May issue includes a regulator group test which put 13 regulators through their paces in the cold March waters of Sweden. As usual the testers rigged each first stage with two demand valves to check whether a pair of divers would be able to breathe comfortably off it simultaneously. Regs on the Rocks will be loaded into the Equipment Group Tests section of Divernet at the end of May.

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