Ice Diving with a Rebreather

Ice Diving with a Rebreather
The dive may be nippy, in more ways than one
By Alex Deas


Lessons from Another Man's Life

A member of the Rebreather World dive forum, whom I have never met, very kindly sent me a video of the 1998 and 1999 North Pole Dive Expedition. In the 1998 expedition, a diver dies after just 8 minutes. The diver was on a rebreather.

This video, with all its graphics, posed the question of whether we have thought carefully enough of the hazards of diving in very cold water with a rebreather. It stimulated testing, modelling and safety reviews. The conclusions were rather startling: that sports rebreathers are fundamentally unsuited to very cold water.


The Attraction

The exothermic heating from the scrubber may suggest that a rebreather is a suitable tool for diving in cold water. This turns out not to be the case.

The condensation that forms in a rebreather is almost pure water. Think of the rebreather as a still: distilled water forms on the breathing hoses, sensors , injectors and mushroom valves. As pure water, it will freeze at 0C, and become slightly sticky below 4C. If the environment is colder than 4C, that spells trouble.

Diving with a sports rebreather in water below 4C, poses special hazards. The risks increase with reducing temperature, as shown below.


Temperature Risk

• Above 4C -> Low risk
• Below 4C -> Significant risk of death
• Below 0C -> High risk of death
• Below -4C -> Almost certain death

The risks are caused by the following phenomena:

• The moisture in the breathing loop is almost pure water, so freezes at a higher temperature than sea water. The water can freeze in the breathing hoses, on the mushroom valve, or in the scrubber.
• The oxygen sensors do not perform correctly at very low temperatures. This will lead to large errors in PPO2.
• The scrubber efficiency drops as the square of the temperature. At around zero, the scrubber stops working. This is a problem if the equipment has been stored in very cold conditions.
• The expansion of injected gas in the humid environment of the rebreather will cause ice to form on the injector nozzle. This can block the injector, so the injector is heard to fire by the user (if a solenoid design), but is not injecting gas.
• The mushroom valves are particularly liable to freeze shut when condensation on them is near freezing.
• Counterlungs freeze together when the diver takes a deep breath. Rebreathers with an exhale only counterlung fare better in this, and do not cool the gas as much so lower temperatures are needed to freeze the mushroom valves shut.
• “Dive reflex” causes a large increase in blood pressure when the head is in cold water.
• Risk of over contraction of any silicone oil used to equalise pressure at depths rupturing electronic housings.
• Risk of inappropriate materials cracking with mechanical shock in a cold environment.
• LCD displays lose contrast in very cold conditions.
• Some integrated circuits, particularly Flash memories and DRAM, do not function well in cold conditions.
• Batteries will go flat much faster in cold conditions than in warm, and their internal resistance rises even when fully charged. This creates more power supply noise, and will cause equipment malfunction if there is any under performance in the power regulators.
• Risk of shock on entering very cold water.
• Risk of dry suit leaks are much more serious in very cold water.
• Risk of mechanical damage due to ice forming and expanding during equipment storage.

The only way around this is to use heating to keep the rebreather heated. It requires around 20C due to the speed at which ice can form on barriers such as the breathing hoses, or around objects where the gas flow is the fastest, such as mushroom valves and gas injectors. This heating requirement implies umbilical diving: 100W is needed - too much for batteries.


Safety Implications

For diving in very cold water, it is necessary to have a SIL rated heating system in the counterlungs, sufficient to keep the loop temperature above 20C, and to have active monitoring of the gas flow so that any blockage can be detected.

Equipment should be stored in a warm location, and at all times when not in the warm location, the equipment should be operating to maintain its temperature.

EN14143:2003 requires the equipment to be tested with storage to minus 30C, for material suitability. Some dives are in environments colder than this, such as in Russia in winter and polar dive expeditions. The equipment is wet when it comes out of the water, so chill factors become an issue also, reducing the effective temperature of the surface of the equipment.


Open Circuit

Open Circuit SCUBA in comparison with a rebreather has few hazards for ice diving, if the normal safeguards are carried out: filling cavities with oil, use of dry gas and ice free regulators. Even then, free flows are common. Using multiple regulators gives redundancy.


Credits

• To Andrey (Faceless), for sending that video, and triggering a detailed test and safety regime.
• To the 1998 and 1999 Polar Expedition film-makers "Conquest of the Artic", from Videodive and all those in the Moscow University Dive Club.
• To Andrey Rodjkov in paying the ultimate price to impart knowledge to another generation.

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