Ice diving
Ice diving is a type of penetration diving where the dive takes place under ice. Because diving under ice places the diver in an overhead environment typically with only a single entry/exit point, it requires special procedures and equipment. Ice diving is done for purposes of recreation, scientific research, public safety and other professional or commercial reasons.
The most obvious hazards of ice diving are getting lost under the ice, hypothermia, and regulator failure due to freezing. Scuba divers are generally tethered for safety. This means that the diver wears a harness to which a line is secured, and the other end of the line is secured above the surface and monitored by an attendant. Surface supplied equipment inherently provides a tether, and reduces the risks of regulator first stage freezing as the first stage can be managed by the surface team, and the breathing gas supply is less limited. For the surface support team, the hazards include freezing temperatures and falling through thin ice.
Environment
Ice diving is underwater diving in water partly or completely covered by ice which may be an overhead obstacle to surfacing at some point of the dive, making ice diving a subclass of penetration diving. This can be in fresh or seawater, and the salinity of the water constrains the possible temperature range of the water. In fresh water the coldest water is in contact with the ice, at, and the warmest may be, some distance below the ice, which is the temperature at which fresh water reaches its highest density. In sea water the temperature can be a little lower, around, depending on salinity. Air temperatures can be considerably lower.Hazards
Hazards of ice diving include the general hazards of underwater diving, the specific environmental hazards of penetration diving, in particular the hazard of not finding the exit area, and some hazards that are more specific to the low temperatures. There may also be hazards specific to the particular dive site.- Low environmental temperatures can cause:
- *Frostbite:
- *Hypothermia:, which can reduce the mental acuity, reasoning capacity and cognitive function of the diver, possibly leading to accidents and work errors. More severe cases can be incapacitate the diver and are directly life-threatening.
- *Non-freezing cold injury
- *Regulator freezing:
- Entrapment by moving ice: When diving through a natural gap in ice cover, it may be possible for the separate floes to move relative to each other, under the influence of wind or water movement, which can change the geometry of the entry and exit area, creating a risk of closing the gap through which the diver must exit the water.
- Falling through weak ice cover: The ice layer may be flawed or insufficiently thick to support the load of the dive team.
- Slipping on ice: Diving gear is heavy out of the water, and the water on the diver's exposure suit can quickly freeze, reducing mobility and traction, and ice is an inherently slippery material.
- Wild animals like sharks and polar bears:
Regulator freezing
When air expands during pressure reduction in a regulator, the temperature drops and heat is absorbed from the surroundings. It is well known that in waters colder than use of a regulator to inflate a lift bag, or to purge a regulator underwater for just a few seconds, will start many regulators free-flowing and they will not stop until the air supply to the regulator is stopped. Some cold water scuba divers install shuttle type shut off valves at each second stage regulator so if the second stage freezes open, the low pressure air can be shut off to the frozen second stage allowing them to switch to the alternative second stage and abort the dive.
The most familiar effect of regulator freezing is where the second stage demand valve starts free flowing due to ice formation around the inlet valve mechanism that prevents the valve from closing after inhalation. Besides the problem of free flow from second stage icing, a less known problem is free ice formation, where ice forms and builds up inside the second stage but does not cause the regulator to free flow, and the diver may not be aware that the ice is there. This free ice build-up inside the second stage can break loose in the form of a sliver or chunk and pose a significant choking hazard because the ice can be inhaled, which may trigger laryngospasm. This can be a particular problem with regulators having ice-shedding internal surfaces that are teflon coated, which allows the ice to break free of the internal surfaces and helps to prevent the regulator from free flowing by clearing the ice. This may be helpful in keeping the demand valve mechanism free to move, but the ice still forms in the regulator and has to go somewhere when it breaks loose.
With most second stage scuba regulators, ice forms and builds up on internal components, and the gap between the valve lever and fulcrum point is reduced and eventually filled by the build-up of ice that forms, preventing the inlet from fully closing during exhalation. Once the valve starts leaking, the second stage components get even colder due to the cooling effect of the continuous flow, creating more ice and sometimes an even greater free flow. With some regulators the refrigerating effect is so great, that water around the exhaust valve freezes, reducing exhaust flow, increasing exhalation effort, and producing positive pressure in the valve body, making it difficult to exhale through the regulator. This may cause the diver to loosen their grip on the mouthpiece and exhale around the mouthpiece.
With some regulators, once the regulator starts free-flowing the flow escalates into a full free-flow, and delivers air to the diver at temperatures cold enough to freeze mouth tissue in a short time, causing frostbite. The effect increases with depth, and the deeper the diver is, the faster the breathing gas will be lost. In some cold water fatalities, by the time the diver's body is recovered there is no gas left in the cylinder, and the regulator has warmed up and melted the ice, destroying the evidence, and leading to a finding of death by drowning due to running out of gas.
Mechanism of icing
When the high pressure gas passes through the regulator first stage, the pressure drop from cylinder pressure to inter-stage pressure causes a temperature drop as the gas expands. The higher the cylinder pressure the greater the drop in pressure and the colder the gas gets in the low pressure hose to the second stage. An increase in flow will increase the amount of heat lost and the gas will get colder, as heat transfer from the surrounding water is limited. If the breathing rate is low to moderate the risk of ice formation is less.The factors that influence ice formation are:
- Cylinder pressure: - The temperature drop is proportional to the pressure drop. See general gas equation.
- Breathing or flow rate: - The heat loss is proportional to the mass flow of gas.
- Depth: - Mass flow is proportional to downstream pressure for a given volumetric flow.
- Water temperature: - Rewarming of the expanded gas and the regulator mechanism depends on water temperature, and the temperature difference between the gas and the water.
- Duration of flow:- During high flow rates heat loss is faster than rewarming, and gas temperature will drop.
- Regulator design and materials: - The materials, arrangement of parts, and gas flow in the regulator affect reheating and deposition of ice. The thermal conductivity of the regulator components will affect the rate of heat transfer.
- Breathing gas composition: - The amount of heat needed to raise the temperature depends on the specific heat capacity of the gas.
The cold inter-stage air enters the second stage and is reduced to ambient pressure, which cools it further, so it chills the second stage inlet valve components to well below freezing and as the diver exhales, the moisture in the exhaled breath condenses on the cold components and freezes. Heat from the surrounding water may keep the second stage regulator components warm enough to prevent the build-up of ice. The diver's exhaled breath at, does not have enough heat to compensate for the cooling effect of the expanding incoming air once the water temperature is much below, and once the water temperature drops below there is not enough heat in the water to rewarm the regulator components fast enough to keep moisture in the divers exhaled breath from freezing if the diver is breathing hard. This is why the CE cold water limit is at which is the point at which many scuba regulators start retaining free ice.
The longer the gas expands at a high rate, the more cold gas is produced, and for a given rate of reheating, the colder the regulator components will get. Keeping high flow rates to as short a time as possible will minimise ice formation.
The air temperature above the ice may be considerably colder than the water under the ice, and the specific heat of air is much less than that of water. As a consequence, there is less warming of the regulator body and inter-stage gas when out of the water, and it is possible for further cooling to occur. This increases the risk of second stage icing, and the gas in the cylinder may be cooled sufficiently for condensation of residual moisture to occur during first stage expansion, as the expanding gas may cool below the dew point specified for high pressure breathing gas, which could cause internal icing of the first stage. This can be avoided by restricting breathing from the set in the cold air to a minimum.
A similar effect occurs with the second stage. Air which has already expanded and cooled through the first stage expands again and cools further at the demand valve of the second stage. This cools the components of the second stage and water in contact with them may freeze. Metal components around the moving parts of the valve mechanism allow heat transfer from the surrounding slightly warmer water, and from exhaled air from the diver, which is considerably warmer than the surroundings.
Second stage freezing can develop quickly from the moisture in the exhaled breath, so regulators that prevent or reduce contact of the diver's exhaled breath with the colder components and the area where the cold gas enters will usually build up less ice on critical components. The heat transfer qualities of the materials can also significantly influence ice formation and freezing risk. Regulators with exhaust valves that do not seal well will form ice quickly as ambient water leaks into the casing. All second stages can develop ice when the inlet gas temperature averages below and this can happen in water temperatures up to. The ice that forms may or may not cause a free flow, but any ice inside the regulator casing may present an inhalation hazard.
A second stage freeze is also likely to happen with the valve open, causing a free flow, which may precipitate a first stage freeze if not immediately stopped. If the flow through the frozen second stage can be stopped before the first stage freezes, the process can be halted. This may be possible if the second stage is fitted with a shutoff valve, but if this is done, the first stage must be fitted with an over-pressure valve, as closing the supply to the second stage disables its secondary function as an over-pressure valve.
Cold water function testing is used to compare a regulator's performance in cold water against various standards, mainly the U.S. Navy Experimental Diving Unit's unmanned cold water test procedures, and European CE open circuit standard EN 250 of 1993. Testing may include failure modes and effects analysis, and other issues relating to manufacturing, quality assurance and documentation. The introduction of a complete computerised breathing simulator system by ANSTI Test Systems Ltd in the UK made possible the accurate breathing simulator testing at all realistic water temperatures that is the current practice.