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PUSHING THE LIMITS An analysis of equipment for safer recreational diving By Andrew Whitehead Published
in Dive Log Australasia
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Recreational scuba diving is all about enjoying the underwater world but within a variety of constraints. Some of these include the following:
- Keeping bottom time to within no decompression limits.
- Staying above the recreational depth limit of 30 metres.
- The limited capacity of the scuba cylinder.
- Performing a 3-5 minute safety stop at 5 metres.
- Exiting the water with 30-50 bar in the tank in case of emergency.
However divers who push the limits should have the appropriate training, equipment and experience so that the risks are minimised. The most well known forms of diving injuries are Decompression Illness (DCI) and Air Embolism though the incidence of these problems in recreational diving is relatively rare.
Decompression Illness
At the “SAFER LIMITS 2000”, Hyperbaric Workshop, held at the Wesley Hospital, Brisbane, September 2000, it became clear that diving is considered to be a very safe recreation these days compared with other adventurous activities. This opinion was based on the number of DCI cases treated in Australia compared with the total number of dives per year. One of the main reasons for this is that most divers now use a dive computer, rather than tables or nothing at all. It would seem that there is evidence to suggest that the use of the dive computer has made diving safer.
Dive Computer
The dive computer has become as much an integral part of scuba diving as the buoyancy compensator. As such, its basic functions should be covered in an Open Water course. In an Advanced Open Water course, time should be given to identifying and explaining the built-in functionality of each diver’s computer so that they can make full use of its capabilities. As with all these electronic gadgets, there is a considerable difference in price, features and functionality between a basic dive computer and, for example, an air-integrated Nitrox computer.
Air Embolism
Diving fatalities due to an air embolism are rare but can result from a rapid ascent due to a serious problem known as an ‘out-of-air emergency’. Basic steps such as dive planning, buddy procedure, frequent monitoring of depth, time and air supply, should prevent these incidents from even starting. It is likely that the real cause of the problem is inadequate training and experience for diving at depth since the training agencies regard diving from 30 to 40 metres as ‘deep diving’.
Deep Diving
Divers contemplating diving beyond the recreational limit of 30 metres should attend a comprehensive Deep Diver speciality course to learn the necessary skills and to use the additional safety equipment that should be used. In particular, this course should cover gas consumption at depth, and include at least four dives using safety equipment such as twin tanks, pony bottles, sling tanks, hang tanks, high performance regulators, reels, lift bags, etc. However, recreational divers must still keep bottom time to within no decompression limits.
Limited Air Supply
Students in their Open Water course learn that the total air capacity required for a dive depends on the following factors:
- breathing rate
- depth
- bottom time
- work level
- safety margin
Divers should be able to reduce their breathing rate by keeping reasonably fit and having adequate training and experience for the dive. The depth is determined by the dive site – say, for example, 30 metres.
So let us assume that a recreational diver has a Surface Consumption Rate of 12 litres per minute and uses the common alloy 88 cu ft tank (11 litres) filled with air to 230 bar. Diving to 30 metres on air is likely to place the diver close to the Doppler no-decompression limit (20 minutes) as well as to the limit of the air supply at the end of the dive (allowing for the ascent and safety margin).
Greater Air Capacity
Air capacity can be increased by using larger scuba cylinders such as 12 or 15 litre steel cylinders. A secondary air supply can be carried in the form of a 3 litre pony bottle, a 5 litre sling tank or twin cylinders. These extra tanks are fully redundant air supplies complete with their own regulator and submersible pressure gauge. As such they make excellent (though expensive) safety devices.
Although the larger tanks and/or secondary air supply will provide a substantial safety margin for diver or dive buddy, there is still the overriding no-decompression limit on bottom time.
Extending Bottom Time
According to the US Navy Dive Tables, the Doppler limit for a 30 metre dive on air is 20 minutes bottom time. Planned bottom time may be extended by using Nitrox (within its own limits) instead of air. Nitrox is oxygen enriched air, or air with a greater than normal percentage of oxygen. People use Nitrox because it may increase bottom time, reduce the chance of DCI, reduce post-dive fatigue and help to sharpen awareness during the dive.
Divers who wish to use oxygen enriched air must attend a Nitrox Diver specialty course which should cover oxygen toxicity, partial pressures, nitrogen toxicity, dive planning, blending and analysing gas mixes. The use of Nitrox pushes you towards using better equipment and having a more disciplined approach to diving. As Bret Gilliam (TDI) says, using Nitrox is not technical diving, just safe recreational diving.
Now consider a 30 metre dive on a Nitrox mix. Using the recommended maximum partial pressure of oxygen of 1.4, a 36% mix would be suitable to a maximum operating depth of 30 metres. The Bhulmann Nitrox 36 Decompression Table suggests a 30 minute bottom time with a one minute safety stop at 6 metres. However, for the diver to enjoy the extra bottom time, a 12 or 15 litre tank would be required.
Nitrox Gear
The most common (and cheapest) method of blending Nitrox is the partial pressure method. This involves adding pure oxygen to the tank first and then filling it with double-filtered air. We all learned at school that oxygen is a highly inflammable gas. Consequently a Nitrox tank must be oxygen-cleaned, have an oxygen compatible valve, and be labelled with the distinctive green and yellow Nitrox band. Typically Nitrox tanks are steel 12 or 15 litre cylinders with high-pressure DIN valves that are rated to 232 Bar.
These days, new high performance regulators are either Nitrox-compatible or ready for Nitrox use, with a DIN or yoke first stage. A Nitrox dive computer can be set in the field for one or more gas mixes. The computer should have a depth alarm which should be set to the Maximum Operating Depth of the gas mix being used. An air-integrated Nitrox computer should display the remaining bottom time under unchanged diving conditions.
Diving Overseas
In remote areas of the South Pacific, it can be difficult and expensive to obtain quantities of pure oxygen. In addition, the weight limits on commercial aircraft mean that divers have to travel light and leave a lot of gear at home. Airline check-in staff are also very suspicious of 7kg steel pony bottles! The lightweight alternative is to take two regulators (yoke first stage) and stainless steel tank bands. Two hire tanks can be bolted together and the BCD bolted to the bands. By attaching a regulator to each tank, you have two independent scuba units. Swapping regulators from time to time ensures that you always have a secondary air supply available. Poseidon regulators can be used left-handed on the second tank because they are never upside down!
About the Author
This particular article describes a path that the author followed over a number of years to replace his older gear with better, safer equipment. The author is an experienced recreational diver, not a professional diving instructor. He is also a financial systems consultant, technical writer and webmaster. Further information and contact details can be found in his website www.diveshow.com.au