Scuba enthusiasts dive for all manner of reasons. Some like the beauty and calming effect of seeing the amazing sea life, and others like the opportunity to meet new friends from all over the planet or the physical and psychological benefits it provides.
A group of adventurous divers likes to “push the envelope” and engage in cave diving, deep diving, wreck diving and other types of “technical diving.”
When I arrived at the boat, it was obvious that the divers fell into that category.
The schedule called for a “double dip,” two successive dives, on the purposely sunk wreck of the 510-foot long, 84-foot wide USS Spiegel Grove, which lies in 130 feet of water near Dixie Shoals off Key Largo. Dive depths range from 60 to 130 feet with the majority between 80 and 90 feet of water.
There were some serious technical divers as well as expensive dive gear on the boat. The gear being assembled by the divers (standard scuba equipment, nitrox filled tanks, and rebreathers) was enough to put a smile on the face of any dive-shop owner.
Recreational scuba divers use a mask to enable them to see underwater, a scuba regulator and tank to provide the air (regular filtered air — not oxygen) they need at different depths, fins that allow them to swim efficiently, some sort of exposure protection (wetsuit) to keep them protected and warm, a dive knife to be used as a tool (not a weapon), a depth gauge and timing device (most of which are now available in underwater computers), and a compass for navigation.
Added to this are a buoyancy-control device to help them float at the surface or to maintain neutral buoyancy under water (like a fish) and lead weight to help counteract the buoyancy characteristics of their bodies and wet suits.
In 1943, Jacques Cousteau and his partner Emilie Gagnan coinvented a demand valve system that supplies divers with compressed air when they breathe. The exhaust gas is discarded in the form of bubbles: this is called an “open-circuit” system.
This standard scuba setup has worked well, and continues to do so, since the advent of recreational diving.
The equipment has improved and been adapted to a broad range of recreational divers, computers have made diving safer and scuba courses have evolved from a sport for only young, athletic, expert swimmers and ex-military divers to an activity that can be enjoyed by many.
There are, as new divers learn, limits for time, depth and rates of ascent to avoid decompression sickness or the bends (the formation of nitrogen bubbles in the body that cause extreme pain, paralysis or even death). Going too deep while diving on air can also lead to nitrogen narcosis, which is similar to being intoxicated — not a good condition to be in during a scuba dive when good judgment and coordination are needed.
Along came nitrox. (Nitrox has actually been in use for several decades. Both the U.S. Navy and commercial diving companies have used it since the 1950s.)
Enriched air nitrox (EAN) refers to a nitrogen/oxygen mix in which the oxygen concentration is higher than the approximately 21 percent found in air.
A higher percentage of oxygen and the corresponding lower percentage of nitrogen give a diver more time at depth without getting the bends. For example, a diver using 32 percent nitrox can stay approximately 30 minutes longer at 60 feet of sea water (90 minutes instead of 55 to 60 minutes) depending on the tables used.
Nitrox works best in midrange diving, usually considered 50 feet to 100 feet deep. Diving with nitrox is great, but divers need to be careful. Deep diving with too high a mixture of nitrox can lead to convulsions.
Then came technical diving, which allows a diver to safely exceed recreational dive limits using advanced procedures and equipment. Examples include dives deeper than 130 feet, exceeding no-decompression limits, and those not allowing a direct ascent to the surface.
Mixed-gas (Heliox, a 79 percent helium and 21 percent oxygen mixture, and trimix, a breathing mixture composed of nitrogen, helium and oxygen), cave, wreck penetration and some forms of rebreather diving are considered technical diving.
Certain dive associations classify any dive into an overhead environment as a technical dive, while others allow recreational divers to swim a short distance in to an overhead environment if there is a nearby accessible and visible egress point.
Although most of the intentionally sunk ships in the Keys have accessible “swim-throughs,” with easily identified openings, divers should never exceed their training and experience levels when diving on a wreck.
Technical diving uses redundancy, meaning if a piece of equipment fails there is a backup available to safely complete the dive.
Many technical dives require decompression stops during which the diver may change breathing gas mixes (from different tanks) at least once. These decompression stops during ascent are necessary to allow gases, such as nitrogen, to be released from the diver’s body.
Mixed-gas diving refers to diving with a gas mixture other than air or nitrox. Benefits for diving with mixed gas include avoiding nitrogen narcosis, improving decompression and avoiding oxygen toxicity — going too deep with too high a percentage of oxygen.
Heliox is often used for very deep diving. Helium is not known to have an intoxicating effect at any depth, has a lower density than nitrogen, making it easier to breathe, and it improves decompression for very long dives.
The down sides are that heliox is expensive, has a limited availability and its thermal conductivity is six times greater than that of nitrogen — meaning you get cold faster and need a special suit to keep you warm.
Another mixed gas, trimix, is used for the deepest scuba dives, usually greater than 400 feet. Like heliox, trimix is strictly for nonrecreational use: military, scientific, commercial and advanced technical diving.
Because mixed-gas dives are conducted at great depths, they require detailed planning, sophisticated equipment and support personnel. It is extremely important for the breathing mixture to be properly identified — the wrong mix can lead to a fatal accident.
There are three basic types of rebreathers: oxygen, semiclosed and closed-circuit. The difference is the way they add gas to the breathing loop and control the concentration of oxygen in the breathing gas.
The breathing loop includes a carbon-dioxide-absorbent canister, a way to add fresh oxygen needed by the diver and a design to ensure that gas circulates in one direction. A single fill of a small gas cylinder or cylinders and CO2 scrubber can last, depending on the model, from one to six hours. And gas duration on a rebreather is nearly independent of depth, allowing a diver to spend more time at the deepest portion of the dive.
With decreases in cost and difficulty of operation, rebreathers are becoming increasingly popular with recreation divers. They are great for photography because they don’t frighten fish with exhaust sounds, and they deliver warm, moist breathing gas and a more optimum gas mixture.
Rebreathers have been around longer that scuba gear. Henry A. Fleuss submitted a patent in 1878, and the rebreather was used two years later to close some crucial valves in the Severn River.
For a great comparison of open circuit and rebreather systems visit the History of Diving Museum in Islamorada.