In the underwater world of scuba diving, descending to depths up to 40 meters (130 feet) is considered recreational scuba. When divers exceed this limit, they enter the realm of technical diving.
Technical diving is a more complex, and challenging, form of scuba diving. Technical divers rely on specialized training, equipment, and mixed gases to safely descend beyond the recreational limit to depths that can exceed 90 meters (300 feet).
There are two types of technical diving: open-circuit diving and closed-circuit diving.
Open-circuit diving equipment includes a regulator attached to a pair of cylinders on a diver’s back. While similar to the setup used by recreational divers, these cylinders contain varied concentrations of oxygen and nitrogen, and often helium, that are determined based on the intended depth and duration of a dive. At deeper depths, helium is added to prevent oxygen toxicity and nitrogen narcosis. The latter is caused by breathing compressed gas at depth and can affect a diver’s ability to complete complex tasks. Open-circuit divers exhale into the water; they don’t recycle any of the gas they breathe.
Closed-circuit diving uses the same gases, but the equipment is more complex. It relies on a closed-circuit rebreather, which includes multiple hoses and cylinders, computers, sensors, and a "scrubber" canister, to circulate the breathing gas in a closed loop. This system removes carbon dioxide from expelled breaths, balances the mix of gases so the diver is always breathing the desired blend based on depth, and recirculates the scrubbed and mixed gas back to the diver. Thus, closed-circuit divers recycle much of the gas they breathe.
Both systems require a diver to make decompression stops before returning to the surface. A diver who ascends too quickly risks decompression sickness ("the bends"), which can cause extreme pain, paralysis, or worse. Decompression stops, which depend on time spent at depth, help a diver avert the bends by allowing excessive nitrogen in a diver’s tissues to escape.
During ascent and decompression, an open-circuit diver switches from the cylinders on their back to a cylinder attached to their side (a stage bottle) containing a mix of gases more appropriate at shallower depths. This mix is often enriched with higher concentrations of oxygen (enriched air, or Nitrox) that aids in the off-gassing of nitrogen. While closed-circuit divers may carry a stage bottle as backup, their onboard system adjusts the gas mix and increases the oxygen concentration as the depth decreases.
Both systems have pros and cons:
These systems enable divers to reach greater depths and spend more time underwater. However, they require considerable training, dive planning, and redundancies. To prepare for the possibility of a malfunction underwater, technical divers also dive with backup equipment, such as extra masks, computers, lights, and regulators attached to alternate air sources.
Technical diving can be done for recreational purposes, has military and commercial applications, and is an important tool in the underwater science and exploration toolkit. Much of the ocean — and some of our lakes — lies below the depth limit of the recreational diver, but with proper training and accumulated experience, researchers can access, explore, and collect data in difficult to reach places, such as great depths and caves.
Technical diving lets us explore, document, and monitor shipwrecks and other maritime heritage sites, up close, and sometimes hands on, helping us better uncover and understand our past. It expands our abilities to study marine life not found in shallower water in real time and deploy, maintain, repair, and recover scientific instruments to collect data over time, helping us better understand our ocean and how to sustain it for future generations.
By Ryan Bradley, East Carolina University
Published November 9, 2023
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