The phrase “technical diving” conjures images of deep descents, cave exploration and squeezing through small openings on a shipwreck. However, that’s not how technical divers start their training. In an ongoing series exploring entries into technical diving, we first looked at technical diving for beginners. Here we’ll take a closer look at the initial stages of decompression diving.
Decompression diving for beginners
Just as recreational divers progress through a series of courses offering different levels of training, technical divers must start somewhere. Once they are comfortable with their equipment configuration — sidemount or twinset — it’s time to consider spending more time at depth. The deeper we go and the longer we stay, the more “penalty time” we incur.
In entry-level decompression diving courses, divers learn to distinguish between the working and the decompression part of their dive. They also plan bottom as well as decompression times accordingly. They also learn how to utilize different nitrox mixes up to and including 100 percent oxygen to make their decompression smarter.
What does decompression diving mean?
What does all this mean? Let’s look at an example. Diving to 148 feet (45 m) on air with a bottom time of 25 minutes incurs approximately 55 to 60 minutes of decompression – a long penalty time compared to a relatively short bottom time. A standard twinset of 11-liter aluminum or 12-liter steel tanks would not hold enough gas to complete this dive with a reasonable safety margin, meaning that 1/3 of the total gas supply remains in reserve.
If, however, we bring a dedicated decompression gas, such as nitrox 50%, we can shorten our decompression time noticeably. And, depending on which decompression planning model we used, we will complete our dive in less than an hour. Adding the ‘richer’ gas, with a higher oxygen content, means we accelerate our decompression.
Theory behind decompression diving
How is this smarter? First and foremost — accelerating decompression time isn’t just about getting out of the water faster. It’s about ending the dive in good time, having off-gassed sufficiently, but without taking unacceptable risks. As important as a conservative decompression schedule is, factors such as hypothermia and dehydration are just as important. The risk of both increases the longer we are underwater, so shortening our dive by reasonable means is beneficial.
How do we know how fast is fast enough but not too fast? During courses such as TDI’s Decompression Procedures, students learn about different decompression algorithms. They spend time planning decompression dives with tables (air only) as well as software that allows for acceleration. They learn to account for water temperature and diving conditions, exercise level during the dive and even age and fitness level.
Decompression Procedures qualifies students to dive to 148 feet (45 m) and complete unlimited accelerated decompression stops. It’s usually taught in combination with TDI Advanced Nitrox, which trains divers to use higher-percentage nitrox mixes up to pure oxygen. The Advanced Nitrox classroom work focuses on calculating oxygen exposure, maximum operating depths and best mixes for the planned dive, while the in-water classes focus on perfecting buoyancy control.
Shouldn’t divers be able to control their buoyancy already at this level? Yes and no. Prerequisites for entry-level technical diving training are relatively low, which means divers might still need some buoyancy work. Some divers may also be new to diving with more extensive equipment, and will still be getting the hang of managing the heavier load underwater. To safely switch to gases like EANx50, students must be able to control their position in the water exactly and without much adjustment.
The Decompression Procedures course builds on that by introducing decompression schedules, hard ceilings and lots of problem solving. Later, and with a minimum of 100 dives as a prerequisite, Extended Range Diver takes students to the limits of air diving at 180 feet (55 m) while using two different decompression gases to optimize their decompression schedule.
PADI’s introductory decompression diving classes are the Tec 40 and 45 courses. The prerequisites here are more detailed than TDI’s, including set numbers of dives or nitrox dives to certain depths, as well as PADI’s Deep Specialty, so logging dives is essential.
Tec 40 allows students to complete 10 minutes of decompression stops on gases up to EANx50. The decompression limitation automatically limits bottom time, too. Tec 45 then takes students five meters deeper to 147 feet (45 m) and qualifies them for accelerated decompression. However, course standards prescribe that decompression is completed (on the course) following an air schedule.
The curriculum also stipulates what tasks students must complete on each dive, usually heavily loading the first dive of each course. PADI’s air diving courses finish with Tec 50, adding — you guessed it — another five meters of depth. There’s also an introduction to two different decompression gases.
Of course, more than two training agencies offer decompression diving courses. SSI, for example, offers Extended Range (45 meters with accelerated decompression up to 100 percent O2) and Technical Extended Range curriculums. The latter allows students to dive to 197 feet (60 m) utilizing two different decompression gases. This is 15 feet (5 m) deeper than even the TDI Extended Range qualification. This is controversial course among some instructors and divers due to the narcotic load on air at those depths, as well as the density of the gas a diver is breathing.
These concerns led RAID to introduce Trimix earlier on in their decompression diving courses. There is an option to include it even in the initial Deco 40, which, as the name suggests, allows students to dive to 40 m (130 feet). In the next post within this series, we will look closer at the pros and cons of Trimix diving.