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Physiology is another example. From the first time divers don a tank of air and start exploring the underwater world, they learn that it’s important to control ascent and descent rates. Descend too fast and you may be unable to equalize your ears quickly enough or exceed your planned depth. So far, so simple. As you are moving deeper into technical diving, however, making the most of your limited bottom time becomes crucial and you must balance a safe descent rate with maximized time underwater. What does that have to do with dive theory? Understanding what’s happening in your ears and sinus cavities when you descend and when you equalize will help when it comes to practicing different equalization techniques. Most of those are best practiced on land initially with someone talking you through it, before attempting them underwater. 

Another concept students learn during initial certification courses is the fact that divers’ tissues absorb nitrogen during the dive. The longer you stay at depth, the more nitrogen your tissues absorb, eventually leading to saturation. Granted, most recreational and technical diving stops far short of saturation, but the basic concept remains in place. The longer you stay, the more care you need to take on ascent. In that sense, every dive is a decompression dive — the main difference being that if divers stay within no-stop limits, a slow ascent and a safety stop are enough to fulfill their decompression obligation.

Stepping into technical diving, managing that decompression obligation becomes more complex. While not ideal, a recreational diver could always return directly to the surface. Missing their safety stop means going against strong recommendations and best practice, but, in all likelihood, the diver should still be okay. A technical diver missing mandatory decompression stops has a much higher risk of becoming ill with decompression sickness.

During technical-diving courses, students learn about different decompression algorithms that help them plan and manage their ascent. Their instructor is likely to recommend an algorithm and a degree of conservatism based on their own knowledge and experience, which then becomes the starting point of the new tech diver building their own experience. While this may sound complicated, it’s founded on the basic concept from the open-water diver course: it’s about managing ascent speed.