As new divers, the seemingly arbitrary 20m depth restriction seems kind of foolish; after all, you’ve got your tables right? You just picked up that sweet computer from the shop that’s rated to 100m, so why can’t you go there? Well, to be frank, while deep diving is really an extension of fundamental skills you’ve learned as an Open Water Diver (or equivalent), there are a few things that set it apart and so additional training and familiarization is recommended before you charge down to 40m! The keys for safe deep diving are redundancy, equipment and situational awareness.
Deep dives are different from regular, shallower dives because the environment can change quite a bit on the way down. Where the first 10m of a dive might be decent for vis and light, it might be near night diving conditions deeper. Absorption and turbidity eat away at the colours you can see, and your air supply dwindles faster. In addition to all that, narcosis effects can become more pronounced as well, adding an element of risk all its own to the dive.
If you’re planning a deep dive, plan for all reasonable contingencies. Use redundant equipment for vital things like your depth gauge (probably the most important redundancy). Carry at least one dive light, but a secondary is never a bad plan. Talk to local divers and find out the conditions that you might be facing when you get to the bottom. If it’s a wall dive, watch your depth gauge(s) like a hawk! Some computers can be set for deep stops, so this is a good option as well. Situational awareness is a trait that good divers seek to develop as much as possible over their diving careers. In a deep dive, it’s as important as it would be on a night or wreck dive. Paying attention to your depth, watching your dive buddy and yourself for narcosis and keeping track of your air are the most key things to watch. And finally, brush up on your certifying agencies emergency decompression stop rules and write them on your slate so you don’t forget. You’re already increasing your risk of DCI, so there’s no sense increasing it further by blowing your tables and not taking the needed stop. Safety stops aren’t optional with deep dives!
Planning and conducting deep dives is a big part of diving for me and many other people, particularly the Tec set. While you are able to go to whatever depth you want, it’s good to get the extra training (and master Dalton’s Law) before you just plunge in. 40m is a long way from the surface and a safe dive is a good dive!