Not that long ago, there weren’t any personal dive computers (PDCs) for technical divers. No PDC that could handle multiple gasses. None that could compute a decompression schedule for helium-based gasses like trimix. Back in those dark times, we planned our dives using desktop computer programs like V-Planner, Decoplan, Abyss or something similar. We copied the deco schedules onto slates or into waterproof notebooks (including a few contingency plans). We executed the dives using these tables and a simple bottom timer to track depth and time.
Fast forward to the present. Now we have a choice of high quality, reliable PDCs for technical diving. PDCs that can compute deco schedules using multiple gasses, including trimix and other helium-based mixes. One of the warnings that emerged as technical PDCs gained popularity was that computers make divers lazy and overly dependent on technology. What happens if the computer malfunctions during a dive? The reasonable answer is that using a computer does not eliminate the need for redundancy. Even though we may be carrying a high tech PDC, we still need redundancy in the form of another computer or printed tables.
Unfortunately, it’s too easy to get sloppy and not follow proper procedures. After conducting numerous dives using a PDC, it’s easy to get lulled into a false sense of security. That can be dangerous. It’s usually when we feel safest and invincible that Mr. Murphy pays a call. No matter how reliable your PDC appears, you need to have redundant methods of computing your ascent schedule. You probably still use software like V-Planner to pre-plan your dive. Take the time to write an ascent schedule and a couple contingency plans in your waterproof notebook. Contingencies can include deco schedules for 10′ deeper than planned, a few minutes longer than planned, as well as shallower or shorter than planned. As a last resort, you could even use the good old US Navy Air Deco Tables to figure out a reasonable deco schedule.
It doesn’t matter what your favorite method of redundancy is, just make sure you have some form of backup for your PDC. One of my instructors told me years ago that being safe isn’t hard. We all know what to do. We know the safety procedures and protocols. The hard part is having the discipline to follow those procedures on every single dive. Be disciplined. Don’t enter the water without proper redundancy.