You need one: Your dive plan
This blog post is of course no training standard or replacement for proper hands-on diving training. It developed from a discussion I had with a tec diving friend. It’s what I think about the question of dive planning:
When you do a “recreational” dive, you only need a rather limited plan. You need to know where to enter and exit, how deep you will – approximately – go, and some “lost diver procedures”, what to do when you get separated. Since recreational dives are usually limited to depths shallower than 35 meters or so, you can ascend very quickly, and in case something goes wrong you can go to the surface right away. There, many of your diving emergencies are solved right away, since you have an endless supply of breathable air. You don’t have to plan much for solving emergencies underwater. Donating air is a skill every diver should be capable of, but mastering that skill is not a “plan”.
On a recreational dive you don’t have decompression stops to do, at least you don’t intend to to a dive which makes these deco stops necessary, and if they happen to occur due to a lack of watching your dive computer, they will usually be short.
It’s different when you tec dive
That’s different during technical (or tec) dives, which mostly involve decompression stops due to extended dive times at deeper depths. You will have to wait and breathe for extended times at depths determined by a decompression algorithm. The algorithm can run on a desktop computer or on a dive computer. As far as I know, all the main training agencies teach to plan a tec dive beforehand in great detail, including all times (precise to the minute) and depths (should be held ± 0.5 meters) of the ascent. But, besides an appeal to authority, WHY is this the case?
Let’s say you would simply use a dive computer to find out what kind of decompression you’d have to do. The main issue facing when doing such improvised deep dives is that gas planning will be much harder, if not impossible. If you only find out how long your deco stops will be once you reach that particular depth, you can’t plan ahead and take the right amount of air, nitrox or trimix to breathe! This is even more the case the deeper you go and the longer you stay there, which will cause your deco stops to be longer. Such improvised deco diving also makes it harder to bring the correct amount of gas to deal with possible emergencies, which are not at all calculated by your computer.
Yes, you could “play it by ear”, and take a good amount of extra gas for deco and eventualities. But why not plan properly right away? Alternatively, you could make real, proper deco plan for the time/depth you plan to go on your dive, and get the amounts of breathing gases from that plan. But then, once underwater, you could simply go by what your computer says; Especially if your computer runs the same algorithm as your deco software, you might think that this is a good idea. I think it’s a less bad idea than no planning at all, but it’s still not ideal: relatively minor discrepancies in depth/dive time could cause fairly significant differences in gas needs, especially when you dive deeper and dive with several different breathing gases.
So, in my opinion, there is a major gas planning/logistics reason why it’s not a good idea to run deco dives by a computer only.
Psychology
I believe there is also a psychological reason why proper pre-dive gas planning is important in deco dives. It’s very beneficial to know in advance what will happen during your dive. If you arrive at 6 meters and then find out that you will have to hover there for 25 minutes, that might not be the surprise to make you happy. Even if you do have enough gas, your exposure protection might be insufficient (wetsuit too thin for such a long hover) or the ocean conditions might not be conductive for such a long deco stop (you might drift away from your exit point). It is discomforting if you don’t really know what exactly to expect in terms of deco at the end of your dive!
Compatibility
Finally, pretty much everybody I know who tec dives dives with a deco and gas use plan made before the dive. Even if you think that your less than fully planned out plan dive procedures are safe, you will have to re-learn when you travel and dive with someone else.
Hence, ladies and gentlemen, I must reiterate the age-old credo with force and conviction: plan your dive, dive your plan!
