Tony has posted about decompression diving before, but it’s been on my mind a lot lately after the winter diving this year.
If you’ve done a recreational scuba diving course, like a PADI or Naui Open Water course, you’ve learned how to do “no decompression diving”. Your entire dive is structured so that, should the need arise, you could ascend directly to the surface (in other words, without having to do a safety stop or decompression stop on the way up) without undue risk of getting bent. This is why you learn how to use the dive tables or the dreadful eRDP, and why you have time limits depending on what depth you go to. This kind of diving is considered (and indeed, is) far less risky than “decompression diving”, which involves overstaying the limits prescribed by the dive tables combined with staged ascents. This means that solving a problem by surfacing is just not an option (unless you want to create a whole other problem for yourself).
As a beginning diver, it’s often a moot point that you only have x minutes of bottom time at a given depth: your air consumption is usually so poor that the air is gone long before the bottom time is up! As you dive more and more, however, the time limits really come into play and you may find yourself ascending with lots of air to spare, but because your computer tells you it’s time to go.
Particularly on repetitive dives (I’m thinking of a day when I did three long dives to at least 20 metres, all before lunchtime) you’ll run the risk of going into deco (in other words, incurring a mandatory stop on the way up in order to get the nitrogen out of your body tissues). Diving on Nitrox can help a lot with this, and it’s healthier, but there may well be occasions when you do find yourself in deco even after just one long (or deep) dive.
The first time it happened to me I got a bit of a fright. I’d read my dive computer manual back to front and made sure I understood all the warnings and alarms, so I knew exactly what it was telling me, but I wasn’t happy. We ascended a bit, my computer went out of deco, and after my three minute safety stop it gave me the all-clear to ascend. I had an idea in my head that going into deco was REALLY BAD, and I confessed it shamefacedly to Tony on the boat afterwards. He wasn’t too fussed; and as Grant pointed out, “There are worse things than going into deco – like not doing your deco stops!”
A little reflection, however, convinced me that I hadn’t done anything wrong (except perhaps not forseeing that this was going to happen and mentioning it to my buddy, who happened to be Tony!). I’ve done over 150 dives in the two years I’ve been diving, my air consumption is excellent, and I’m very calm underwater. Tony and I know each other well enough that our underwater discussions can be quite detailed. I’ve since allowed myself to go into deco a few times, never more than a few minutes because I dive on a single 10 or 12 litre cylinder, and I’ve done the stops that my computer requests of me (usually only an extra 1-3 minutes plus a 3 minute safety stop). I can tell you that this can be a little boring, unless you have jellyfish and seals for company, or an entertaining dive buddy!
I am very, very glad that I have my own dive computer now, and make sure that I understand exactly what it’s telling me at each point in the dive. I’ve been on a deep dive, and two young divers who didn’t have computers spent the entire dive below me. I went into deco and got a one minute stop on the way up; they must have incurred a much longer obligation, but didn’t know about it because they didn’t have dive computers. If you’re at special risk of decompression sickness due to age, weight, fatigue or other considerations, a dive computer is even more important. Small changes in depth can make large changes in the amount of bottom time you have remaining, and unless you are glued to your buddy’s side all the time, relying on their computer to keep you safe is risky.
