Safety ProceduresUncategorized

My Analog Dive Computer

In my last article, I discussed the use of personal dive computers and the absolute necessity of carrying some form of back up decompression tables.  I referred to the practice of generating decompression tables using software such as V-Planner and then writing the deco schedule in a waterproof notebook.  In the past week, I’ve received a couple e-mails asking for more information about this practice.  Apparently since the advent of personal dive computers, carrying handwritten back up tables has faded.  Here’s my approach.  Take it for what it’s worth.

As I noted in the previous article, personal dive computers for technical divers are a fairly recent development.  When I got into technical diving in the 90s, the accepted standard was to calculate a decompression schedule for a specific dive using desktop computer software.  Back then we used Abyss or Decoplan.  Once V-Planner came out (around 1999 I believe), many of us switched to that and have used it ever since.  Once a deco schedule was computed, it would be copied by hand into a waterproof notebook.  At that time, the gold standard was Richie’s Wetnotes.  Since then, many other manufacturers have come out with waterproof notebooks of their own and the term “wetnotes” has become the generic term for any such book (much to Richie’s chagrin I’m sure).  The reason these notebooks are waterproof is that the pages are not paper, but thin sheets of plastic.  I carry mine in my drysuit thigh pocket.

WetNotes Waterproof Notebook by Richie
WetNotes Waterproof Notebook by Richie

When developing a dive plan, I start at my computer.  I open V-Planner and start running various permutations of my plan.  For a Great Lakes wreck dive, I usually know the depth and start with a bottom time of 20 minutes.  Then I start fine tuning the variables.  What if I use X for bottom gas?  What if I use Y for bottom gas instead?  How does changing deco gasses affect the plan?  What happens when I vary bottom time?  This process can be an article or two all by itself.  Since we’re talking about how I use a waterproof notebook, let’s fast forward to the point where I have a viable dive plan worked out.  I also work out a couple contingency plans.  You can go crazy with contingencies.  You can make plans for staying longer than expected, shorter than expected, deeper than planned, shallower than planned and plans that account for lost deco gas.  You can come up with more contingencies than pages in the book.  You have to decide which plans are worth carrying with you.  Neatly write the plans you want to take with you in the notebook.  I usually write the deco schedule for planned dive, a five minute longer bottom time and a 5 minute shorter bottom time.  You, of course, can make as many or as few contingency plans as you like.  I write them neatly with a fine Sharpie, clearly identifying depth, time and gas mixes used.  I do this because this notebook becomes a permanent catalog of dive plans.

Handwritten decompression schedule.
Sample deco schedule using runtimes written in author’s waterproof notebook.

When you write plans in your book, you can use either stop times or run times.  Stop times are pretty straightforward:  stop at 90 feet for 1 minute, stop at 80 feet for 1 minute, stop at 70 feet for two minutes and so forth.  Run times refer to the total elapsed time of the dive.  For instance, on the plan pictured above for a 20 minute dive to 190′, I would leave the bottom when my timer reads 20 minutes and make a slow ascent to 90 feet.  When my timer reads 24 minutes, I ascend to 80 feet.  When my timer reads 25, I ascend to 70 feet and so on.  The benefit of run times is that it eliminates the need to do math at your deco stop (looking at your timer, looking at your schedule, adding the stop time to elapsed time).  However, for run times to work, you have to be on schedule.  You have to leave each level at the proper time.  If you leave the bottom at 23 minutes instead of 20, your whole run time schedule is off.

After a couple years of diving, you’ll probably have about every profile you’re likely to need written in that book.  It has become your analog dive computer.  Even if the dive plan changes, you’re pretty likely to have the appropriate schedule, or something close, in your notebook.  I’ve shown up to the boat only to find that our destination has been changed because of weather or some other reason.  While others are scrambling to warm up their notebook computer (or tablet, or phone) to cut new tables, I just have to flip through my “analog dive computer” to find the right deco schedule for the new depth.

I’ll admit it.  I like planning dives.  I like noodling around on V-Planner with different scenarios.  I’m also pretty uptight about having back up plans and back up plans for back up plans.  I use a personal dive computer on every dive.  But I also have my notebook in my pocket with plenty of back up schedules.  Maybe carrying a notebook full of dive plans isn’t your style.  That’s fine.  Just be sure you have adequate redundancy and back ups.  Technical diving is all about the odds.  I try and stack the odds in my favor on every dive.  Dive safely.