Crunching the Numbers

By Karl Shreeves

Sometimes the thrill of tek diving comes from facing the unknown. No one has ever visited this deep wreck. No one has ever dropped so far down this wall. No one has ever pushed this cave so far. Oh, by the way, no dive computers handle helium breathing gas and no one has written a table for the dive you want to do. No worries-you write your own.

Fifteen years ago if you told your buddies you wrote your own tables, they'd have taken you to the nearest mental institution for evaluation. Today, no serious tekhead would consider being without decompression software, particularly when diving the trimix (helium, nitrogen, oxygen) blends needed below 130 feet. And, tek divers commonly use decompression software with air and enriched air, even though their dive computers will handle those gases.

Why Software?

Although there are pre-written trimix tables, it's impossible for a single set of preprinted tables to accommodate all the variables: helium, nitrogen and oxygen combine into more than 170 theoretical trimixes, then decompression time varies with the various trimixes, enriched air and oxygen you use for decompression. Diving air or enriched air, you can use high oxygen nitrox and pure oxygen to get you out of the water faster than your typical dive computer allows (it thinks you're breathing the same gas the whole time), which is why this software comes in handy even for tek diving with air and nitrox.

Consider the profile if you're exploring a wreck or cave for 15 minutes at its deepest depth and know you'll spend 15 minutes 50 feet shallower on the way out (because that's where the wreck/cave goes); computing that 15 minutes spent at a shallower depth will make a huge difference in decompression time compared to treating it all as the deepest depth. Cutting your own tables offers other advantages: you can dial in your own conservatism factor. Set the altitude. Calculate your gas consumption. Pick decompression stop depths for 50, 43, 25 and 19 feet, because that's where there are good rocks to sit on. See how different enriched air decompression blends affect your profile. Calculate emergency tables in case a playful dolphin swims off with your oxygen bottle. Figure out the proportions to use when blending your gas. Stay up till 3:00 am and print 200 sheets of dive tables for just one dive because the software is really cool.


Before you run out for your PC copy of Abyss, ProPlanner, Voyager or other software (Mac versions seem to be an insurmountable challenge for decompression software writers), remember what Robert Heinlein said, 'TNSTAAFL' (There's no such thing as a free lunch). That is, you don't get something for nothing.

For one, outer edge diving is often outer edge decompression theorywise. The programmers pretty well know their stuff and put in the best decompression models they can, but they're just models and the further you get from the hard data they're built on, the mushier the data they give you become.

Then, you can punch in all kinds of variables, but do you know what variables to punch in? How much conservatism should you set? Did you know that some deco theory suggests that too much can be as bad as too little? Do you know when? How do you calculate air breaks--or do you? Do you know what an air break is? All programs use valid decompression models, but different models rise from different assumptions that produce different results--do you understand what the computer's doing? How do you choose the right options for you? Have you ever heard that deco schedule writing is as much art as science? Does any of this make you squirm?

Obviously it takes more than software to cut tables. First, you need training that matches the table you're making, i.e., you have no business crunching trimix tables if you're not trimix certified. Credible decompression software writers won't sell you software above your certification level.

Second, if you're cutting tables for the cutting edge, you need to know more about decompression theory than that a guy named Haldane started it all. Bury your nose in some basics, such as the latest edition of The Physiology and Medicine of Diving, then pay attention to what's going on in decompression research (there's some interesting stuff coming out of NASA), what's being tried in the field, and what's working and not working. See what's on the internet if you like, but remember that crap and gold look identical when typed into a chat room, bulletin board or Web page.

Finally, be responsible. The decompression software writers provide the best they can, try to keep it up to date and have a pretty good track record. But ultimately, you set the variables, make the choices, cut the table and decide to use it. As with any diving, you are accountable. So if you get bent, remember, you made the decisions and accepted the risks.