These are Not Your Granny’s Dive Computers

Technology and Technical Diving… By Steve Lewis

Personal dive computers have gone through as many changes and radical upgrades as their desktop cousins in the past decade, and while this offers potential benefits for all divers, these “fourth generation” machines are a real boon to those of us who dive a little deeper and a little longer.

TechnologySeveral years ago, I wrote a widely circulated article about Personal Dive Computers suggesting strongly that they had no place in tech-diving. While I still require students in my TDI decompression procedures and Helitrox deco dive classes to “switch your computers to gauge mode” – something I’ll explain a little later – my general stance on PDCs and technical diving has changed… about 180 degrees.

Now while I sit here as a contrite victim of the “Never Say Never Syndrome” it begs explaining that few of us could have foreseen the changes in store for PDCs in such a short space of time. For those of you too young or too smart to have experienced the first iterations of dive computers, allow me to frame the choices we have today, in a little history.

Early generations of PDCs shared a few traits, not the least of which was that most ran an unmodified Buhlmann algorithm: no deep stops, no modifications, no user access to adjust conservatism, and certainly no sign of dual-phase alternatives on the immediate horizon. To make matters somewhat more challenging, these puppies were – for the most part – single gas and that gas was air.

To compound a situation in which no allowances were possible for a diver who breathed nitrox on the bottom or oxygen-rich deco-gas during ascent, early dive computers did not play fair once the diver got him or herself into staged decompression, adding time independent of necessity.

Even with the arrival of “Nitrox” computers and then the first gas-switchable, multi-gas computers, the situation with regards to padding the required ascent times was – by and large – nothing short of ridiculous. I recall the tipping point for me was a pretty simple cold-water wreck dive to around 40 metres (that’s about 130 feet) with pure oxygen as a deco gas. My bottom-time was 30 minutes or so, and certainly no more than 35 minutes.

For that kind of exposure, you’ll probably agree with me that a total ascent time of 35-40 minutes would be expected, and even on the most conservative schedule and a complete loss of decompression gas, a total of around 55 – 60 minutes would be fine. I recall getting to 6 metres (20 feet) after a controlled ascent, and a couple of prior stops, making the switch to O2 and having my “bleeding-edge” brand-new multi-gas computer suggest 25-minutes at that depth and an additional 49 minutes, three metres shallower (10 feet for you imperialists out there). For the record, I had had no “depth ceiling” violations on the way up.

Did I mention the water temperature was slightly above freezing?

The following day, that PDC had an accident with a framing hammer. It should be stated that my issue was not really with the computer design or with the software engineer’s interpretation of Professor Buhlmann’s fine mathematics; rather, my issue was with the computer company’s legal department. It was evident after the simplest of research that in an attempt to cover their corporate “ass-etts,” the computer company had followed the advice of someone with a briefcase under one arm, a statute book under the other and no diving experience at all. And that person had mandated that if 45 minutes is good, then 75 must be better: “make it so.”

I decided to use tables and a bottom-timer from that point on.

So what changed? In two words: technology and statistics. Over the intervening period, the design and function of computers completely changed. Many of those changes were driven by a market demanding PDCs that produced sensible decompression schedules, with the remaining changes a natural fallout from smaller and more powerful computer chips and innovations like bright full-color OLED displays, user-replaceable batteries, and more sensitive and remarkably robust ceramic pressure sensors.

Technology2Interlace all that with actual diving experience supplied by scads and scads of highly-respected technical divers conducting – and surviving – dives way beyond the scope of the traditional sport-diving exposures. Add to that direct input from hyperbaric physicians, and the end-result is a workable solution. Welcome to the 21st Century.

Of course, there were many other changes to both equipment and attitude. High on this list is the growth in popularity of closed-circuit rebreather diving using a trimix diluent. There’s no need to get into the nuts and bolts of it here, but let’s just take it as read that you need to be Gottfried Wilhelm Leibniz to keep track of your decompression obligation “free-hand” during, let’s say a cave dive of variable depth with a 10/50 diluent. (If this is Greek to you (and Leibniz preferred Latin) it might help to understand that during a trimix dive, a CCR maintains the partial pressure of oxygen in a diver’s breathing mixture by juggling the proportions of the other two gases. You have my word that it can be a humbling experience to realize at depth that you really only have a moderate understanding of what species of trimix is in the breathing loop at any one time, because all you do know is that it is NOT the same as your diluent. Hence, the need for some computerized help).

I firmly believe that one of the requirements of ANY technical diver (on open or closed-circuit) is to have a pretty comprehensive understanding of basic decompression theory. A bonus is for them to understand the approximate relationships between bottom-time, depth, and gases breathed or set-point. The cherry on top of the ice-cream sundae is for them to be capable of producing a workable decompression schedule armed with nothing but a piece of paper, a pencil and some background information. Hence my insistence that the easy way out of a dive computer is NOT on hand during deco training.

However, with the hardware available to us today, you would have to be a complete Luddite to ignore the convenience, security and reliability of the PDCs on the market right now. And I would like to offer a small thank you to the industrial design team at one particular manufacturer who has designed a unit small enough to leave some real-estate on my arm, with a screen I can read without glasses, and an interface that requires no buttons!

Dive Safe!

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Steve Lewis ( is an active instructor-trainer with TDI/SDI and was a member of headquarters staff for several years. He has written scores of articles on dive safety and skills development and is a regular contributor to several online magazines and discussion groups. His best-selling book called “the Six Skills and Other Discussions” is available at select dive stores and through onLine stores such as Amazon and Create Space eStore via:

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