Personal Dive Computers... Thank God!

“Change is inevitable. Change is constant.”

Benjamin Disraeli — 19th Century British Statesman

There’s a vintage TV comedy sketch by Monty Python called ‘The Four Yorkshiremen.’ You can check it out on YouTube, but the synopsis is simple. Four old guys dressed in dinner jackets and who look like they’ve made it, are talking about how tough they had it growing up. The conversation quickly escalates from pretentious to ridiculous as they try to outdo each other by exaggerating the challenges they overcame as kids. For example: “… We lived for three months in a brown paper bag in a septic tank. We used to have to get up at six o'clock in the morning, clean the bag, eat a crust of stale bread, go to work down mill for fourteen hours a day…”  And so on.

The sketch winds up with Michael Palin’s character saying: “But you try and tell the young people today that... and they won't believe ya’.”

With all that in mind, things really were very different when Michael Menduno first coined a term “technical diving;” a phrase divers throw around these days without much thought. Back then, we really did — to coin a phrase — live in a shoebox in the middle of the road and walk barefoot uphill to school both ways in the rain and snow.

There was plenty of seat-of-the-pants thinking back then, and a lot of misinformation about how to end a dive in as good a shape or better than when it started. Things have obviously changed for the better. Well, mostly for the better. Nothing illuminates the change better than the situation with underwater lights. They are cheaper, more powerful, more reliable, smaller, and brighter.

And we’re spoiled for choice when searching for a high-performance scuba regulator, a sidemount harness, a closed-circuit rebreather, a drysuit that doesn’t leak, even “little” accessories such as DSMBs, reels, spools, and a waterproof notebook.

One of the most profound changes has to do with dive computers.

Divers of all sorts — from those who want to bimble about at six or seven metres for a half-hour, to members of expedition teams opening up new cave passages at great depths a couple of hours swim from daylight — have benefitted not just from new technology, but in huge part from a changed mindset.

Last week, a brand-new diver studying for her very first open-water certification, looked at me wide-eyed when I explained how recently PDCs (personal dive computers) have become a mainstream part of a diver’s basic kit.

“What did you do before?” She asked completely puzzled by an old copy of a floppy plastic version of the US Navy Dive Table I’d handed to her.

And that got me thinking.

Once upon a time, I wrote an article entitled something like, “Dive computers have no place in technical diving.” Now that was long, long ago in a galaxy far, far away. Then, dive computers were designed by lawyers with an unwavering eye and tight, sweaty-palm grip on litigation; or more precisely, on avoiding it. And this fact alone made doing any decompression diving following the advice of a dive computer a real drag.

For example, on one notable occasion, after a moderate dive on a Canadian wreck sitting at a modest depth, the brand-new “nitrox enabled, gas integrated” computer I’d been sent to evaluate, displayed a required stop at three metres for around five minutes. No more than one or two minutes later, my time to surface had jumped from five to 45 minutes! Impossible.

Talking to someone who worked for the manufacturer a few days later, I was told: “if a five or six minute stop is what the decompression algorithm calls for, we pad it… considerably. After all, you must admit if five minutes is good, 45 minutes must be even better…”

It was not necessary to be a rocket scientist, brain surgeon, or hold a PhD in physiology and hyperbaric medicine to work out that thinking about decompression stress in such a simplistic and wrong-headed way was totally bogus. I threw the computer into a junk drawer and went back to trusting a digital bottom-timer and custom tables.

Thankfully, times have changed. Third or fourth generation PDCs — and I am never sure if we are on Gen. 4 or 5 right now — approach the mysteries of decompression stress from a totally different direction. One of informed understanding and regard for best practice.

Diver safety is still paramount, but software engineers seem comfortable following the recommendations of the algorithm loaded into their computers to calculate ascent rate and stops, rather than making stuff up on the fly because a lawyer is breathing down their neck.

Nothing has diluted a diver’s responsibility to understand more about decompression planning than turning on his or her computer. It’s as necessary as ever that every dive team has a robust and sensible bailout plan if Murphy joins them as an uninvited dive buddy.

So where does this leave us?

Well, to begin with, a bunch of the folks who used to eschew dive computers have a different view now in light of those changes; me included.

Nothing has diluted a diver’s responsibility to understand more about decompression planning than turning on his or her computer. It’s as necessary as ever that every dive team has a robust and sensible bailout plan if Murphy joins them as an uninvited dive buddy. And with that in mind, a PDC has now become — for most of us at any rate — the thinking diver’s default tool to keep safe and out of the recompression chamber.

I still teach decompression theory in much the same way as I did in the days of crappy PDCs when they were not worth the real estate they took up on a diver’s wrist.

My dive students need to understand in their own terms my tongue-in-cheek statement that a decompression algorithm is mathematics — a pure science — while physiology is smoke, mirrors, Latin, and alchemy — guesswork, variables and overcomplicated grammar! No disrespect intended towards physiologists, but the complexities of human physiology are not easily captured with mathematical formulas.

Most critically, once they have processed that, it’s important that we accept and are comfortable in the knowledge that we CAN be bent even when our computer says we are not!

Standard warning seen in all Shearwater manuals.

Computers, even the very best, are fallible. They will supply the same ascent profile regardless of how we feel the day of our “big dive.” (Or our little one.) A computer doesn’t know that we’re feeling like a piece of toast left out in the rain for the afternoon, or that we got no sleep the night before, after drinking a half-bottle of rum and smoking three cigars.

In essence, divers are kept safe from decompression stress for the most part by statistics and probability, and common sense. And that common sense extends to their choice of which PDC to wear.

Luckily that has become surprisingly simple. We can very easily find one that has a proven track record; that uses a proven algorithm; that has an intuitive way to program variable conservatism levels for different types of diving (and different days) on land and even during a dive; that has a graphical interface showing tissue loading; and that’s easy to read when we need information in a hurry… like when we’re diving. If it also has good customer support, buy it!

There are divers still who believe as I did once, that PDCs have no place in a technical diver’s toolkit; however, times change. And I have a firm belief now that being a Luddite is not only NOT best practice, but it might result in an unwanted chamber ride!

For more information about the author go to

7 Responses

  1. Good article!

    I am wondering what you think about air integration (AI) in dive computers specifically wireless AI? While I am a firm believer in having redundancy to check the tank pressure the current complaints I see from sections of tech divers about how terrible AI is seems really overblown.

    I’ve never had a serious problem with wireless AI in the decade I have been using it. I tire of the drum beat I hear from them about AI killing you .. I am sure that is right after a diver dies because you used split fins or the wrong mask 🙂


    • Avatar Bari Gowan

      As far as ‘AI’ goes – me neither. I’m new to Shearwater (just bought the Perdix AI), but I’ve been using Oceanic dive computers for years (since the 1980s!). I ‘moved over’ to hoseless operation when they did. Yeah, for sure – in the beginning, and with certain models – there were ‘transmitter lost’ times during dives. But these declined over the years to an insignificant irrelevance.
      I just sold my (Huish replaced) VTX. OLED does NOT work underwater – whatever they say. Haven’t tried my Perdix ‘in anger’ yet, but it’s certainly readable outdoors!

  2. A dive computer is just a device. We feed it information about what we want, and it gives us data. We then need to interpret that data, and apply it to the dive. With variable PPO2 settings, variable Gradient Factors, and a host of other adjustments, we can tailor the information the computer provides us to better fit each particular Dive, as well as our personal preferences and tolerance for risk.

    There is no magic, and no voodoo. It is just a tool, and if you learn how to use it properly, like a journeyman, it will serve you well. What else can you expect from a tool? Can you start the chainsaw on the tailgate of the truck and expect it to go cut down the tree by itself? Can a dive computer fail, like any other tool? Absolutely, so plan for that in whatever way works for you.

    Certainly one of the advantages a dive computer provides, is that we can download the dive record for a detailed examination in debrief. This is where the AI feature is truly valuable. I manage my gas when planning the dive. While diving, I look at my cylinder pressure only to confirm I am, or am not, on my Plan. Whether I do that on a digital device or SPG, is personal preference, however with AI I can review how I used my gas after the fact. This allows me to continue to improve and hone my dive planning skills.



    • Avatar Eddiemack

      You touched on a great point here. Just because a diver uses a computer doesn’t mean the computer data is the dive planner. Divers should still plan the dive, dive the plan. I have a perdix ai, and still use plastic dive tables for planning dives. I use the computer to back up what I already know from the dive plan.

  3. Just as the standard PDC’s have evolved to become better tools with greater reliability so have AI PDC’s with wireless transmitters.
    Of course wireless transmitters can and will still fail but so do SPG’s and HP hoses.
    With proper maintenance, servicing and pre-dive checks I would guess that the failure rate would be negligible for both and fairly even percentage wise although it would be good to have some actual 3rd party independent testing to verify that guess.
    The downside of AI PDC’s and Transmitters is obviously their greater cost but that may well be offset by convenience and better readability for some, especially older divers with presbyopia.
    In some scenarios wireless transmitters may help to reduce clutter and possible entanglement hazards which can be an advantage over an SPG and HP hose.
    Whichever you use you should always know how much gas you have in your tanks anyway, just verifying it by checking your SPG or AI PDC. If either should fail for any reason then the dive should be called and the diver should always have planned sufficient reserves to exit safely.
    If it is an extreme dive and it is critical to know exactly how much gas you have to the nearest psi or bar then you could run both for redundancy but in my opinion that is overkill for no real world advantage and some possible disadvantages by increasing possible failures points.
    Most people tend to be a little conservative and resistant to change but I predict eventually SPG’s will go the way of the J valve especially if the cost of AI PDC’s starts to come down.

    Steve Bogaerts

  4. Avatar Barron Moore

    Good to see this, Often DIR standards vary according to “personal preferences and tolerances”. Divers including myself choose what is best to take or not on dives. Two computer have kept a good handful of dives from being called. I have kept my preference of slating profiles including contingency profiles. The slate pages are my “tables” and the second computer is in gauge mode with compass on the scooter.

  5. Nice Article!