But I can’t be bent, my computer says I did everything right
By Steve Lewis
With thanks to Neal W. Pollock, Ph.D., Research Director, Divers Alert Network
If you are a certified diver, chances are you know a few things at least about decompression stress since part of your initial training (and a lot of what followed in more complex programs hopefully), explained the vagaries of breathing compressed gas underwater.
The issue with diving – at least for this discussion – is that as a diver descends in the water column, he or she has no option but to breathe compressed gas; and the inert gas contained in whatever is being breathed is stored in the diver’s body. This is sometimes called inert gas uptake. At the end of a dive, on the way back to the surface, the process is reversed and is called inert gas elimination or more simply decompression. These two processes are part of every dive… even seemingly benign sport dives to shallow depths for short periods of time. Every dive really is a decompression dive.
Managing inert gas uptake and decompression within safe limits when diving is second only to making sure one has something other than water to breathe. If we “get it wrong” and remain at depth too long, ascend too rapidly, breathe the wrong gas, or simply have a bad-luck day, we run a higher than usual risk of suffering decompression sickness (DCS). Getting bent, the colloquial term for DCS, is a collection of disorders caused by a portion of the inert gas stored in a diver’s body bubbling out of solution too rapidly. The consequences of being bent run the gamut from nausea, fatigue, mild joint pain and dizziness all the way through paralysis and death.
The uptake/elimination cycle is a complex one. For example, it’s believed the speed of inert gas uptake is different (faster) than the speed of inert gas elimination.
Not only are uptake and elimination NOT lineal mirror-images of each other, but several variables are thought to play important roles in the uptake/elimination processes as well. I used the term: “… are thought to play important roles” because the variables – the bio-physical processes at play within a diver’s body – are complex and not completely understood. In a word, the factors governing decompression safety are effectively capricious. We might say with some authority that because of its complexity and variability, DCS is the bête noire of divers and diving. It certainly scares the bejesus out of me.
In the vast majority of recreational dives, the inert gas in question is nitrogen, but when a second inert gas is introduced into the breathing mix – helium for example – a whole new array of complications is unleashed. Diving with two breathing gases – oxygen and nitrogen – presents us decompression challenges: diving with three magnifies the challenge considerably.
An ally in the fight for information about and a better understanding of gas uptake and elimination is the dive computer.
Personal dive computers (PDCs) have evolved astonishingly rapidly in the past 20 years. The current generation does a very good job of tracking the mathematical prediction of inert gas uptake and elimination even when the person wearing the device is diving deep, for long periods of time, and breathing multiple flavors of gas. However, a PDC offers no iron-clad safe-guard that its user will not suffer a DCS episode.
Accepting the ever-present risk of DCS and understanding the erratic character of this risk, is a pre-requisite of becoming a responsible and informed diver, regardless of whether your dives take you to 10 metres or 100 metres; or last for 20 minutes or 200.
A personal dive computer – like any computer big or small – is very good at crunching numbers. It excels at calculating gas uptake based on depth, time and breathing mix; and, with the help of a decompression algorithm, showing users how fast or slow to ascend, where to stop in the water column, and for how long. However, this is all theoretical. Decompression theory is just that; and a decompression algorithm is simply a mathematical model that postulates what happens within a diver’s body when he or she is diving. That’s right, deco theory is woven throughout with guesswork: some of it informed, some not so much so.
The shortcoming of any decompression algorithm and therefore of any dive computer is that the relevance of its calculations to you and me are limited because it cannot adequately account for the numerous biophysical variables particular to us as individual divers. You and I may be similar perhaps, but certainly we are not the same. We can wear the same brand and model of PDC and dive very similar profiles breathing the same flavor of nitrox (or trimix), but the two of us will most certainly on-gas and off-gas at different speeds and with different levels of efficiency. And those differences will vary from day-to-day, dive-to-dive. One of us might get bent while the other is free of any signs or symptoms.
Adding yet another complication is that there are simply dozens of dive computers on the market and several substantially different decompression algorithms at their core… some models of PDC are capable of running more than one algorithm. This makes it close to impossible to give useful suggestions detailing the pros of each and how to work around the cons. Nevertheless, there are a few recommendations that apply to PDC use generally.
Number one, read the user’s manual. Have the computer beside you as you do so and play with it. Get to know what your new tool is capable of and how to activate any bells and whistles it may be fitted with. Learn what button does what and how to access the type of information that is going to help you stay safe on your dive. Absolutely have as an end goal for your “getting to know you” session, understanding what algorithm your computer uses, how to adjust conservatism factors, and the potential effects of more or less conservatism on you and your health.
Number two, use it according to the guidelines in the user manual and whatever common-sense you have been gifted with. In attempts to reset their PDC because of a recent transgression (usually something that came close to getting the user bent), I have witnessed divers pulling out batteries, hanging computers in the water “to decompress,” and even leaving their PDC on the boat for a dive to “cool off.” None of this is a good idea. Seriously.
Number three, all late generation dive computers deliver warnings when their users misbehave. These take the form of audible alarms (bells and buzzers) or visual warnings (flashing colors, symbols or messages). Some combine both. However yours is designed to deliver its warnings, take note of those warnings and modify your behavior accordingly. I once shared a decompression station and an annoyingly long stop with someone whose PDC chirped ceaselessly at him (and anyone else within earshot). He had NOT read the user’s manual and therefore was unable to switch it to any of the three decompression gases being used on the dive. His computer wanted him to return to about 30 metres and stay there for a long, long time.
Number four, understand that a PDC, even one with a four figure price tag, it not a panacea. At most, and following the best possible scenario, all a dive computer can supply its user with is an approximate guide to their decompression status; and a rough guess at their proximity to decompression stress.
Paraphrasing Neal Pollock, a computer, even when used correctly, provides no more than superficial protection from DCS: just the very first-level of information. We need to dig a little deeper into what affects decompression, and understand a little more about our PDC than when to change its battery if we want to mitigate the risks of decompression sickness.
Dr. Pollock, Research Director at Divers Alert Network and a researcher at the Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Center, tells us there are more than two dozen factors influencing decompression safety. These include the obvious, such as time and depth, as well as the less obvious and less easily defined and quantified such as epigenetics, atmospheric pressure, and pre-dive exercise.
Essentially, Pollock’s research underscores the difficulty of producing a “magic silver bullet” capable of protecting us completely from DCS. He also suggests that often divers who suffer DCS look for some way to shift blame. They tell us their incident was “unearned.” “Hey, I did everything right… exactly the same as many times before.” They moan because their computer did not warn them. In fact, one often hears a diver express confusion because their dive computer did not get bent and they did.
He says if we fail to recognize errors in our behavior, our pre-dive preparations, or the influence of our personal makeup and fitness to dive, “and we refuse to take personal responsibility,” the learning process breaks down.
Pollock explains that many of us focus on only a small part of the overall picture regarding decompression safety. He uses the example of hydration. Divers routinely blame poor hydration for causing their DCS, but few have a realistic handle on what constitutes good hydration, and fewer yet on the many other factors that contribute to deco stress.
“Proper hydration may play a role in decompression safety, but throwing back a half-litre of water immediately before diving does nothing except make you pee,” he explains. “The “hydration” goes right through without any appreciable effect.”
So where does this leave you and me?
If your diving exposures are mild, you are certified to use and indeed use the appropriate nitrox for your dives, you behave responsibly and cautiously, and follow the best practices suggested by organizations such as DAN, chances are good you will never experience DCS.
If your diving is a touch more radical, and you routinely conduct staged decompression dives, the advice is to dive especially conservatively. Research and understand all the many factors that may have an impact on your safety, and plan accordingly.
Most of all, take responsibility for your actions and don’t make a challenge out of who can get out of the water fastest. Better to enjoy a slightly delayed post-dive beverage with your mates than spend hours in the chamber wondering why it is you’re bent but your computer isn’t.