Hi Guys and Girls,
In my 27 years of diving (military, commercial & recreational) I have seen a few things and learnt a few lessons. I was even fortunate enough to have spent a few years in experimental diving and as a recompression chamber operator treating bent (and worse) divers.
It is not so important what brand of computer you buy, but how the algorithm operates. It is critical to your health and wellbeing what the algorithm stops you from doing, not what it allows you to do.
Any computer or table that allows you more time in the water increases your chances of meeting me, the chamber operator who MAY be able to fix you. It is as simple as that.
I have lost count of how many times I have heard the line "But .... my computer said it was ok". Now, THAT is unfortunate.
If you spend more time in the water and are not bent, you do have a higher concentration of microbubbles circulating in your blood. Microbubbles themselves do not cause a bend for the purposes of this explanation. They are usually removed over time by a combination of metabolism (uptake of oxygen by the body's cells) and gas diffusion (oxygen and nitrogen - or helium removed via gas exchange in the lungs). A higher concentration of microbubbles means a higher percentage risk of there being the occasional larger bubble - this means a bend. Your less conservative computer (or table) allows you to go back diving sooner and for longer while you are on that liveaboard holiday ... with a higher concentration of microbubbles remaining. At the end of the next dive, there are even more microbubbles than after the first dive. That means an even higher risk of a bend inducing bubble set. And so on. The more conservative computer allows more dissipation of microbubbles, less generation of extra microbubbles and therefore less chance of larger bubble formation.
Many many people get bent on liveaboards because they want to maximise the dive time for the dollar spent. That is an understandable mindset, but it is the mindset that brings most divers undone. It is on a liveaboard when you are doing 4 or 5 dives a day that you need conservatism the most (or a Dive Instructor or Divemaster for that matter). A conservative computer means that you go home to your family and you don't meet me, the chamber operator.
Another issue, that is not detected by dive computers, is the bend caused by multiple ascents. This type of bend is usually cerebral/spinal and is particularly nasty. Bottom time is irrelevant and it is a function of increasing and decreasing ambient pressure on the body many times over a short period of time. I have seen Dive Instructors (and fish farm divers) getting bent because they did up to 16 ascents from 8-10 metres in a 2 hour period.
A dive computer only operates as a function of depth and time. Nothing else. Generally, they cannot make allowances for gas consumption (those that claim to just can't as not all the gas in the cylinder goes into your body). They cannot make allowances for body work rate, metabolic efficiency, lung efficiency or volume, physical fitness, body fat, cold, dehydration, age, patent foramen ovale etc. To allow for all of these variables, conservatism is built into the algorithm. Some young & fit divers will think they need less conservatism than a middle-aged, unfit and overweight diver. I have seen just as many young & fit divers in good physical condition with DCI. Whilst I agree that being in good physical condition does help in many ways, it will not save you from a bend if you push the limits.
The moral of this story is that there is more to safe diving than blindly following the computer that gives you more time in the water. Diving Instructors have a duty of care to their students (and other divers) to ensure that they are suitably knowledgeable about the issues and to only issue advice that will not get them hurt.
The single best piece of advice I can give to a student is to be conservative - that way they will have a long and enjoyable diving career. To advise otherwise will ensure a short and tragic one.
Having said all that, I do dive with a computer these days. In my experience the best tabulated deco times are stipulated by DCIEM. They are the most scientifically accurate tables available today (based on algorithm modelling and manned testing), particularly for repetitive diving. I used these tables as a baseline to compare various algorithms for no-d, decompression and repet diving times.
Nitrogen and helium, as inert gases, do behave and diffuse differently within the body. DCIEM have tables for each.
I found that the Suunto RGB model to be the most appropriate 3 years ago, but there may be others around now that are similar. I use a Suunto D6 with the default (or normal) personal settings for conservatism. Deep-stop is ALWAYS on as a staged decompression is far better physiologically than one long shallow stop.
Dive Safe!
No Pressure
