COMPUTERS, DEPTH TIMER, TABLES

If you asked most people why they use a particular computer the answer will often be along the lines of  “I use the xyz computer, I like it and I haven’t had any problems with it”.  Fair enough you might say, but the decision of which computer to use/buy isn’t quite the same as other kit.

The computer is a piece of equipment that many people rely on to tell them how long they can spend at a given depth; and how long and at what depth to do their decompression stops.  Therefore, HOW the computer calculates this has a DIRECT bearing on the profile of their dive and therefore the probability of them getting Decompression Illness. It is therefore somewhat alarming that the vast majority of divers don’t understand how their computers calculate their No Stop Time, their stop depths and their stop times and the off gassing during surface intervals.

The vast majority of people don’t know what assumptions have been made in the model and the model’s limitations.  And yet these same divers are advising others on what computer to use.  I find that both strange and worrying.

With few exceptions, modern dive computers are reliable bits of kit and most now have reasonably well laid out displays.  Most of them also perform the same functions, be it just for air, or for Nitrox and/or Trimix as well.  The primary consideration then when deciding which to use should be “it uses the xyz a model, I understand that model and I am happy with how it calculates my decompression and with the assumptions made in the model”.  Alas, this is rarely the case.

Since the functionality of most modern dive computers is by and large the same, the issue with regard to dive computers is therefore which decompression model they use and whether they factor in deep stops. This is a huge topic in itself and is probably best discussed and taught by way of a lecture/discussion session.

That said, I will mention at this point that the only dive computer (at the moment) that I am aware of, that utilises the latest theories on bubble mechanics (Varying Permeability Model) is the Suunto Vyper.  The Aladin and most other computers use a Buhlmann algorithm (either ZHL16c or ZHL8) BUT allow tissue saturation's to reach close to the M-Values.

Computers that allow for gas switches are a bad idea since they may allow for a change in the dive plan or bottom time “on the fly”, but have you planned for this in your gas requirements?  An extra couple of minutes of bottom time on deep dives may cost you 10 to 15 minutes more decompression.  Such computers also invariably lead to less planning of the dive and all the attendant risks that involves. For technical dives you are better off using a simple UWATEC depth timer.  These simply show max depth, current depth and dive time.  They cost 80 Euro and the battery lasts about 5 years. It has no seals to the battery compartment and thus has to be disposed of when the battery runs out.  No failure points therefore.

Uwatec depth timer