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Question
Posted : February 18, 2008
What other features to look for in a dive computer for beginning technical diving? Particularly there are models allowing two and three switches on the market, do people use three mixes without using trimix or more likely just two?
I am about to buy a new dive computer and I would like to get one that would be useful for beginning technical diving. It strikes me that spending a lot of money on a trimix one would not be sensible at this stage, so I was looking at ones that allow nitrox gas switches and a gage mode, figuring that this will allow me to use the computer up to the point that I start using trimix (if indeed I do start) and as a gage beyond.
Answer
Posted : April 7, 2008
Wow an interesting post and one that yields a number of important points…

[1] I’m of the school of thought that uses table creation software (eg ProPlanner from Delta-P) and then writes these tables onto slate and then dive that plan. I use all standard computers in gauge mode and I use a mixed gas computer as BACKUP. An odd arrangement – but think about it I know if the gas computer has failed or that something has gone badly wrong with planning as my tables should be the most conservative… If the computer shows loads more deco then THINK in water and work out why…

[2] I cut the following tables (a) DEPTH of Sea Bed + 2m, and a total bottom time of the PLANNED dive. (b) DEPTH of Se Bed + 2m and 10 mins to total bottom time for those dives you get to the bottom of the shot line and think where on earth is the wreck – or something is wrong and I’m aborting dive, and (c) DEPTH of Sea Bed + 2m, and a total bottom time of the PLANNED dive + 5 mins.

[3] The Technical dives you may have three gases such as AIR, Nitrox 32 and Nitrox 75% and these three gases are a reasonable combination for many dives that technical computers can handle.

[4] There is a growing trend for dives below 45msw to use normoxic trimix and as such you may have two or more gases for these dives. Personally I like doing 45msw+ dives on trimix as you are much clearer narcotically.

[5] I have seen many leading technical divers using two of the same computers and doing no dive planning using table generating software. Not sure I like the idea of this personally. I believe in diversity and I use a D-Timer (UUWATEC), Stinger (SUUNTO) and a full VR3 (Delta-P) as my devices on technical diving. I keep the Stinger in gauge mode for the trimix or multi-gas diving. Suunto do a range of new computers that are multi-gas and these are very good as your primary computer to use as a computer (non-gauge mode) until later with trimix and you can then use it in gauge mode. Others are emerging or have been around for a while now.

[6] Cut your tables onto a small 60mm by 30mm (wide) slate that you drill a hole top and bottom of this and enough bungie between these hols to go around your hand. On the top side put your main run time table and on the back your 5 mins longer at depth. In your pocket put the bug out 10 mins bottom and a copy of these two plans. This back of hand slate is very very effective and can easily be flipped over even with cold hands… Email me direct for a photo of this or post a reply and I’ll put a photo on my website.

[7] Don’t put too much reliance on these computers – please. They don’t know your Cardio-Vascular fitness, nor Hydration, nor other physiological aspects of your body. They are all based on a mathematical model (several different types which have though had a lot of dives done with them…) A computer will constantly calculate these theoretical tissue models for your given dive profile, but take care to add contingency for your deco as most models only just have you surfacing with tissue compartment(s) that have not long left 100% saturation. By diving to tables created by software like ProPlanner you will be building in a good amount of contingency given your profile will not exactly follow nor never exceed the chosen plan. Obviously you will follow your training for deviations from the plan – but you will dive conservatively to such a table plan.

[8] See my other post on Dive Computers as not all the algorithms are fully implemented in terms of each theoretical tissue compartment being fully calculated each time – some use look up tables or approximation tables.. so again take care on multi-day diving blindly following a computer. I’m not knocking the computers, virtually all I’ve seen over the last couple of decades have been really useful, accurate and good – but use them wisely as a tool. Batteries fail with low temp – often suddenly – don’t be in a situation that your screwed because you don’t have a solid plan for the dive and know what deco etc you have to do…

Hope these ramblings are what you were after, let me know. Cheers Andrew.
Answer provided by ADM Diving

Andrew Dawson-Maddocks
Additional Comment

Posted : April 7, 2008 by - Cuvavu
Thank you, that is all very useful and exactly what I was hoping for. The back of hand slate sounds a really good idea, not one that I have come across before, but one that I will definitely look at implementing. A photo would probably help, though, if you can manage it. Once again thanks a lot. Now I need to find my credit card and get me a dive computer :)
Additional Comment

Posted : April 7, 2008 by - ADM Diving
I'll work add a photo to my tech tips page of my Library link on my website www.admdiving.co.uk this will give you a photo.

Yes it is a neat idea one i've picked up myself from training and other divers over the years. It is so simple and easy to make a few of them.

safe diving and happy to help further. (sorry for the delay in replying to the Question initially) - too much fun diving.

cheers Andrew.
Additional Comment

Posted : April 8, 2008 by - ADM Diving
Another useful thing I have done is to mark on my two delayed SMB reels the following depths in msw 3, 4.5, 5, 6, 9, 12, 15, 18 and 25. I used a permanent marker pen and used a short mark (about 1 cm) for each 1m and a long mark (about 3 cm) for each 5m and then made up each marking in the same way as roman numerals !! the 4.5m mark is in a different coulour so it is obvious.

Even if you had to labouriously count (if you were solo) you can at least do your deco safely and reasonbably accurately. Personally I don't think it is a good plan to deco as a solo diver - deco is best done with a buddy or indeed the support of the team.

The good thing as well you know if you use one of these reels for wreck penetration you will know how far you are inside the wreck (or if venturing seriously in if your training and experience allows - you will know when you exceed 25m in and when you are 25m from your exit upon return).

Hope this other gem you find useful. I have used it once to prove a dive computer was wrong and to be discounted on a dive - the computer went back for maintenance and the transducer had indeed failed.

I'm all for these useful quirky tips for Safer diving. Best regards, Andrew.
Additional Comment

Posted : March 6, 2009 by - Iain3
Re. marking the line on your reel to help with deco stops, I think I'll try that, sounds a very simple and effect idea.

Would you take into account how far under water the line starts? i.e do you put a 3 meter mark three meters down the line and do any 3m deco stops a little above that mark bearing in mind that you have at least a foot or more of DSMB under the water before the line starts, or do you estimate and put the 3m mark at 2.5 meters down the line to allow for the distance between the top of the line and the waters surface? What depth the line starts will vary depending on how full the DSMB is, or how much water got in.
Additional Comment

Posted : August 24, 2009 by - ADM Diving
Hi Iain,

I used a paperclip which i poked through the line without damaging it as i always tie on in the same way...

Bradly speaking doing your deco (consistently) a half a metre either deeper or shallower largely doesn't matter - so long as two rules are never broken

[a] you NEVER exceed the MOD for a deco gas eg pure Oxygen of 6m (or as prescribed by your training agency), keep the DSBM upright but do not pull hard on it such that you will vary your depth as you ascend and then drop down as you put the unecessary tension back on the line (very very bad for helium mixed gas diving) and

[b] you always have dived the plan conservetively so when you are using the marked line and counting you have conservitism in your diving profile and the deco you do, using no timers and a marked line is always 20% more at each stop, and 40% more at the shallow stop (gas supply dependant)...

hope that helped,

best regards,
Andrew
Additional Comment

Posted : January 23, 2010 by - Tonks
the new suunto helo 2 technical dive computer allows for upto 8 switchable dive gas mixes with trimix. prices at around 695 pounds or cheaper if you know a man down the pub :-)
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The views expressed by the author are their own and the publishers accept no liability for the advice and views expressed by the author, or other users, which are provided as a general service to divers. Users are warned that secondary posts are the views of other users and may not be technically correct.
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