Trimix dive computers and planning

[craig chamberlain]

Hi.
I have recently started tech diving and when i am 18 i will continue to the TDI avanced trimix level. (not all in 1 go, i will gain more expirience between each course). i have started to save up for a dive computer for tech as i have a suunto gekko now and am thinking of upgrading so i might aswell get a trimix capable computer. i understand that a lot of tech divers use a bottom timer and depth gauge and write their plan on a slate, but what if you have to change your plan during your dive? Anyway i think that i would probably do that and still have a trimix computer just to be on the safe side. I have looked at a few trimix computers like the liquidvision and the suunto hel03 and the NHeO3 and the shearwater predator and so far i think that the HNe03 would be the best option for me considering the price. If anyone has any suggestions then that would be great.

i would be looking for a computer that is trimix cabable with any ratio of each gas e.g from 8 - 100 and including 100% O2, and any amount of helium and nitrogen.
also i would want to be able to use it for recreational diving with just 1 tank of air or nitrox.


Also i was wondering why some computers and not "compatable" with some deco planning software aprt from the reson that they use different algorithms. surely if your compuetre says your deco is over then it is over?

Thanks very much everyone##!!!!!!!!!!!

really appreciaite it

 

[Red Sea Shadow]

surely if your compuetre says your deco is over then it is over?

!!!

 

[evil_xander]

Cannot comment on the computers, as I use bottom timer for trimix dives, but just one thing to clarify: deco is not exact science.

When your computer says deco is done and your body disagrees you end up on O2, in a chamber, or worse. So, whom do you trust? Also, computers and water do not mix so well sometimes, batteries have limited duration, different computers in the team mean probably different deco times (= no team basically) & etc. The one between your ears should be more or less okay, providing you use proper gases and have adequate level of knowledge & skills.

You (hopefully) have variations of the plan and an idea how to adjust it accordingly on the fly (ie. ratio deco system or similar method).

 

[craig chamberlain]

so why do different computers use different algorithms? is there a "best/safest" one? how does deco software work out your deco stops? (like hoe long for and how deep)?

 

[evil_xander]

Why? Simply because they can use a different one.

I think that you better discuss these things with your instructor. Learning deco diving online is not really a good way to start (for your own safety). Equipment questions are one thing but where you're aiming seems like you're not familiar at all with deco theory. If you need a good reading material, I can recommend the book 'Deco for divers' by Mark Powell.

 

[craig chamberlain]

yeah i am not yet deco trained as i am not 18 but am curious about deco and algoriths and thanks for the recomendation.

 

[Jax]

Craig, you are asking a question with a tremendously involved answer.

I recommend http://www.amazon.com/Deco-Divers-Mark-Powell/dp/1905492073 . . . it covers all aspects of deco, including the various agorithms and how they work. It will help you make a much more informed choiced.

Safe dives!

 

[FWADIVER]

I'm sure my opinion will be unpopular but here is my $0.02 and you get what you paid for it: The 2 schools of thought are run tables and a bottom timer, or fly your computer and use tables as a backup. I currently prefer to fly my computer (shearwater) with tables as a backup. During training you will likely do tables and use your computer as a timer, and whether you go tables and timer, or tables as a backup depends a lot on who you dive with, who your instructors and mentors are. My justification for flying the computer is that if all hell breaks lose, and you were to blow all the schedules on your table, the computer will still work out your deco for you. There was a incident a while back involving some divers doing tables and ratio deco in a cave, where they had an emergency, blew their tables, lost track of everything they were doing during the dive in the rush to get out, once out of the overhead, they basically had people passing them o2 bottles left and right without any real idea of how much deco they had left. At least if someone in this instance had a computer it would give you an idea of how long you had to stay down. I would never rely solely on the computer, and the shearwater allows you to use it to plan dives which is what i do. I write my sked down for +/- 5 mins and +/- 10 feet using the shearwater to plan (which allows it to take into account my current loading) on my slate, but generally fly the computer using the slate as a backup for when the computer dies.

 

[boulderjohn]

Yes, there are several schools of thought (more than 2, actually) on how to do this. I believed I have dived with people who used every one of them at one point or another.

If I were you, I would not give this question a second thought right now. You have a lot of learning to do. It is not just academic learning--there are physical skills required that will take a long time to master. All your initial training will almost certainly be without a computer, so you don't need one right now.

It will probably take you a lot longer than you are expecting to get through your trimix training, unless you have really good access to the water, lots of time to do a lot of diving, and a whole lot more disposable income than I do. You are probably looking at a number of years before you are at the point where you really need to make this decision. If you purchase a computer right now, when you complete your training several years from now, one of the following will be true:

• You will decide that, yes, you do want a computer, but not the one you already purchased before you really knew what you were doing.
• You will decide that you don't want the computer after all.
• You like the brand computer you have now, but you wish you had a newer, better one.

Technical diving is extremely expensive. There are many important purchases to be made that you will need right away. I would not spend a dime on a purchase you don't have to make until you have to make it. If you decide then that a computer is what you want, you will be able to make a good sound judgment on the latest and greatest version available to you.

 

[TSandM]

Craig, one of the big differences between usual recreational diving and technical diving lies in the planning. On a recreational dive, you have enormous flexibility to execute your dive, within broad parameters. As you start getting into staged decompression diving, this becomes less and less true. A big part of technical diving lies in the pre-dive plan, to make sure you have enough gas to do the dive AND to cope with reasonably anticipated contingencies. As someone said to the resort owner up in Canada two weeks ago, "Everybody here has technical training, and the one thing we know how to do is to keep a schedule." A lot of the in-water training in technical diving is ensuring that you retain enough awareness to keep to your schedule, despite distractions and stresses. You really cannot execute a staged decompression dive the way you do a recreational computer dive, by going down and hanging out until your computer shows you the amount of deco you are willing to do.

But there is no magic bullet on decompression. The actual physiology of gas in the human body is not really very well understood. Different decompression algorithms make different assumptions about gas diffusion in and out of tissues. Some take into consideration bubble formation, and what is theorized to occur when that happens, and some do not. They generate different ascent profiles, and the differences can be minor or quite striking, depending on what approaches you are comparing. Most dives are executed safely, if the schedule is kept, and some are not. To my knowledge, there aren't any long-range studies of ANY of the decompression programs, to see if there are significant differences in DCS rate between them. As our Rick Murchison here on SB says, every time you do a staged decompression dive, you are performing an experiment on your own body -- keep careful notes!

One big problem with using dive computers for this type of diving is when buddies are using different ones, or different settings. What do you do when you think you have a three minute stop at 20 feet, and your buddy signals for a 15 minute one? Did you bring enough gas to do that? Did you bring enough deco gas to share with your buddy, if he runs out or loses his?

All that said, I use a Liquivision X1 computer, and I allow the VPM software to run. It doesn't occupy much display space, and I can ignore it if I want to. My dives are preplanned, using a different approach from V-planner, but one which is generally pretty similar in output. The computer is, as FWADIVER describes, a fallback if everything else goes completely wrong, and neither I nor my team can remember what we have done and compute what we need to do next. I've never needed to use it for that purpose, though, and I'd have a hard time recommending that someone spend over a thousand dollars on a bit of insurance you really shouldn't ever need. (I bought mine for the OLED display, not for the trimix computer capability.)

Get Mark Powell's book. It's very good reading, and I think it will greatly expand your understanding of all these issues. Another book I would highly, highly recommend is Steve Lewis's Six Skills book. (He's Doppler here on SB.) It will also expand your range of thinking about technical diving and the issues pertaining thereto.

I'm glad you are asking questions. Just don't spend a lot of money on shiny trinkets until you have the answers!