Gas switching computers

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Deefstes

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Hi all, while doing my homework on dive computers I started wondering about this whole idea of "gas switching".

Take the example for instance of the Suunto Vyper compared to the Vytec. Essentially there are three difference between the two. The Vytec has air integration (sure, that'd be great but it still costs almost the price of the computer for the transmitter), deep stops (sure, I'd like that too) and gas switching (huh?)

For these three features (of whih the first one is useless unless you fork out the additional dough to get a transmitter) the Vytec comes at a price significantly more expensive than the Vyper.

Now can someone explain to me please how often divers actually use this gas switching function? I'm only OW and would like to further my qualifications one day but currently, in my limited understanding, I just don't understand the purpose of gas switching.

If you were switching between Trimix (normoxic or advanced) and other gasses (such as air or EAN) it would make perfect sense because you would typically be using a different bottom gas than a travel gas or deco gas. However, this computer does not support Trimix. So basically you can only switch between gasses of various oxygen levels.

How often do divers do that? I mean use different gasses for diving and for deco when the bottom gas is only air or EAN?
 
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The gas switching is not really a good idea for recreational diving. But, gas switching normally refers to the switching to a higer O2 level of gas for deco stops and clearing of Nitrogen faster so as to minimize the length of deco stops. by using the deco gas if you will and a computer and following NDL rules sometimes a diver will have less deco time because as you accend your computer credits you time and NDL. I think by switching to a higher o2 content gas you clean up faster and I think reduce your SI somewhat before the next dive.
 
Sure, but how many divers actualy do this? Is this common practice? Common enough to warrant making a dive computer to cater for it or is it realy just a gimmick to sell the computer?
 
No I dont believe that it is a gemmick to sell the computer. Again, it is not recomended using a 40 cf to off gas on while accending while diving within recreational limits. But after some advanced nitrox and maybe some intro to tech diving and some pool time practicing switching it would be possible. I also believe that most upper end computers have the gas switching available, IE 2 or 3 gas computer. The 3 gas computer are for the serious deep divers that spend lots of time at the bottome and have to carry 4 or 5 bottles and stage a bunch of bottles for deco stops.
 
When I do deco dives, I will switch to 100% oxygen to finish off. If the computer does not know you have switched, then it will make you stop in the water as if you are still breathing the back gas which will be considerably longer, depending upon the dive profile you have just been through. I don't know how many divers switch gases, as a total percentage, but I know that up here in New York a lot of divers are doing it after diving a wreck. Incidentally, I go off pre-cut tables and put my computer in gauge mode - I do not have a two gas computer yet.
 
What to buy is going to be dictated a lot by where you think you want to go. I would recommend that should you choose to increase skills, get your basic nitrox (if you haven't already), do some diving....go to advanced nitrox...where more than likely you are going to get some exposure to deco procedures. Chances are you will not be diving computers during class and the instructor will have you using tables. You can decide after that class whether or not you will be moving to trimix (and again, most instructors are going to have you using tables during that class as well). A higher end gas switching computer is a pretty significant investment (anywhere from 1300 dollars or more)...and I would suggest that it be one of your later investments instead of one of your first.

A gas switching computer is generally used for divers doing diving which would incorporate using multiple gas mixtures, such as tech/deco type diving. There are varying degrees of what is available insofar as capabilities ranging from a computer that can accommodate 2 mixtures of EAN that can be switched, to computers that can have a menu of 10 different ones, ranging from AIR to any mixture of EAN and trimix, the gas ratios and gas selections being set up by the user according to what they will actually be using, and can actually be adjusted and switched underwater. Further, the computer is calculating the diver's deco obligation and profile based on diver's depth and what gas is being used, with a recalc performed as the diver switches from one gas to another on the computer. To answer a specific question you asked...you could use a gas switching computer if you were doing a deco dive using EAN....for example, EAN 26 to 140' and then using some level of EAN high O2 content gas for deco.

The value to me is that using a gas switching computer is a convenience. It shines where you are not diving a true "square" profile, but where you may be at different depths during the course of your dive, as you are credited with times you were shallower and debited when you were deeper. On a true square profile, that is for example going to 200' and staying at the depth for 20 minutes and then beginning your ascent, it really offers no advantage (other than ease of use) over using a program like V-Planner (or the dive planning feature on the computer) to cut tables. Either way you can use the dive computer or program like V-planner to calculate different gas mixtures impacts on dive runtime. It is simplier to ensure that if you do use laptop software and a dive computer, that both use the same algorithms.

A diver might use a gas switching computer, but the wise diver using one will have his dive plan, including deco plan entered onto a slate or a wetnotes book as a backup, planning his dive and diving his plan. Generally I will cut 3 tables as backups:

1) My planned dive (let's go with 200' for 20 minutes)

2) A "bailout" plan...so I would cut a table for 12 minutes at 200'. If I had to call the dive early, if it is 12 minutes or less, I will use this table for my ascent plan. Anything 13 - 20 minutes I would use table #1 for ascent.

3) A table that shows 5 minutes more and the next greater 10' depth (210 for 25) so that I have a plan should something happen that I dipped below 200 or stayed longer then 20 minutes.

Granted...that is anal, but personally I would rather be safe than sorry. I can't imagine much more trying on a person than to have a computer failure and not have backup tables immediately available for use...instead trying to calculate them on the fly. It can be done...but why not error on the side of safety.
 
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I dive a 3-gas computer, but use it only as a backup in terms of planning. I create the plans SlowRain discussed above, and use them. And by the way - it clears just around the time of my tables. Whichever is more conservative I follow.

Why? Redundancy - I've found more than one set of deco tables on the bottom of Lake Travis.
 
SlowRain, I agree with you 100% even though I am not a technical diver. You're reasoning makes sense to me and should I get to that point one day I would think that having three tables with me as you described is a very good idea.

Having said all that, you didn't really answer my question. Not to worry though, I think my question have been answered already. It seems to me that, when you get to the point where you'd be switching between gasses during a dive, a Suunto Vytec would probably not be the ideal computer for you anyway.

What it tells me is, don't bother with the Vytec, get the Vyper. If one day I do start switching between gasses, I'd probably want to get a computer that can deal with Trimix as well.
 
Another computer to consider would be the DiveRite Duo or one of its clones. Relatively inexpensive <$400,easy to use and handles 2 nitrox mixes. It's a perfect cave diving computer.

Early ones had some problems,supposedly fixed now but I am not convinced.The Tusa IQ 700 is the same computer but with a 2 year warranty.

Probably the 2 leading Trimix computers are the Liquivision X-1 (around $1,800) and the Shearwater Pursuit ($1,400) Serious tools for big dives,overkill for most.
 
I second the Nitek Duo. It's a perfect cave diving 'puter. Cave dives are the ultimate in sawtooth profiles so it is not easy to cut tables. I'm not a big fan of calculating deco on average depth so I use the Duo as my primary and I'll deco out on the back-up tables in my pocket if the Duo failed me. But it's great to switch to O2 and watch your deco obligation shrink up. It is also a great computer for the Advanced Nitrox/Decompression Procedures level dives w/ one decompression gas.

I just picked up the Shearwater Pursuit for trimix diving and so far it is great. A lot bigger than the Duo, but much more capable. 5 OC gases, user-adjustable gradient factors, and forward-looking TTS (time to surface). It will estimate your deco based on the mixes you have set up in the computer instead of basing it on the current gas (unlike the Duo). Great computer for mix dives, but overkills for anything else. My Duo is still the weapon of choice for anything w/ air or nitrox.
 
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