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Why we dive standard gases

Discussion in 'DIR Diving' started by Gareth Burrows, Oct 7, 2010.

  1. Gareth Burrows

    Gareth Burrows Super Moderator
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    There's been some discussion on gas choice here over the past couple of days, so I thought I would clarify the DIR position on gas selection, both for bottom gas for reactional divers, and decompression gas for technical divers.

    DIR divers use the same gases. Any ten DIR divers which gas they will use at any given depth between 1 and 100 metres, and they will all give you the same answer. For example, between the surface at 30 metres we breathe EAN32. Between 30 metres and 48 metres we breathe 21/35. A little deeper we breathe 18/45 trimix. Deeper still and it's 15/55 trimix. Very deep indeed and it's 10/70 trimix. Our decompression gases are equally standardised. 100% Oxygen, 50% and 21/35 trimix are the three we use unless diving something genuinely extreme.

    So why do this. Many people are taught the strategy of "optimum gas". This means picking a "best" mix for a given depth. for example, if you pick 37% nitrox for a dive to 28 metres, this maximises the amount of Oxygen you can have in your gas, and thus limits the amount of Nitrogen. Makes sense to me. Equally, with a decompression gas there is a strategy that suggests something similar, which is why people end up diving gas mixes like 70% and 80%. Nothing wrong with that, it's a tried and tested strategy used by the vast majority of divers

    But it's not what we do.

    so we dive standardised gases. 20 metre dive? EAN32. 25 metre dive? EAN32. 12 metre dive? EAN32.

    why would we do this.

    Well, let's think about what happens when we standardise a gas.

    1. Mixing the gas becomes a doddle
    For example, 32% is 32 bar of O2 and topped up with air. That was easy wasn't it. It gets better. 21/35 is 70 bar of Helium topped up with, wait for it, 32%. 18/45 is 90 Bar of Helium topped up with 32%, 15/55 is etc etc etc. Mixing like this becomes easy, especially once you find a nice friendly shop that banks 32%. you lean how to mix it and then unless you are doing top ups you dont need tables or calculators etc etc. In my BSAC club we have O2 and Helium and mixing the standard gases for a large amount of cylinders is easy because you're putting the same thing in all the cylinders.

    2. We learn patterns in decompression limits
    Using EAN32, for example, we can do a dive to 30 metres for 30 mins. At 25 metres its 45 mins. At 20 metres its an hour. Ooh, that's nice. Think about that. As long as we now round our dive time up to the next largest 16 minutes, and as long as ascent at the DIR rates, then we know how long we can dive - without tables or computers. You could develop patterns like this with any gas you chose as long as you either ran enough numbers or did enough dives. The benefit comes not from the actual gas, but from the standardisation of that gas. dive the same gas enough, and you'll get a very accurate feeling for the maximum allowable dive time in the recreational range, given any combination of depth and time.

    3. Decompression cylinders can be the same size
    If you use 80% deco gas on one dive and 50% deco gas on another dive, you are either going to have to limit your decompression or buy different cylinders. You are on the 50% a lot earlier and thus obviously need a lot more gas. Standardising the decompression gases means you can have dedicated cylinders, permanently marked up with the appropriate labels, which every technical agency recognises as being the safest way to do things.

    4. It's easy to maintain safe limits.
    All our limits are the same for every dive. In addition to our minimum decompresion limits, our maximum operating depth is the same for every dive. We don't have to think "whats the Max operating depth for this gas" - its the same for every recreational dive. this extends into technical diving as well. Every standard gas has a prescribed range in which it works.

    5. Out of Gas!
    If a decompression gas fails, recalculation is easy because you are not diving a gas that might be new to you. You're diving the same decompression gases you always dive. What's more, everyone else is diving the same thing anyway so you can take someone else's gas and just carry on regardless.

    6. It permits ratio deco
    Ratio deco, something I will write about in the future, allows DIR divers who have been trained to the technical level to execute any dive without the need to rely on computers or tables, and perform the required decompression whatever the depth and time happens to be. to a large extent, this is only possible because so many thousands of dives have been done using the same gases the decompression can be accurately predicted. This would, again, be true for any gas, not just the gases that GUE and UTD select, but the research would need to be redone.

    7. It adds a safety factor to gas switches.
    When do you do your gas switch on an accelerated decompression dive. If you are breathing 70% you might do it at 12 metres. If you are breathing 80% it might be 9 metres. If you follow a strategy of "best mix" this depth could be different on every dive, which we believe could lead to confusion if there was a problem on ascent. We change at either 21 metres. Or 6 metres. On every decompression dive. That brings a certain comfort and familiarity. It also means if someone tries to switch at a different depth, there is no confusion or communication - they are wrong and you just slap the reg out of their mouth.

    8. It makes gas planning simpler.
    what's the depth? that's the complexity of the gas planning. We don't need to run numbers through computers, or work out PPO2 limits or CNS limits or OTUS or Maximum operatiing Depths or anything else for that matter. Stick to the standard gases in the ranges they are prescribed and you avoid all complications like this. Once we know the depth, everyone on the team knows what gases we will use. It just doesn't need discussion.

    now DIR doesn't have the monopoly on standard gases. Most of the technical divers I know from all agencies use standard gases these days for the above reasons. Most recreational divers use a standard gas - air. The only thing that different about DIR is that we only ever use standard gases, and we accept the gases that are defined by DIR because we recognise that the benefits described above vastly outweigh any cons.
    What's perhaps more interesting is why DIR chose the gases it did. But that's a different story altogether.
     
    starwatcher likes this.
  2. neilh

    neilh Well-Known Member

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    I think the ad for the London Recompression Chamber that appeared at the end of your post for me was a nice touch for a post about gases and deco :)

    It also helps with shops that don't bank 32%. Several shops that I've used regularly over the years have commented that they like it when I come in cos they know there'll only be a handful of mixes that I'm after.

    There's another benefit here as well and that's around dive planning. For the exploration stuff we did out of Eyemouth last year I spoke to the skipper about various marks and options we could look at and agreed a range. We didn't know what the weather would let us actually get out to and dive so I didn't have depths I could tell people to prepare for. However I could tell people we'd be doing an 18/45 dive on one day and a 21/35 on the other - with options for 32% dives on each. That immediately told everyone the depth ranges we'd be working with and what deco gases to bring along.

    That was nice :)
     
  3. bottlefish

    bottlefish Super dooper member
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    Garfs Edit :Really sorry bottlefish, accidentally pressed EDIT rather than QUOTE. Think i have repaired your post but shout if i messed it up - somewhat new at this. oops.


    [Edit: rephrased, my original opening was, er, inccurate! Sorry!!]
    ==================================================

    Sorry Garf, but felt compelled to comment on a few things...

    Mixing gases is a doddle nowadays anyway, the gas monkeys do it all for you ... unless, of course, you're out on expo somewhere, but then that's a whole new world of pain....

    No argument there.


    I'm not sure of your logic here, or what you are saying? I use Ali 80s for my deco/bail out as standard, they're (for me) the most comfy to dive with... it doesn't matter if I only need 500 litres of the gas inside, it'll still be an ali 80 pumped to 200'ish bar, downsizing to a smaller cylinder doesn't make sense.

    That does mean there's a benefit in sticking to the same gas for mutiple dives, as you can re-use (e.g. if I needed 60 bar for a deco dive, had a 200 bar fill, then try and re-use it for the next dive).... so I'd state that as a possible benefit for standardising.... but once you get to topping off/refilling, then it's all back to the gas monkey's magic.

    I'd argue the opposite, in much the same way that I'm not a great fan of rich right, lean left. Provide a system that requries checking, then you have no choice but to check, provide a system that's "always the same", complacency can slip in... on the flip side, if a tech diver isn't capable of monitoring their depth and sticking to their plan, then they shouldn't be diving tech.
    (07-10-2010 09:48 PM)Gareth Burrows Wrote:

    You're making quite an assumption here, that a team that doesn't opt to use standardised gases, also allows each team member to select their own gas. That (in my very limited experience ) is rarely the case. Optimum has choice is made within the team, based on the team parameters, for the team as a whole. So, in a lost gas/out of gas scenario, the options for grabbing someone else gas are just the same... and the lost gas scenario will already be preplanned on a slate on your arm.

    Or, you could just plug in the new gas (whatever flavour it was... ) into your VRX and let it do the work for you.... (but that's a whole new bub fight!! )

    Yep

    Sorry Garf, but this one is, I believe, utter bollox.

    (Again, in my limited experience... ) the only safe way to do a gas switch is through a notox, any other vodoo bull crap that pretends to make it safer is just that. 6 metres, 21 metres are just numbers, you need to refer to gauges to know when you are there. You should also be referring to your plan, to see when and how long, what you are going to do there.

    Yes, gas planning using ideal gases can take time, there are a lot of what if scenarios to throw around (just ask GLOC the amount of emails we sent back and forth trying to sort out gases for the Southwold last year!!), however I think it's a good exercise, you start to understand patterns in the answers you are getting back, which in turn helps you understand how the choices you make can affect you. A little bit like sticking to standard gases I guess.... also, for some people, the planning process is a part of the package, they enjoy it (or perhaps that's just me ), to take it away would be to spoil some of the fun.

     
  4. DaveS

    DaveS Play nice. Enjoy UKD

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    Quote strings corrected just so that it reads as I think it was meant to. Content not change :)
     
  5. Gareth Burrows

    Gareth Burrows Super Moderator
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    I don't have gas monkeys either in my shed or in my club gas room unfortunately. I've also been on several trips in the UK where we've had to do it ourselves. This one isn't the end of the world, but it's definitely easier if you know all the figures without having to look them up. If course, you could achieve this with any gas you dive regularly.

    Take a person who has an AL7 and uses 80% as his deco gas most of the time. Now he wants to dive 50%. He or she will find that the numbers don't add up. Now he moves up to an AL80 and everything is hunky dory. As you say - at this point it's irrelevant you can just dive an AL80 for everything and you know the bottle is big enough. But is that really scalable? I wouldn't want to swim around with 3 AL80s. Personally, on a dive like you did with GLOC last year (The southwold you refer to later) I would be using 3 bottles. I know that at 70 metres (I think thats the Southwold depth?) An AL80 is perfect for the bottom gas, an AL7 for the 50% and an AL40 for the O2. The numbers just add up. If suddenly decided i wanted to breathe a bottom stage, a deep deco gas, and 80%, I honestly have no idea what size bottles would be appropriate, I'd have to go away and work it all out - and that's my point. Now I'm not saying that's wrong, but I personally believe my approach is easier for me.

     
  6. bottlefish

    bottlefish Super dooper member
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    At least you have a shed!! I have to go to the lock up if I want some cave time :(

    Seriously, thanks for the thought out reply (note to Mods, we really could do with a thanks button....), and yep, reckon your edit/quote left my post intact..

    One small correction in your reply, Gloc and I sadly never made that dive on the Southwold, the weather gods conspired against (I couldn't make it on the subsequent trip)
     

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