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Ratio deco and multiple dives

Discussion in 'Technical Diving' started by hawk, Jun 14, 2016.

  1. hawk

    hawk Doing It Rong
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    Let me start this (as any troll would :)) and say that I am not trolling here but am curious .....

    How does ratio deco factor in multiple dives? I think I understand the basic principles and I like the logic of standard gases but I haven't seen how it considers surface intervals and multiple dives.

    By way of background, I understand deco theory, do deco dives and am qualified to do deco dives by a couple of agencies. I tend to plan using Vplanner for my dive planning and run my OSTC2 on 30/85 (which I am sure I have read is the basic assumption used to validate ratio deco??)

    I have recently been involved in diver training (can you guess which agency :)) where maximum dive times have been calculated based on previous dives, surface intervals involving a drive over a mountain and then a subsequent dive at a higher altitude than the first ad it got me wondering ....
     
  2. Zubar

    Zubar Active Member
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    I think you need a small bag of bones and to know the necessary incantations to make it work for multiple dives. :woot:
     
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  3. becky9

    becky9 Diving bore!

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    Lol nah min 1hr, up to 3hrs double shallow stops from 9, 3-4hrs 1.5x - tbh if its an odd kind of time I tend to check with baltic or somat for sanity check, some ppl will double regardless. Its all about patterns. x
     
  4. hawk

    hawk Doing It Rong
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    LOL - post of the week :)

    Thanks Becky - is that based on 32 or does it work for all gases / depths?
     
  5. becky9

    becky9 Diving bore!

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    Its based on mix 21/35, 18/45. It will work on others but you need to look at the numbers - 25/25 eg is fine. If we dive 32 (or 30/30) we generally dive min deco profiles which are a tiny bit different - any time over the mdl we add at 6 - or we do it with 100% - general rule of thumb 1hr at 30 - 10 at 6 on O2 x


    Edit: I should also say based with 50% deco bottle on average 51m depths - doesn't have to be max - just average x
     
  6. hawk

    hawk Doing It Rong
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    Thanks - switching at 21m presumably?
     
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  7. becky9

    becky9 Diving bore!

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    Yes I'm sorry that too :D x
     
  8. becky9

    becky9 Diving bore!

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    I should also say that ascent rates are also bound in (getting out of dodge ;) ), but should you look at the numbers in planning software you should see where it works and where it falls down or needs tweaking x
     
  9. becky9

    becky9 Diving bore!

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    Tbh I like it, I'll try and convince Kiril and JJ next time I see them :D ;) x
     
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  10. hawk

    hawk Doing It Rong
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    I think we can assume that anyone even vaguely interested in this topic would understand the importance of controlled ascents. You gooists work on 3m per min after half depth don't you?
     
  11. Iain Denham

    Iain Denham Active Member

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    Whats the advantage/reason for using ratio deco (from someone who doesn't really know much about it) rather than just putting your dive into say V planner and dive the plan?
     
  12. becky9

    becky9 Diving bore!

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    Not quite, thats a min deco profile. 9m/min till we get to 75% of the deep portion of the ascent -((AvDepth+21)/2 -rounded up to nearest 3m), then we slow to 6m/min till we hit the gas switch at 21. The 75% (80%ATA) used to be where we would do a 'deep stop' but we don't do those anymore. Its variable ascents rates for the win ;) x
     
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  13. becky9

    becky9 Diving bore!

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    Because of the patterns it means they can be adjusted on the fly very easily - and very easily remembered. Theres nothing wrong with doing planned dives, its just that if you can see patterns and they are easily remembered it becomes very flexible - effectively the whole team knows them without us all having to pore over the planning software or doing 'trust me' dives.

    Edit: Putting it simply - its easy - pushing the easy button !! x
     
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  14. Iain Denham

    Iain Denham Active Member

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    Ahh Ok understood.

    If I need to put "proper planning" in place I just use V planner, if the dive changes it I have it on my phone so can change it that way, but I can see the reasoning behind it.

    So I assume then you look for dive profiles and try to find patterns so you can remember them such a your 32,30,30 would that be classed as RD if you were adding a min BT and a min stop time (subject to assent rates) or is it more complicated than that, I am not a trimix diver?
     
  15. becky9

    becky9 Diving bore!

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    Its more that we can actually do it on the fly during a dive.

    Sorry my fault. I think I'm muddying the water a bit by throwing numbers in.

    by way of explanation the 32 and 30/30 are gases - more specifically back gases. The former regular 32% nitrox and the later a recreational trimix hence 30/30. We would generally dive these on what we call Minimum deco. Its basically what many would call no-deco/non stop diving.

    Min Deco

    These would be recreational dives with no mandatory deco stops.
    This gives us a max no deco limit of 30minutes at average depth of 30m, rounding up for deepest.

    We would still do an ascent where we would ascend to half the max depth rounded to nearest 3m at 9/m a minute - then we slow that down to 3m a minute - this normally takes the form of 30secs actually ascending and then a 30secs pause. So every mulitple of 3m we pause for 30sec and then ascend 3m in 30secs. This makes it manageable, and the keeps the team together. We repeat until we surface.


    Now for dives shallower than 30m - for every 3 metres shallower than 30m our average depth is we get an extra 10 minutes playtime (an extra 10 minutes no deco limit). So:

    27m we get 40mins,
    24m we get 50mins
    21m we get 60mins
    18m we get 70mins .... and so on until we get to 9m where our no deco limit is unlimited(!)

    Whilst there are some analogies to Ratio Deco, Minimum Deco isn't what is commonly referred to as Ratio Deco.

    Outside this - whats not normally taught on recreational course is provision to diving the same recreation gases (32%, 30/30) but going beyond the no deco limits.
    What we would do in these cases is to stop at 6m for the amount of time we spend over the deco limit. For example at 30m we know we can stay for 30minutes but what if we stop for 38 minutes? Easy - we are 8 minutes over our deco limit so we can do those at 6m. We would have a deco stop of 8 minutes in addition to the normal min deco ascent. This is utilizing our existing backgas.

    For completeness - I also refered to 100% or O2 above... this is just using accelerated decompression using a stage of O2 at 6m to cut short our stay at 6 - potentially we could do 1 hour at 30m and trade off 30 mins of deco on backgas for 10 minutes of deco on O2. This is purely punching the data into planning software and finding out that it works and planning intermediate dives on that basis of those numbers. I would recommend that you have the correct training in gas switching protocol and buoyancy skillset before using this proceedure - high PO2 can be DEADLY.



    Ratio Deco

    This is what we talked about above, the main thing is knowing how its derived, that its fundamentally observing patterns within existing deco algorithms and building a set of guidelines around them. For a lot of the diving we do these observations have already been made and the guidelines formulated. We then use these as a basis to do our deco, tweaking for personal preference. team preference, conditions etc.....

    For the dives I've done up to now, this is based of average max depth of 51m, single deco gas of 50% and max unadjusted deco of 30minutes Unadjusted comes in for repeat dives - where you need to extend shallow stops or lost deco gas proceedures (very similar really).


    I won't go into the details of how to use these guidelines as Garf has done it previously - but its important to remember they aren't a direct replacement to following a computer. They are not rules, and heaven forbid using them as such. The underlaying understanding of what it is important.

    Sometimes however we find situations where the guidelines aren't quite ideal or we have to use a different gas. In these cases we can then refer back the planning software, tweak some more or build new plans based on what deco we need to do and where we want to do it. If we do this right we should be able to lever our new plan into our existing guideline framework. We can use the software to test our plans to see if they are workable and fit our needs - pretty much in the way most people would normally use planning software with the exception that we would more than likely want to manipulate the plan to fit into how we would do our deco.






    Forgive any mistakes lol. I'll read it back when I've posted :D x
     
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  16. Dave Whitlow

    Dave Whitlow Well-Known Member

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    Isn't that what dive computers are for?

    I'd love to understand why RD is considered better than having a computer on each arm. Can anyone explain?
     
  17. Wibble

    Wibble Fish don't talk
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    Great explanations Becky.
     
  18. becky9

    becky9 Diving bore!

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    People stare at a box for one thing, not all people, but many do. I've seem smart people who are good divers doing utterly bone things with computers. For some people its easy to follow rather than to understand. Comments such as 'its tracking my nitrogen loading over a weeks diving' spring to mind.... smart people with no idea.

    With a team all following the same plan irrespective of what differing computers would be ordinarily telling them in a computer knows best model.

    Some people only use it as a guide - which ok fair enough - but if push comes to shove and you know better why use a computer and ignore it? in that case why bother at all?

    I have no problems with computers, its the lack of understanding and blind ignorance that they promote thats a problem.

    x
     
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