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Deco gasses: 80% vs 100%

Discussion in 'General Scuba Diving' started by Wibble, Jul 1, 2018.

  1. Wibble

    Wibble Fish don't talk
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    Currently planning some dives in the Normoxic range, 60ish metres. I generally use two deco gasses, 50% and 80% as they optimise (i.e. minimise) the dive time and keep the CNS levels down.

    I'm curious as to why one would choose 100% as the rich gas as this has a minimal affect on decompression time but at the cost of dramatically increasing the CNS O2 loading and increasing backgas consumption in the case of a 50% failure (can't get on to O2 until 6 metres, whereas with 80% you can get on to it at 9 metres).

    I do use 100% for shallow / short decompression dives, such as 30 to 40 metres with the main benefit of only paying for the actual O2 used, not for a whole cylinder when there's been a small topoff.

    Example for a 65 metre 30 min dive (26 min BT) on 18/45 with 50% and 80%:
    • Dive time = 103mins, CNS = 57%
    With 100%:
    • Dive time = 100mins, CNS = 92%
    This gets considerably larger when considering a bottom stage to extend bottom times and requiring air breaks from the 100%

    So why 100% over 80%
     
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  2. Harvey-NG

    Harvey-NG Member

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    Before GUE divers come along and spoil the fun (only joking, I'm contemplating joining them to be honest) this is an interesting question, which I'm sure many divers will have an answer for - this is my answer and my opinion, please feel free to disagree! Personally I should say, as someone who does light deco dives every now and again, I just use 50% as it's cheap (ish) comparatively, and since I don't dive on any form of mixed gas, there's not much need for me to break out any 100% just yet.

    The difference in deco time for different combinations of deco gases varies a lot depending on the profile. The arguments for using just 50% and 100% is perfectly valid, as is the argument for alternative blends. It really depends on the profile. If 100% reduces your deco time by a negligible amount, don't use it. The only reason deco gases are used is so we don't spend hours decompressing on bottom gas, so if the time benefit isn't there, then don't spend the money. This is why I only use 50%, the hassle and cost of also carrying 100% is not worth the time saved for the short deco dives I do (plus I'm only trained to use up to 80% I must add, so my opinion should probably be taken with a pinch of salt.) Regardless, I don't carry 80% or 100%, 50% works well for what I do.

    This is my view: minimise time spend in decompression without risking any form of decompression illness / sickness, and use the appropriate gases to do so. If 100% is necessary, then use it. If your training specifies that you should only use 50% and 100%, then do so. Otherwise, use whatever gas is appropriate, if 100% is not necessary, then don't use it, it'll be a waste of money. If it doesn't save time, then don't use it. Personally, I blend gases myself in my BSAC club so the cost is less, but if a gas is expensive, the time benefit and cost must be weighed up. Physiologically, and the effects on the way gases behave, there is no reason why 100% should be used over 80%, however for serious decompression there will be a (albeit slight) time benefit to the 100%.

    The choice is yours, 80% is less 'strict' but will slightly increase your deco time although it is cheeper, but 100% is easy to blend, perfect if needed on the boat in an emergency, and will push your decompression time to the minimum, there isn't a gas more suited to decompression, but it's more expensive. There's literally hundreds of arguments both ways, neither gas is a bad option in my opinion.
     
  3. Harvey-NG

    Harvey-NG Member

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    Ok so I've now reread your question, and the benefit of 100% over 80% is that it pushes your deco to the absolute minimum time needed, there is obviously no better gas than 100% O2 (or more to the point 0% N2 and He) for off gassing. For minor decompression dives (all dives are decompression dives but we'll save that for another day) the benefit is only slight, the choice - including myself depends on training limitations and ultimately cost.

    Any knowledge I have about decompression theory beyond 80% O2 blends is theoretical though, so if someone more qualified has an opinion, I'd like to hear it, as I'm sure Wibble would as well.

    (Read deco for divers by Mark Powell, it's the bible for this stuff.)
     
  4. Wibble

    Wibble Fish don't talk
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    GUE opted for standard gasses, configs and protocols as it makes teamworking more effective. I was told during some of my training that it was thought about by people with immensely more diving experience than myself so I didn't need to think about it.

    Getting away from agency-specific constraints, when planning a dive one has to balance a multitude of considerations. For shallower dives which have shorter decompression times, the differences between 80% and 100% aren't huge. Example 60mins at 30m on 32%:
    • No deco gas - dive time = 99mins, CNS=36%
    • 50% - dive time = 91mins, CNS=43%
    • 80% - dive time = 79 mins, CNS=42%
    • 100% - dive time = 79 mins, CNS=52%
    Interestingly the same 30m dive for 60mins on air comes out with the 100% taking longer than the 80%.
    • No deco gas: 157 mins (and using more than a twinset - oops!)
    • 50%: 107 mins
    • 80%: 95 mins
    • 100%: 97 mins
    The challenge of deeper dives is your deco obligation starts much deeper, so getting on to 50% at 21 metres means you can off-gas the helium, and increasing this to 80% or 100% minimises your overall time at the 6 metre stop (which is about half of your overall deco time)

    My question is still why would one use 100% for a deep dive where this trashes your lungs (hence the need for "air breaks") and pushes the CNS (Central Nervous System O2 loading) through the daily limits? This is especially the case for an "advanced" trimix level dive where the overall decompression time will easily exceed 2 hours.



    (I appreciate the answer is use a rebreather, but it's interesting to learn about dive planning on the safety of the surface. Also this applies to rebreather dive planning)
     
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  5. Vanny

    Vanny Active Member

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    Can’t think of a reason to use 100% for all the reason stated. Maybe if you didn’t have the availability to mix an 80% or whatever you could just decant knowing the mix would be 100%

    Re ccr it is a consideration for bailout , a positive is it gives a separate alternative supply of O2 to drive the unit in the event of the primary supply failing , but equally 80% is viable for that. After that the restrictions around CNS and MOD become a negative. Personally I have access to a deep gas and a 70%.
     
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  6. Alex Denny

    Alex Denny Active Member

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    80% also has a greater margin of error if your buoyancy is off and you drop a little too deep on the stop...
     
  7. JasonP

    JasonP Active Member

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  8. jb2cool

    jb2cool Moderator
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    George is a little shall we say polarising and strong willed in his views so take that with a pinch of salt. If you agree with it then by all means ditch the 80% and use O2 instead but there will certainly be those who disagree with a lot of what he says.
     
  9. splinter

    splinter Active Member

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    I'm sure he's got some very good points, but his ranting puts me off listening to anything he has to say.

    Sent from my SM-G900F using Tapatalk
     
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  10. Wibble

    Wibble Fish don't talk
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    For what it's worth, I did a 35m dive with most around 34m on 32% with 60 mins on the bottom and ~20 mins of deco on 100% at 6m (Frognor - a truly excellent dive). Was the first time I've seen my Shearwater ding a CNS warning. 32% is a bit rich for that depth.

    Filling the 100% isn't easy if you don't have access to a booster. So probably 80% is easier to get a full fill.

    I've also had mixed success with "only paying for the O2 I've used" when filling 100%.
     
  11. Dave Whitlow

    Dave Whitlow Well-Known Member

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    Whilst I'd not put it in the same eloquent terms, and would strongly disagree that 'real diving' involves caves, I do agree 80% is a mix few benefits and a number of disadvantages and is a mix I have never used in any of my cylinders and one I never plan to use.
     
  12. Nick Ward

    Nick Ward Active Member

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    I guess an interesting follow on question, is why some agencies limit max O2 to 80%?
     
  13. JasonP

    JasonP Active Member

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    Because they don't trust their divers to have sufficient control of their buoyancy.

    However, do any of them still do this?
     
  14. Wibble

    Wibble Fish don't talk
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    CNS?
     
  15. Nick Ward

    Nick Ward Active Member

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    and yet, min Gold standard on the B&TW is mandatory (+/- 0.5m for non BSACers)
     
  16. Tribal Chestnut

    Tribal Chestnut Well-Known Member
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    BSAC ADP I believe.
     

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