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Dive tables for accelerated decompression

Discussion in 'Advanced Scuba Discussions' started by 60plus, Sep 3, 2019.

  1. 60plus

    60plus Barracuda

    # of Dives: 25 - 49
    Location: Cumbria UK
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    Oh dear me, more flak. Some of you read far too much into things. Firstly I never said that diving sea caves without qualifications is safe - it is not. It is however the case that in the instance of that particular cave I was prepared to accept the level of risk that some consider unacceptable.
    Regarding decompression diving I also know that you should be trained and build up to it gradually, not just leap off the boat with two tanks and hope for the best. [Edited by Moderator]

    To get back to diving. There will be times when I want to go a bit deeper or stay down a bit longer than I can on a single tank. Several members of my club have chosen to do this by using CCR. However my interest in diving is such that I do not want to spend that amount on training or diving equipment. A basic decompression / technical diving course is not that expensive but the equipment is not available at many of the locations I dive.
    I have seen a technical diving instructor dive as I have described earlier in this thread, an air tank on his back and nitrox hung on his D rings. Not only does he dive like this he dives solo like this at times. I do not know the % nitrox he was using but it would be less than 40%. I only spoke to him briefly and he said his reason for doing it was convenient when trimix was not available. To me this did not seem unreasonable and I thought it may enable me to have worthwhile dives to the 40m + wrecks at Scapa flow for example. This instructor was the only time I have come across anyone doing it, but it aroused my interest and that is why I posted to see if tables were available so I could assess the dive / bottom time advantage before pursuing it further.
    More generally if people only did what they had learned from others we would still be living in the stone age or possibly not that advanced.
     
  2. kafkaland

    kafkaland Manta Ray

    # of Dives: 200 - 499
    Location: Saline, Michigan
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    I didn’t realize that this is the same guy until you pointed it out. Kudos to your good memory, Marie. And as to the OP, I think we’re well into trolling territory. He couldn't get away with advocating cave diving without proper training, and now he’s trying the same with deco diving.
     
    Soloist, rjack321 and Marie13 like this.
  3. mac64

    mac64 Barracuda

    # of Dives: 5,000 - ∞
    Location: Ireland
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    You can do it, you’ll gain redundancy from the 12ltr of 38%O2 at a cost of high PPO2. For 30min bottom time at 45meters the 38% O2 will get you out of the water 6min earlier than if to did the dive on air. You’ll be changing gas at 26 meters, the 15 ltr will be empty but the 12 will have plenty. A terrible dive plan with serious risks. If for any reason the 12 ltr fails you won’t make it back. To mitigate the risk of emptying the 15ltr you would have to use the 12ltr of 38%O2 at 45meters and the PPO2 world be over 2 bar
     
  4. KenGordon

    KenGordon Rebreather Pilot

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    For Scapa you might be better off just diving best mix for each morning dive, although that can be pricy. You’d be looking at deco times about the same as your bottom time. If you only have one deco gas you ought not to be planning dives that can’t be done on backgas in any case as loss of deco gas might be difficult. Backgas deco for these dives is a bit tedious, there is nothing to look at and the shot can get quite busy.

    Yorkshire are running an ADP course at Stoney in November, and Southern at NDAC. These are £160 and £150 respectively, plus B&B etc of course. ADP covers many configurations.

    ADP Stoney Cove 16th November 2019 17371

    Your problem here, on SB, is kind of the reformed smoker attitude. They will have started out being told that deco is the work of the devil, or at least Democrats, and when they get to do it they have to do it in a particular way. Anything that doesn’t conform to that is clearly mad etc...
     
  5. barth

    barth DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Netherlands
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    (We can have a long discussion about what is safe for a dive to 45 meter/150 feet.)

    (I would do the dive with 2 x 12 liter backmount with 21/35 trimix and 1 x 11 liter stage with ean50 for deco. I would have a buddy with the same equipment. Both would at least be gue tech1 trained.)

    (But I do understand that other people make other choices.)

    I did try to make a diveplan for you.

    IMG_7396.PNG IMG_7395.PNG IMG_7394.PNG
     
  6. RayfromTX

    RayfromTX Student Of Gas Mixology Staff Member ScubaBoard Sponsor

    # of Dives: 200 - 499
    Location: Hill Country of Central TX
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    @60plus I'm a new student tech diver taking courses that I would recommend for you for the diving you are talking about doing. A lot of what we are learning is "what will you do when this happens?" It's a little eye opening and I've been hanging with some highly trained and skilled divers for a few years now talking around these subjects. So much so that I'm not really learning much new about deco algorithms, gradient factors, gas planning or mixing. Indeed, I'm learning a little but not a lot about the gear required for these setups. My wife is learning a lot about that stuff. What I'm learning a lot about is contingencies. There are a lot of what ifs when you can't ascend directly to the surface that just don't have to be accounted for when you can surface in an equipment failure or other problem(physiological or health issue) that arises.

    Beyond that there are actual skills and muscle memory things that need to be learned. Things that are easy at the surface or at 30m are different deeper. Things that are obvious sitting around the table can lead to poor choices on the fly underwater. Nobody thinks more clearly with elevated levels of N2 that I have ever heard of. I hope this is being clearly articulated both for you and for others that will read this thread in the future.

    I will end with this note. Diving is actually more dangerous than we may think. Surfacing repeatedly and safely doesn't mean it's not actually quite dangerous. We know lots of people, even some of the best instructors, that get seriously hurt and/or killed doing this activity the right way. They get bent and never fully recover or die later as a result of their injuries. They disappear without a trace. These are people that do things safely and have thousands of dives. Some we learn from and others we never actually know what happened. Training and having the right attitude with regard to risk taking can help a great deal. We have seen members come on the board and act cavalier about training and experience and pushing the envelope and we have watched them die. That is one reason for the dramatic responses that can occur in threads like yours. You should always remember though that even if you decide to do this right and get the training you need first in order to greatly reduce your risk, this is still a far more dangerous activity than many people acknowledge and dramatically more so when we go deeper and carry more gear and incur deco obligations.
     
    chillyinCanada, Lorenzoid and Soloist like this.
  7. Marie13

    Marie13 Great Lakes Mermaid

    # of Dives: 200 - 499
    Location: Great Lakes
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    The OP has no reason to be ignorant on the topic. Two excellent books were written by a fellow Brit, Mark Powell - Deco for Divers and Technical Diving: An Introduction. But if the OP doesn’t like to read...

    I read those two books multiple times and also went through TDI’s Advanced Nitrox and Deco Procedures textbooks last winter, actually doing all the associated homework - math formula work-ugh! - and answering all the questions. Got myself MultiDeco and ran dive plans for the wreck dives I would do. I know enough to be dangerous. Did I act upon this knowledge? No, because doing even minimal deco dives without the training was more risk than I was comfortable with, and also because the local folks I dive with don’t take foolish chances, nor enable others. I’m taking the classes next year.
     
    WinfieldNC and Soloist like this.
  8. KenGordon

    KenGordon Rebreather Pilot

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    OP, since you are being taken to task for ignoring advice by some people, I offer this

    Class details

    Although Loch Long might be better

    Class details

    which is advice ignored by some posters, who have also ignored the advice to get on with things such that might have some personal experience to back up their posts.
     
  9. 60plus

    60plus Barracuda

    # of Dives: 25 - 49
    Location: Cumbria UK
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    Ken, thanks for your positive approach and advice. I am actually away diving in Madeira for 10 days that cover the course dates, however I would not be ready to sign up for a course yet in any case. I have probably already covered the GUE fundamentals course contents as I am qualified to use nitrox and dive to 40m. Post #44. I am not ignorant of decompression procedures and some of the other similar assertions you make are also inaccurate. I talk to other divers and get different responses, this is because what they wish to get out of diving is different. If I was going to do a lot of deep diving the answer would be obvious - get trained and buy the required equipment, but most of my diving is less than 30m with a few dives between 30m and 40m. There are however a few times when I feel I am missing out by not being able to spend a reasonable amount of time in the 40 to 50m range and I am just looking for the most cost effective and acceptably safe way to achieve this. No point enrolling for training only to find that I do not wish to spend the required amount on equipment. Barth post #50 that is exactly the type of information I was looking for when I started this thread - thank you. Someone in an earlier post suggested I would be exposing myself to a PO of 2 by using nitrox 32 at 45m. If at 45m my air failed I would immediately start my ascent, and in a few kicks would be at a depth where nitrox 32 gave a PO of 1.4 or less.
     
  10. kafkaland

    kafkaland Manta Ray

    # of Dives: 200 - 499
    Location: Saline, Michigan
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    No. Just no. I’m not exactly a fan of GUE, but they will teach you way more in fundies than this. Including why diving to 40m without helium in the mix is not necessarily a good idea.
     
    peocro likes this.

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