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Thread: Gas Planning

  1. #1
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    Default Gas Planning

    The Ginnie fatality thread and a few conversations with others got me thinking. We talk a lot about conservative planning however we don't go into details much. I figured I'd post how we plan and execute dives, I hope others do as well. This post will basically be about the dives we did on the trip we just got home from in NFL. All dives were in Peacock Springs State Park.

    Day 1: We had been out of the caves for a few months so we decided to do a warm up dive at Orange Grove. While we were gearing up we talked to a couple of friends that mentioned that the distance tunnel was exceptionally clear. We are comfortable doing multiple jumps on a single dive so doing a single jump, to the distance tunnel which we have done a lot, was well within our comfort zone of a warm up dive.

    We dive thirds, which does not mean that we turn the dive at our thirds. Getting all the way to thirds just simply is not something that we do. On this dive we started the dive @ 3700 psi however we had a real jam when we were tying off which took us a while to sort out. When we entered the cave we were down to 3300 psi. We made the jump and enjoyed the distance tunnel then turned the dive at our normal 2600 psi. Normally we would have probably allowed our gas to get down to 2400 psi since we started the dive, in the cave at 3300 psi however it was the first dive of the trip and we were still getting ourselves sorted out. Better off having more gas to exit.

    Day 2: P3 to the spring tunnel. Another dive that we do a lot. We know this area very well, just like the Spring Tunnel. We decided to stage this dive to have more time in the Spring tunnel. This is where we apply conservative gas management. P3 is normally a slight siphon until you hit the spring tunnel. We dive our stages to 1/3's however we shut down the stage @ 2200 psi in P3. We look at the time that it took to dive the stage, which was 20 minutes, we then swim the stage another 20 minutes in before we drop it.

    Here is the reason that we do this. If the stage is dropped when 1/3's is reached then there is 2X the gas to get out on the stage. There is a stage for each diver and each diver is going to maintain more than 2X the gas they use on their back gas, so there would be more than 4X the gas needed to get out. Look at dive incidents and how many of them had a stage 50 to a couple of hundred feet away or the exit within a couple of hundred feet. Better to carry the stage further in and be able to get to it if SHTF.

    With a double stage it is simple to manage with time. If you breath the 2nd stage for 20 minutes then you simply drop it 20 minutes after you drop the first stage. Past 2 stages it is easy to jot down the time on a slate and keep it straight, especially if you are doing varied depth on a particular dive.

    We also try to drop stages past any type of restriction instead of before. With P3 & the Spring Tunnel there is a nice rock just after the bedding plane that is a good place to drop the stage even though it is a few minutes early the rest of the Spring Tunnel doesn't have a lot of good places to drop.

    Once on back gas we started @ 3700 psi and turned the dive @2700 psi. Once back to the stage we go onto the stage and preserve our back gas as much as possible. If things go wrong it is easy to leave an empty & buoyant stage, it can always be picked up later when the issue has been sorted out. Back gas helps maintain buoyancy control and is easier to swim once a stage is dropped that is empty.

    Day 3: We decided to go back to Orange Grove and do a single stage to Woody's room. X had not been there before and it had been at least 15 years since I had. OG goes from an outflow cave to a siphon so we again shut down our stages 200 psi early. We carried them just before the jump to Woody's Room. Again there is a nice rock that is easy for other divers to avoid the stages. The jump to Woody's doesn't have a good place to leave the stage and there would be a fairly long swim before there is a good place to drop. Since this dive was new to X and, for the most part, new to me since it had been so long since I had done it. We turned our dive @2700 psi. We were very slow and careful going in to get a good orientation after the jump.

    When we got back to our last cookie we took the time to orient ourselves to the other line leading into the passage and a third line on how it was run. We were still above our third's so we had plenty of gas to do this and "theoretically" had enough gas in our stage to exit without our back gas. We were also within 50 feet of Challenge which we had a last resort option if anything really went wrong.

    We almost always exit well above our last third, normally we have 1500-1800 psi left in our back gas without a stage. With a stage we often won't get below or first third. We also have had a few dives that we have dipped into our last third. Sometimes we do a dive that gets a little side ways and it takes time to sort it out. We often dive low viz systems or places that may be clear on the way in that will be a mess on the way out. Just the exhaust bubbles sometimes cause silt on the ceilings to rain down and drop the viz to near zero on the way out.

    We have situations like this where one or the other has forgotten to switch to the other side of the line causing us to swim into the rock and it takes time, when using touch contact, to back up and get it sorted out. We have also been in normally clear systems when someone else has has had a bad day and the viz drops to nill. For us the caves aren't going anywhere and we can always take more gas if we want to get a little further.

    One thing that we always do is look at how much gas we have left when we end the dive. By that I mean exit the cave. We often swim around in OW and have fun at the end of a dive. We like to ascend supper slow even when we don't have deco so blowing off gas after getting to OW is no problem because we have plenty by the time we get there. When we have a dive that gets close to or past our last third we spend a lot of time sorting out how it happened and adjust our planning accordingly moving forward.

    We never look at being well above our last third as an opportunity to push it more. When that happens it is always because the dive went off without a hitch and nothing happened to cause us to use more gas. The closer we are to our second third the happier we are. There are a few systems that we dive 1/6's simply because it could get ugly if something went wrong during the dive. There is a siphon or two that I know that 1/8's is about right without being too conservative.

    Bobby

    The Light Dude
    Innovation through exploration

    Local Zip Code Diver

  2. #2
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    Ironically enough, I agree with you on the topic of too many people dying close to a stage bottle. I strongly disagree with diving 1/3rds on the stage. 1/2+200 is IMO the way to go-- keep the extra with you in your back gas at all times.

    -James Garrett
    http://www.jamesg.net
    Quote Originally Posted by Sl├╝dge View Post
    ...AL...he's just about worthless for anything other than giving you extra gas.

  3. #3

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    Quote Originally Posted by jj1987 View Post
    Ironically enough, I agree with you on the topic of too many people dying close to a stage bottle. I strongly disagree with diving 1/3rds on the stage. 1/2+200 is IMO the way to go-- keep the extra with you in your back gas at all times.
    depends on what you're doing with the stages and backgas. It's one thing if you're diving with the backgas as your reserve on a DPV if you aren't planning on poking around a lot without stages, it's another if you are planning on using the stages to get somewhere, then use the backgas to poke around after you've dropped the stages. Two different strategies for two different goals.
    Trying to create an easy rule for all situations with gas planning doesn't make sense to me


  4. #4

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    I'm with Bobby and Tom on this one.

    The short version is that diving stages to 1/2 +200 and keeping all the reserve in your back gas works fine, provided that you are going to actually able to carry the stages until they are down to 1700 psi on your way in. If it is not feasible to do that, then you are in effect leaving penetration gas in the stages, and are needlessly limiting your penetration, or you are carrying a stage some place where it isn't a good idea.

    In some cases diving the stages to thirds and diving the back gas to thirds as well just works better, and provides for a more efficient use of your gas without compromising your reserve.

    -----

    I'm also in agreement with Bobby that diving to "thirds" should seldom mean actually diving to thirds - for two separate reasons.

    First, the concept of Rule of Thirds gas planning was developed with a team of three in mind.

    With a team of three, if one diver suffers total gas loss at the point he reached thirds, the reserve gas carried by the other two divers is twice what the out of air diver used to get to that point. In contrast, with a team of two in the same scenario, the other team member's reserve gas is equal to what the out of air diver used to get to that point - and it will not be sufficient to get the diver out, given the increased stress and slower exit while sharing gas.

    If you're diving "thirds" in a team of two, you are kidding yourself if you think both of you will survive total gas loss near your turn point.

    Second, I recognize that some places will result in an exit that is potentially slower than your entrance. Woody's Room is a good example as the passage leading to it is low, and unless someone has been there recently, you may have to scrape the silt on the floor to get in there, and you'll have zero viz or very low viz for a fair bit on the way out.

    I almost always dive 1200 psi "thirds" even if I have a 3900 psi fill, just because what ever I have over 3600 psi creates additional reserve gas. I then seldom use that full 1200 psi third, increasing the turn pressure based on the dive conditions and the possible increased exit requirements. For example, some restrictions are easier to negotiate in one direction than another. Consequently, if I am not very familiar with and confident that a restriction will not pose additional delay on the way out, I'll pad the reserve by increasing the turn pressure. Same with silt, and/or complex navigation.

    The end result is that I'll usually end a dive to "thirds" with 1500 to 1700 psi remaining. With a 3900 psi fill, 1500 to 1700 psi remaining is 38% to 44% of the total gas in reserve, and with a 3600 psi fill it's 42 to 47% in reserve - very close to "quarters".

    If I need more penetration distance, I'll bring a stage, or two.

    NACD Cave DPV Cert # 666: Cave DPV Anti-christ

  5. #5
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    Quote Originally Posted by Bobby View Post

    Here is the reason that we do this. If the stage is dropped when 1/3's is reached then there is 2X the gas to get out on the stage. There is a stage for each diver and each diver is going to maintain more than 2X the gas they use on their back gas, so there would be more than 4X the gas needed to get out. Look at dive incidents and how many of them had a stage 50 to a couple of hundred feet away or the exit within a couple of hundred feet. Better to carry the stage further in and be able to get to it if SHTF.
    This way, you have the same amount of gas you'd have available if you dived 1/2+200, but less of the gas is shareable. Unless there is a specific reason, I dive the stage to halfs. If I have to drop it earlier due to cave configuration, then so be it, but I will reserve enough gas in the backgas for exit with a lost stage either way. This becomes a lot more interesting if the stage you might lose is also a deco stage, but that's why we carry very large amounts of gas on what most would consider short dives (also a team of 2).

    As was stated above this post, thirds is not for teams of two. Also, if there is a lot of percolation, we increase the limit further. This has meant that on a 3500 psi fill, we have had dives where we used only about 750psi in the backgas, and 600 in the stage (on the way in). As you noticed yourself, just bad visibility has lead to situations where you exited below your last third. Given that you turned before even reaching true thirds, there would not have been enough gas in your team to get out from max penetration if gas sharing (of course, there is a limit to how many things can be expected to go wrong, so I might be judging a bit too harshly). I assume you're aware of this, given your last paragraph.

    Factors that influence how much gas I take (all of these increase the reserve):
    - team size (2: fourths, 3: close to thirds)
    - visibility
    - percolation
    - restrictions (even minor, they will take time to navigate in an air-share scenario)
    - flow (makes everything more complicated, even going out, although our caves are low-flow overall)
    - depth (a minute of problem solving takes a lot more gas at 70m)

    There isn't a one-size-fits-all approach that we use (or I haven't found one yet), so here is an example of a dive that I already considered fairly aggressive: vertical shaft to ~20m, 10 minute descent to 45m, 5 min at this depth, then another 10 minute slow descent to maximum of 69m.

    Gas used was:
    - D18 (15/55) - 230 Bars
    - Al80 (21/35) - 230 Bars
    - Al80 (50%) - 210 Bars
    - Al40 (O2) - 200 Bars

    We descended on the Al80, and used pretty much exactly halfs on it to get to the end of the 45m. From the D18, given that we were in a team of two, we used 50 Bars (230 - 30 for the stage, divided by 4). Visibility was good, so we kept this plan exactly as discussed; the cave has very little danger of percolation, and the flow is barely noticeable.

    If you look at the decompression obligation, then a loss of deco gas would have made this a fairly aggressive dive plan, nonetheless. There were other teams in the water and a safety diver actually checked on all teams at some point. Residual gas was, after gas breaks etc., 130 bars in the D18, empty 21/35 stage, 110 bars in 50%, 75 bars in O2 (this amount of deco is the max we do for this o2 size).

    Others went for solo dives with an RB, single Al80 and a DPV for further penetration than we did. Clearly, they assume a maximum of loss-of-DPV OR RB, not both, although I would guess that the latter would already make this a very non-trivial emergency.


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    Quote Originally Posted by data2 View Post
    - D18 (15/55) - 230 Bars
    - Al80 (21/35) - 230 Bars
    Do you fill ALs and steels to the same pressure? You overfill the ALs and underfill the steels? Can I ask why?


  7. #7
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    Quote Originally Posted by bent View Post
    Do you fill ALs and steels to the same pressure? You overfill the ALs and underfill the steels? Can I ask why?
    Filling issues, and the steels are at their working pressure. Normally, I keep the ALs at 210 max. I wouldn't recommend 230, and is definitely not conservative


  8. #8
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    The reason that I personally prefer to spread my gas around rather than use 1/2+2 is for failures. I like SM passage and sometimes small passage is rough on gear. I have had failures on my BG from squeezing through small stuff. It is mentally reassuring when you get back to the stage and know you have plenty of gas to get out, even if a BG cylinder is compromised. Nothing wrong with 1/2+2 I personally feel more comfortable having my gas split up so that a single failure doesn't put a lot of pressure on due to the gas in that particular cylinder. Damage or failure can also happen on a stage so being conservative and having stages the furthest into the system to maximize their usefulness is my approach.

    Bobby

    The Light Dude
    Innovation through exploration

    Local Zip Code Diver


 

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