+- MNScuba.com (http://www.mnscuba.com/forum)
+--- Thread: Redundancy question (/showthread.php?tid=623)
Redundancy question - Freedive WI - 05-01-2003
This is NOT meant to be another rebreather vs. DIR question so please don't take it as such.
What, or should I say how, do you plan you redundancy for a trimix rebreather dive- especially when wreck diving?
I know that you have triple back-ups on your sensors and a single back up on your hand-sets.But, what I want to know about is bail out gases and deco gases.
What do you carry and how much? What do you do in a catastrophic failure- like tearing an inhalation hose on a wreck penetration dive?
Also, how are some of you backing up your V3's?
The idea of having a ton of warm gas to breath with minimal deco is a nice thought, especially when you can run it all through a computer to gain maximum benefit on your deco profile. I've read about some nice deep dives in cayamn with long times at various depths and then decoing out on a reef. It sounds really nice.
That doesn't mean that I will dive this way because of my previous experience with dive gear malfunctions. ( I just had two spereate dive watches fail on me while freediving a month ago >
) I just like to read about people who really do it plan out their safety.
I have seen the navy manuals wher they buddy breath on Mk15's. Do you practice that kind of gas sharing on the Inspirations??
Jon
Re:Redundancy question - Inspirationdiver - 05-01-2003
In my opinion, one of the biggest advantages the Inspiration has is its many options for bailout. If the electronics were to completely fail or if the scrubber breaks through, you can still stay on the loop and use a semi-closed technique. This is easily done by exhausting all the gas out of the loop, and then injecting new gas, repeat this every 4-6 breaths. By doing this you can still effectively conserve 4-6 times the gas you would have used on OC.
I do however plan my bailout gases for the worst case scenario. I plan on losing the loop at the end of the bottom time. This is the point where the most decompression would be required. I always bring enough gas to be able to complete all the deco stops on OC. For most dives up to 350 ft a pair of steel 45's will usually cover all needs. You can imagine how much less weight this is when compared to the gas requirements on OC. I also don't even plan on using any of this gas, that means I can carry the same two stages without having to fill them, compare that with how much deco gas you go through on OC.
Re:Redundancy question - Freedive WI - 05-01-2003
That answers some of the questions.
What do oyu do if you loose a hose and what do you do if your computer shuts down? For the VR3 users.
Jon
Re:Redundancy question - john j - 05-01-2003
The worst case failure would be something that causes the rebreather to be completely useless when the maximum decompression obligation is present. A failure of that magnitude would be something like a total flood of the system or if the scrubber chemical completely failed or got bypassed. On the Inspiration, these failures would be rare as long as the user followed proper procedures and did maintenance. I have never had a flood on the Inspiration (so far) but have had 2 floods on the Dolphin rebreather. Both of these were user error and I knew I had a flood right away on the way down so there was no deco obligation at all. Electronics failures, oxygen cell failures, battery failures, a partial system flood, etc. would not necessarly require the user to abort use of the rebreather entirely so that's why I say that the worst case failure is something like a total flood where you have really no other option but to go completely to OC bailout. The system is pretty tolerant of water so it would take a major failure to totally flood it.
On a typical trimix dive that I do, I carry a side cylinder of bottom mix and one of air. Typically a 40ft3 or 80ft3 cylinder of each, but soemtimes more than that. If the worst case failure happened I would switch to OC bottom mix, alert my buddy and then we would both follow the best route back to the surface and switch to OC air at 150ft, of course doing deco stops as required along the way. When I switch to bailout, I consider my first obligation is to hook up with my buddy so I can get his gas if I need it on the way back. I also have up to 25ft3 of oxygen in the rebreather and up to 25ft3 additional on board diluent bottom mix too that I can breathe from OC. The VR3 allows you to switch to OC bailout and tells you what stops to make on the various gases. This can all be planned in advance so you can calculate gas requirements plus add 1/3 extra as usual and choose your tanks accordingly. If the dive is very deep or very long, you can image that the bailout requirements get quite high. In this case, we have either brought larger cylinders, staged cylinders along the return path, used a team approach where each buddy team may have to swap OC stage bottles if someone has a catistrophic failure, or we may employ all of these techniques. Depends on the kind of dive we are doing. The "team" approach to bailout gas is going to get some real flaming by some members on this group I'm sure, and I can understand that, but it is reality and it is a personal choice. In its defense, I can tell you there are some very experienced and well known divers using that technique.
I don't currently carry a spare VR3, but I do have a spare bottom timer and depth gauge of course. I have had 2 VR3 failures and in those cases I just followed my buddies VR3 deco becasue we had all the same gases and same profile. I sometimes carry spare tables or if I have done a certain dive a few times, I know what the stops are and would write them down or do them from memory in the worst case.
I have practiced CCR buddy breathing too and it can be done. It is a pretty advanced skill though and I wouldn't use it unless all other options were gone. It wouldn't be so bad if you had a deco line or a ledge to hold on to but if you were in mid water, buoyancy control while doing that would be really hard.
Re:Redundancy question - igotoofar - 05-01-2003
Great post John, I think the idea of team diving would work well as long as all members respect the team. :-*
Re:Redundancy question - Freedive WI - 05-01-2003
Thanks for the replies so far John and Inspiration.
My worst case would be penetrating a wreck and cutting a hose on something sharp inside which would negate using any of the rebreathers characterisitcs- like semi closed mode.
I am all for a team apporach to problem solving and am glad to hear that you use some form of it.
You mentioned that you have 25 cuft of bottom mix. Is that enough to get you through your deep stops and up to your air bottle (150') or do you carry a third stage on deep dives- 02, air, bottom mix? Let say diving the Gunilda or the Kamloops.
You mentioned snagging gas form your buddy. Can you plug your rebreather into his bottles? Or, are you just talking about grabbing his stage?
Thanks again for the replies.
Jon
Re:Redundancy question - DRE - 05-01-2003
One more question: why do you use air as a bailout, instead of a 32% or 50%, which would be more advantageous in terms of deco - what's the reasoning behind choosing air?
Re:Redundancy question - igotoofar - 05-01-2003
Not being a CCR diver, I would think the MOD would be the determining factor. EAN32 could only be used at 132ft or above. To not carry air would leave you with a non-breathable gas if below that! If you are below the Air Depth of 218ft you would then have to have something different than Air.
Re:Redundancy question - john j - 05-01-2003
Yes-if you cut a hose bad enough, you would probably not be able to use the rebreather at all and you might start having a bad day. I have hose protectors on my unit which are just ballistic grade nylon covers to help protect the hoses. The Halcyon rebreather as another example has some really tough material on it's hoses for this purpose, but I'm told that if it rubs on your ears it can actually scrape them so they bleed, ouch.
Sorry, I was a little vague in my description of where the gas is. Inside the shell of the rebreather I have a 25ft3 bottle of oxygen and a 25ft3 bottle of bottom mix. In rebreather jargon, we call these the "on-board" gasses meaning that they are mounted inside the rebreather. In addition to that, I would carry a 40-80ft3 bottle of bottom mix and a 40-80ft3 bottle of air, each side mounted using the standard d-rings and clips like everyone else does. All of the tanks I listed above, both the on-borad and side mount tanks have "whips" attatched. These whips are hoses of whatever length you want with a quick connect fitting that plugs into the counterlungs of the rebreather. I can plug in any two tanks I want to at any given time. Oxygen from the on-board cylinder is always plugged in on the right counterlung and the left counterlung will have either on-board diluent or diluent from a stage bottle plugged in (we would call that "off-board diluent". Some people dive with the on-board diluent plugged in and others dive with the off-board diluent plugged in. The advantage to running from the off-board diluent cylinder is that if you have to give it to your buddy, you can then plug in to your on-board diluent and you will have a full 25ft3 cylinder to use. All of my stage bottles have both standard OC regulators and whips on them so I can breathe OC off them or plug them into my rebreather. If I pass one of my stage bottles to my buddy, he can plug his rebreather into the whip or he can breathe off it using the OC regulator if he needs to. This system of whips is really flexible becasue we can all plug into each other's bottles, or into bottles that we have stashed along the route or we could even have whips hanging from a boat if we wanted to. Stage bottles can be mounted on any side you want or all on one side if that's your gig. I don't normally use a scooter so I mount one bottle on each side, which balances me out nicely.
For the Gunilda for example, we might each carry a 40ft3 bottle of bottom mix and one of air and have an 80ft3 bottle of oxygen on the line at 20ft. Could even bring an extra tank of bottm mix down and clip it to the line at the bottom and leave it there until the last divers come up. Lots of things you can do. Some people carry 2 different trimixes-one in the on-board tank and a different mix in the off-board tank. I was trained like that but don't see a strong advantage so I will be carrying the same mix in both tanks this year. Some people like to carry nitrox 36 or 50 in the other off board tank. Calculations I did convinced me that overall, the best way to go is air in that tank for dives in the 200-450ft range. You could probably recite specific reasons why it might be better to use EAN 36, or EAN 50 or something else but I'm saying that overall, when I considered everything, in my opinion air was the best compromise. Actually if you look at it, for wreck dives in general that I do (maybe not true for caves or long wreck penetrations) you don't need a huge amount of bottom mix bailout because the deep stops are pretty short and you will soon be at a depth where you can safely breathe air. Then of course your stops get progressively longer so the gas you need the most of is the air or EAN 36, EAN 50, whatever you choose to have in the other stage bottle, plus some oxygen too.
The total system failure is really what I am most concerned about. The other failures like electronics failures, cell failures, etc which people might initially think are really bad actually are not a major problem if you practice and follow your training. You would still abort the dive but unless you totally flood the unit, you have hours of gas available, even if you lose one or two tanks of gas for some reason.
How are you using team appraches to gas sharing in you diving?
Re:Redundancy question - john j - 05-01-2003
This is great, some nice calm debate and discussion. I hope we can keep this going.
This is a good question and I've thought a lot about it. I haven't done the analysis for OC deco but I have for CCR bailout/diluent choices. I ran a bunch of dive plans in the 150-450ft range for various bottom times in the 15-30 minute range, then plotted the total deco times as a function of what gas I would use for the second bailout/diluent gas. I plotted FO2 for the second bailout/diluent gas on the X axis and total deco time on the Y axis. It turns out to be a "u" shaped curve where both high FO2's and low FO2's for the second bailout/diluent gas cause increased deco times. The first bailout gas of course is the bottom mix so you stay on that until you reach the mod for the second bailout gas and then you switch. If you use an oxygen rich gas for second bailout, the MOD is shallow so you have to stay on the helium mix longer, so deco time increases. If you use an oxygen lean gas like air for the second bailout gas, the total deco time begins to increase again becasue the N2 gradient is less during your deco. According to the data, the lowest point of the curve was around EAN 34 or so, but when compared to air, it only gave a few minutes of reduced deco even for a long deep dive. Since air is easily available and cheap, plus you can breathe it deeper than any of the nitrox mixes I concluded that overall, air was the best compromise for the second bailout/diluent gas. I don't know how this would work out for a fully OC dive. It seems strange and I didn't believe it at first either so I checked the data again and it is correct. Again, it is a compromise so if you oly look at certain aspects of it you could easily argue against it. I am looking at the big picture here.