Disabled deco gas on backup computer.

[rjack321]

I've always planned on sharing deco gas if mine or my buddy's were to fail or be inaccessible. Has happened once actually, an LP hose oring failed on a buddy's O2 bottle. I did most of my O2 time (about 66%), then he did most of his. Net we both did enough deco since the 66% on backgas was 2x longer than the remaining O2 time.

Planning to compensate for a failed deco gas with only backgas doesnt get you far.

 

[Zi55ou]

Planning to compensate for a failed deco gas with only backgas doesnt get you far.

Which is why we plan for being able to complete the dive using only backgas and without access to a buddy's deco bottle. Obviously the scenario you describe is much less unpleasant than having to do masses of air deco potentially on your own which is why we are using our second computer to decide, while everything is rosy, if we are comfortable doing that amount of air deco.

Perhaps I'm not getting your point but isn't the logic to plan for two failures in the system?

 

[CptTightPants21]

Which is why we plan for being able to complete the dive using only backgas and without access to a buddy's deco bottle. Obviously the scenario you describe is much less unpleasant than having to do masses of air deco potentially on your own which is why we are using our second computer to decide, while everything is rosy, if we are comfortable doing that amount of air deco.

Perhaps I'm not getting your point but isn't the logic to plan for two failures in the system?

There really is no need to make it as complicated as deliberately leaving a gas unset on your computer.

Pre dive planning checks should account for no deco gas contingency. But for one deco gas dives, simply diving and leaving a third of gas as reserve will cover any lost deco gas contingency. Another rule of thumb is is to take your current deco time and multiply it by 2.4 to get an estimate of what the back gas deco time will be--it depends on the profile and back gas, but actual deco time will be around 2.4 or a little less around 2.1/2.2. If you don't like either of those two methods, you can always go into your computer and deactive the deco gas mid dive to see what the accumulated deco gas will be. Redundant systems such as two dive computers are supposed to match and verify one another. I don't think it is a good practice to deliberately set them to different parameters and have them showing different values, especially when the information you are trying to gather has several easy calculation work arounds.

 

[rjack321]

Which is why we plan for being able to complete the dive using only backgas and without access to a buddy's deco bottle. Obviously the scenario you describe is much less unpleasant than having to do masses of air deco potentially on your own which is why we are using our second computer to decide, while everything is rosy, if we are comfortable doing that amount of air deco.

Perhaps I'm not getting your point but isn't the logic to plan for two failures in the system?

No the logic is to dive with a buddy who carries enough deco gas to share.

In cold water, drifting in a current for at least twice as long as planned, or for instance in a cave with 50mins of O2 time you are going to be in a world of hurt trying to complete the dive on air deco.

I dont know anyone planning for 2 "major" failures. Like 2 lost deco gasses or OOA plus a lost deco gas

 

[JohnnyC]

Oopsies backgas deco sucks. Hedge your bets and sling a bottle of 50 as well as your O2. Losing 2 deco gases is pretty statistically unlikely. If you're doing short enough dives where backgas deco volume is enough, you're gonna be fine carrying a 40 of both 50 and O2, so it's not like it's a burden.

It also hedges your bets against any buddy issues. Team bailout sucks, especially in open water. Lose your buddy and your only deco gas and you're drifting under a sausage having to do backgas deco isn't an ideal situation to be in.

 

[doctormike]

Maybe I'm missing something, but I'm concerned about the underlying premise of the OP's question. It's not that common to realize in the course of a dive that you have lost your deco gas. That might be the scenario where you are staging your deco bottles. But in a cave dive, you should be able to find them on the way out, and in the ocean that's not such a great idea (as in "The Last Dive").

So I'm wondering why you would continue a dive once you had realized that you no longer had the capability to complete you dive plan and now were in contingency mode? Personally, if that happened to me (say with a blown LP hose on my deco bottle), I would just end the dive at that point.

Yeah, yeah - two failures. We don't plan for that. But I wouldn't spend time figuring out how much deco I'm "comfortable" doing now that I no longer have a contingency option, just to make my dive a bit longer. I would minimize my deco obligation and just get out of the water as soon as reasonably possible.

 

[rjack321]

My understanding was that the OP was keeping the backup set on air only and not exceeding the deco time he could comfortably complete the dive without the deco gas. Then used the primary computer and the deco gas once he got to 30ft and could switch to the working bottle. So the deco gas was just a "bonus"

 

[doctormike]

My understanding was that the OP was keeping the backup set on air only and not exceeding the deco time he could comfortably complete the dive without the deco gas. Then used the primary computer and the deco gas once he got to 30ft and could switch to the working bottle. So the deco gas was just a "bonus"

Right, I understand that. We all do lost gas scenario, which means that we can complete all required deco assuming that we have to deco on our backgas at the end of the dive, with the maximum inert gas loading of the planned bottom time. My point is that lost gas is a contingency for when you ascend fully loaded and THEN find out that you don't have the option of accelerated deco. It's less than ideal, and it involves an increased risk if anything happens to your back gas (yes, I know, two failures).

But the OPs scenario was losing your deco gas during the dive, and then making the decision to KEEP on loading inert gas. So you are voluntarily increasing your deco obligation while already in a failure mode and working on contingency. I wouldn't do that, no matter how comfortable I was doing a longer deco on my back gas. There is no reason not to end the dive at that point.

If you were in a cave with two lights, and one failed, would you continue on with one, to the point of maximum penetration that you felt comfortable doing with only one light, or would you turn the dive and exit? How is this different?

 

[CptTightPants21]

No the logic is to dive with a buddy who carries enough deco gas to share.

In cold water, drifting in a current for at least twice as long as planned, or for instance in a cave with 50mins of O2 time you are going to be in a world of hurt trying to complete the dive on air deco.

I dont know anyone planning for 2 "major" failures. Like 2 lost deco gasses or OOA plus a lost deco gas

Just to clarify, in a buddy dive there is a contingency against two major failures--failed manifold and lost deco, or two lost deco, etc and those care covered by buddy's deco gas, your 2nd deco gas if dive requires it, and the 3rd of backgas/min gas reserve. A solo dive can handle one major failure, maybe 1.5 depending on time and circumstances

Oopsies backgas deco sucks. Hedge your bets and sling a bottle of 50 as well as your O2. Losing 2 deco gases is pretty statistically unlikely. If you're doing short enough dives where backgas deco volume is enough, you're gonna be fine carrying a 40 of both 50 and O2, so it's not like it's a burden.

It also hedges your bets against any buddy issues. Team bailout sucks, especially in open water. Lose your buddy and your only deco gas and you're drifting under a sausage having to do backgas deco isn't an ideal situation to be in.

Carrying a 2nd deco bottle for a dive that doesn't require it is an added complexity and a waste of space. Doing backgas deco on a dive that only required one deco gas in the first place isn't the end of the world--you are basically are just doubling your deco time. If the dive plan is such that doubling the deco is too much, I would question the dive plan and see if there shouldn't be a 2nd deco gas included already. The space taken by a completely unused unnecessary 2nd deco bottle could be used for a bottom stage or just left blank for decreased drag. A bigger dive that requires two deco gases will have the backup deco mix already included. For dives that only require one deco mix, the back gas reserve is the contingency. At the end of the day it is a personal choice. Look at the dives you are doing and look at the ones you would like to attempt, play around with the lost deco scenarios--is adding a 2nd bottle worth the effort vs the potential increased backgas deco?

 

[JohnnyC]

Just to clarify, in a buddy dive there is a contingency against two major failures--failed manifold and lost deco, or two lost deco, etc and those care covered by buddy's deco gas, your 2nd deco gas if dive requires it, and the 3rd of backgas/min gas reserve. A solo dive can handle one major failure, maybe 1.5 depending on time and circumstances



Carrying a 2nd deco bottle for a dive that doesn't require it is an added complexity and a waste of space. Doing backgas deco on a dive that only required one deco gas in the first place isn't the end of the world--you are basically are just doubling your deco time. If the dive plan is such that doubling the deco is too much, I would question the dive plan and see if there shouldn't be a 2nd deco gas included already. The space taken by a completely unused unnecessary 2nd deco bottle could be used for a bottom stage or just left blank for decreased drag. A bigger dive that requires two deco gases will have the backup deco mix already included. For dives that only require one deco mix, the back gas reserve is the contingency. At the end of the day it is a personal choice. Look at the dives you are doing and look at the ones you would like to attempt, play around with the lost deco scenarios--is adding a 2nd bottle worth the effort vs the potential increased backgas deco?

Carrying another deco gas is not complicated. Really, 2 deco gases is hardly a blip on the radar of complexity nor effort. If a diver is so cavalier about their technical diving that they're willing to ignore planning and peg their runtime to their available backgas deco, 1) they probably shouldn't be doing decompression dives, and 2) might as well carry a backup since losing their deco gas is their big concern rather than actually doing it safely. If a technical diver can't handle two deco bottles they shouldn't be doing technical dives. We tell divers all the time that carrying a redundant source of gas is a good idea. In this case, they're carrying a redundant source of deco gas.