SeaJay
Contributor
Okay, I'm only newly DIR, but I think I can answer your question.
...Or rather, elaborate on the other's correct answer of "DIR teaches a balanced rig."
The original question posed by faye was, "What's the DIR stance on redundant bladders?" (Forgive me for paraphrasing.)
The stance is, "No redundant bladders."
However, the question was later elaborated on, and again I'll paraphrase and say that the question was, "Well, how do they recommend I handle this situation?"
The situation, as best as I can tell from reading this thread, was this:
Salt water, open ocean dive
AL80 doubles rig
Backplate and harness
3 mm wetsuit
Properly weighted and trimmed for the dive
Here's how the situation would unfold, if the diver was diving the above rig:
The short version is that the diver would be properly balanced throughout the dive and buoyancy would never be an issue, even with a complete bladder failure.
The long version works like this (and you don't have to be a chemical engineer to understand this):
Let's assign some numbers to your rig so as to illustrate the way this would happen. These are common numbers that I personally have experienced:
Me: Relatively neutral in salt water
3 mil wetsuit: 8 pounds buoyant in salt water at the surface, 70% of that lost at 100'
Backplate: 6 pounds negative
Tanks: 2.2 pounds negative (each) at the beginning of the dive, 4 pounds positive completely empty
Weight belt: 10 pounds on the belt
Okay, let's say that at the beginning of the dive, I jump in the water and immediately sink to the bottom, uncontrolled. I have a complete and catastrophic bladder failure (unlikely, since it was tested on the boat.) If the problem is that my valves are turned off, then I simply reach back and turn them on. (Taught in DIR-F.) Likely, I can do this before I hit 20 feet. Of course, this would never happen, since I've responsibly tested my gear before getting in the water. However, assuming that it did happen, then let's see how the weight would work out:
Me: Relatively neutral
3 mil wetsuit: At 100', my wetsuit is (30% of 8 pounds) 2.4 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, 4.4 pounds negative
Weight belt: 10 pounds negative
That puts me at 18 pounds negative. I can actually swim this up, buy why? I'd drop my belt and surface 8 pounds negative, at 30 fpm. Interestingly, much of this would be offset with my lungs (never holding breath, but filling them with each breath). Deco would be a nonissue, since I would not need a deco stop. I'd have only been in the water a matter of seconds. We're also assuming that the bladder failure is COMPLETE, and that there is no assistance from the bladder whatsoever. This is highly unlikely. The errant sharp object or swordfish would make it leak a good bit, but it would still hold some air. Even the elbow suddenly coming off would not produce a catastrophic failure... You could simply hold some buoyancy with it by placing your body horizontally, which you should be anyway.
At the surface, in that instance, here's how that would turn out:
Me: Relatively neutral
3 mil wetsuit: 8 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, 4.4 pounds negative
Weight belt: Gone
That puts me at just 2.4 pounds negative, easily swimmable. The difference would be the buoyancy change in the wetsuit. Of course, don't forget that I'd be just 2.4 pounds negative with two full tanks of air on my back. I'm in no danger of drowning. And again, we're assuming that you don't have even one pound of help from the wing.
Of course, this scenario would never happen anyway, since:
1. I tested my gear before I got in the water.
2. I entered the water with a full air cell.
3. If the problem was valve-related, then I'd simply reach back and turn them on.
4. I also have the option of dumping air out of my tanks before surfacing... There is no emergency if I'm on the bottom with two full tanks. I could technically do this as a last resort instead of ditching my weights. (There's as much as 12 pounds ditchable there.)
However, the scenario that I think you're more apprehensive about is the scenario where there is a deco obligation. Here's how that would pan out at 100':
Me: Relatively neutral
3 mil wetsuit: 2.4 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, around 5 pounds positive (remember, they're not totally empty yet)
Weight belt: 10 pounds negative
That places me at 8.6 pounds negative, and I'd swim that up. At 20 feet I plan to do a stop... But now I have this situation:
Me: Relatively neutral
3 mil wetsuit: 4.8 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, around 6 pounds positive (they're getting empty)
Weight belt: 10 pounds negative
That places me at a little over 5 pounds negative, which is very managable, either with just lung power (the average lungs from full to empty can produce as much as 9 pounds of bouyancy) or with simple kicks (not as pretty, but just as effective). I have not bothered to ditch weights. And still, we're assuming NO help from any air cell at all, which again, is unlikely.
At the surface, here's how it would pan out:
Me: Relatively neutral
3 mil wetsuit: 8 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, approaching 8.8 pounds positive
Weight belt: 10 pounds negative
That leaves me almost perfectly neutral... At least within a pound.
Thus, a redundant bladder is not necessary... And since a redundant bladder increases task loading with no benefit, DIR preaches that single bladders are a better choice. Additionally, a redundant bladder may actually give you a false sense of security, since as someone else mentioned, if you have a tear in one, you're likely to have torn the other as well. Two bladders does not mean that one will always work. In fact, in the specific examples that you stated, such as dangerous marine life or an errant sharp object, a redundant bladder would have failed just as easily as a single bladder.
Things get even simpler when using single tanks, since there is about a 6 pound swing instead of a 12 pound swing from full to empty tanks, and so the diver will be diving with even less weight.
The same math above can be applied no matter what your circumstance.
My recommendation, if you want the real answers to your question, is to pose them at your DIR-F class. They do this stuff for a living, and are much better at it than me, and can help you to understand even more.
...Or rather, elaborate on the other's correct answer of "DIR teaches a balanced rig."
The original question posed by faye was, "What's the DIR stance on redundant bladders?" (Forgive me for paraphrasing.)
The stance is, "No redundant bladders."
However, the question was later elaborated on, and again I'll paraphrase and say that the question was, "Well, how do they recommend I handle this situation?"
The situation, as best as I can tell from reading this thread, was this:
Salt water, open ocean dive
AL80 doubles rig
Backplate and harness
3 mm wetsuit
Properly weighted and trimmed for the dive
Here's how the situation would unfold, if the diver was diving the above rig:
The short version is that the diver would be properly balanced throughout the dive and buoyancy would never be an issue, even with a complete bladder failure.
The long version works like this (and you don't have to be a chemical engineer to understand this):
Let's assign some numbers to your rig so as to illustrate the way this would happen. These are common numbers that I personally have experienced:
Me: Relatively neutral in salt water
3 mil wetsuit: 8 pounds buoyant in salt water at the surface, 70% of that lost at 100'
Backplate: 6 pounds negative
Tanks: 2.2 pounds negative (each) at the beginning of the dive, 4 pounds positive completely empty
Weight belt: 10 pounds on the belt
Okay, let's say that at the beginning of the dive, I jump in the water and immediately sink to the bottom, uncontrolled. I have a complete and catastrophic bladder failure (unlikely, since it was tested on the boat.) If the problem is that my valves are turned off, then I simply reach back and turn them on. (Taught in DIR-F.) Likely, I can do this before I hit 20 feet. Of course, this would never happen, since I've responsibly tested my gear before getting in the water. However, assuming that it did happen, then let's see how the weight would work out:
Me: Relatively neutral
3 mil wetsuit: At 100', my wetsuit is (30% of 8 pounds) 2.4 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, 4.4 pounds negative
Weight belt: 10 pounds negative
That puts me at 18 pounds negative. I can actually swim this up, buy why? I'd drop my belt and surface 8 pounds negative, at 30 fpm. Interestingly, much of this would be offset with my lungs (never holding breath, but filling them with each breath). Deco would be a nonissue, since I would not need a deco stop. I'd have only been in the water a matter of seconds. We're also assuming that the bladder failure is COMPLETE, and that there is no assistance from the bladder whatsoever. This is highly unlikely. The errant sharp object or swordfish would make it leak a good bit, but it would still hold some air. Even the elbow suddenly coming off would not produce a catastrophic failure... You could simply hold some buoyancy with it by placing your body horizontally, which you should be anyway.
At the surface, in that instance, here's how that would turn out:
Me: Relatively neutral
3 mil wetsuit: 8 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, 4.4 pounds negative
Weight belt: Gone
That puts me at just 2.4 pounds negative, easily swimmable. The difference would be the buoyancy change in the wetsuit. Of course, don't forget that I'd be just 2.4 pounds negative with two full tanks of air on my back. I'm in no danger of drowning. And again, we're assuming that you don't have even one pound of help from the wing.
Of course, this scenario would never happen anyway, since:
1. I tested my gear before I got in the water.
2. I entered the water with a full air cell.
3. If the problem was valve-related, then I'd simply reach back and turn them on.
4. I also have the option of dumping air out of my tanks before surfacing... There is no emergency if I'm on the bottom with two full tanks. I could technically do this as a last resort instead of ditching my weights. (There's as much as 12 pounds ditchable there.)
However, the scenario that I think you're more apprehensive about is the scenario where there is a deco obligation. Here's how that would pan out at 100':
Me: Relatively neutral
3 mil wetsuit: 2.4 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, around 5 pounds positive (remember, they're not totally empty yet)
Weight belt: 10 pounds negative
That places me at 8.6 pounds negative, and I'd swim that up. At 20 feet I plan to do a stop... But now I have this situation:
Me: Relatively neutral
3 mil wetsuit: 4.8 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, around 6 pounds positive (they're getting empty)
Weight belt: 10 pounds negative
That places me at a little over 5 pounds negative, which is very managable, either with just lung power (the average lungs from full to empty can produce as much as 9 pounds of bouyancy) or with simple kicks (not as pretty, but just as effective). I have not bothered to ditch weights. And still, we're assuming NO help from any air cell at all, which again, is unlikely.
At the surface, here's how it would pan out:
Me: Relatively neutral
3 mil wetsuit: 8 pounds positive
Backplate: 6 pounds negative
Tanks: Combined, approaching 8.8 pounds positive
Weight belt: 10 pounds negative
That leaves me almost perfectly neutral... At least within a pound.
Thus, a redundant bladder is not necessary... And since a redundant bladder increases task loading with no benefit, DIR preaches that single bladders are a better choice. Additionally, a redundant bladder may actually give you a false sense of security, since as someone else mentioned, if you have a tear in one, you're likely to have torn the other as well. Two bladders does not mean that one will always work. In fact, in the specific examples that you stated, such as dangerous marine life or an errant sharp object, a redundant bladder would have failed just as easily as a single bladder.
Things get even simpler when using single tanks, since there is about a 6 pound swing instead of a 12 pound swing from full to empty tanks, and so the diver will be diving with even less weight.
The same math above can be applied no matter what your circumstance.
My recommendation, if you want the real answers to your question, is to pose them at your DIR-F class. They do this stuff for a living, and are much better at it than me, and can help you to understand even more.
