Balanced (DIR) Buoyancy help needed -very technical-
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yeah ignore that, you still need wing lift regardless of whether it is ditchable or not.
x-tek isn't a bad rig, the plates are beautiful, but they are just very expensive for what they are and I don't really like their wing shapes, too wide, but that's just me. Dive Rite, Deep Sea Supply, and Halcyon are all Made in USA which is nice as well and I try to support that when I can
I am not the expert who has posted the criteria many times stating that the rig should be able to float itself. I have simply noted that it is typically repeated as gospel here on SB and so I addressed it in my post. I believe this principle was even mentioned earlier in the thread. If you accept that gospel (that the rig needs to be able to float itself, unattached from the diver), then it logically follows that it makes a difference to wing lift requirements if you hard attach lead to your rig versus putting it on a weight belt that is on your person instead of on your rig.
I feel like the REAL point of my analysis was that no matter how you slice it, the OP only needs a wing of 20 or a little more pounds of lift capacity. It doesn't matter how much ballast he's carrying, as long as it is the correct amount for him to be neutral at the end of his dive with an empty wing.
That said, I recognize my relative inexperience, so if I am saying something here that is incorrect, I wish someone would help me out and explain what is wrong and why.
If you're taking your rig off in the water to hand up to a divemaster so you don't have to climb up the ladder with a 120 on your back--something that people with back problems may very well do--the rig should be able to float itself.
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couple scenarios
All weight on the rig-wing has to be able to pick up the whole rig, in this case 22lbs+10lbs of gas+anything else that is clipped off to it=32lbs minimum, so choose a 35-40lb wing.
Some weight on the diver-wing can be slightly smaller, but not by that amount of weight, only the amount of weight that is required to sink the diver itself but you then remove any amount of leeway that you have to keep your head above the water in an emergency without ditching weight, which I don't recommend especially if you're diving over coral which can cause some very permanent, very nasty damage to the reef.
Here he has 22lbs ballast, plus weight of gas which is 10lbs, so he needs a 35lb wing minimum. Technically he could remove the 8lbs from what it takes to sink himself and he could get away with a 30lb wing which would not be that unsafe, but if I was diving in the open ocean, I would go for a 35-40lb wing in this situation. Now, problem is the Scubapro X-Tek 30 is a really good wing design, it's essentially a copy of the Halcyon Eclipse wing which is quite good, BUT the 40 is a terrible design for single tank and will cause all sorts of problems due to the wrapping of the wing, and the 30 may or may not actually be 30lbs of lift depending on how they measured it. Most manufacturers measure total wing capacity, not when it is mounted on the plate with a wing, on the diver. I don't know how Scubapro does theirs, but I wouldn't risk it since you're already cutting it close with that amount of lift.
All weight on the rig-wing has to be able to pick up the whole rig, in this case 22lbs+10lbs of gas+anything else that is clipped off to it=32lbs minimum, so choose a 35-40lb wing.
Some weight on the diver-wing can be slightly smaller, but not by that amount of weight, only the amount of weight that is required to sink the diver itself but you then remove any amount of leeway that you have to keep your head above the water in an emergency without ditching weight, which I don't recommend especially if you're diving over coral which can cause some very permanent, very nasty damage to the reef.
Here he has 22lbs ballast, plus weight of gas which is 10lbs, so he needs a 35lb wing minimum. Technically he could remove the 8lbs from what it takes to sink himself and he could get away with a 30lb wing which would not be that unsafe, but if I was diving in the open ocean, I would go for a 35-40lb wing in this situation. Now, problem is the Scubapro X-Tek 30 is a really good wing design, it's essentially a copy of the Halcyon Eclipse wing which is quite good, BUT the 40 is a terrible design for single tank and will cause all sorts of problems due to the wrapping of the wing, and the 30 may or may not actually be 30lbs of lift depending on how they measured it. Most manufacturers measure total wing capacity, not when it is mounted on the plate with a wing, on the diver. I don't know how Scubapro does theirs, but I wouldn't risk it since you're already cutting it close with that amount of lift.
AJ
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Agree, it does matter how you distribute the weight. You want the diver and the rig together to have enough buoyancy to get to the surface safe and stay there. Be aware however that if you wear a heavy weight belt on your body, you could become negative buoyant without your BCD. This is one of the reasons I do not wear a weight belt, but have all my weight attached to the rig. If I take off the rig while boat diving, I will always stay afloat and the rig by itself will too with the wing filled.
As I understand the OP, it took him 20 # to sink himself (in salt water) WITHOUT any rig on - only his pony bottle. If the pony bottle is 2 # negative, then I see where you get 22# from.
But, once he adds a 6# BP, harness/rigging, and regulators, he's not going to be carrying 22 # of lead. If he errs on the heavy side, I guess he might be carrying 12 # of lead.
So, yes, as I said before, if he puts all his lead on the rig itself, I can see where he would need a wing 30+ pounds.
But, the OP said he is 20 # positive in his wetsuit without his rig on. So, if he put 10 or 12 # on a belt, then even if he separates from his rig, he's still going to be 8 - 10 # positive. You said weight on the diver can make the wing smaller but only by the amount that is required to sink the diver. Well, he needs 20 # to sink. So, if I am using what you said correctly, then putting the 10 - 12 # on his belt still leaves him safe and it still means his wing only needs to be able to support less than or right around 20 # to float his rig.
Is the difference that you're only allowing for putting 8 on a belt because that's what it takes to compensate for his personal buoyancy but not including his wetsuit buoyancy? Why can't he include his wet suit buoyancy in that number, thus moving all 10 - 12 pounds of his lead to his belt and still being 8 - 10 # positive at the surface (and still at least slightly positive even at depth)?
---------- Post added January 8th, 2016 at 11:41 AM ----------
Sure, but that doesn't seem to be a factor here. The OP only needs 10 - 12 pounds of lead and he already checked it would take 20 pounds to sink him in his wetsuit, without his BCD (and assuming he holds onto his pony bottle).
uh oh, it never even occurred to me! I think you may have just found my major problem with so much weight.
also I agree with you. In my original post, the last image has in red the final ballast. If I were to wear all my personal lead needs on my belt I could actually get away with a 20# wing. What I'll likely end up doing the more I think about it and calculate stuff is:
Rig (13#): -2 pony slinged, -2 steel tank at end of dive, -2 regs, -1 STA, -6 harness with BP
On me (8-10#): in two ditchable belt pockets.
I'll then test it out in the ocean again, making sure I flood the suit. I get all my gear mid next week. I buy American myself, but in a different sense; I'm more of a buy local guy. Although I get better deals online I buy at the LDS, which is a platinum Scubapro dealer (ocean safari), which means they only carry that exclusively. I understand why they do it (higher margins than the other stuff), but I too wish they were selling American scuba products. Trickle down economics does work sometimes.
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you are forgetting to factor in wetsuit compression.
When we discuss the lift required to float the rig at the surface, we are assuming that the diver is neutrally buoyant on his own, and all of the weight is attached to the rig, i.e. no weight belt because most people are moving to integrated weights of some sort and prefer not to ditch. This equates to the worst case scenario at depth which is a full tank, and full wetsuit compression.
Where you are missing right now is that you aren't factoring in wetsuit compression which is one of two things a BC has to compensate for. It has to compensate for any delta in unit buoyancy caused by exposure protection as well as the weight of the gas carried. In this case it is 11lbs of gas *backgas and pony*, and somewhere around 10lbs of buoyancy introduced by the wetsuit as worst case during compression. Now, the rest of the weight that he is carrying is being used to make him neutral so the bc isn't compensating for it, however in order to remain comfortably at the surface you need a buffer of 10 or so pounds of lift to keep your head above water. In that case a 30lb wing is close, but I would want to verify that the wing actually has 30lbs of lift. Pretty easy on a scale with your rig on, just have a buddy fill it with water and weigh before and after it is filled.
The other reason to consider a bit more wing lift in this circumstance is how thin the wetsuit is and while the OP obviously has bioprene and the USMC have likely given him some more cold tolerance than others, if you decide to add a 5mm hooded vest or go up in wetsuit thickness, you have no buffer in the wing which requires purchasing a new wing. Reason I initially suggested the DSS wings is that the LCD40 is narrow enough that there is no taco around the tanks and since he's using a FX120, there is no concern with length. Going from the LCD 20 to the 40 you go from a 16x22 wing, to a 17.5x26 wing, and the LCD30 is a 16.5x26 so you aren't really getting a significantly larger profile.
When we discuss the lift required to float the rig at the surface, we are assuming that the diver is neutrally buoyant on his own, and all of the weight is attached to the rig, i.e. no weight belt because most people are moving to integrated weights of some sort and prefer not to ditch. This equates to the worst case scenario at depth which is a full tank, and full wetsuit compression.
Where you are missing right now is that you aren't factoring in wetsuit compression which is one of two things a BC has to compensate for. It has to compensate for any delta in unit buoyancy caused by exposure protection as well as the weight of the gas carried. In this case it is 11lbs of gas *backgas and pony*, and somewhere around 10lbs of buoyancy introduced by the wetsuit as worst case during compression. Now, the rest of the weight that he is carrying is being used to make him neutral so the bc isn't compensating for it, however in order to remain comfortably at the surface you need a buffer of 10 or so pounds of lift to keep your head above water. In that case a 30lb wing is close, but I would want to verify that the wing actually has 30lbs of lift. Pretty easy on a scale with your rig on, just have a buddy fill it with water and weigh before and after it is filled.
The other reason to consider a bit more wing lift in this circumstance is how thin the wetsuit is and while the OP obviously has bioprene and the USMC have likely given him some more cold tolerance than others, if you decide to add a 5mm hooded vest or go up in wetsuit thickness, you have no buffer in the wing which requires purchasing a new wing. Reason I initially suggested the DSS wings is that the LCD40 is narrow enough that there is no taco around the tanks and since he's using a FX120, there is no concern with length. Going from the LCD 20 to the 40 you go from a 16x22 wing, to a 17.5x26 wing, and the LCD30 is a 16.5x26 so you aren't really getting a significantly larger profile.
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