Help with Doubles

[Searcaigh]

Just curious: What class are you taking where you're diving doubles, that is not "an actual tech course"?

In my neck of the woods (or kelp) IANTD also recommend doubles for Advanced Recreational Trimix

 

[tbone1004]

Okay, good. So there is some potential benefit. Note the keyword "potential".



If the DR bungees are providing tension when the wing is deflated, then they must be providing even more tension when it is inflated. I'm sure they are not ideal springs, but, as long as they aren't stretched to the limit of their elasticity, being stretched twice as long should yield ROUGHLY double the tension, right? So, how can it be that they would provide no help in equalizing the sides of a wing when horizontal?

Because this bungee isn't doing what you think it is doing. The OMS bondage wings, the old Dive Rite Travel wing, etc. are under constant tension, continuously trying to restrict the size of the wing. The Classic, Nomad, Rec wings from Dive Rite now have a Gusset Control System that is designed to be loose if you need the full volume of the wing, and tight if you want to restrict the volume of the wing.. If you dive it loose, it does basically nothing, it is there so you can restrict the size of the wing for streamlining if you don't need the lift. You are not supposed to inflate against the compression of the bungee, though you obviously can. If you restrict the wing, you take a 65lb classic wing and can take it down to 40lbs if you like the shape of the classic better than the Rec, or you don't want to have multiple wings etc etc. It's a completely different design protocol than what the OMS wings used and people seem to be blissfully ignorant that while both wings have an elastic control system, they are 100% different in design and function.

And, for that matter, the pressure differential at, just say for example, 100' between the high and low sides of a wing (when the diver is on his side) isn't that great. A DR Classic is 26" wide, so, call it 2'. That's 4 ATA on the high side and 4.06 ATA on the low side (if I've done my maths correctly). That's 1.5% greater pressure at the bottom than at the top. You're saying the bungees are short enough to control the wing when it's empty. With 5 or 6 (looking at a picture of a DR Classic) lengths of bungee pulling across the "tube" (attempting to compress) on the high side, you're saying that would make no noticeable, practical difference in how much air is in the high side versus the low side (as compared to the same wing with no bungees)? When it's only fighting a 1.5% pressure differential?

1.5% is more than enough of a pressure differential. Think about it. With equal pressure on each side of the wing being put on it by the bungee, it is no different than equal pressure being put on it by the water. Net pressure differential is what causes the air to be pushed to one side or the other, whether that pressure is 0 with no bungee system, or 100lbs on each side, the net differential on the wing when level is 0 so the air will balance equally, the only difference is the total pressure of the system which is irrelevant for balance, only the OPV. Upset that pressure by any amount greater than 0 and it will start migrating towards the path of least resistance.

 

[Michael Guerrero]

Here is a picture of the wing. This might help make things clear. The bungee is only on the bottom half of the wing, and it is a donut style, though the bottom channel is small.

View attachment 217957

 

[AfterDark]

Did this person explain to you how these bungies keep air from moving from one side of the wing to the other? How does the bungie cord on one side of the wing know to keep air out, while the other side knows to keep it in?

Well the way it works on my wing is the bungies on one side / end are tighter than the other which restricts the air on one side / end by not inflating as much as the other. This is how I solved my trim issue. When surfacing swimming I was sinking head first. I tightened the 2 lower bungies on both sides which allowed the upper part of the wing to inflate more than the lower section. Viola normal trim! On and below the surface.

 

[tbone1004]

and by doing that you are choosing to restrict the total capacity of your wing. While that is not a permanent restriction, when you use gusset control type systems to alter the shape of the wing you are sacrificing usable capacity of the wing. Which is OK and what it is designed for, you are not however supposed to be inflating against the bungees during normal use otherwise you defeat the purpose of the GCS in the first place

Watch the video to understand what it is actually supposed to do. Again, this is not designed to be used for anything other than altering the shape of the wing by reducing lift capacity. You aren't supposed to inflate against the bungee.

[video=youtube;Mo6nAbWM4zo]https://www.youtube.com/watch?v=Mo6nAbWM4zo[/video]


This is the bondage style wings

 

[Searcaigh]

Thanks guys for the explanation of the Dive Rite wing, I might give it a go this weekend with my small doubles and report back.

 

[halocline]

OP, it sounds a little like your main problems are:

1. It's possible LP95 doubles aren't the best fit for you, you might need longer and skinnier tanks. This is where some in-person mentorship from someone with excellent experience and knowledge can be very useful. You should also try to borrow a set of AL80s, maybe LP85s, and maybe HP100s to see how they work for you.

2. Once you get your harness adjusted, it could very well be just lack of experience. Doubles are way more mass on your back and at first can feel pretty 'tippy'. This is one of the often-touted things about sidemount. But, with a little practice, as Tobin mentioned, you can get used to the extra mass and actually use it to your advantage. once you get your trim dialed in it's very stable.

As far as bungeed wings, I avoid them. The first thing I did with my dive rite rec wing was remove the bungee and throw it away. On that wing I believe it's there in an attempt to make the wing more useable for single tanks, which really means more sell-able to divers trying to hedge their bets and only get one wing. That never works well. I would take the bungee off your wing, it's almost certainly not helping anything.

 

[stuartv]

Because this bungee isn't doing what you think it is doing. The OMS bondage wings, the old Dive Rite Travel wing, etc. are under constant tension, continuously trying to restrict the size of the wing. The Classic, Nomad, Rec wings from Dive Rite now have a Gusset Control System that is designed to be loose if you need the full volume of the wing, and tight if you want to restrict the volume of the wing.. If you dive it loose, it does basically nothing, it is there so you can restrict the size of the wing for streamlining if you don't need the lift. You are not supposed to inflate against the compression of the bungee, though you obviously can. If you restrict the wing, you take a 65lb classic wing and can take it down to 40lbs if you like the shape of the classic better than the Rec, or you don't want to have multiple wings etc etc. It's a completely different design protocol than what the OMS wings used and people seem to be blissfully ignorant that while both wings have an elastic control system, they are 100% different in design and function.

Okay, that makes sense. But, in that case, why is it bungee instead of non-stretchy line? So the user CAN still inflate to full capacity, if needed?

1.5% is more than enough of a pressure differential. Think about it. With equal pressure on each side of the wing being put on it by the bungee

Yes, but if all the air is in the left half of the wing (for example), then it won't be equal pressure from all the bungees. Oh, and just for the record, the person who told that he uses the bungees because they aid in balance does use the type of OMS wing like you posted pictures of.

 

[tbone1004]

if you restrict it the way dive rite shows in the video, you will likely pop the OPV before you can get all of the air into it. You could use line if you want, but it is less flexible in lengths so you have to cut and knot etc etc every time you want to change something slightly. Bungee flexes so is easier.

Yes you are correct that the pressure increases, but neither surgical tubing or bungee follow Hookes law. In either application from OMS or Dive Rite, the bungees are not stretched significantly enough to have a meaningful difference in their elastic rate.
With the OMS wings because it is silicone tube, they can't stretch that much, and with the bungee, it is only able to provide so much pressure. The OMS wing I pictured has 12 pieces of thick hose restricting it, the Dive Rite classic wing has the equivalent of 14 pieces of 1/8" bungee. Stop thinking that both are trying to do the same thing because they are completely different. I'm not going to look up the elongation properties of the silicone bungee that OMS uses vs the regular bungee Dive Rite uses, but you can if you want.

The bungees that OMS uses are an equipment solution to a skills problem. There is nothing wrong with standard wings for lateral balance, you just have to have it put in the right spot and know how to use it. All those bungees do in the OMS wing is put more pressure on the bladder than they need to have under all circumstances. The Dive Rite will only put pressure on it once you exceed the adjusted amount of air it can hold which is going against the design parameter of the wing in the first place.

The Rec wing first got the gusset control so people could have one wing to rule them all, it doesn't work, it's a decent doubles wing for small doubles, but it's a crappy singles wing. It's shape is all wrong for most people though, not a good solution. The Nomad got it so you could restrict the volume and control the shape if you didn't need all of the lift with small sidemount tanks, it is a REALLY useful feature on the XT. The Classic wing has it so if you need the shape of a doubles wing for doubles, but don't need the 65lbs of lift because you're diving AL80's, LP72's, LP45/50's on standard manifold spacing etc. It prevents Dive Rite from having extra SKU's, and allows guys in doubles to not having to buy multiple wings if you only occasionally dive the smaller cylinders. It's a good solution, but it's 100% different than the OMS style that was designed to restrict wings that were built far too big and it created all sorts of potential issues. Many people dive them and haven't had an issue, fine, but that doesn't mean it's a good idea or a design. The Dive Rite one is not as good as having the right wing for each circumstance, but you have to understand that the bungee is designed to restrict the total volume of the wing when in use and you are supposed to adjust or remove it for your lift requirements.

 

[KevinNM]

I extruded an DIN o-ring in the pool last night and actually figured out what was going on, what to do and got it done. Lost a huge amount of air before I figured out what was going on, but hey, not bad for a first time.