Balanced (DIR) Buoyancy help needed -very technical-

[Usmc0656]

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.

I actually looked at the LCD and fell in love with it but the "fad" towards donuts pushed me more towards the Torus. I called Toby and he's a very smart man, but I think he's just a bit frustrated discussing the same thing with people over and over again. You mention the word "zippers" or "donut" or "pinch flats" and he will amaze you with a lot of information, wanted or not. Unfortunately my sentence was a trifecta, "since the Torus doesn't have zippers, if I were to get a pinch flat how much would it cost to send it in for repair? Also, could I purchase another donut bladder and try my hand at stitching it myself if you guys were out of business by some catastrophic event?". LOL big mistake. I didn't need convincing, as I can see the merits of each design, but it didn't matter to him. If something happens to my wing I just wanted to know I can send it in and pay XX dollars. I also discussed my buoyancy issues with him which also seemed to frustrate him a little bit. He has a funny deep "here we go again" breath/sigh he makes right before you hit on one of those subjects that he has an auto response for . Anyways, it is what drove me to the pool to prove it to myself before ordering, and eventually led to this post and my recent purchase from somewhere else (I went to the dive shop to fill my pony bottle and told them what I was about to do and they made me a quote). He may just have been having a bad day though, and we're all entitled to one... not that he was mean or anything... just a tiny bit irritated? Absolutely great and knowledgeable guy though, as well as very helpful. I also had spoken to DGX (Geoffrey), and since I live close by (hour and a couple minutes away) I wanted to drive by and talk to someone, look at stuff, kick tires, shake hands, ya know?.... but they told me I couldn't and to just call in and speak to Geoffrey instead. Kinda lost my sale there as I'm a visual guy. Great guy also to speak to and very knowledgeable. I would wager that Tobin is a lot more technically proficient in diving details though. Anyways, I digress.

The buoyancy loss at depth for a worst case 8 pounds for the suit and I'm ok with without calculating that in because I'm making myself neutral at the safety stop (with near empty tank). The 8 pounds is a click or two on the wing at depth, and in a catastrophic failure I can swim it up (I think). BUT if there were a drastic failure of the wing at depth on a full tank and I absolutely had to go up... I'd actually be really heavy... 8 pounds from suit loss and quite a few pounds of gas for a total of ~16 pounds negative. Can I swim that up comfortably while already stressed out from the catastrophic failure? This is why I want the 10 pound ditchable on my belt. You know what people don't do that I think they should? Figure out how much weight they can swim up from 60 ft without having a heart attack. I just realized I have no idea how much I can lift without too much exertion. Why don't the agencies put that as a requirement? It certainly would curve the number of OW severely overweighted (too much lead, not lipids) students. I'm doing the AOW course with SDI now, and maybe we will. This Saturday we're doing something with reels in the pool... exciting.

 

[tbone1004]

Tobin can certainly be a bit grumpy at times, but it is all in your best interest. He has been around for a long time and seen a lot of things so you either love him or hate him, not much middle ground. I am partial to horse-shoe wings personally. I own both donuts and horseshoes and both have their place, but for single tank diving, I much prefer horse-shoes. I also much prefer to have zippers on my wings, so the couple DSS wings I have and the ones I recommend are the LCD's. The reality is that in well over a thousand dives, I have never had a pinch flat, but I learned backplates and take very good care of them, especially on boats. The zipperless wings don't really bother me, but some have issues with them, and to be honest I've never actually unzippered my DSS wings because I've never had to. I've also found that one of the common places for bladders to get pinched is when they get caught in a zipper....

Regarding kicking the weight up, day one for our students is to recover a diving brick *10lbs* and return it to the side of the pool in just a bathing suit. You can kick up 15lbs with fins. It's not fun, but you aren't going to die from so long as you can breathe. The reason the agencies don't do it is because they want diving to be fun and something everyone can do, despite the physical requirements that go with it, they wouldn't make nearly as much money if they had tough swimming requirements. Most of them used to, then did away with it in the 80's and 90's when they were trying to make it more accessible. No different than the American Red Cross removing the treading water requirement from lifeguard training and forcing you to use a tube while putting someone on a backboard in deep water.

 

[stuartv]

you are forgetting to factor in wetsuit compression.

I think I was covering that part. I noted that he measured his wetsuit and it only added 8 pounds of buoyancy, so that is the max he could lose. 8 # (MAX theoretical) for suit, 8 # for gas (in an FX120 - 8.17 #, to be exact, according to specs I found online) and 1 # for gas (in an AL13) means 17 # negative for worst case scenario (at bottom with full tanks and 100% wetsuit compression).

And wetsuit compression doesn't come into play for the other factor (floating your rig).

Regardless, I understand your points now. Thank you for clarifying.

I really like my 23 # wing for diving that same rig and wetsuit as the OP, and I believe I am totally safe doing so (at least as far as wing lift capacity needs are concerned - ha ha!). But, I will agree that if I were to wear my 5mm suit and a hood I might indeed switch to my 30 # wing. However, there is no way I would switch to a bigger-than-30 wing for single tank diving if a different option is to simply put some or all of my lead on a weight belt, stay with the 30, and still be safe. For that matter, if I ever actually dive my 5mm suit (still sitting unused), I will run the numbers and if I can use my 23 # wing safely by using a weight belt, I will.

The OP said it takes 20 # plus a full pony bottle to sink him in his 3/2 wet suit. Assuming he leaves the pony bottle clipped to his rig and puts 12 # of lead on a belt, he could get out of his rig and still be 10 # positive. No need to ditch weight to be safe, right? And it seems like that would let him use a 30 # wing safely (even if he ends up going to a 5mm and a hood it doesn't matter for floating his rig if it's on his belt). Given what you said about how nice the SP 30 is and how sucky the SP 40 is, that seems like it would be an option for him to consider. MORE than enough for now, in his current exposure protection.

 

[dmaziuk]

You know what people don't do that I think they should? Figure out how much weight they can swim up from 60 ft without having a heart attack. I just realized I have no idea how much I can lift without too much exertion. Why don't the agencies put that as a requirement? It certainly would curve the number of OW severely overweighted (too much lead, not lipids) students. I'm doing the AOW course with SDI now, and maybe we will. This Saturday we're doing something with reels in the pool... exciting.

Not sure how realistic the 60ft number is: you have to be living someplace where it's deep enough and warm enough and safe enough to try. I suspect if one's life depended on it, one wouldn't have a problem swimming up with 16 lbs. I can lift two bricks from a 13 ft pool (bathing suit, no fins), but I am a swimmer. With USMC training you probably can too: it's not that different from swimming in full gear. Having a heart attack while you're at it is a somewhat different issue, though.

 

[stuartv]

The buoyancy loss at depth for a worst case 8 pounds for the suit and I'm ok with without calculating that in because I'm making myself neutral at the safety stop (with near empty tank). The 8 pounds is a click or two on the wing at depth, and in a catastrophic failure I can swim it up (I think). BUT if there were a drastic failure of the wing at depth on a full tank and I absolutely had to go up... I'd actually be really heavy... 8 pounds from suit loss and quite a few pounds of gas for a total of ~16 pounds negative. Can I swim that up comfortably while already stressed out from the catastrophic failure?

I would imagine you could do it without TOO much trouble. Also remember that as you go up your suit expands so you get more and more help as you go.

But, if you're diving in the ocean (or maybe anywhere, but especially the ocean) it's a good idea to carry a SMB (Surface Marker Buoy). Which means learning how to use it and practicing with it before you need it. I think they are generally 20 - 35 pounds or so of lift (mine is 30). In the unlikely event that you actually lose 100% of the lift from your wing, while you are at max depth (so, without most of the buoyancy from your suit), while also having full tanks, you could inflate your SMB to help you "get up". I say it's unlikely, but you COULD jump in, go straight down to 130' and immediately pull the corrugated hose right off your wing and lose all the air inside. So, unlikely but not impossible.

 

[tbone1004]

Faber FX120's hold 134cf of gas at 3600 which is a common fill, 128 at working pressure, and the AL13's hold 13.9cf, so 13lbs of gas. It is splitting hairs, but that's why you have to use the water volumes of the bottles because the indirect measurements we use are all rough estimates and nominal values.

Either way, a 30lb wing can work if he has a belt on and it won't be unsafe, just tight. Remember that staying with that wing means you have to have the balance on your belt, so with the gear you have, you basically can't put any weight pockets on that rig. 13lbs of gas+2lbs of tank+3lbs of regulators+6lbs of plate and harness=24, plus any lights and anything else you have, add 3lbs if you have SS cam bands and a SS STA, etc etc and you're right at the limit of what it can float.

 

[dumpsterDiver]

One thing that might be missing from the discussion is the dive objectives. Sounds like people are - once again trying to be super technical or precise, but I like a buffer.

If you grab a bag of lobsters or have to assist an overweighted Diver who was stupid enough to have no ditchable lead, having 10 or 12 lbs of extra lift capacity might be very welcome. BTW wearing ditchable lead on belt is smart _ try an elastic rubber freedive belt and you may be extra happy. Especially if you waist is not smaller than your hips.

 

[stuartv]

Faber FX120's hold 134cf of gas at 3600 which is a common fill, 128 at working pressure, and the AL13's hold 13.9cf, so 13lbs of gas. It is splitting hairs, but that's why you have to use the water volumes of the bottles because the indirect measurements we use are all rough estimates and nominal values.

Either way, a 30lb wing can work if he has a belt on and it won't be unsafe, just tight. Remember that staying with that wing means you have to have the balance on your belt, so with the gear you have, you basically can't put any weight pockets on that rig. 13lbs of gas+2lbs of tank+3lbs of regulators+6lbs of plate and harness=24, plus any lights and anything else you have, add 3lbs if you have SS cam bands and a SS STA, etc etc and you're right at the limit of what it can float.

I was using the specs published here. Maybe this will be helpful to others.

http://www.indianvalleyscuba.com/se...ction/information/CYLINDER SPECIFICATIONS.pdf

 

[dumpsterDiver]

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.

This is 180 degrees to.my thinking. If you have to hand a tank up take the belt off first. You will be safer and the tank lifted will appreciate 10 lb lighter tank. Plus if dolphins show up or whale shark, you can jump in and snorkel with the weight belt, generally my weight belt are never too heavy to. Snorkel with.

With a really thick suit I may be adding some non ditchable lead to the scuba rig.

 

[tbone1004]

Blue Steel Scuba - Cylinder Specs
XS Scuba Luxfer Aluminum Scuba Cylinder Specifications
from the distributors directly. the indian valley is close enough, but the faber FX series are very very far off from their nominal values.

The 120 is actually 128, the 100 is 108, the 133 is 142, and the 149 is 159...