Balanced (DIR) Buoyancy help needed -very technical-

[Usmc0656]

I won't totally discount what insta-gator is saying, but it needs some significant amendment.

Way back in the day, we established neutral buoyancy in a similar manner when using a 7mm wet suit. We'd weight ourselves so that with all the air dumped from the BC or wing, and with only 500 psi in the tank (a normal end of a recreational no decompression dive pressure) we would just barely float at eye level with full lungs and then sink as soon as we exhaled.

Combined with the compression of the wet suit, that left us neutral in the same condition but with normal lung volume at 10-15 ft, so the diver could easily hold a safety stop.

The critical difference here is floating at eye level with full lung volume, then sinking when you exhale.

If you instead weight yourself so you can descend from the surface with full lungs, you are now over weighting yourself by about 10-11 pounds, reflecting the buoyancy the average adult gets with a 5 liter lung volume.

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Another factor that screws up this approach is nervous divers who unconsciously fin or fan their hands when vertical at the surface. Lazy instructors, rather than addressing this underlying problem, will over weight the diver. This actually aggravates the "nervous fins" issue, as the nervous diver is still unconsciously trying to get his or her head the same distance above water, and now has to fin harder to do it. More weight is applied, more finning occurs, and the cycle continues until in the extreme you have a diver in a 3mm or 5mm wet suit with 40 pounds of lead on a weight belt.

Once the diver is comfortable in the water, the diver discovers he or she is massively over weighted, and once that excess weight is removed, maintaining neutral buoyancy in the water is suddenly much easier as a much smaller volume of air is managed, requiring far smaller changes.

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If the diver started with a new wet suit, that wet suit will lose buoyancy over the course of the first dozen or so dives due to some permanent collapsing of the neoprene that occurs. The degree to which this occurs depends on the thickness and the quality of the neoprene used, but it happens to some degree with all wet suits. The end result is that the diver will probably need to reduce weight once the suit has some dives on it.

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I've never put any stock in fixed formulas for how much weight you need based on body weight as body types vary far too much in terms of buoyancy.

For example last week two of us were in the water with similar body weight, the same suits, same 22 lb wings, can lights and side mounted LP 85 tanks and no lead weight. The other diver could easily maintain neutral buoyancy with some wing volume to spare and I was at least 10 pounds too heavy, and was still 2-3 pounds short of neutral buoyancy at full lung volume. At the end of the dive with 1200 psi in the tanks (1/3 capacity), I could finally manage neutral buoyancy with minimal lung volume and a fully inflated wing.

Body mass and density vary, making any fixed formula a waste of time.

I hadn't seen this post before my latest one. This answered some of my leftover questions, thank you. I like that you're a KISS philosophy kind of guy/gal. The formulas I never give much weight to also unless I fully understand them. People try to use "constants" when they aren't really constant, and because they don't understand the details they end up getting very off answers on those rare occassions that they don't apply (like my bodyfat). For example, the 2.5% difference between salt water and freshwater is pretty much accepted, but it would never apply in the Dead Sea because there is significantly more dissolved minerals/salt and is actually heavier (denser). Also, the convection effect, temperature, currents, air exchange, depth (at a mile, water compresses almost 1%, which is tiny), and evaporation even changes the density differences in the same body of water at various depths. It's so small and actually constant for everyone that 2.5% applies pretty regularly. Now, 6-7 pound suggestions are based on average factors which excludes the people that sink like a rock, and those that can perform a high jump if they drop weight at a safety stop.

Thanks again to everyone. I'm learning a ton from you all.

 

[DA Aquamaster]

I have one one more confusion. I read that being neutral at the safety stop is what a balanced rig is all about, but the recreational stuff requires that you establish buoyancy at eye level on the surface. Those are not the same thing...or are they?

As tbone indicates it is, roughly speaking, the same thing.

The general principle is that if you are floating eye level at the surface with full lungs, you are slightly positive as you're floating about 1/2 of your head (about 5 to 7 pounds in weight) above the surface. At half lung volume you're essentially neutral at the surface, and thus with a little wet suit compression at 10'-15' you can maintain normal buoyancy with a normal breathing cycle.

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Dry suits complicate the issue significantly as dry suit buoyancy depends on how much air you maintain in the suit. If you're warm water diving with just a wicking layer, you need just enough gas to prevent a suit squeeze. If however you are diving below 100' on a lake Superior wreck dive where the bottom temps are 35-39 degrees F, you'll have much heavier undergarments that have to be properly lofted to insulate. That requires a lot more gas in the suit, which results in a lot more buoyancy. That buoyancy won't change with depth, as you'll ad or dump gas to maintain a constant volume, but it does vary based on water temp.

To further confuse the issue, many divers still run a minimum suit volume, just short of a suit squeeze with heavier undergarments. They won't fully loft and thus won't offer full insulation value, but it works just fine in "cold" water in the 50-65 degree range. But put them in the water on an ice dive, and the more experienced cold water divers will add some gas to the suit and ensure they have enough weight to allow for that.

 

[Usmc0656]

the faber tank buoyancy does not include valve or first stage, so you are down around -1 to -1.5 depending on the valve. Not a big deal, but every couple pounds helps.

The confusion stems from the difficulty of getting deep enough in a pool as well as the difficulty of weighting someone while they are submerged. It is "close enough". The issue again comes back to exposure protection. Drysuits don't change buoyancy regardless of depth, your 3/2 won't change nearly as much as a 7mm farmer john, so everything is different depending on the specific circumstances.

Balanced rigs are very simple and very complex at the same time and really only work under very specific circumstances. I.e. the last time I dove a balanced rig was in 2010, I do not have the luxury of being able to dive in a balanced rig because I do not dive with single tanks and I cave dive in warm water, so I will always be overweight, unfortunate fact of life. Going away from al80's isn't as cut and dry as you would think, the time they get away from them is typically when they buy their own tanks because they realize that while a Faber FX100 is twice the cost of an AL80, it's about the same size height, diameter, and weight, but it requires 5 less lbs on their weight belt, and it holds almost 50% more than an al80 *al80's hold 77.4cf, the FX100 holds around 110*. So you cut 5lbs off of your total rig weight, and you can dive for significantly longer. The head thing is fairly arbitrary but most of the weight of your head is submerged when you're at eye level, and since we are measuring with a micrometer, marking with chalk, and cutting with an axe, it's close enough. There are far too many variables in diving to ever dive a perfectly balanced rig, that's what experience is for where your lungs will be able to automatically compensate for a significant amount of adjustment without you realizing it.

I didn't know about the valve differences in buoyancy. I got it off the product page which had included the valve in the description. I guess they borrowed that table and didn't concern themselves with annotating that small but important detail. Thanks.

 

[tbone1004]

yeah if you look at the blue steel page it has a note saying in salt water tank only. Again, easiest way to do this though is with a luggage scale and just hang your rig with everything clipped onto it, including fins into a pool. Will give you the actual total ballast.

If you have a buddy and you want to get your actual buoyancy, you can put your full rig on, with what you know to be more lead than you need, and clip off a reel of some sort and hang from it. BC fully empty, and just breathing in a relaxed fashion like you would on a safety stop, If you can get to 10ft great, but I prefer to do it shallow since you want to still be able to descent if you have to from suit compression. Have your buddy look at the luggage scale and subtract any volume of gas in the tank from that number. The difference is how much extra lead you're carrying. This one is quick and fairly easy but you need a buddy to help.

 

[motorref]

USMC0656 - I'm glad I saw this, and I while may not be as technical as the others, I can relate what works for me: I typically dive 2-3 times a year in warm, salty water, and occaissionally in a semi-local quarry here in VA.
Like yourself, I'm older (05/57), former military and bigger than most people (6'1" 260#) and has done more 12oz curls than anything else; if I have to wear a wetsuit I need a 2XL to get up over my shoulders and get my arms in, but in water temps above 76-78 degrees I never use a wetsuit - rash gaurd and bathing suit are all I need for an hour in the water, even down below 60 ft.
When I dive oftern I typically use 14 -16 lbs with an AL80 in salt water, and as little as 6-8 in fresh; when I don't dive for a few months I find that I need 18 pounds (or more!); I think this is because I get excited and "forget" to control my breathing as well as I normally do.
My suggestion is to simply practice with different weight (and a buddy that can carry any excess that you want to dump), but the key is practice, practice, practice; don't over-analyze it.
I have a very good friend that is new to diving as well, and like yourself a former Marine (there are no EX-Marines!) so I've sent the link to this thread to him, to see if it'll help him any (he's a really big guy).

Good luck!
KevinL

 

[dmaziuk]

I have spent a lot of time pondering this question. What I have come up with as a way of explaining it to myself is this:
...

I suspect the other reason is it's a rule of thumb to get you started. You'll fine-tune it as you keep diving. But yeah, I think suit compression at 10-15' vs Al80's buoyancy shift from full to empty works out to something you should be able to control with just your lung volume.

 

[boulderjohn]

So, the Rec Rule of Thumb "adds" 6 pounds at the surface by telling you to take a full breath and you should be floating at eye level.

I don't know about other agencies, but PADI teaches you to hold a NORMAL breath for a weight check. There is a big difference. (If a PADI instructor said something different, that instructor was wrong.)

I have always found the weight check to be a pretty simple process. I have never gone to anything remotely like the calculations being advocated by some in this thread. I have also found that every system other than a simple weight check that I have seen, including especially the many weight check calculators on the Internet, will overweight you. I have seen some that will have me wearing 3-4 times as much weight as I know I need.

Instructors tend to overweight students in pool instruction, often significantly, because of the need to have them rest firmly on their knees during instruction. If they are having the students do the skills on the knees in the open water as well, they will also have them overweighted for that reason. When I posed for pictures for the PADI Undersea Journal for an article on teaching students while they are neutrally buoyant, I had to do shots of skills both horizontal/neutral and kneeling/negative. I was a little overweighted when I did the horizontal/neutral pictures, as I usually am while instructing. Then I I posed for the kneeling/negative pictures, something I had not done in years. I could not do it with that weighting. In order to demonstrate the skills on the knees, I had to DOUBLE my weight, and remember I was already overweighted.

 

[stuartv]

...with my 3/2 XXXL Scubapro Everflex. Yeah... i'm a very large boy.

...
I decided on a SS plate (6 pounds), HP120 (-9 full, neutral empty), regulators (2 pounds), handlight and knife (2 pounds), and my H2O EAS2 13cf pony bottle (-2 pounds).

Incidentally, I am not quite as large, but I wear a 3/2 XXL SP EverFlex. I dive with a DSS SS BP (size Large, so 6 #), and a single HP120 (old skinny neck PSTs, so true 3500 psi). I normally carry at least one light (DGX 600) and 2 or 3 cutting devices (Eezycut Trilobite, EMT shears, and DGX Titanium Tek knife). I don't carry a pony on single tank dives, though.

With all that, in salt water, I need no extra ballast at all and I am what I would say is virtually perfectly weighted. If I don't control my breathing, I might even be just a little light. At most, I would need to add 2 pounds.

My personal buoyancy is pretty close to neutral in the pool. I haven't measure mine as exactly as you did, but I can sink to the bottom if I exhale all the way and float easily with a full breath. With your personal buoyancy being 7 pounds positive in fresh water, I would think you would not need more than 10 to maybe 12 at the most pounds of ballast for salt water.


Further calculations:

Assume you are weighted to be neutral at the surface at the end. Some people aim for neutral at the SS, but, for the sake of this discussion go with me here. Neutral at the surface with an empty tank. That would actually mean you're carrying a little bit more weight than you need to be neutral at your SS.

That means you start the dive ~9 pounds negative (the weight of the gas in your HP120) at the surface.

According to your measurement, your suit is 8 pounds positive. I think a reasonable estimate for loss of buoyancy at the deepest recreational depths is 75% (roughly). That means you might lose 6 pounds of buoyancy at the bottom. Obviously, you CAN'T lose more than 8 pounds and you will certainly lose less by some amount.

So, if you get in and go straight down to 130' (the worst case for Recreational diving, assuming you stay within your limits), you will arrive at the bottom (9 for gas + 6 for suit crush =) 15 pounds negative.

That means that your wing only needs a max of 15 # of lift capacity to let you be neutral at any point during your dive. As you breathe your tank down, it will become less negative, so needing less lift. If you go more shallow, your suit will be more buoyant due to less crush, so again needing less lift. Max depth (I.e. max suit crush) at the very start of the dive (tank full) is the worst case for how much wing lift you need (aside from what you may need in order to float your rig at the surface).

The other factor that is recommended to accommodate with your wing lift is the ability to float your rig at the surface. I don't want to debate the importance of that, so let's just assume you should make sure you can float your rig. In your case, your HP120 is -9 (when full), BP is -6 #, regs are -2 # (?), assuming lights, etc. are attached, so another -2 #. So, your total rig is 19 # negative at the start, more or less, right?

That would indicate your wing needs a minimum of 19 # of lift. Going with the two numbers (15 at bottom at start and 19 to float rig), you go with the larger number.

IF you are carrying 10 pounds of lead as ballast, and IF you attach that lead to your rig in such a way that you can't ditch it, then your wing lift would need to be at least 19 + 10. But, why do that? Wear it on a weight belt, so it's not attached to your rig. Then a 20+ # wing can support your rig and the 10 # on your weight belt still leaves you 5 # positively buoyant (personally) without your rig. Ditch the weight belt (if necessary) and you are back to 15 # personally buoyant (between your body and your wet suit).

Any way you slice it, it seems that a wing with something over 20 # would work for you (unless you want to hard attach your ballast to your rig). I dive basically that same rig with a Hog 23 # wing and I love it.

Now, all that said, I am relatively a beginner and tbone knows his shtuff. So, I hope he will come back to this thread and explain what is wrong with my calculations here. I have re-read what he said and I don't really understand why he would recommend a 30+ pound wing.

---------- Post added January 7th, 2016 at 03:21 PM ----------

I don't know about other agencies, but PADI teaches you to hold a NORMAL breath for a weight check. There is a big difference. (If a PADI instructor said something different, that instructor was wrong.)

I have never taken any PADI course.

My "normal" breaths are sometimes very full, sometimes very shallow, and sometimes in between - and I mean that completely seriously. As the only legitimate purpose is to give you a number to start working from to then dial your weighting in, I am not sure I see how it makes a "big" difference. The instruction I received did not teach me to get that number and then just stay with it.

If I start with a number on my first pool dive and then check my weighting and refine it all the way up to my final OW check out dive, does it really make a "big" difference if I started the first pool dive 2 pounds overweight or 8? Isn't most anyone's weighting requirement likely to change even just from the first pool session to the final OW check out anyway, so regular checks (actually checking ability to hold a stop at the end of the dive, not just the "rule of thumb" check at the surface with a full tank) during the OW course are a good idea and will quickly do away with any "big" amounts of extra weight?

 

[Usmc0656]

First of all, I appreciate each and every single contributor. This is very educational and I'm glad I created an account on this forum. People have also been awesome sending me PMs with additional information. There's a lot of experience here and it's awesome to be able to tap into it.

I went to the ocean today and it was actually calm surf. I took a weight belt, suit, and my pony. I put 20 pounds on my belt and just assumed the pony was 2 pounds (H2O EAS2 with a 13cf tank)... and it was perfect. Exhaled all the way I would sink to the floor. Half to nearly full lungs I would rise until about my mask broke surface. Anything in between would make me suspended in the water. Granted, I was only in maybe 6 ft of water and sitting indian style. With all your help I was able to feel comfortable with the calculations of what I needed to order so that I can put most of my weighting requirements on my rig itself. I had already decided on a Faber HP120 and the Mk25/S600 setup, but to that I ordered today a Scubapro X-Tek 40# singles wing with SS plate, and basic hogarthian harness. I know the tech community almost doesn't see any gear that's scubapro (that's not regulators) because there's such good gear already out there (DSS, Dive Rite, DGX, Hog, etc). I just wanted to be a walking scubapro billboard. When it comes in I will finalize my guestimations and confirm/reject them. So far they have been exact though.

 

[stuartv]

I went to the ocean today and it was actually calm surf. I took a weight belt, suit, and my pony. I put 20 pounds on my belt and just assumed the pony was 2 pounds (H2O EAS2 with a 13cf tank)... and it was perfect. Exhaled all the way I would sink to the floor. Half to nearly full lungs I would rise until about my mask broke surface.

So, with a tank that's neutral when empty, a SS BP , harness/rigging, and regulators that all add up to around 9-ish pounds, you'll need somewhere between 10 and 12 pounds of lead. That sounds about right to me. Even said that in my last post.

Also as I said in my last post, it still seems like you only need a wing with just over 20 pounds of lift - unless you affix all your lead to your rig so that it is not ditchable. Then you would need a wing that is at least 30 pounds.

I'm still not clear on why anyone thinks you would need a wing in the 40 pound range. Or, to put it another way, I wish I understood what is wrong with the calculations I did above.

Bottom line: It's been said on here many times that wing lift only needs to be enough to handle 2 things - floating your rig and covering maximum loss of buoyancy. If you put your lead on a weight belt, instead of your rig, then your personal buoyancy has no affect at all on the lift required to float your rig and a 20+ # wing will float a SS BP with a single steel tank. And your maximum change of buoyancy is also unaffected by your personal buoyancy. All that matters is how much gas you are carrying and your exposure protection. You already tested your wetsuit and the most you could lose from that is somewhere less than 8 pounds. Your gas is ~9 pounds. So, again, a 20+ pound wing is enough.

As for weight on the rig versus a belt, well, I like having the most compact and streamlined wing I can use and still be safe. So, I would much rather have 10 or 12 pounds of lead on a weight belt than put it on my rig and have to have a wing that is 10 to 12 pounds bigger.

---------- Post added January 8th, 2016 at 09:40 AM ----------

I went to the ocean today and it was actually calm surf. I took a weight belt, suit, and my pony. I put 20 pounds on my belt and just assumed the pony was 2 pounds (H2O EAS2 with a 13cf tank)... and it was perfect. Exhaled all the way I would sink to the floor. Half to nearly full lungs I would rise until about my mask broke surface. Anything in between would make me suspended in the water. Granted, I was only in maybe 6 ft of water and sitting indian style.

I forgot to ask. When you got in for this test did you submerge and pull your suit open at the neck (front and back) to make sure the suit was completely flooded with no air trapped in it? When I don't do that, it seems like I am usually about 2 pounds or so (total guesstimate) light at the start of my dive because of air trapped inside the suit.