Balanced (DIR) Buoyancy help needed -very technical-

[Usmc0656]

The problem is that i've always worn about 26 pounds of lead for diving in jacket style scuba. It was completely unscientific how I got that number, so I decided to dive DEEP into the calculations to see what the heck is going on.


I've read a lot and understand that the three most important factors are:

1. Enough lift for floating the rig solo.
2. Enough lift for compensating for buoyancy changes in suit (failures, compression, etc).
3. Be able to swim up in case of catastrophic wing failure.

The first thing I wanted to figure out was my PERSONAL buoyancy. Both solo, and with my 3/2 XXXL Scubapro Everflex. Yeah... i'm a very large boy. After I EASd from the corps I decided to drink my fitness away and only PT when I'm doing 12 ounce curls. I'm obese and just wanted to get that out of the way. To establish my buoyancy I:

1. Went to a pool in just shorts and found that the lead that made me buoyant on a mid breath was 7 pounds.
2. Then I put the suit in the water and tried to hold it down with weight until it wasn't rising or falling... that was 8 pounds.
3. THEN I put on the suit and confirmed that 15 pounds was buoyant for me. The question then became... how do I calculate the conversion to salt water.

I did some digging and, with some scientific understanding of how displaced water weight works for objects in the water, I went ahead and figured out how much fresh water my fat ass is displacing. Turns out it's 4.5 cubic feet for myself, and about .2 cubic feet for the suit. The conversion then is straight forward. How much does 4.7 cubic feet of salt water weigh (each cubic foot is about 64 pounds)? Here is a summary of my calculations:





It seems like my 15 pounds in fresh water converts to 22.5 pounds in salt water (those numbers on the right should be negative). I then started to create calculations like crazy to try to figure out what weighs what and I found someone else that’s about as crazy as I am and created a calculator for us. 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). I plugged in the numbers into the calculator and found that I could not reach 22 pounds negative as a whole (for me to be neutral) at the stopping point (on the spreadsheet, the column that says "3m end") UNLESS I add 18 pounds of additional weight. Please see the parts I circled in red. Without it, it ends up being only 4 pounds negative in total ballast, which seems like it will make my ass shoot out of the water and high-five the moon.





The question now is on the wing size… I ALMOST purchased a 25 sized wing thinking minimalist DIR… but when I started crunching numbers it seemed like that would be a really really bad idea. Even though the numbers up there are inflated I’m guessing I should subtract the 22 pounds I need for myself and the suit to know what’s actually the real ballast? If that’s the case then I arrive at this…





This means I’ll only need 19 pounds of lift to stay with my head above the water at the start of the dive (predive, that’s the 10 pounds at the top). So… I only need 19 pounds… but then I can’t float the rig by itself…. So a 40 pound wing is almost exact? This just sounds insane to me. Am I doing something wrong? Sorry about the long way around, but I’m sure a fellow newbie can learn a lot from what I posted here in detail, and I know they can learn from any feedback the community can give me.

 

[AJ]

Don't know the answers, but some things to consider:
22 pounds
Why are you calculating 8 puonds positive lift for your suit, but forget about positive lift for yourself?

Tank
Tank buoyancy does not change during the dive, the weight of air it contains does. Don't see that in our spreadsheet.

Floating rig
Why would you want your rig to float by itself? No need for that, just inflate your BCD and it will float anyway.

Neutral buoyancy
I consider me and my gear as a whole that needs enough buoyancy to dive safe. The buoyancy check I do is go down to 3m/10ft with all the air out of my suit and wing and 30 bar or so in my tank. When I exhale I should stay down on the bottem. When I inhale I should be rising. Thats neutral buoyancy.

Lead
Lead is what keeps the whole system (you and gear) down. The amount of lead should be determined by the buoyancy of the whole system, not by your buoyancy (suit and yourself) and the rig apart. I think thats the main flaw in your calculations.

 

[Insta-Gator]

How To Perform a Simple Buoyancy Check


Initial weight
If you don’t know where to begin, take about 10% of your body weight. If diving in tropical waters with a thin wetsuit, subtract 4-6 pounds; if diving in cold water with lots of exposure protection, add 4-6. This will give you a starting weight to tweak.


Enter the water
Begin at the water surface with full diving equipment and an inflated BCD. Ideally, cylinder should be nearly empty (600 to 1000 psi) resembling your end-of-dive profile.


Hold a normal breath (with your regulator in your mouth) and deflate your BCD
At this point you should find out if you are properly weighted. If you sink, you are overweighted, if you bob out of the water, you are underweighted. An ideal weight will keep you approximately eye-level with the surface of the water.


Repeat
Based on feedback from the previous step, adjust your weight accordingly and repeat until you float at eye-level.


Compensate for your cylinder (or use spreadsheet and cylinder specifications)
If you are doing this check with a full aluminum cylinder, you should add about 4 pounds to compensate for the end-of-dive when the cylinder will be more buoyant.

Men should add 6-7 lbs (about 3 kg) if moving to salt water, or subtract 6-7 lbs (about 3 kg) if moving to fresh water.

 

[tbone1004]

ok so quick summary. Ignore insta-gator, I apologize, but you can't use that. Want proof? I weigh 275lbs and I can bare tank breathe on an al80, I sink... I certainly don't need over 30lbs of lead in the ocean. Hell, the only time I need 30lbs is if I'm in the ocean, with a drysuit, in a stab jacket *they all float*, and an AL80.... His description is taken from a number of different recreational weighting strategies that will almost always result in you being overweighted, promise, it's just a fact of life. Further proof saying you should add 4lbs to compensate, when the tank holds over 6lbs of gas, so if you only factor in 4lbs, you are planning on always having over 1000psi in the tank. Not realistic. It comes from lazy instructors not wanting to properly weight their students. Further proof, when we weight students for OWT, they're in AL80's, stab jackets, and usually 5mm wetsuits, and we RARELY put more than 8-10lbs on them, very very rarely will we have an overweight student that needs more *we have rigorous swimming requirements that tend to weed out very out-of-shape/obese students*. If we weighted them for 10% of their body weight, they'd be pinned to the floor.

you=7lbs
suit=8lbs*this sounds like too much, but we'll go with your numbers
net body=15lbs

Who's HP120 are you using? I think only the PST E7-120 is neutral and that's a fairly rare beast and I don't actually trust that number, I'd get a cheap luggage scale and just hang your rig from the edge of the pool to get the total number.

Don't add in the knife and light for your buoyancy, only lift. You could drop them.... So 10lbs negative against 15lbs positive, you should have 5lbs of lead on in the fresh stuff. You have to add back in 13lbs for the weight of gas and the other stuff you're carrying, so 13+10+8=31lbs for your wing. 30lb wing should be fine because you shouldn't see total compression of suit at depth.

Now, to adjust for the salty stuff you have to figure out how much water you actually displace which is kind of a bitch to do exactly, but the easiest way is to hop on a scale. Make sure to subtract for whatever air is in the tank though it won't really matter, and multiply total weight by 0.025. Reason instagators 6-7lbs doesn't work is because in a configuration like yours, even sans wetsuit, my total weight will be well over 300lbs requiring 8lbs of lead. Fun stuff.

Salty adjustment for you should be around 8-9lbs depending on how big you are, I'm 270 but wear XXL, I know guys that weigh less but are shorter and a bit rounder that float considerably and wear xxl's as well, so you need to hop on the scale and figure it out. Total ballast you need should be around 25lbs, and a 40lb wing is as small as I would go. If you don't already have a backplate, I would call Deep Sea Supply and get one of their large plates which are wider than normal and may feel more comfortable and also get a pair of the weight plates to bolt onto them. You'll gain an extra pound in the plate *"normal" backplates are only 5lbs btw, but with the harness they get up to about 6*, and the bolt on plates will give you an extra 8lbs on the plate for a total of around 15lbs of ballast. You can put the rest on a weight belt, weighted STA, trim/weight pockets, etc etc, but at least most of it is on the plate for better balancing. If you already have a plate, then there are some other ways to get that much lead onto the rig. The LCD40 from Tobin is also one of the nicer large wings out there because it is longer rather than wider than most wings which gives you a better trim profile and it is much less prone to tacoing like many other 40lbs wings. He also actually measures the volume with the wing on a rig with an 8" diameter tank to give you a worst case scenario unlike most others that will measure total water volume of the wing and not put it on a rig to get that number so you don't actually know what you have unless you fill it with water while it is on your own rig.

 

[archer1960]

The problem is that i've always worn about 26 pounds of lead for diving in jacket style scuba. It was completely unscientific how I got that number, so I decided to dive DEEP into the calculations to see what the heck is going on.


I've read a lot and understand that the three most important factors are:

1. Enough lift for floating the rig solo.
2. Enough lift for compensating for buoyancy changes in suit (failures, compression, etc).
3. Be able to swim up in case of catastrophic wing failure.

The first thing I wanted to figure out was my PERSONAL buoyancy. Both solo, and with my 3/2 XXXL Scubapro Everflex. Yeah... i'm a very large boy. After I EASd from the corps I decided to drink my fitness away and only PT when I'm doing 12 ounce curls. I'm obese and just wanted to get that out of the way. To establish my buoyancy I:

1. Went to a pool in just shorts and found that the lead that made me buoyant on a mid breath was 7 pounds.
2. Then I put the suit in the water and tried to hold it down with weight until it wasn't rising or falling... that was 8 pounds.
3. THEN I put on the suit and confirmed that 15 pounds was buoyant for me. The question then became... how do I calculate the conversion to salt water.

I did some digging and, with some scientific understanding of how displaced water weight works for objects in the water, I went ahead and figured out how much fresh water my fat ass is displacing. Turns out it's 4.5 cubic feet for myself, and about .2 cubic feet for the suit. The conversion then is straight forward. How much does 4.7 cubic feet of salt water weigh (each cubic foot is about 64 pounds)? Here is a summary of my calculations:

It seems like my 15 pounds in fresh water converts to 22.5 pounds in salt water (those numbers on the right should be negative). I then started to create calculations like crazy to try to figure out what weighs what and I found someone else that’s about as crazy as I am and created a calculator for us. 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). I plugged in the numbers into the calculator and found that I could not reach 22 pounds negative as a whole (for me to be neutral) at the stopping point (on the spreadsheet, the column that says "3m end") UNLESS I add 18 pounds of additional weight. Please see the parts I circled in red. Without it, it ends up being only 4 pounds negative in total ballast, which seems like it will make my ass shoot out of the water and high-five the moon.




The question now is on the wing size… I ALMOST purchased a 25 sized wing thinking minimalist DIR… but when I started crunching numbers it seemed like that would be a really really bad idea. Even though the numbers up there are inflated I’m guessing I should subtract the 22 pounds I need for myself and the suit to know what’s actually the real ballast? If that’s the case then I arrive at this…





This means I’ll only need 19 pounds of lift to stay with my head above the water at the start of the dive (predive, that’s the 10 pounds at the top). So… I only need 19 pounds… but then I can’t float the rig by itself…. So a 40 pound wing is almost exact? This just sounds insane to me. Am I doing something wrong? Sorry about the long way around, but I’m sure a fellow newbie can learn a lot from what I posted here in detail, and I know they can learn from any feedback the community can give me.

I didn't look at all your calculations in detail, but your conversion from fresh to salt is pretty easy: take the total weight of you and your rig that is neutral in fresh water (probably 325 - 350 lb or so), and add 2.5% of that amount in extra lead.

The final check is simply to get in the water fully dressed and rigged up with ~500 psi in your tank (i.e. at the end of a dive), and see if you are neutral at some shallow depth that you choose with your *BC completely empty*. 15 ft is a common value because that's where safety stops are done; I use 6 ft because of all the shallow water shore diving I do; I want to be able to stay down and look at things in the shallows.

If you need air in your bc at your chosen test depth, you're over-weighted. If you can't stay down, you're under-weighted.

 

[DA Aquamaster]

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.

----

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.

 

[Usmc0656]

I thank you all. I have a blue steel Faber HP120. It's -8.8 full and +0.25 empty, but I like the suggestion of verifying it. The .025 after weighing is kinda what I did. Fresh water is 62.4 pounds per square foot, which translates to 64 pounds salt (2.5% more). I didn't know the plate can be bolted on weight. If that's the case, then it seems the 22 pounds was a good target weight. What I'll do is go to the ocean today and verify my calculations from the fresh water.

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? The eye level thing leaves a good chunk of head above the water that weighs you down. Is that to simulate an empty AL80's positive buoyancy when empty? I mean, there has to be a reason they suggest that. The time anyone gets into anything but an AL80 is when they're going advanced and doing deco dives. For advanced though the best advice, in my opinion, is to establish buoyancy with a near empty tank submerged... Since we have so many different types of tanks out there with different buoyancy characteristics. No?

 

[Patoux01]

The time anyone gets into anything but an AL80 is when they're going advanced and doing deco dives.

In many places, an Al80 is anything but common. (France, Switzerland, probably most of other european countries as well)

The main reason for that eye level thing is: you don't have to think. You're overweighting the guy anyway, therefore he'll get down whatever happens.

 

[tbone1004]

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.

 

[stuartv]

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?

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:

The final objective is, as you say, to be neutral at 15' with an empty tank. And the general rule of thumb given to new recreational divers assumes (I believe) diving with an AL80.

Backing up from that, if you were able to determine your weighting to be neutral at 15' with an empty BC and a full tank (AL80), you would then simply add 6 # to your weight to be neutral with an empty tank.

Backing up from THAT, you could try to determine your weighting at the surface, with an empty BC and a full tank. From there, you know that you need to add 6 # to accommodate an empty tank AND make an adjustment to reflect the buoyancy you will lose at 15' due to suit compression. Again, I think the Rec Rule of Thumb is assuming no dry suit, and further assuming that, whether it's no wet suit at all or a 7mm wet suit, the rule of thumb gets you "close enough."

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. The weight of the part of your head that is above water is ROUGHLY equivalent to 6 pounds. Any buoyancy lost from wet suit compression would, essentially be over weighting.

IOW, I think the Rec Rule of Thumb works to be "close enough" by having an assumption of an AL80 (which tells you the weight of gas to compensate for) and the average weight of a human head (thus giving a ROUGH number for the amount of weight you are floating when you are at eye level).