Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool

[rsingler]

Attached to the bottom of this post are 2 large Excel spreadsheets called Buoyancy Estimator v25d.xlsx (for current Excel), and Buoyancy Estimator 25d.xls (for Excel 97-03) that are designed to answer many of the questions suggested by the topics mentioned below. Taking the time to look at data makes for some surprising answers!
EDIT: Spreadsheets removed in favor of new version.
See sticky thread Optimal Buoyancy Computer in Advanced forum.

Background:
As a large number of long threads have documented (Have you ever had to dump your weights? , weights advice please , Formula for estimating wing size? ), there is great interest in the fine points of buoyancy. The issue of beginning divers being significantly overweighted has occupied many discussions. The notion of diving a “balanced rig” has engendered long discussion (Question about “balanced rigs” and having all ballast unditchable ), with definitive comments made on both sides of an argument for and against releasable weight.

“Neutral buoyancy” is also discussed at length. Add Additional Weight For Shallow Dive? , Perfecting my buoyancy

We often differ about a proper buoyancy check. Float at eye level, yes. But how big a breath to hold? Is tank empty or full, and if full, do we add lead to compensate for air breathed or not? I think many beginning divers do not understand the purpose of the check. I also think many advanced divers do not know the size of the change in wetsuit buoyancy from 15 feet to the surface, and its implication.

A third topic: that of choosing the proper BCD bladder size, also generates significant interest. For BP/W divers particularly, staying streamlined and avoiding “taco’ing” makes choosing the right bladder size for a wing somewhat important.

Redundant buoyancy is a familiar topic for doubles and technical divers, but should also be important to a large group of cold water wetsuit divers who may not understand the implications of wetsuit compression at depth. That concept is certainly not taught uniformly to beginning Open Water students, and yet, redundant buoyancy should be carried by at least some of them.

How much weight can you swim up? That topic, too, has been brought up again and again. BCD or Wing failure, or drysuit flooding certainly make the question appropriate, but it would be nice to quantify the problem, wouldn’t it?

All of these issues can be fleshed out with the data produced in the spreadsheets below. I think it might be worth your time to go exploring.

I apologize to those of you who do not use Excel. Importing this into Google Sheets, for example, will break some of the interconnected links among the pages of the spreadsheet. Several other spreadsheet programs may or may not recognize the internal links. But for those of you who can open an .xlsx or .xls file, you may find some very interesting data. This spreadsheet is AN EDUCATIONAL TOY, and I have placed disclaimers throughout. While the formulas used in the spreadsheet approximate what we think is happening, there are just too many individual variables to think of this as the sole planning tool for your dives. You still need to use your training to properly weight yourself as you were taught.

Enjoy!

Dive Safe!

EDIT: Minor update to certain cell headings and explanatory notes. No formula changes.
Security updated in Scenarios.xlsx. New spreadsheets loaded.
EDIT: Formula error identified by @stepfen corrected. Thank you! New spreadsheets loaded.
EDIT: Major upgrade based upon the past six months' worth of comments and suggestions.
Variables now in place for salt water and fresh water. European tanks added. Formulae updated considerably. As a result, the "Scenarios" spreadsheet does not quite track with the new version, although they are similar enough that the educational value makes them worth leaving in place. To fully see the changes, please read members' comments from over the past six months. The new spreadsheets are found both here, and again on page #6, for those that jump to the end.
EDIT: More fresh water correction improvements. Version 24 deleted. New version uploaded.
EDIT: Error found in v25 Hood buoyancy computation. Corrected spreadsheet v25b loaded.
EDIT: Upon request, reserve drysuit buoyancy formula improvements were accelerated and loaded as v25c below.
EDIT: Formula error corrected. V25d loaded. Sorry!
EDIT: Spreadsheets removed in favor of new version in Optimal Buoyancy Computer thread

 

[rsingler]

Comment, part II:

I have learned a lot by playing with the numbers. For example, did you realize that your buoyancy may change by over 10 pounds in some configurations between a 15 foot safety stop and the surface!?

As another grey-area example, have you seen the statement, “You shouldn’t dive steel tanks with a thin wetsuit”? Why might that be true? And if true, WHEN is that true? This toy will allow you to look at that.

The spreadsheet will also answer the garden variety question, “How big a wing should I buy?” More important, it will show in the “failure” areas the buoyancy implications of any given choice.

Don’t forget the weight of your rig without you in it. Some equipment configurations will float just fine with you in your exposure suit, but by themselves will sink despite a fully inflated bladder. Once again, having the data makes some things apparent that weren’t apparent before. For example, splitting your required weight between the integrated pouches and a few pounds on a weightbelt may now allow your rig to float without buying a bigger wing or a larger size BCD than will fit you comfortably. I’ve seen this latter problem with size Small BCD’s and thick wetsuits. Splitting weight allows the rig to float itself, and the spreadsheet will demonstrate that a 25-30 lb bladder size is still adequate despite carrying significant lead.

Finally, two entire pages are devoted to the concept of ditching some weight at depth. This has been a personal interest of mine, given the vigorous discussion for and against, and a LONG Scubaboard thread about balanced weighting, whatever that may mean to you.

Let me make some definitive statements based upon what I’ve learned or confirmed by playing with this tool.
1) You probably don't need as big a wing as you bought.
2) Partial weight ditching works! (assuming you know how much negative buoyancy is too much for you to swim up)
3) I can dive a steel tank with a two mil wetsuit and a steel backplate just fine, thank you! How can I say that? Because at the end of the dive with my steel tank and my backplate, my buoyancy is +0.5 lb! If my bcd or wing fails, I only have to swim up a maximum of 10#, assuming the failure occurred at the very beginning of the dive. You can solve a lot of SB arguments with a little data.
4) Redundant buoyancy requirements are either zero or very small with wing failure using a drysuit (which makes sense to most divers).
5) Redundant buoyancy requirements are either zero or very small with ditchable weight after a drysuit flood (which may not be intuitively obvious).
6) With a torn wing or BCD, there is a predictable depth at which you are neutrally buoyant after ditching modest amounts of weight during a wetsuit dive. At that depth, you can rest after swimming up against the negative buoyancy from your carried weight and failed BCD, offgassing in preparation for final ascent, and only then continue an ascent finishing with positive buoyancy of a known amount. In other words, after ditching a pre-planned amount of weight, there is NO "runaway ascent."
7) The standard neutral buoyancy check doesn’t quite work for NorCal 8mm wetsuit divers. If I’m floating at eye level with an empty BCD and 500 psi tank, then I’m more than 5 lb negative at my safety stop. With an almost 9 lb buoyancy change from 15 ft to the surface, I’m either negative at my safety stop, or very buoyant at the surface at the end of the dive. That shift explains why so many divers have such trouble in the last 10 feet.
8) None of all this really even applies to the tropical vacation diver in a 2mm shorty, so let’s not start arguing about it. Whatever fails, you can swim it all up to the surface, unless you have grossly overweighted yourself. In any case, at the surface ditch weight, or get out of your rig, or whatever you like.

Some of you reading the above may be completely unsurprised by what I’ve said. All the same, being able to quantify the issues is an eye-opener, even for experienced divers. It was for me.

If you’re not comfortable in Excel, take heart. The number of user-changeable cells is pretty small, and the spreadsheet is password protected (PW: scuba) to keep you from messing things up inadvertently. The primary thing you need to know is that there are “tabs” at the bottom of your screen that will walk you through the process. Left to right, they bring up separate spreadsheets, including two text-only pages that explain what you can learn and what the assumptions are in the calculations. Then selecting tabs to the right of the first two brings up separate spreadsheets that
1) you use to input your personal configuration (including selecting from a list of EDIT: now over 150 different tanks!), and then
2) enter your carried lead to show how much lift you’ll have at the surface vs at depth, with and without an equipment failure, and
3,4) show the buoyancy implications of ditching weight of varying amounts at depth for both wetsuit and drysuit.
Finally, the “Calcs” page at the end is an unmodifiable sheet that shows how your personal buoyancy calculations were derived.

If you ARE comfortable with spreadsheets and are having fun playing, you may get frustrated by having to enter data on two different pages than where some results are displayed. We did that to avoid having a HUGE single page that you’d have to keep scrolling all over to see. We got lost with a one-page product. But with wider computer displays, if you’re comfortable in Excel, try the View|Open New Window, and then View|Arrange All (Vertical), and also maybe View|Zoom (80%). That way, you can keep a portion of the Initial Data Entry spreadsheet open to change wetsuit thickness or shift from single to double tank, while immediately seeing the Lift vs. Depth results change. Or with a given equipment configuration, have the Lift vs. Depth and a Weight Ditching page open side by side to see the effects of changing carried lead.

If you have a question, don’t hesitate to PM me (start a Conversation) by clicking on my username at the top of this post, and I’ll be happy to help you navigate. As you might guess, this toy has undergone significant modification (many thanks to @kmarks , @-JD- , and @johndiver999 ). There are LOTS of ways to approach these questions, so I also have also attached a sample “scenario” spreadsheet that extracts data from the various pages and combines them side-by-side so you can see what you might find out by tinkering. That spreadsheet is not user-alterable, and is just a sample set of scenario results. If you’re Excel-savvy, go ahead and unlock the spreadsheets and feel free to tinker with the formulas. Don’t forget about your “Personal Buoyancy” entry on the Initial Data Entry sheet. It’s a way for big heavy-boned muscular divers with no body fat to get meaningful results with the same tool as a fluffier diver.

In short, if it’s light grey, you can put your own data in, without unlocking anything.

REMEMBER, this is a theoretical tool that should not be used as the sole tool for weight planning purposes. You still need to do a real buoyancy check.
To evaluate its usefulness to you, see how close its calculations come to what you normally use for carried lead with your own personal gear configuration. If it’s way off, then feel free to ignore the whole thing.

 

[rsingler]

Comment, Part III:
Before you enter your own data, take a peek at the current entries. It’s set up for a 154lb, 5'9" diver of average muscle mass wearing a backplate and 30# wing and a thick wetsuit. He’s carrying 14# lead with his steel 95 cu ft tank. He's weighted to be zero buoyant at 15' which means he floats at eye level with a FULL tank (a common NorCal technique with thick neoprene). On the Lift vs. Depth spreadsheet, that shows as +2.9 lb of buoyancy at a full tank (because he’s supporting a portion of his skull weight above the surface). If he were 0 lb. buoyant at the surface, he would be just awash (but that’s not how we measure neutral buoyancy, right?).

Examining the results, we see that at a 15 ft stop with a near empty tank, he’s about 0.4 lb negative. But if he weighted himself by standard convention (floating at eye level with a 500psi tank) with 20# of lead, he’d be +3.2 lb. buoyant at the surface (floating at eye level), but -6.4 lb. at his safety stop! Now that’s easily managed with 3 liters of air in his BCD. But, we can see a flaw in the typical “neutral buoyancy check” for an average guy with a lot of neoprene: being +3# at the surface with an empty tank means you’ll be very negative at the safety stop. That’s why some cold water divers “argue” about how to measure neutral buoyancy – because they know they’re a little overweighted using an empty tank. They’ll use a full tank and not add the weight of the air they’ll consume. It’s not that one point of view is right and one is wrong, it’s that using thicker neoprene changes the dynamic of the neutral buoyancy check due to wetsuit expansion between 15 feet and the surface.

What else do we learn from this sample diver?
Due to wetsuit compression, he loses 27# of buoyancy at 100 ft!! That means that early in a dive, with a BCD tear and loss of buoyancy, he’d be 20# negative. That might be a bit much to swim up for an out of shape diver. If he has redundant buoyancy, he could “hang” off an SMB, or hang off a buddy, but at 20# negative, either prospect is a slightly precarious fix. Now it all gets a bit better as you hit 15 feet, because wetsuit re-expansion means you’re only 6.7# negative. That’s much easier to swim up. But again, we have a potential issue: if you’ve self-rescued with an SMB or lift bag, are you able to keep from an uncontrolled ascent as the air in your bag expands?

Instead, consider the Weight Ditching page. For our sample diver with a BCD fail at the beginning of a dive, what happens if he ditches 7# of lead? At 80', he converts a 19# negative scenario to a 12# negative situation and is able to swim up with moderate effort. His wetsuit re-expands and at 16 feet, he is now 0# buoyant, despite still carrying 7# of air in his full tank. At this depth, he can just float there and catch his breath from the effort of ascending. He can offgas for as long as he likes. Then he can start a gentle ascent to the surface, where he will arrive 10lb. buoyant. And if he exhales fully on the way up from 16 feet, temporarily dumping another 4-6 lb of buoyancy, there is no risk of rapid ascent. Once on the surface, he is easily able to float with a torn BCD. At the price of a $60 weight pocket, he has converted a potentially dangerous equipment malfunction to a non-problem.

This data is why I’ve become a convert to partial weight ditching planning for some equipment configurations. This diver might have
“FULL: -7#/16 feet
1500psi: -3#/16 feet”
written on a slate, and be ready for a catastrophic BCD malfunction with or without redundant buoyancy. In fact, I would submit that this form of self-rescue would be smoother than either hanging from an SMB or depending upon a buddy with whom you have not practised.

Anyway, take a look at the other scenarios on the extra spreadsheet, and play with your own data in Buoyancy Estimator v25d. Enjoy!

Safe diving!

 

[AfterDark]

I haven't had a chance to do much with it yet but the short time I did generated some questions.

Does the dry suit buoyancy data and assumptions pertain to all types of dry suits? Is the trilaminate the same as the crushed neoprene in the assumptions for example? Also undergarments, I know I use less with my neoprene suit then other divers use that dive trilaminate suits. Or are these stupid questions, because I haven't used it enough yet?

 

[Kevrumbo]

 

[rsingler]

I haven't had a chance to do much with it yet but the short time I did generated some questions.

Does the dry suit buoyancy data and assumptions pertain to all types of dry suits? Is the trilaminate the same as the crushed neoprene in the assumptions for example? Also undergarments, I know I use less with my neoprene suit then other divers use that dive trilaminate suits. Or are these stupid questions, because I haven't used it enough yet?

No, the spreadsheet does not apply UNIFORMLY to all drysuits. True crushed neoprene (e.g. DUI CF200) has some negative buoyancy apart from the positive buoyancy created by the liner airspace. A laminate suit is considered neutral by the spreadsheet. And coming next in buoyancy, so-called crushed neoprene in less expensive suits, which is really just super-compressed neoprene, may actually have some positive buoyancy apart from the liner buoyancy.
The sheer number of variables, plus an inability to test each of them at both the surface and flooded was just beyond our capabilities.
I'd recommend inputting a fudge factor in the personal buoyancy cell (since that won't change much with depth) so that you can compare your actual weight requirement from your experience, with the predictions made by the toy. That fudge might be a little negative buoyancy with crushed neoprene, zero with tri-lam (our assumption), and a little positive with compressed neoprene.
There is already a small fudge factor based on our measurements, hiding in cell I17 of the Calcs page.
Hope this helps!

EDIT: The new version of the spreadsheet: Optimal Buoyancy Computer, now allows a selection of any of four types of drysuit fabric: trilam, crushed neoprene, compressed neoprene, neoprene, and computes buoyancy differently for each drysuit. See the User's Manual for additional info.

 

[kelemvor]

FYI you can get a free trial of Microsoft Office 365 (includes excel) here: Free Trial Download, Try Microsoft Office 365 Products

Also, if you're any kind of a student or teacher then you can have it free. I doubt scuba teachers (or students) qualify unless it's through a university Office for Students, Teachers, & Schools | Office | Microsoft

 

[rsingler]

Thanks @Kevrumbo !
Nice video.
So if we put a hypothetical diver in the water with a 6mm wetsuit and a 5mm vest, and tweak the fixed buoyancy so that she's +3.5 lb at the surface (holding the top of her skull above the surface) with a steel tank and 13 lb of weight, the toy spreadsheet says she's -13.5 lb at 27 meters/85 ft.
The video shows that she's unable to swim that weight up effectively, (though the video seems to show a little artistic license with her effort and finning technique). If she drops her 13.2 lb (6 kg), weightbelt, the video shows that she's neutral, as predicted almost exactly by the spreadsheet.
The spreadsheet predicts that she'll hit the surface 8.7 lb positively buoyant, confirming the video's concerns about an uncontrolled ascent.
But if she dropped 3# of weight with that full tank, she'd only be 10.5 lb negative. Inhaling fully will add another couple pounds of buoyancy and she might be able to make some headway with an ascent. Her wetsuit would begin to expand, making things easier. At 22' she'd be neutral and could rest there.
For her ascent, she's now 3# lighter, but having offgassed with her full tank for as long as she wanted, she can probably finish her ascent safely despite being light.

What's my point?
First, thank you for the video, and one more data point confirming the spreadsheet's accurate prediction of a 17 pound loss of wetsuit buoyancy at depth.
Second, since there are hundreds of cold water divers in Monterey Bay, CA going below 85 feet in wetsuits, we need a plan for them other than "buy a drysuit". (Me, I dive a drysuit.)
My recommendation would be a 'lil ol' SMB tucked away to help with redundant buoyancy. But absent that, the spreadsheet shows that you can probably self-rescue safely with partial weight ditching, assuming that your buddy has left you.
This is all a theoretical exercise, since there are lots of ways to skin a cat, and what we are debating is not a single failure (BCD fail), but a TRIPLE one (no BCD, no buddy, no sausage). I've just given you a solution to a triple failure, and what I am disinclined to accept is that you can't safely dive to 85 feet in cold water in a wetsuit.

Again, nice video!
Different conclusion for me, though.
Thanks @Kevrumbo !

 

[diversteve]

Excel is also available online - free use.

You can upload and save your changes but printing requires generating a .pdf output first.

Microsoft Excel Online - Work together on Excel spreadsheets

 

[stepfen]

Hi there. I am experimenting with the calculator as I hope I will get my first BP/W soon and I think I fount a problem with the latest version (v22).

In the "Initial Data Entry" tab if you choose any of the first 2 tanks (Generic Al or LP Steel) the Sum of Tank Buoyancies and Total CU Ft air carried (Cells 30F and 31F) are calculated as 0 !

I think the mistakes are in cells 34K, 34L, 35K and 35L.

For example 34K now is:
=IF(B34>0;H36*B34;"") , while I think the correct one should be:
=IF(B34>0;H34*B34;"")

Similar for the rest of the cells 34L, 35K and 35L.

Keep in mind that it is my first time "playing" with this tool so it might be that I am missing something - in which case I beg your pardon and sincerely apologize in advance.

Huge thanks and congrats for the great work BTW