Info Optimal Buoyancy Computer

The Optimal Buoyancy Computer
A tool to help nail buoyancy and improve safety, before you splash​

1) How much lead should I carry with my new wetsuit?
2) How big a wing should I buy?
3) Will my BCD support my lead, both at the surface and when my wetsuit is compressed at depth?
4) Will my BCD support my rig without the help of my wetsuit/drysuit, if I doff it at the surface in an emergency, or underwater due to an entanglement?
5) How do I balance my rig?
6) How might partial weight ditching help me deal with an emergency? Will it really result in a runaway ascent?
7) How does the neutral buoyancy check change with thick neoprene?

I’m excited to announce the release of the Optimal Buoyancy Computer.
Designed to answer a variety of buoyancy questions, it provides accuracy directly proportional to the precision of your data input. Starting with as little as your height, weight and suit thickness, you can get ballpark weight requirements quickly. With additional information, you can compare equipment configurations, and plan for self-rescue after hypothetical equipment failures.

This tool is an Excel spreadsheet, and is a revision of a tool originally released in Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool , which was itself a revision of a toy spreadsheet first introduced in this thread: Advice on lift capacity for BP&W in April, 2018. After months of user suggestions, this new tool uses a simpler, modified data input system, and produces both simple and complex analyses of buoyancy. It works in both metric and Imperial units, salt and fresh water, and with both U.S. and European tanks.

Included is a 50-page user’s manual to lead you through the more complex parts of the tool, and a Quick Start section to get you going with minimal familiarity with spreadsheets. Additionally, the manual discusses the theory behind the more complex buoyancy calculations, whether you need help with Excel or not. If you are not facile with Microsoft Excel, the manual will take you through it all, step by step.
Here's the Table of Contents:

Download the .xlsx file for current versions of Excel. Use the .xls file for Excel 97-2003. Other spreadsheet programs may or may not recognize the internal links, but trial versions of Excel are available for free. You will see a generic Excel warning about possible viruses - don't worry, there are none! Click "Enable Editing", and save a copy. After saving, you will be able to edit the data fields for your use.

Many thanks to @stepfen , @johndiver999 , @kmarks , @Akimbo and the many others who have made suggestions and comments along the way.

NOTE: If you are using Excel 2003 and download the .xls file, extensive protective formatting is not functional. Thus, when you are diving a wetsuit (for example), you may be able to see drysuit "data" on the same page. The data for the "other" suit is NOT accurate under those conditions and should be ignored. With current versions of Excel, this information is blanked out for safety.

As each new version is uploaded, the count of downloads returns to zero. We are currently at over 2000 downloads of the tool, counting repeat customers! Thank you for your interest!

WARNING: These spreadsheets are experimental tools using formulas created by amateur divers for educational use only. Numerous assumptions regarding buoyancy have been made based upon only partially tested equipment configurations. The information herein is for your personal educational use and should not be relied upon to determine the adequacy of a given equipment configuration. Consultation with a dive professional regarding equipment, weighting and performing a neutral buoyancy check should all be strongly considered before diving a new equipment configuration. Note specifically that the practice of ditching weight at depth is a controversial one, and the theoretical data in this spreadsheet should not be considered a recommendation of that practice.

---

Selected for the ScubaBoard Knowledge Base.

This thread was selected for the ScubaBoard Knowledge Base on 22 November 2021. Special rules discouraging off-topic and counterproductive replies apply after this date.

---

 

[rsingler]

Thanks for constantly making improvements! I'm starting by using your new personal buoyancy feature (which is much improved over the last version) to estimate my lead needs. But, I plan to visit the pool and collect some data and then reverse calculate my personal buoyancy.

Please critique my plan.

WOW! Very ambitious, and this should enable you to use the tool for all future configurations accurately.

Two comments:
1) Since you're going to all that trouble, and if the pool is warm enough, could you grab some empty soda bottles and/or weights and a mesh bag? If you directly measure your personal buoyancy using the new method, you can compare that with the correction you were going to apply to what the tool gave you without PB applied. That'll tell us that the science is right.
2) I would recommend using the Lift Tab to generate your recommended numbers, instead of QuikResults. You can input the depth of the pool as your dive depth, and you'll get accurate numbers with which to determine neutral buoyancy. There is so much happening between 15 ft and the surface, that you'll want to be as precise about your test depth as possible. There aren't many pools where you can reach 15' and still have water beneath you.

Additionally, relying on QuikResults adds another wrinkle. The prominent numbers in that tab are "weight required to hold a safety stop (be neutral) after breathing your tanks down to 300psi." If instead, you enter your actual tank pressure as the starting pressure in the Tanks Tab, and perhaps 500 psi less as the ending pressure in the same tab, you'll have accurate rig buoyancy for both your scale test and the short "dive". Then, using the Lift Tab, you can fiddle with weight required to give you "0" buoyancy at whatever depth you like.

Please send me a copy of your spreadsheet with your numbers entered and let me know how it turns out!

Ladies and gentlemen, I believe we have an enthusiast here.

 

[Ready4Launch]

"...or hang your BCD from a luggage scale at the surface of a swimming pool."

Shouldn't that be based upon whether you're calculating for fresh or salt? Or, is there a correction being made in the background based upon Diver&Dive tab? Seems that correction is being made and called out on the Pers Buoy tab with the note: "NOTE: This does NOT have to be the same water type as your dive!"

And, for BP/W rigs, is this really necessary?

Last, I think you really need to edit page 7 of the manual for the different characterizations of each figure. Neutrally buoyant should be neutrally buoyant and not both, positively and neutrally.

 

[Ready4Launch]

Also, forgot to ask, for my Jet Fins, should they be entered under the Other Fixed Weight field?

 

[rsingler]

"...or hang your BCD from a luggage scale at the surface of a swimming pool."

Shouldn't that be based upon whether you're calculating for fresh or salt? Or, is there a correction being made in the background based upon Diver&Dive tab? Seems that correction is being made and called out on the Pers Buoy tab with the note: "NOTE: This does NOT have to be the same water type as your dive!"

And, for BP/W rigs, is this really necessary?

Last, I think you really need to edit page 7 of the manual for the different characterizations of each figure. Neutrally buoyant should be neutrally buoyant and not both, positively and neutrally.

You're right - your bcd will displace a different volume of water if you measure its weight submerged in salt water compared with fresh. But if for a 70 kg average-sized person the salt/fresh difference is 3-5lb, for a bcd the difference is only an insignificant number of ounces. Perhaps I should have explained that, because of sharp minds like yours. But the manual is already too wordy. So I just left out discussing where you measure your equipment displacement. My bad...

And as for BP/W's, you're right. You already know the weight of the plate and fittings. But if you haven't thought about your buckles and D-rings and only know your plate weight, a dip in the pool with a luggage scale can be revealing.

I'll take a look at the graphic again. Thanks for the suggestion!

 

[rsingler]

Also, forgot to ask, for my Jet Fins, should they be entered under the Other Fixed Weight field?

Yes.
Actually, any un-ditchable weight field would work (e.g., trim pockets field, too), but Other Fixed Weight is the most logical.

 

[Darcimus]

How do I account for dry gloves? Do I add a set weight for the gloves themselves then add based on the liner thickness?

 

[rsingler]

Glove buoyancy is computed solely based upon liner thickness. The glove itself is assumed to be very close to neutral. When you enter glove liner thickness, the program computes buoyancy based upon the assumed surface area of your hands from hospital burn formulas.

 

[Darcimus]

OK so basically 1%. Cool.

 

[gr8jab]

WOW! Very ambitious, and this should enable you to use the tool for all future configurations accurately.

Two comments:
1) Since you're going to all that trouble, and if the pool is warm enough, could you grab some empty soda bottles and/or weights and a mesh bag? If you directly measure your personal buoyancy using the new method, you can compare that with the correction you were going to apply to what the tool gave you without PB applied. That'll tell us that the science is right.
2) I would recommend using the Lift Tab to generate your recommended numbers, instead of QuikResults. You can input the depth of the pool as your dive depth, and you'll get accurate numbers with which to determine neutral buoyancy. There is so much happening between 15 ft and the surface, that you'll want to be as precise about your test depth as possible. There aren't many pools where you can reach 15' and still have water beneath you.

Additionally, relying on QuikResults adds another wrinkle. The prominent numbers in that tab are "weight required to hold a safety stop (be neutral) after breathing your tanks down to 300psi." If instead, you enter your actual tank pressure as the starting pressure in the Tanks Tab, and perhaps 500 psi less as the ending pressure in the same tab, you'll have accurate rig buoyancy for both your scale test and the short "dive". Then, using the Lift Tab, you can fiddle with weight required to give you "0" buoyancy at whatever depth you like.

Please send me a copy of your spreadsheet with your numbers entered and let me know how it turns out!

Ladies and gentlemen, I believe we have an enthusiast here.

I've incorporated your feedback into my plan. Finally getting to the pool this Saturday to take all these measurements. I delayed because I've been loosing 4lbs/week (almost 70 lbs so far), and I wanted to be closer to my November vacation weight to make things as accurate as possible.

I'll share all the data collected and my spreadsheet sometime this weekend!

Cheers!

 

[gr8jab]

Hello all! I made it to the pool today, and here is the data I collected and my feedback.

POOL TIME

Setup:

me: 5'10" at 230lbs
swimsuit
short sleeve rash guard
3mm booties
HOG aluminum backplate
HOG 35lb wing
2 small ditchable weight pockets on harness
4 small non ditchable trim pockets on backplate
Original Nautilus Lifeline
Deep 6 fins
80CF Al rental tank​

Personal buoyancy. It was really hard to estimate 1/3 full lungs. I think this is where some of my sources of error might be in this process, but I'm hard pressed to find a different method. I found that I was able to stay at eye level with no weight or lift at what I think was 1/3 full lungs. I was wearing a swim suit and short sleeve rash guard, and mask/snorkel, nothing else.

The pool had a max depth of 9', which is probably another source of error, since at that depth there are a lot of changes in buoyancy in short distances. I wish I was able to test in 15' of water instead. Wearing my setup as described above and with 2850psi, with NO ADDITIONAL weight, I was able to sink with a full exhale and gently descend. While at the bottom of the 9' pool (where I tried to hover just off the bottom, so I'm calling it 8') I couldn't take a shallow breath without going positive. I added 2 lbs and felt very comfortable, able to ascend and descend at will, within my normal breathing range. I think 1 - 1.5 lbs would have been right on the nose perfect. I spent considerable time getting all the air out of my rig, both from the bladder and general trapped air.

Later, after the dive, I hung my rig in the water via a luggage scale. My son was in the water and took more time twisting and turning the rig to get out all the trapped air. It measured 2.4 lbs (sinking).

SPREADSHEET TIME

I plugged my numbers into the spreadsheet, including the 2lbs of ditchable weight. It said I was -6.2 buoyant at 8' depth. But, I'm pretty sure I was around neutral. I'm not sure if I'm looking at the right numbers. Please check me. Please note that it is very possible that I would not have added those 2lbs if I had 15' of pool depth to experiment. These shallow depths of my pool are probably the most error prone for the spreadsheet.

The Quick Results did recommend only 0.3 pounds of weight. That is less than 2lbs from what was probably optimal, so I think that was a good estimate.

I have attached my spreadsheet with my numbers for everyone to check.

CONCLUSION

I plan to keep these numbers and use the Quick Results page to estimate my weight for my Florida trip in November. I will use my experiences at deeper depths in November to fine-tune my personal buoyancy number, and will then use the spreadsheet for future trips with different wetsuits!

I hope to add my camera and flashlight buoyancy soon, as that might offset some of the weight I'll need in salt water.