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Optimal Buoyancy Computer

Discussion in 'Advanced Scuba Discussions' started by rsingler, Mar 23, 2019.

  1. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
    3,563
    4,064
    113
    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. :)
     
  2. Ready4Launch

    Ready4Launch Barracuda

    # of Dives: 100 - 199
    Location: United States
    211
    49
    28
    "...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.
     
    rsingler likes this.
  3. Ready4Launch

    Ready4Launch Barracuda

    # of Dives: 100 - 199
    Location: United States
    211
    49
    28
    Also, forgot to ask, for my Jet Fins, should they be entered under the Other Fixed Weight field?
     
  4. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
    3,563
    4,064
    113
    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!
     
  5. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
    3,563
    4,064
    113
    Yes.
    Actually, any un-ditchable weight field would work (e.g., trim pockets field, too), but Other Fixed Weight is the most logical.
     
  6. Darcimus

    Darcimus ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 25 - 49
    Location: Regina, Saskatchewan
    149
    78
    28
    How do I account for dry gloves? Do I add a set weight for the gloves themselves then add based on the liner thickness?
     
  7. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
    3,563
    4,064
    113
    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.
     
  8. Darcimus

    Darcimus ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 25 - 49
    Location: Regina, Saskatchewan
    149
    78
    28
    OK so basically 1%. Cool.
     
  9. gr8jab

    gr8jab Manta Ray

    # of Dives: 100 - 199
    Location: Oregon, USA
    629
    372
    63
    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!
     
    Doctor Rig, Jcp2 and rsingler like this.
  10. gr8jab

    gr8jab Manta Ray

    # of Dives: 100 - 199
    Location: Oregon, USA
    629
    372
    63
    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.
     

    Attached Files:

    Doctor Rig and chillyinCanada like this.

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