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Thread: Calculating Weight Requirements

 


  1. #1
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    Kingpatzer's Avatar
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    Calculating Weight Requirements

    I've seen a few posts where folks comment on how there's a number of ways to calculated weight needs.

    Unfortunately, I've never actually seen any of these spelled out except the PADI "take a breath and hold it." method.

    Can folks either point me to a page that has the various methods spelled out, or could you spell out your favorite method here?

    Thanks

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    hudson's Avatar
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    This is probably overkill (the description, not the method) but...

    The usually taught "float at eye level" method can be very inaccurate in my experience, so it's best to simulate the exact conditions where you need to be neutrally buoyant.

    Let's first define the correct amount of weight: you should be neutrally buoyant below the surface with a near empty tank, half a breath (err on the "too much" breath side to gauge this). Also need to define "near empty tank". That is just above the minimum psi you always plan to surface with (500 is a good number)

    Now you have two alternatives, get down to 5 or 10ft with a full tank or with a near empty tank. The full tank method will only work if you are diving cool or cold water. If you use a full tank, you need to add the weight of air you carry in the tank once you are done.

    In either case, ballpark your needs, then add some more weight in your pockets or clip them to your d-rings. Carry all the equipment you normally do. How much "extra" weight you want depends on what your ballpark figure is, about half your estimated figure would be safe. If you can, have 2 pcs of each denomination adding up to the total extra you want. Find a shallow spot, either a shore dive or a training platform for example. Get all the air out (it's best to swim around a bit to get rid of trapped air), practice a good half-breath in horizontal position and start removing weights till you attain neutral. Compensate for the air in the tank if using a full tank. Voila, done. If you find yourself losing buoyancy control above this depth on ascents, add a pound or two.

    After that you may want to work on your trim, shifting weight around up and down your body, around your weight belt etc to find the perfect arrangement to leave you motionless in perfect trim. Anything you end up clipping high on your BC can then be transferred to your cambands using weight pockets.

    Numbers for ballparking (these may be way, way off, please feel free to correct)

    Self buoyancy: -1 to 4lbs
    3mm wetsuit 4-6lbs
    7mm wetsuit 16-22lbs
    7mm Boots, hood etc 1lbs each
    Crushed neoprene drysuit with med-weight underwear 32-40lbs (less for trilam, more for neoprene)
    First stages -2lbs
    BC: +1lbs or more if not BP/W
    Tanks: refer to a tank spec chart
    Last edited by hudson; January 27th, 2008 at 01:29 PM. Reason: fixed wording to avoid misunderstandings about shallow water buoyancy

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    herman's Avatar
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    There are several ways weight can be approximated but in the end it comes down to trial and error. You can measure the requirements for each piece of you gear and add that up, then add what you need for your body...trouble is the only real way to know the correct weight is to measure it....put you BC in the pool, get all the air out of it and add weights until it sinks....and then that only is valid for fresh water, you have to add more for saltwater. To add to the confusion, everyones body is different and believe it or not but expereince level also plays a big role in the weight needed. IMO it's not worth the effort, might as well just put it all on and do the 1/2 breath-eye level routine. It's dead on and easy to do. If your not sure, error on the overweighted side, it's a lot easier to deal being overweighted than it is under (don't forget to add weight for the air used in the tank). The biggest help is to make sure you log your weight, equipment worn, how it worked and any "next time" suggestions. On each dive tweek the weights until you have a log book entry that works for every dive combination you have.
    herman

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    I think the ball park method with a full tank at the surface works partly because in addition to your full breath, dry BC, dry wetsuit, air pockets, etc.... You have about the same amount of buoyancy to compensate for the weight of the full tank. If you are in the upright position, staring straight ahead, your mask should be just under the water. Remember, this is done with a full breath, and during a breathhold with a completely empty BC. If you exhale, you should sink below the surface. As you dive, all the little trapped air bubbles in your bc pocket, your bc jacket, your wetsuit, your hood, boots, etc. get compressed and wet, then you lose this buoyancy. This is a crude method, but it works for rental gears. Most likely, you will be overweighted.

    If you did this method with a wet, just dove wetsuit, or no wetsuit at all, it might underweight you. Also your maximal inspiratory volume varies between people.

    My opinion is, the best method is with a wet, doved wetsuit, wet doved gear, and a 500 psi tank at the surface is best. And this is done with an average breath, and not with a full breath.

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    Quote Originally Posted by Kingpatzer View Post
    I've seen a few posts where folks comment on how there's a number of ways to calculated weight needs.

    Unfortunately, I've never actually seen any of these spelled out except the PADI "take a breath and hold it." method.

    Can folks either point me to a page that has the various methods spelled out, or could you spell out your favorite method here?

    Thanks
    Thanks for asking the question. I'm such a crazy mixed up kid (cold water, warm water, salt water, fresh water, 3 mil shorty to farmer Janes - and I'm now 30+ pounds less than I've ever dived). I'm dreading the "how much weight do you need?" question next Saturday.

    I know I've seen a quick estimate based on weight and type of wetsuit. A starting point would be good if anyone can point me to it.

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    I've always thought the manufacturer's of exposure protection and diving accessories could do divers a great service if they published the buoyancy characteristics of their products (brand new of course) for each size. That would help divers get in the ballpark for weight when adding/changing pieces -- although trial and error will always be necessary to some degree.
    3 1-week dive vacations a year just isn't enough. I need a 6 month sabbatical!

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    jim T.'s Avatar
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    I've got a doubles question. If you do your 10-15' buoyancy checks using 500psi GAUGE pressure, you really have 1,000 psi. I did my buoyancy checks using 500 psi gauge pressure. Should I also be checking it at 250psi GAUGE pressure which is really 500psi?

    I usually make sure I'm at my safety stop with 500psi gauge pressure, but wondered about
    using 250psi instead or in addition?

    I've gotten different answers from different doubles divers. What's the consensus?

    Thanks.

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    With a steel tank, full 3 mm wetsuit, 5 mm boot, a beanie 1 mm cap, I can dive with 6 lbs in fresh water.

    With aluminum tank, same set up, I need 11 lbs in salt water.

    With a 7mm/5mm full wetsuit, boots, and cap, I need 16 lbs in salt water.

    With a 7/5 full suit, and a 7 mm corewarmer, I need 22 lbs fresh, and likely about 5 lbs more in salt.

    With a drysuit, I need 22 lbs in freshwater with 200gm insulation.

    With nothing at all, and a steel tank, I need no weight in fresh water.... But about 5 lbs with an aluminum tank.

    You can not estimate weight needed unless you know the habitus of the diver. Obese divers have more fat, fat is buoyant, and you need to figure that in.

    You also need to know the type of BC one is wearing .... Some are negative (steel or AL backplate), and some are positive (plastic moulded backplates).

    The type of tank affects your buoyancy also. If you were neutrally buoyant, and dove with a neutral BC .... Steel tanks are negative when empty, while aluminum tanks are neutral to POSITIVE 6 lbs when empty.

    You can not pass judgement until you know the type and manufacture of a tank. So called "neutrally" buoyant aluminum tanks are simply manufactured with an extra 4 to 6 lbs to compensate for the positive buoyancy characteristics of the AL tank. Unfortunately, this makes the tank heavier.

    If you make yourself neutrally buoyant with your BC (unfortunately, we will ignore the reg), you can calculate how much weight you need to add or subtract by looking at the buoyancy characteristics of the following types of tanks:

    Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

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    Quote Originally Posted by hudson View Post

    Let's first define the correct amount of weight: you should be neutrally buoyant at 10ft with a near empty tank, half a breath (err on the "too much" breath side to gauge this). The other alternative is to try for neutral buoyancy at the surface, but you have no reason to hold a depth between 10ft and 0ft, and in my experience if you are neutral at 10 ft you will have a nice and very slow ascent to the surface. Also need to define "near empty tank". That is just above the minimum psi you always plan to surface with (500 is a good number)
    In shore dives significant time can be spent near the exit point safely burning air while exploring in shallow bright water.

    There is no need to reduce weights so much that you cannot be in control until you surface.

    When in doubt a diver is far better off with 2-4 extra pounds than 2 too few.

    Remember to set your weight so that you bob vertically at eye level at the end of your dive with an empty BC, an average breath, your feet still (crossed) and about 500 PSI in your cylinder. A deep breath should get your mask out of the water and a deep exhale should sink your mask. Do all of this while breathing from your regulator. The end of the dive is the defining moment for your weight requirement and you want just enough to let you stay down in the shallows with a light cylinder.

    You can make the same test pre-dive with a full cylinder and add 5 pounds to compensate for the buoyancy gain you will experience as you breathe the tank down. Be sure to repeat at the end since you are apt to have some stowaway buoyancy (trapped air) in your gear early in the dive. You are safer being two pounds heavy than 2 pounds light.

    Pete
    My ever growing collection of assorted ramblings on scuba topics can be read here.

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