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Thread: Question for those in the know! 85lb wing too big?

 


  1. #21
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    Quote Originally Posted by DevonDiver View Post
    Go figure how I achieve neutral buoyancy on 4 tank dives with a <30lb wing (sidemount) several times a week...

    Even accounting for the fact I'm in (max) a 3mm suit...
    I don't really see the problem. It is just physics. A 3mm suit is probably around 7 lb buoyant. 4 tanks (I'll assume Al80s) gives you 24 lb of gas. You probably do not use all 24 lb But I like to be able to plan on using it (not much sense in having unusable reserves). So, figure your actual gas swing is more like 20 lb (leaving 500 psi in each tank). Actual compression on your 3mm suit is less than 7 lb. 5 lb of buoyancy lost as you approach 90 ft might be a more typical figure. So that would be 25 lb of buoyancy swing on a <30 lb wing. That is using most of the capacity but not a real problem. You have room to spare - just not an excess. If you really start to push the limits of your wing you still have a bit more neutral buoyancy capacity just in managing the gas in your lungs. So, a few pound on the heavy side of neutral is still quite manageable between breathing control and just a little pitch in body position.

    Or do you see something wrong with my interpretation of the physics?
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  2. #22
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    Quote Originally Posted by awap View Post
    Or do you see something wrong with my interpretation of the physics?
    Nope, I see that you've presented a very rational reason why 30lb is sufficient for most technical divers (warm-water/al80s).... and an even more prudent justification as to why a diver in that position wouldn't require more than a 40lb wing, at the most.

    As you say, a wetsuit doesn't lose total buoyancy - especially not within recreational, or even entry-middle level technical depths. Likewise, the suit isn't compressed on the surface - so pre-dive flotation isn't an issue. Upon reaching depth, you're already burning through gas... have a few lbs spare...and can compensate a few lbs more with breathing (not actually compensation, as we're breathing more fully at tech depths anyway right?).

    As an aside, not sure where you get your figures for cylinder buoyancy from, here's the figures I use: Scuba Diving Cylinder Buoyancy Details

    Catalina S80 - 1.8lbs negative when full @ 3000psi (4 x cylinders = 7.2lbs) / 4lbs positive when empty (4x cylinders = 16lbs)
    Luxfer 80 - 1.4lbs negative when full @ 3000psi (4x cylinders = 5.6lbs) / 4.4lbs positive when empty (4x cylinders = 17.6lbs)

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    Quote Originally Posted by DevonDiver View Post
    As an aside, not sure where you get your figures for cylinder buoyancy from, here's the figures I use: Scuba Diving Cylinder Buoyancy Details

    Catalina S80 - 1.8lbs negative when full @ 3000psi (4 x cylinders = 7.2lbs) / 4lbs positive when empty (4x cylinders = 16lbs)
    Luxfer 80 - 1.4lbs negative when full @ 3000psi (4x cylinders = 5.6lbs) / 4.4lbs positive when empty (4x cylinders = 17.6lbs)
    I use a different site but the same data. I take the difference between full and empty buoyancy to get the weight of the gas which is what causes the change and requires compensation. e,g., 4.4 (empty) - (-1.4 (full)) = 5.8. Rounded off to 6 lb. Tank weight is immaterial as long as you are weighted correctly (and not overweighted just from the tank(s)). Pretty soon I may remember that 13 cu ft is a pound - maybe.
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    not if you're raising a Volkswagon...

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    Quote Originally Posted by awap View Post
    I use a different site but the same data. I take the difference between full and empty buoyancy to get the weight of the gas which is what causes the change and requires compensation. e,g., 4.4 (empty) - (-1.4 (full)) = 5.8. Rounded off to 6 lb. Tank weight is immaterial as long as you are weighted correctly (and not overweighted just from the tank(s)). Pretty soon I may remember that 13 cu ft is a pound - maybe.
    The relative weight of gas doesn't matter, except for it's contribution to the buoyancy of the diver - so the +/- figures are most telling.

    The diver is 'most' negative upon reaching the bottom on descent. This is the point where they have the most gas and the least buoyancy from their wetsuit. If they multi-level upwards and with each breath they take.. they become more buoyant as the dive progresses. In technical diving (where gas consumption is high) that happens quickly.

    So (using Luxfer AL80):

    4 tanks are -5.6lbs
    Carrying weights to compensate for 5/6ths (tanks at 500psi) empty tanks later in the dive -14.6lbs
    Total negative buoyancy at this point: -20.2lbs


    The other factor is wetsuit buoyancy. This is dependent on wetsuit size (S/M/L/XL etc) and wetsuit thickness (3mm/5mm etc). The loss of buoyancy at depth doesn't matter - it's a decrease, but doesn't contribute towards negative buoyancy in any way. What matters is the amount of weight needed to be worn to compensate for its maximal positive buoyancy (an issue on descent and ascent at shallow depths).

    So, to exceed a requirement for 20lbs of compensating weight (bringing total negative buoyancy beyond the threshold of a 40lb wing), the wetsuit needs to be more than 20lbs/9kgs un-compressed buoyancy.

    According to this chart, 20lbs/9kgs is on the outside limits for wetsuit buoyancy. More buoyant than 7mm with over-vest and hood....



    In the study/article linked above, they measured and recorded that a 5mm suit only provided 2.63kg/5.8lbs!

    Also add to that consideration the total loss of buoyancy from the suit - which is an easily calculated fraction, based on ambient pressure.

    DEPTH - POSITIVE BUOYANCY

    Surface - 20lbs (un-compressed)
    10m - 10lbs (50% compressed)
    20m - 6.6lbs (66% compressed)
    30m - 5lbs (75% compressed)
    40m - 4lbs (80% compressed)
    50m - 3.3lbs (84% compressed)
    60m - 2.85lbs
    70m - 2.5lbs
    80m - 2.2lbs
    90m - 2lbs
    100m - 1.8lbs

    That positive buoyancy needs to be factored into the overall calculation - subtracted from the total negative buoyancy. Realistically, on all but the deepest trimix dives, a 20lbs suit doesn't need 20lbs of compensating weight. For the 'average' tech diver (up to ER certification), they would always retain a significant amount of positive buoyancy from their wetsuit (retaining ~15% positive buoyancy)..which is noteworthy when considering the overall premise that 40lbs of bladder lift is sufficient.

    Bear in mind also, that when calculating surface flotation requirements, only the weight of gas is a factor - the wetsuit is uncompressed and balanced against the weight carried to offset its buoyancy loss at depth.

    So, to continue the calculations:

    Total negative buoyancy at this point: -20.2lbs
    Total weight needed to offset a 5mm wetsuit: -5.8lbs
    Less retained buoyancy in the suit (@50m): 0.96lbs

    TOTAL compensation required from wing bladder: 25.04lbs

    Add to that, consideration of respiration derived buoyancy/compensation:

    Average tidal volume of diver (respiration) : +/- 3.3lbs (buoyancy variation upon normal breathing)
    Average vital capacity of diver (respiration) : +/- 8.25lbs (buoyancy variation upon maximal breathing)

    END RESULT:

    A technical diver with 4x Luxfer AL80, in a 5mm wetsuit, diving to 50m, needs a maximum buoyancy provision of 25.04lbs, of which ~4-5lbs can be offset in the short/medium term by a deeper breathing pattern.



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    Quote Originally Posted by DevonDiver View Post
    The relative weight of gas doesn't matter, except for it's contribution to the buoyancy of the diver - so the +/- figures are most telling.

    The diver is 'most' negative upon reaching the bottom on descent. This is the point where they have the most gas and the least buoyancy from their wetsuit. If they multi-level upwards and with each breath they take.. they become more buoyant as the dive progresses. In technical diving (where gas consumption is high) that happens quickly.

    So (using Luxfer AL80):

    4 tanks are -5.6lbs
    Carrying weights to compensate for 5/6ths (tanks at 500psi) empty tanks later in the dive -14.6lbs
    Total negative buoyancy at this point: -20.2lbs


    The other factor is wetsuit buoyancy. This is dependent on wetsuit size (S/M/L/XL etc) and wetsuit thickness (3mm/5mm etc). The loss of buoyancy at depth doesn't matter - it's a decrease, but doesn't contribute towards negative buoyancy in any way. What matters is the amount of weight needed to be worn to compensate for its maximal positive buoyancy (an issue on descent and ascent at shallow depths).

    So, to exceed a requirement for 20lbs of compensating weight (bringing total negative buoyancy beyond the threshold of a 40lb wing), the wetsuit needs to be more than 20lbs/9kgs un-compressed buoyancy.

    According to this chart, 20lbs/9kgs is on the outside limits for wetsuit buoyancy. More buoyant than 7mm with over-vest and hood....



    In the study/article linked above, they measured and recorded that a 5mm suit only provided 2.63kg/5.8lbs!

    Also add to that consideration the total loss of buoyancy from the suit - which is an easily calculated fraction, based on ambient pressure.

    DEPTH - POSITIVE BUOYANCY

    Surface - 20lbs (un-compressed)
    10m - 10lbs (50% compressed)
    20m - 6.6lbs (66% compressed)
    30m - 5lbs (75% compressed)
    40m - 4lbs (80% compressed)
    50m - 3.3lbs (84% compressed)
    60m - 2.85lbs
    70m - 2.5lbs
    80m - 2.2lbs
    90m - 2lbs
    100m - 1.8lbs

    That positive buoyancy needs to be factored into the overall calculation - subtracted from the total negative buoyancy. Realistically, on all but the deepest trimix dives, a 20lbs suit doesn't need 20lbs of compensating weight. For the 'average' tech diver (up to ER certification), they would always retain a significant amount of positive buoyancy from their wetsuit (retaining ~15% positive buoyancy)..which is noteworthy when considering the overall premise that 40lbs of bladder lift is sufficient.

    Bear in mind also, that when calculating surface flotation requirements, only the weight of gas is a factor - the wetsuit is uncompressed and balanced against the weight carried to offset its buoyancy loss at depth.

    So, to continue the calculations:

    Total negative buoyancy at this point: -20.2lbs
    Total weight needed to offset a 5mm wetsuit: -5.8lbs
    Less retained buoyancy in the suit (@50m): 0.96lbs

    TOTAL compensation required from wing bladder: 25.04lbs

    Add to that, consideration of respiration derived buoyancy/compensation:

    Average tidal volume of diver (respiration) : +/- 3.3lbs (buoyancy variation upon normal breathing)
    Average vital capacity of diver (respiration) : +/- 8.25lbs (buoyancy variation upon maximal breathing)

    END RESULT:

    A technical diver with 4x Luxfer AL80, in a 5mm wetsuit, diving to 50m, needs a maximum buoyancy provision of 25.04lbs, of which ~4-5lbs can be offset in the short/medium term by a deeper breathing pattern.


    While the weight characteristics of the tank can not be totally ignored, I suspect if you run your same logic with a Catalina Al80 (S80, 4lb, -1.8lb) you will get the same results. at least it would work the3 same for me as the only difference would be about 1 lb of lead on my belt. In fact, if the diver were wearing a 7mm suit, you might get the same results with most of the steel 80's as long as the difference is compensated by the amount of lead carried. In that case, it is the buoyancy of the diver, not the buoyancy of the tanks that matters. In warm water, the steels would grossly overweight and the tank characteristics might dominate in wing selection. In my case, as a single tank diver; the switch between Al and Steel is usually just a matter of adjusting my added weight (including changing from a heavy to a light plate).

    But you are correct in the sense that if you were to try to go with 3 HP 80's rather than Al 80s in warm water, light suit; you would probably be looking at a different wing requirement even though the gas swing is about the same. But the reason would be that those tanks, even with no added weight, leave you overweighted. But if you were diving those conditions with the Al tank and 24 lb of lead on your belt (remember that thread) then the Steel 80s would just offset most of the lead and the same wing should work for both.
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    I am fairly certain that manufacturers' stated lift capacities are all over the place, making it more complicated to apply any precise lift-need calculations to actual wing selection. I have two dive rite wings, a travel wing and a rec wing. The travel is listed at 30lbs lift, the rec wing at 45. Yet I'm sure that the rec wng is at least twice the size, fully inflated on tanks, as the travel wing....no way are those figures accurate in actual use. I suspect the rec wing has more than 45 and the travel wing less than 30.

    So probably the best way to choose a wing, short of being able to really test the lift capacity yourself, is to ask around with experienced divers and take your best guess...

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    For me, a 40# wing with a drysuit is more than sufficient for double HP100s and a 40 ft^3 deco bottle. I can't imagine more than doubling that for a rig with less ballast!

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