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Weighting for descent vs bottom

Discussion in 'Basic Scuba Discussions' started by mi000ke, Oct 26, 2016.

  1. mi000ke

    mi000ke Manta Ray

    # of Dives: 100 - 199
    Location: Massachusetts & Grand Cayman Island
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    (Apologies for the long post.)

    I am pretty familiar with the issues involved in proper weighting – I actually constructed a spreadsheet for myself that computes my net buoyancy at beginning, midpoint, and end of dive at 0, 33, 66 and 99’ (accounting for suit thickness, compression, air consumption, net tank weight, body buoyancy, and equipment).

    So I computed my proper weight to both get me below the surface after a good exhale (I’m a sax player and know how to clear my lungs pretty well) as well as get neutral at my safety stop (I can also compute neutral at surface at end of dive as well).

    Anyway, my correct weight to get me down as well as to get close to neutral at the end is 16 lb (I weigh 150 lb and have always dived a 5mm full suit and when water temp < 75, a hood and boots as well). Actually I can do 15 # without the hood and boots but it’s easier to work with even numbers. I have done 25 dive with my “proper” weighting, and it seemed to work just fine.

    I am doing my AOW this week, and did PPB for my first dive so my weight and trim would be good for the rest of the course. Now, as an aside, I was taught and always seem to observe that most people descend feet down and just try to sink naturally once they purge their BCD and exhale (but after a search here, discovered that a lot of people duck dive down head first with finning).

    Well my instructor insisted I could use less weight, so we tried 14 lb and after purging my BCD and a good exhale I just continued to float – and my instructor observed that I was pretty still. So we agreed that 16 lb was correct.

    Well on my third dive of the course we were down with another group and their guide swam over and (as he later explained) thought I had too much air in my bcd (we were at about 95’) and that I was not able to hover motionless fighting the weight, so he took out 4 lbs. – a 25% reduction. And that was perfect for the rest of the dive. But of course at the bottom with the compression of my 5mm I would need much less weight.

    So the conundrum was/is, do you weight for the bottom (or some depth below the surface), meaning much less weight (and then have to figure out how to get yourself down), or for the top meaning more weight but making your descent easier?

    The other instructor (the one who pulled my weights) suggested it would be better to underweight and pull myself down the anchor line if needed rather than weight for the easy descent (even using the eye level at the surface test).

    Well he insisted I do the next dive using 12 lb, and it turned out that if I did a duck dive (again which I was not taught to do, but did in desperation) and finned really hard I was just barely able to get down that 5-10 feet where pressure took over and I was fine. The rest of the dive was buoyantly perfect and he was correct about improved hovering. I was of course more buoyant at the end, but had no problem holding my safety stop depth without having to hold onto the line. I just surfaced with an empty BCD.

    So (finally) my questions are, should we under-weight and work really hard (or use an anchor line) to get ourselves down? Is it supposed to require some work to get down? And for those divers who are able to do an easy descent, are they actually overweighed? Or is my situation that unique?
     
    ScubaJill likes this.
  2. Diver0001

    Diver0001 Instructor, Scuba

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    My philosophy is that control of the dive means from the surface to the surface. No part of the ascent should be uncontrolled and therefore setting your buoyancy to any depth below the surface is only going to set you up to finish every single dive with some kind of uncontrolled ascent in the last few metres. I know some people suggest that but I personally think it's wrong thinking. You do not maintain control over your buoyancy for *most* of the dive, you control your buoyancy for *all* of the dive.

    R..
     
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  3. Bob DBF

    Bob DBF Solo Diver

    # of Dives: I just don't log dives
    Location: NorCal
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    Actually you were not under weighted if you maintained a safety stop at 15', 300 to 500# of air in your tank and an empty BC. That is what I, and others, believe to be a perfect weight. That being said a few more pounds, for a specific reason, is not a problem, just remember you are over weighted.

    Learn to duck dive, its a matter of technique and is easy once you figure it out, find someone that knows how and work with them a while. One problem is that a Jacket BC can be 4 or 5 # more buoyant at the surface, more if it is not completely deflated, from air pockets in the material and how it is put together, these air spaces fill once underwater, the deeper you go the faster they fill so you loose that buoyancy once you surface. Also your wetsuit is affected by depth (pressure) and may not rebound to the same buoyancy after the dive, the amount is depth dependent.

    I like the feeling that, at the end of a dive, I can float on the surface in my gear with an empty BC.


    Bob
     
    kablooey likes this.
  4. Beau640

    Beau640 Solo Diver

    # of Dives: 200 - 499
    Location: Michigan
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    Short answer is you should be weighting yourself for the end of the dive, with an empty tank at the safety stop.

    For proper weighting, we have multiple ways to estimate it. You can use multiple formulas that are out there to estimate how much weight you will need based on your weight and the exposure protection you choose. Then you can do a "buoyancy check" at the start of the dive which is when you are at the surface with an empty BCD and exhale - you should slowly sink. However this does not confirm that your weighting is correct.

    The way you verify that you have the correct weight and your ultimate goal of weighting is to have the absolute minimal amount of weight possible so that when you are at 15ft at your safety stop with 500psi or less in your tank, you can hold the safety stop with an empty BCD.

    The fastest way to do this is at the end of your dive make sure you breathe your tank down to 500psi. When you are at 15ft, if your bcd is not empty then you have too much weight. When you completed your safety stop go to the surface, hand off 1-2lbs, and go back down to 15ft (may have to duck dive down at this point because the goal is to hold 15ft not descend easily to 15ft with an empty tank). Keep doing this until you are able to easily hold a safety stop with a completely empty bcd at 15ft. That is your minimal weight.

    All of the estimates we have are just ways to estimate that weight you would need at 15ft with an empty tank. That weight is your minimal weight that you verify at the end of the dive for the exposure protection and gear you have.


    In terms of weighting at the beginning of the dive, if you have your minimal weight for an empty tank, you should be able to get down because at the beginning your tank is full. I will say however that it takes a lot of practice to learn how to get down with that minimal weight. A couple of reasons for this is that at the very beginning of the dive before you've had any suit compression, you will have temporary air spaces. Small amounts of air are frequently trapped in places in your wet suit between your skin and the suit just from putting it on and will get vented when under pressure/compression and will no longer be an issue if you go down (You can see this if you do multiple dives without removing any of your exposure protection at all/not tearing any of it down. Your second dive will be easier to get down because that temporary air is gone).

    The thing that takes time and really needs practice though is breathing at decent. Frequently divers will exhale and the descent will start, but just as their head gets under the water they inhale and they pop right back up or don't think they can descend. You do have to do a little breath hold at the beginning if you have the minimal weight. Exhale and as you are going down don't breath in until you are a few feet under the surface. then do small breaths, not giant full breaths until you are further down and suit compression has taken effect and you are far enough below. Or just duck dive down and force yourself down a few feet if you can't get the hang of the breathing :p


    TLDR: weight for the end of the dive with an empty tank at the safety stop of 10-15ft and an empty bcd. that is the proper/minimal weight you should have.
     
    Last edited: Oct 27, 2016
    Dogbowl, stuartv and northernone like this.
  5. Banyan

    Banyan Divemaster

    # of Dives: 200 - 499
    Location: Homestead, FL
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    As everyone is saying, you weight for the safety stop with an empty tank. If you're able to do that with 12#, then you need that or less. Reasons for inability to sink initially may include not completely exhaling, unconscious finning, air pockets in the BC, and air pockets inside the wetsuit.
     
  6. BRT

    BRT Giant Squid

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    How much air was in your tank at the end of the dive with 12lbs?
     
  7. redacted

    redacted Guest

    Off you can hold a SS at the end, it should not be necessary to pull yourself down at the beginning of the dive. You may need to do a better job emptying your HDD and getting all the air out of your suit.
     
  8. Diver0001

    Diver0001 Instructor, Scuba

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    I don't know where this talk about weighting for the safety stop at the end of the dive comes from. This means that the last 15 ft of every dive involve an uncontrolled ascent, which is something you want to avoid.

    Of COURSE you want to be weighted right for the 15 ft stop but also at 10ft and 5ft or even 2ft. Cutting back on the couple of pounds of weight it takes to maintain control throughout the *entire* dive from surface to surface is like having breaks on your car that stop at some point AFTER the stop sign. Someone said above that the perfect weight was with 500# in the tank. What if the diver gets delayed and he has 300# in the tank? Cutting the margin so thin will mean that he will not be able to hold his safety stop if he has to use any of the reserve air he has...... think about what you're saying here.

    Anyway, I think this is wrong thinking. Obviously we are dealing here with two different paradigms but I think the difference in how much weight you need to be in control all the way to the surface isn't worth the potential issues involved in making an uncontrolled ascent through the last 15ft of every dive. Seriously.

    If you think about it, no technical diver on the planet would weight themselves to make an uncontrolled ascent over the last 15ft of every dive for obvious safety reasons so why are you recommending for recreational divers to do just that? It's bad advice if you ask me.

    R..
     
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  9. dmaziuk

    dmaziuk Orca

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    The 1st 10 feet tend to be "bouncy" especially after a long(-er) break when your gear is bone-dry. It's worse in salt water. I'm taking an extra couple of pounds for the very first tank of the trip, and leaving it behind for the rest.

    If you can hold a safety stop breathing normally, you should be able to control your final ascent with just your lungs -- but people's lung sizes vary as does the weight of their gear, some might carry an extra pound or two for that.

    I tend to carry a couple of pounds over the "peak performance" weight: I like not having to control my breathing for buoyancy while also controlling it to frame, and positioning myself for, a shot. But then I can swim in speedos across the bottom of a 12' deep diving well carrying a 10 lb brick, so I am fairly comfortable with an extra pound or four... YMMV.
     
  10. guruboy

    guruboy Divemaster ScubaBoard Supporter

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    I don't know about you but my lungs are not enough to compensate for the buoyancy changes of a 7mm full wetsuit
     

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