Do experienced divers need to carry less weight?

[AJ]

So I think that completely ties with my experience -- as I dived more, and gained a better understanding of buoyancy and mechanics, I shed more lead. It's probably the same for other divers as well.

Yep, me too. At first I was afraid to go up unplanned. Nowadays I hate being over-weighted. I shed weight to the point that even a deep breath wille take me up. I love the feeling of weightlessness in stead of having to pump air in my wing and suit. It took me quite some dives to reach that kind of buoyancy control however.

 

[boulderjohn]

- Tank buoyancy mechanics: I had ingrained the habit of carrying extra weight while diving AL-80 tanks (mainly because I'm told that's what I need to do, not because I understand the swing in buoyancy from negative to positive for an AL tank). When I switched over to ST tanks, I took some weight off, but not enough. And I certainly didn't realize that ST will cause less of buoyancy swing towards the end of the dive.

...and you are still learning.

There is no difference in the buoyancy swing between aluminum and steel tanks. None whatsoever. It is one of the great myths of scuba.

Your buoyancy depends upon the relationship between 1) the amount you weigh, including all your gear, and 2) your total volume. Changes in buoyancy during a dive occur because of a change in volume or a change in weight. Changes in volume occur when you inflate or deflate your BCD, your thermal protection compresses or decompresses, and when you breathe. Changes in weight occur when you pick something up, drop a weight, or lose air weight from the scuba tank. When we talk about the buoyancy swing of a scuba tank during a dive, we are talking about the loss of air. Period. Losing 6 pounds of air from an aluminum tank creates the same swing as losing 6 pounds of air from a steel tank, because your volume does not change in either case.

People get confused because they know an aluminum tank goes from negative to positive as air is removed, whereas steel tanks remain negative. That doesn't matter. It is the weight and volume of your total package that determines buoyancy, not one small part of that package.

You need extra weight with an aluminum tank because it is lighter than steel, not because of a difference in buoyancy swing during the dive.

Aluminum tanks do impact trim in that most will become lighter in the tail as they lose weight.

 

[tursiops]

Tank buoyancy mechanics: I had ingrained the habit of carrying extra weight while diving AL-80 tanks (mainly because I'm told that's what I need to do, not because I understand the swing in buoyancy from negative to positive for an AL tank). When I switched over to ST tanks, I took some weight off, but not enough. And I certainly didn't realize that ST will cause less of buoyancy swing towards the end of the dive.

Yes, you need extra weight if using AL80 than Steel, all else being the same, because a Steel tank weighs more in the water than does an AL tank.
No, it is not because of buoyancy swing, which has ONLY to do with the weight of the air you've used over the course of the dive; 80 cuft of air weighs about 6.4 pounds, so if you've used 2500 psi of your 3000 psi AL 80 tank, you've used 2500/3000 of 77.4 cuft (an AL is not actually 80 cu ft!) or about 65 cuft, which weights about 5 pounds. THAT is why you want to be weighted 5 pounds heavy at the beginning of the dive, so you are neutral at the end. It has nothing to do with the kind of tank you are using....you'll still use up many cuft of air and it weighs something.

 

[Ski_Lounge]

@boulderjohn & @tursiops Ah indeed, noted! Thanks for the explanation, I do have so much more to understand and incorporate, appreciate the feedback.

 

[boulderjohn]

More thoughts on experience and weight...

• The newer diver, overweighted, is diving within the depth limits of training, swimming at about 30-45 feet. With all weight at the waist, the diver swims at a 45° angle, kicking constantly. Because the diver is overweighted, the BCD has a lot of air. The diver ascends a few feet and is suddenly heading toward the surface, because that excess air reacts to the change in pressure, and the change in pressure is far more significant at shallow depths. "Rats!" the diver says. "I keep heading to the surface. I must need more weight!" A DM, infact, may provide that extra weight, giving professional support to the notion.
  MORAL: Adding weight makes it worse. The diver is going to the surface because of too much weight, not too little. If the diver can descend, the diver has enough weight. Going back up unintentionally is a skill issue, not a weight issue. (Not talking about an empty tank weight shift.)
• A newer diver and a more experienced diver are following a DM on a reef. Both are swimming along nicely, but they come to a large coral formation that will force them to swim over it. As the newer diver reaches the top of the coral formation, he realizes his BCD is pulling him to the surface. He dumps air from the BCD and comes back down on the back side of the formation, where he needs to add air back quickly to avoid crashing on the bottom. "I need more weight," he thinks. When the more experienced diver comes to the same coral formation, she realizes she will become more buoyant as she ascends, so she exhales as she does. At the top of the formation, she levels off, reaches the back side, and inhales to maintain buoyancy as she descends.
  MORAL: Experienced divers anticipate changes in buoyancy as they change depth and take proactive steps, usually through breathing, to prevent problems less experienced divers attribute to being underweighted.

 

[Storker]

It just seems like apprehensive people won't fully exhale in water..

Rather that, IMNSHO. IME it's all about the breathing.

 

[Storker]

I've seen instructors over-weight divers who could easily dive with 2 lbs
Yet they stuff them with 6 and 8 lbs

Well, >90% of my dives have been dry. I even certified OW dry. And I'd rather be overweighted than underweighted (within reasonable limits). If I'm somewhat overweighted, I can deal with that. If I'm underweighted, I'll have serious issues holding my safety stop, and worst case I'll even have problems submerging.

So if I were teaching a class, I'd rather my students were somewhat overweighted than underweighted. When they're certified, they'll have all the time in the world to tune their weighting. Which they should do if they're truly independent divers.

Up here, a kg or two too much isn't significant.