Do experienced divers need to carry less weight?

[drk5036]

When one muscle contracts, there is basically an opposing muscle which stretches. One would seem to offset the other?

When the muscle expands, it shortens in length. There is almost no change in volume, just a change in shape (as was mentioned above).

 

[tursiops]

I said I thought that your muscles got firmer and bigger when you tensed up, hence a tense diver is more buoyant than a relaxed diver.

So... where does this extra volume come from?

Just to be sure, I did a small search. One of the papers I found (R. J. Baskin, P. Paolini: "Volume change and pressure development in muscle during contraction", The American journal of physiology 213(4):1025-1030, 1967, DOI 10.1152/AJPLEGACY.1967.213.4.1025) did in fact measure a volume increase of muscles during contraction. Up to a whopping 3x10^-5 mL/g, or some 0.003%. I don't think that a volume change of 0.003% would have a significant effect on weighing.

When one muscle contracts, there is basically an opposing muscle which stretches. One would seem to offset the other?

When the muscle expands, it shortens in length. There is almost no change in volume, just a change in shape (as was mentioned above).

OK, I'm always ready to accept facts over speculation.
But, then, why are can you descend easier if you are relaxed? Is it really ALL due to the lung volume?

 

[Marie13]

I remember a post from dearly departed Lynne (TS&M) where she wrote that some guy online was giving her a hard time for needing something like 30lbs. From photos she was a petite woman, but she liked to be warm in cold PNW waters and I think wore really thick undies. She said she dove with this guy once and after seeing she really DID need that seemingly large amount of weight, he shut up about it.

While a lot of divers ARE overweighted, other people really do need a lot of weight.

 

[johndiver999]

I said I thought that your muscles got firmer and bigger when you tensed up, hence a tense diver is more buoyant than a relaxed diver.

OK, I'm always ready to accept facts over speculation.
But, then, why are can you descend easier if you are relaxed? Is it really ALL due to the lung volume?

As I mentioned in my post, it just seems like apprehensive people won't fully exhale in water.. just my little observation... nothing scientific. Funny yesterday, I jumped in the pool. I happened to notice that I took a big breath in when I jumped - without thinking about it.. seems like a natural response.

 

[johndiver999]

If muscular contraction/relaxation was an effective means to adjust displacement/buoyancy - then it should be easy to demonstrate while holding your breath. You could test it without scuba gear, if lung pressure was a concern. I wonder if you could actually make a change if you manipulated the diaphragm which might actually affect the lungs, rather than skeletal muscles.

 

[boulderjohn]

I remember a post from dearly departed Lynne (TS&M) where she wrote that some guy online was giving her a hard time for needing something like 30lbs. From photos she was a petite woman, but she liked to be warm in cold PNW waters and I think wore really thick undies. She said she dove with this guy once and after seeing she really DID need that seemingly large amount of weight, he shut up about it.

In Puget Sound, Lynne wore a Whites Fusion with their MK3 undergarment. When I dived with her there, I did the same, the only time I ever wore my MK3s. I was shockingly buoyant--I couldn't believe it. I did not think Seattle had enough weights to get me under water.

 

[hedonist222]

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

 

[boulderjohn]

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

6 and 8 lbs????

1. Years ago I spent a week diving with an operation on Ste. Maarten and got to know the crew pretty well. On day I saw one of their instructors doing basic buoyancy exercises with a petite woman who I knew was taking an AOW class, and what they were doing was not part of the planned dive. I asked the instructor later what was going on, and she said the woman had asked for an unreal amount of weight. She had talked her down to something more reasonable and then seen immediately on the dive that the woman did not have a clue about buoyancy.

Through one of the most incredible coincidences in world history, that young woman had the seat next to me on the return flight, and we talked about it. She had recently gotten her OW through a chain store dive operation in LA, and they had given all students 20% of their body weight for the pool sessions. They give this 100 pound woman 20 pounds. Incredulous, I said I could not understand how they could do the buoyancy exercises with that much weight. She said no one could do those buoyancy exercises at all. The instructor told them not to worry about it--skills like hovering were for true experts only, and to get certification, all they had to do was give the buoyancy exercises a futile try. The instructor had them wear more weight in the OW dives, and told them to wear that extra weight whenever they were in the ocean.

2. On a boat dive in Bonaire, a young woman clambered on the boat ahead of her boyfriend. She had a weight integrated BCD, and the weights were bulging out of the pockets, obviously far more than the pockets were designed to hold. Other divers started talking about it with her. She said she was newly certified, and she was following her instructor's recommendation on weighting. Her boyfriend, who had not yet gotten out of the water, insisted on it. People acted quickly to take out half her weight before the boyfriend got back on board and told her not to tell him until later. She did just fine on the second dive.

3. While I was doing a dive as a part of my IDC in Key Largo, I saw a woman literally crawling on the sand on her hands and knees, with so much weight she could not possibly swim unless she fully inflated her BCD. The look on her face was pure misery. When I saw that, I vowed that none of my students would ever look like that.

Instructors put unreal amounts of lead on students. Giving 6-8 extra pounds to a diver who needs 2 is closer than I believe most come.

 

[Bob DBF]

If one is experienced in weighting oneself properly, yes. Experience as in dive count or certification, maybe. One can have excellent control of their buoyancy whilst overweighted, so how one dives is not necessarily an indicator of proper proper weighting.

 

[Ski_Lounge]

Reflecting on my personal experience when I started diving couple of years ago.

- Complacency and ease: I don't really think I knew what it meant to be "properly weighted" initially. Not blaming my instructors, they were great. But until I started paying more attention to my buoyancy much later, my weighting consisted of "I'll grab the same amount of lead I used last time". Which, uh, was whatever I used in my OW course (aka, very overweighted). As others have mentioned here, some OW instructors do overweight their students (rightly or wrongly).
- Situational awareness: So I'm diving with ~20 lbs of lead, but it was not obvious to me or my (equally new) diving buddies how large that was (for the conditions we dove). To @boulderjohn Bonaire dive boat story, I wish I had buddies like that!
- 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. [EDIT: Striking this incorrect section off, as pointed out further below.]
- Descend mechanics: In my AOW course, the instructor (GUE trained, very big on buoyancy and trim), asked all of us to shed 10 lbs of lead. I did it, but to be honest, I struggled descending from the surface (because I was not kicking, mostly I was used to dropping like a stone). I mis-associated "not being able to drop like a stone" with "not having enough lead"; breaking that mental association still took more dives. In a 5mm or 7mm wetsuit (especially a new one), it was sufficient to just get to down to 10-15 ft by kicking, which reduced my buoyancy enough to continue descending without requiring additional weight (but I had yet to learn that).

Maybe I'm wrong about this, but it seems that more experienced divers need to carry less weight than newbie divers.

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.