Why Do Relaxed Divers Need Less Weight?

[RobPNW]

I hear this all the time. After a few days of diving you relax and need less weight. Or when a diver gains experience, they need less weight. Is it simply because you're breathing easier and presumably exhaling more thoroughly? Or is there something about tension in the body that makes a person more buoyant?

 

[inquis]

Is it simply because you're breathing easier and presumably exhaling more thoroughly?

Yep. A related aspect is breathing more slowly, allowing yourself time to exhale further and slightly sink ("undoing" the slight rise) before starting the cycle again.

Our muscles are mostly made of water, which doesn't compress when squeezed. Tense and relaxed muscle fibers have about the same volume (and therefore similar buoyancy).

ETA: newer divers often have "happy feet" (and hands). Combined with diagonal trim, it's a recipe to go up unless more weight is added.

ETA2: thought of another related factor: more experienced divers often move more slowly and/or efficiently, requiring smaller breaths, which lowers the "average" amount of air during an entire breath cycle. There are likely other related factors that ultimately decrease the average amount of air in the lungs.

 

[boulderjohn]

In my early tech diving training, I noticed something that might be telling. On our dives, we would go deep, go through gas switches, etc., before getting to the main part of our bottom time, when we would be swimming along a wall at a specific depth on cruise control for a while. We, of course, made sure our buoyancy was good for that depth as we started. At that point, I would invariably go into my Zen diving mode in which I tried to relax totally. For the first few times I did that, as soon as I went into that relaxed state, I would rapidly lose depth and have to add air to my wing to compensate for the loss of buoyancy.

 

[Norwegian Cave Diver]

When I get into bed and put my head on the pillow I take a breath and allow my mind and my body to relax. I do the same in the first few seconds after I put my head under water and start the dive.

 

[dmaziuk]

It's a mystery. Try floating on your back in the pool and you'll find that you can only do it if you relax. There is no physics explanation for it that I know of.

 

[RobPNW]

I know that whenever I busy myself with any task loading in the water I start drifting up. Must be breathing too shallow.

 

[Wallowa]

If a relaxed diver and a non-relaxed diver [? working/anxious] have the same non-compressible volume and the same weighting when in a neutral buoyancy state; only the lung volume and rate of breathing can change the buoyancy. The rate and extent of lung volume changes will dictate the rate of buoyancy variations. Diaphragm vs chest breathing will change the extent and rate of the changes. Experienced divers tend to Zen out [good term] and breath slower and often with less total lung volume exchanges, neither big inhales or exhales; same reason they will probably get more BT from an equal volume of gas carried. Since the assumption is the divers are "equal" other than in experience; thermal induced changes of respiration will be equal.

Noteworthy is if they are dealing with hypercapnia [normally from shallow/breathing exchanges in lungs] it increases breathing rate but not necessarily depth/extent of volume changes [tachypnea] and can come from regulator performance or activity level or anxiety. Are they over-breathing the reg or working hard or frightened. Concentrating of a task can lead to less gas exchange or even holding your breath.

My views and I could be wrong..............................

 

[inquis]

I know that whenever I busy myself with any task loading in the water I start drifting up. Must be breathing too shallow.

I know what you mean, but I see the term "breathing shallow" typically used for short breaths centered about nearly empty lungs. (Picture a safety stop when underweighted.)

Generally, people talk about the lower, middle, and upper third of the lung capacity for the center/average of the cycle (lower is best). My understanding is that anxiety or task loading shifts the center up, causing you to drift up if you don't adjust the external buoyancy aids (wing or drysuit). It may also shorten the excursions away from the center, but that actually reduces the buoyancy oscillations.